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J Appl Physiol 86: 441-449, 1999;
8750-7587/99 $5.00
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Vol. 86, Issue 2, 441-449, February 1999

Vasomotor responses of soleus feed arteries from sedentary and exercise-trained rats

Jeffrey L. Jasperse and M. Harold Laughlin

Departments of Medical Physiology and Veterinary Biomedical Sciences, University of Missouri, Columbia, Missouri 65211


    ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References

Our goals were to determine the nature of endothelium-dependent and -independent vascular responses in isolated soleus feed arteries (SFA) and to test the hypothesis that these responses would be altered by exercise training. Exercise-trained rats ran 30 m/min, up a 15% grade, 1 h/day, 5 days/wk for 10-12 wk, while sedentary control rats were confined to normal cage activity. SFA were isolated, cannulated, and pressurized at 90 cmH2O. After a 1-h equilibration period, the dose-response relationships to constrictors, endothelium-dependent dilators, and endothelium-independent dilators were examined. SFA developed spontaneous tone, demonstrated myogenic reactivity by maintaining vessel diameter in the face of large changes in intraluminal pressure, and constricted in a dose-dependent manner to norepinephrine and potassium chloride. SFA dilated in a dose-dependent manner to the endothelium-dependent dilators acetylcholine and increased flow and to the endothelium-independent dilator sodium nitroprusside. SFA did not dilate to the putative endothelium-dependent dilators bradykinin, substance P, and clonidine or to adenosine. Dilation to acetylcholine was attenuated markedly by arginine analogs and less by 20 mM KCl, but it was unaltered by indomethacin. These results indicate that SFA respond to a number of vasoactive substances, consistent with the hypothesis that SFA participate in the control of vascular resistance. However, exercise training does not appear to elicit a stimulus adequate to alter vasomotor responses in SFA.

vascular smooth muscle; endothelium; microcirculation; acetylcholine; norepinephrine


    INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References

THE CONTROL OF VASCULAR RESISTANCE in skeletal muscle is dependent on the coordinated regulation of vascular diameter in a number of vascular segments (25). Traditionally, this has been thought to occur primarily in the arterioles that reside within the muscle. However, a number of studies have demonstrated that feed arteries, which lie outside of the muscle just proximal to the arterioles within the muscle, play an important role in control of vascular resistance (7, 12, 25, 33). Feed arteries are active sites of blood flow resistance and dilate during muscle contraction (12, 33). Feed arteries in vivo dilate to application of sodium nitroprusside and demonstrate a reactive dilation on release of upstream arterial occlusion (33).

Exercise training has been reported to increase maximal blood flow capacity in skeletal muscle (17). Furthermore, training alters the control of vascular resistance in skeletal muscle such that regions composed primarily of highly oxidative fibers receive more blood flow during exercise in trained vs. sedentary animals (2). Training-induced adaptations in vascular control mechanisms may contribute to these changes in blood flow. After short-term daily exercise, isolated second-order arterioles from the rat gracilis muscle have been reported to exhibit enhanced responses to the endothelium-dependent agents acetylcholine, L-arginine, and flow (9, 28). Responses to the endothelium-independent agents sodium nitroprusside and sodium nitrate were unaltered as were responses to norepinephrine (28). Long-term exercise training has also been reported to cause alterations in vascular control in the rat spinotrapezius muscle (13-15), but these adaptations are dependent on both the arterial branch order studied and the duration of the training protocol (15). However, the gracilis and spinotrapezius muscles are not primary antigravity muscles, and adaptations to exercise training in these muscles may differ from those in a primary locomotory muscle. The present study, which utilized the soleus muscle, is the first to examine the effect of long-term exercise training on vasomotor responses of the feed arteries of an antigravity muscle used in locomotory activity.

The purposes of the present study were twofold. First, we wanted to examine the nature of vasomotor responses of soleus muscle feed arteries. Second, we wanted to determine whether exercise training induces changes in feed artery vasomotor responses that might contribute to previously observed training-induced alterations in soleus muscle blood flow (2). We examined vasomotor responses in vitro so that responses to various stimuli could be isolated from the broad range of factors present in vivo that may also be altered by exercise training and may confound interpretation of the results. We selected an exercise training program that has been shown to result in increased oxidative capacity (4) and blood flow capacity (18) of the soleus muscle. On the basis of previous studies (3, 9, 14, 15, 28, 31), our hypothesis was that exercise training would attenuate vasoconstrictor responses and increase endothelium-dependent dilation.


    METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References

Animals. Male Sprague-Dawley rats (150-175 g body wt) were obtained (Sasco) and housed two animals per cage in a room with controlled temperature (24°C) and light (12:12-h light-dark cycle) conditions. The rats were given food and water ad libitum. All experimental procedures were approved by the Institutional Animal Care and Use Committee of the University of Missouri.

Exercise training. Experiments were completed in 55 sedentary and 61 exercise-trained rats in 7 groups. In each group, 25 rats were ordered from the breeder. On arrival, rats were brought to the laboratory and familiarized with the motorized treadmill. Rats that were unwilling to run or ran poorly were removed from the study group. The remaining rats were randomly divided into sedentary control and exercise training groups. The training group ran 5 days/wk for 10-12 wk. Rats ran up a 15% grade, and running speed and duration were increased gradually during the first 3 wk until a speed of 30 m/min and a duration of 60 min were reached. Rats continued to run 5 days/wk for the duration of the 10- to 12-wk training period. Sedentary rats were confined to normal cage activity during this period.

Oxidative enzyme capacity. After the soleus feed arteries had been removed, the soleus muscle was dissected free of surrounding muscle and stored at -70°C until it was processed. Citrate synthase activity, a marker of oxidative enzyme capacity, was measured spectrophotometrically by using whole muscle homogenate according to the method of Srere (27).

Preparation of arteries. Rats were anesthetized with pentobarbital sodium (50.0 mg/kg), and the soleus feed arteries were carefully isolated, removed, and transferred to a Lucite vessel chamber containing cold (4°C) MOPS-buffered physiological saline solution [PSS; containing (in mM): 145.0 NaCl, 4.7 KCl, 2.0 CaCl2, 1.17 MgSO4, 1.2 NaH2PO4, 5.0 glucose, 2.0 pyruvate, 0.02 EDTA, and 3.0 MOPS, pH 7.4]. Arteries were cannulated on one end with a glass micropipette filled with PSS-albumin solution (1 g bovine serum albumin/100 ml). The artery was tied securely to the pipette by using 11-0 opthalmic suture. The artery was flushed, and the other end was cannulated and tied. The electrical resistances (LCR Bridge Circuit, model LCR-740, Leader Electronics) of pipettes were 150-250 kOmega . The resistances of pipettes were matched (±0.5% during most experiments but ±2.0% during some nonflow experiments). After cannulation, the vessel bath was transferred to the stage of an inverted microscope (Nikon Diaphot 200; ×20 or ×40 magnification; spatial resolution with either magnification is <1 m) coupled with a video camera (Javelin Electronics, Los Angeles, CA), video monitor (Sony), video micrometer (Microcirculation Research Institute, Texas A&M University, College Station, TX), and Macintosh/MacLab data-acquisition system. Luminal diameter and pressure were continuously monitored throughout the experiment and recorded on the computer. The bath was gradually warmed and maintained at 37°C for the duration of the experiment. Luminal pressure was set at 60 cmH2O initially and raised to 90 cmH2O halfway through the 1-h equilibration period. This pressure was selected as similar to the normal in vivo pressure in rat soleus feed arteries, on the basis of in vivo rat soleus feed artery pressure measurements made by Williams and Segal (32). Micropipettes were connected to independent reservoir systems, and pressure was measured through sidearms connected to low-volume-displacement pressure transducers (Electromedics). Arteries were pressurized by elevating both reservoirs to the same level. The bath solution was replaced every 15 min during the equilibration period.

Experimental design. This study had two purposes: 1) to characterize vasomotor responses of the rat soleus muscle feed artery, and 2) to determine whether exercise training induces changes in the vasomotor responses of soleus feed arteries. The approach used to accomplish these purposes was to compare vasomotor responses in feed arteries isolated from normal sedentary and exercise-trained rats. Because the literature indicates that training may alter vasoconstrictor (3, 31) and vasodilator responses (3, 14, 15), our experimental design was selected to allow comparison of both vasoconstrictor and vasodilator responses between sedentary and exercise-trained rats. In the first phase of experiments, we examined responses to the vasoconstrictor norepinephrine, to the endothelium-dependent dilator acetylcholine, and to the endothelium-independent dilator sodium nitroprusside. In the second phase of experiments, we examined vasomotor responses to additional agents in each category. We examined constrictions to KCl and myogenic responses; responses to the putative endothelium-dependent dilators bradykinin, intraluminal flow, substance P, and clonidine; and responses to the putative endothelium-independent dilator adenosine. Finally, in the third phase of experiments, we characterized the mediators of endothelium-dependent dilations to acetylcholine with the use of the nitric oxide synthase inhibitors NG-nitro-L-arginine methyl ester (L-NAME) and Nomega -nitro-L-arginine (L-NNA), the cyclooxygenase inhibitor indomethacin, and 20 mM KCl to counteract the effects of endothelium-derived hyperpolarizing factor (EDHF) (1) .

When more than one intervention was studied in an artery, the vessel chamber was rinsed numerous times with fresh PSS, and a 15- to 30-min recovery period was allowed before administration of the next agent. Drug order was matched in feed arteries from sedentary and exercise-trained rats. The following guidelines were used to determine the order of interventions during an experiment. Mechanical interventions (flow or myogenic responses) were performed first, followed in order by endothelium-dependent vasodilators, KCl, norepinephrine, adenosine, and sodium nitroprusside. In arteries in which inhibitors of endothelium-dependent dilation were used, the acetylcholine dose response in the presence of inhibitor was the last intervention.

Vasoconstrictor responses. We examined vascular responses to two vasoconstrictor stimuli, KCl and norepinephrine. KCl and norepinephrine were selected as vasoconstrictors to allow comparison of constrictions resulting from activation of voltage-gated calcium channels (KCl) and receptor-mediated mechanisms (norepinephrine). Concentration-response relationships were determined from measurements of changes in diameter produced by replacement of the bath solution with isosmotic solutions of 20, 40, and 80 mM KCl or by cumulative addition of norepinephrine (10-9 to 10-4 M) to the bath.

Myogenic responses. Myogenic responses were examined by measuring diameter responses to changes in intraluminal pressure (0-135 cmH2O). Changes in intraluminal pressure were obtained by raising or lowering both reservoirs (connected to the cannulating pipettes) an equal distance. This elicited a change in intraluminal pressure without causing flow through the artery. Beginning at 90 cmH2O, pressure was raised in 15-cmH2O increments to 135 cmH2O, then decreased in 15-cmH2O increments to 0 cmH2O, and then raised again incrementally back to 90 cmH2O. The myogenic curve was performed twice in each artery: once in normal PSS with calcium present (active curve) and again at the end of the experiment after a 1-h incubation in calcium-free PSS (passive curve).

Vasodilator responses. Vasodilator responses were determined by using the same feed arteries. Sodium nitroprusside and adenosine were selected because they produce vasodilation through direct effects on the vascular smooth muscle by activation of guanylate cyclase and adenylate cyclase, respectively. Acetylcholine and bradykinin were also selected as vasodilators because they mediate vasodilation indirectly, signaling release of endothelium-derived relaxing factors such as nitric oxide, prostacyclin (PGI2), and/or EDHF. In the presence of steady-state vasoconstriction due to spontaneous tone (or induced by previous administration and washout of KCl), vasodilator responses were determined for sodium nitroprusside (10-10 to 10-4 M), adenosine (10-9 to 10-4 M), acetylcholine (10-9 to 10-4 M), and bradykinin (10-12 to 10-6 M). Responses to substance P (5 × 10-8) and clonidine (1.8 × 10-5) were also examined because these agents have been reported to produce endothelium-dependent vasodilation (23). Vasodilator dose-response curves were obtained for each feed artery.

Flow-induced dilation. Flow-induced vasodilation was examined by measuring diameter changes produced by intraluminal flow (i.e. flow-induced vasodilation). Flow was induced by raising one reservoir while lowering the other an equal distance. This has been shown to cause flow through the vessel lumen without changing intraluminal arterial pressure (11). Perfusate flow was measured with a ball flowmeter (Omega Engineering), which was calibrated by using a perfusion pump (model A99, Razel). A subset of the sedentary rat data for flow-induced dilation has been presented in a previous report (8).

Endothelium-derived mediators. In the third phase, experiments were conducted to determine 1) the identity of endothelium-derived mediators in soleus feed arteries and 2) whether exercise training alters the relative importance of each in mediating endothelium-dependent dilation. In these arteries, an acetylcholine dose-response curve was performed under control conditions and then repeated in the presence of one of three inhibitors: 1) either 300 µmol L-NAME or L-NNA to inhibit nitric oxide production by nitric oxide synthase (3, 28), 2) 5 µmol indomethacin to inhibit production of PGI2 by cyclooxygenase (19, 21), or 3) 20 mM isosmotic KCl to counteract the effects of EDHF on vascular smooth muscle cells (1). After feed arteries were incubated in the inhibitor for 15-20 min, the acetylcholine dose-response curve was repeated. Control experiments (n = 8) indicated that successive (at least up to 4) acetylcholine dose-response relationships did not differ from each other.

To confirm that acetylcholine-induced dilation was endothelium dependent, the acetylcholine dose-response was repeated in two arteries after perfusion of the lumen with air. This procedure has been shown to abolish endothelium-dependent responses (29). After the initial acetylcholine dose-response curve, the artery was untied on one end and 5 ml of air were flushed through the arterial lumen. The artery was again filled with PSS-albumin, cannulated, and secured to the pipette. The artery was allowed a 30-min equilibration period for development of spontaneous tone. Then the dose-response relationships to acetylcholine and the dilation to 10-4 M sodium nitroprusside were determined.

Passive diameter. At the end of all experiments, feed arteries were incubated for 1 h at 90 cmH2O in calcium-free PSS containing 2 mM EDTA for determination of passive diameter. The maximal diameter obtained during this incubation was used for data analysis purposes as described below.

Solutions and drugs. Bovine serum albumin (United States Biochemical; >98% pure) was used in the PSS-albumin perfusate. Acetyl CoA used in the citrate synthase assay was purchased from Boehringer Mannheim. NaCl, KCl, and glucose were obtained from Fisher Scientific. All other reagents were obtained from Sigma Chemical.

Statistical analysis. A total of 55 sedentary and 61 exercise-trained rats were used in this study. When more than one feed artery from a rat was used in an experiment, data obtained from feed arteries of the same rat were averaged and counted as one observation. Thus n is equal to the number of rats for each experiment.

Responses to various interventions were measured in actual diameter (µm) and are presented either as actual diameter or as relative to the possible change in diameter to normalize for possible differences between groups in beginning and maximal passive diameters. For constrictors these data are calculated as percent possible constriction = [(DB - D)/DB] × 100 and for dilators as percent possible dilation = [(D - DB)/(DP - DB)] × 100, where D is the measured diameter, DB is the beginning diameter before the intervention was started, and DP is the maximal passive diameter measured after 1 h of incubation in calcium-free PSS. We were concerned that other methods of expressing the data would yield different results from those for the method chosen, but expression and analysis of the data as percentage of control diameter did not change any of the conclusions of the study. Responses between groups were compared by using two-way repeated-measures analysis of variance with one within comparison (dose) and one between comparison (group), followed by Tukey's multiple-comparison post hoc test. Student's unpaired t-tests were used to compare body and muscle weights, citrate synthase activity, number of feed arteries, feed artery passive diameter, percent spontaneous tone, and maximal and EC50 responses.


    RESULTS
Top
Abstract
Introduction
Methods
Results
Discussion
References

The efficacy of the training program is demonstrated in three ways (Table 1): 1) body weights of exercisetrained rats were 10% lower than those of sedentary control rats; 2) the soleus muscle weight-to-rat body weight ratio was higher in exercise-trained rats; and 3) citrate synthase activity, an indicator of muscle oxidative capacity, was 33% higher in the soleus muscles of the exercise-trained rats.

                              
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  		Table 1.   Characteristics of rats, soleus muscles, and soleus feed arteries of sedentary control and exercise-trained rats

Soleus muscle weight did not differ between groups, nor did the number of medial feed arteries per soleus muscle. The passive intraluminal diameter of soleus feed arteries averaged 205-216 µm and was not altered by exercise training. Arteries from sedentary and exercise-trained rats both developed 32% spontaneous tone.

Vasoconstrictor responses. Soleus feed arteries constricted in a dose-dependent manner to increasing concentrations of norepinephrine (Fig. 1). The maximal constriction induced by norepinephrine was 66 ± 2 vs. 73 ± 5% in arteries from sedentary and trained rats, respectively. The response to norepinephrine was not different between soleus feed arteries from sedentary control and exercise-trained rats.


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  		Fig. 1.   Dose-response relationship of soleus feed arteries to norepinephrine. Values are means ± SE. Sed, sedentary (n = 9); ExTr, exercise trained (n = 15). Beginning diameters: Sed, 189 ± 16 µm; ExTr, 138 ± 14 µm. There were no significant differences between groups.

Increasing doses of isosmotic, extracellular KCl caused dose-related constrictions in soleus feed arteries (Fig. 2). Soleus feed arteries from sedentary and exercise-trained rats constricted similarly. Maximal KCl-induced constriction in arteries from sedentary (47 ± 6%) and trained (60 ± 8%) rats did not differ, nor did EC50 values for the response (sedentary = 29.6 ± 4 mM; exercise trained = 27.8 ± 2 mM).


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  		Fig. 2.   Dose-response relationship of soleus feed arteries to KCl. Values are means ± SE. Sed, n = 10; ExTr, n = 7. Beginning diameters: Sed, 202 ± 13 µm; ExTr, 209 ± 8 µm. There were no significant differences between groups.

Myogenic responses. The response of soleus feed arteries to changes in intraluminal pressure is presented in Fig. 3. When arteries had tone (active curves), incremental increases in intraluminal pressure from 90 to 135 cmH2O did not cause any change in the steady-state arterial diameter. Incremental reductions in intraluminal pressure from 135 to 30 cmH2O also did not cause significant changes in the steady-state diameter of feed arteries. A further reduction in pressure to 15 cmH2O caused intraluminal diameter to decrease slightly, and pressure reduction all the way to 0 cmH2O caused intraluminal diameter to decrease markedly (sedentary: total diameter reduction 86 µm; exercise trained: total diameter reduction of 108 µm). Raising pressure again to 15 cmH2O increased intraluminal diameter, and an increase in pressure to 30 cmH2O increased diameter further. Additional incremental increases in pressure back to 90 cmH2O did not elicit further significant changes in intraluminal diameter. There were no significant differences in the active responses to changing intraluminal pressure between feed arteries from sedentary and exercise-trained rats.


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  		Fig. 3.   Responses of soleus feed arteries from sedentary and exercise-trained rats to changes in intraluminal pressure under active and passive conditions. Values are means ± SE. Sed, n = 12; ExTr, exercise trained n = 12. Diameter significantly different from diameter at previous pressure for each curve: * Sed active, P < 0.05; + ExTr active, P < 0.05; # Sed passive, P < 0.05; ddager  ExTr passive, P = 0.054.

The myogenic curve was repeated after a 1-h incubation in calcium-free PSS (Fig 3: passive curves). In the passive state, arteries were at a larger diameter because they no longer had developed tone. Similar to the active curve, incremental increases and decreases in intraluminal pressure did not elicit changes in intraluminal diameter until low intraluminal pressures were reached. Feed artery diameter decreased when pressure was reduced to 30 cmH2O and decreased further as pressure was reduced to 15 and 0 cmH2O. As pressure was incrementally increased, diameter increased significantly again as pressure was raised to 15, 30, and 45 cmH2O. Further increases in pressure did not elicit further significant increases in diameter. Similar to the active curves, no differences between feed arteries from sedentary and exercise-trained rats existed in the passive responses to alterations in intraluminal pressure.

Pressure-diameter data were analyzed to determine whether arterial diameters were similar at a given intraluminal pressure during both increases and decreases in pressure. No hysteresis existed in the diameter response to pressure during either the active or passive curves in arteries from either sedentary or exercise-trained rats.

Endothelium-independent vasodilator responses. Sodium nitroprusside produced a dose-dependent dilation in soleus feed arteries (Fig. 4). The dilatory response did not reach a plateau by a dose of 10-4 M, a concentration usually considered to elicit maximal vasodilation in vessels. There were no differences in the responses to sodium nitroprusside of arteries from sedentary control and exercise-trained rats.


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  		Fig. 4.   Dose-response relationship of soleus feed arteries to sodium nitroprusside. Values are means ± SE. Sed, n = 9; ExTr, n = 17. Beginning diameters: Sed, 130 ± 10 µm; ExTr, 95 ± 8 µm. There were no significant differences between groups.

Soleus feed arteries did not change diameter in response to application of adenosine (10-9 to 10-4 M) (n = 9; data not shown). Furthermore, feed arteries did not dilate to the stable adenosine analog 2-chloro-adenosine (10-9 to 10-4 M; n = 8; data not shown).

Endothelium-dependent vasodilator responses. Soleus feed arteries dilated in a dose-dependent manner to increasing concentrations of acetylcholine (Fig. 5). This response was maximal by a concentration of 10-5 M and a higher concentration did not elicit further dilation. Feed arteries from sedentary control and exercise-trained rats dilated similarly to acetylcholine.


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  		Fig. 5.   Dose-response relationship of soleus feed arteries to acetylcholine. Values are means ± SE. Sed, n = 48; ExTr, n = 54. Beginning diameters: Sed = 119 ± 5 µm; ExTr = 106 ± 4 µm. There were no significant differences between groups.

The responses of soleus feed arteries to other putative endothelium-dependent dilators were also examined. Bradykinin (n = 8) over a concentration range of 10-12 to 10-6 M did not change the diameter of feed arteries. In addition, feed arteries failed to dilate to 1.8 × 10-5 M clonidine (n = 2), an alpha 2-adrenergic agonist, and to 5 × 10-8 M substance P (n = 2). The viability of the endothelium in these cases was confirmed by the fact that feed arteries dilated to acetylcholine after failure to dilate to clonidine or substance P.

Flow-induced dilation. The response of soleus feed arteries to increases in intraluminal flow is shown in Fig. 6. Soleus feed arteries were very sensitive to increasing flow, dilating to the lowest increases in flow. The response to flow was not different between arteries from sedentary control and exercise-trained rats. In arteries from both groups of rats, significant increases in diameter occurred with increases in flow below 16 µl/min. Although arteries tended to dilate slightly with further increases in flow, these dilations were not statistically significant.


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  		Fig. 6.   Dose-response relationship of soleus feed arteries to intraluminal flow. Values are means ± SE. Sed, n = 14; ExTr, n = 9. Beginning diameters: Sed, 131 ± 12 µm; ExTr, 117 ± 14 µm. There were no significant differences between groups.

Endothelium-derived mediators. The acetylcholine dose-response relationship was also studied in the presence of various inhibitors to determine the mediators of endothelium-dependent dilation and whether the relative contribution of different mediators was altered by exercise training. Inhibition of nitric oxide synthase with either L-NAME or L-NNA attenuated dilation of feed arteries to acetylcholine by >50% (Fig. 7). These data indicate that a large proportion of the endothelium-dependent dilation of feed arteries to acetylcholine results from nitric oxide release. The attenuation caused by nitric oxide synthase inhibition did not differ between arteries from sedentary control and exercise-trained rats, indicating that exercise training did not alter the dependence of the response on nitric oxide.


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  		Fig. 7.   Dose-response relationship of soleus feed arteries to acetylcholine before and after nitric oxide synthase inhibition with NG-nitro-L-arginine methyl ester (300 µM) or Nomega -nitro-L-arginine (300 µM). Values are means ± SE. Sed, n = 12; ExTr, n = 10. Beginning diameters: Sed, 122 ± 12 µm; ExTr, 114 ± 10 µm; Sed + arginine analog, 113 ± 5 µm; ExTr + arginine analog, 116 ± 12 µm. There were no significant differences between groups before or after NOS inhibition.

Inhibition of prostaglandin production by indomethacin did not alter the dose-response relationship of feed arteries to acetylcholine in either the sedentary control or exercise trained group (Fig. 8). These data suggest that acetylcholine-induced dilation of soleus feed arteries is not dependent on prostaglandin production.


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  		Fig. 8.   Dose-response relationship of soleus feed arteries to acetylcholine before and after cyclooxygenase inhibition with 50 µM indomethacin (Indo). Values are means ± SE. Beginning diameters: Sed, 116 ± 11 µm; ExTr, 87 ± 8 µm; Sed w/Indo, 131 ± 14 µm; ExTr w/Indo, 101 ± 11 µm. There was no significant effect of cyclooxygenase inhibition, and there were no significant differences between groups before or after cyclooxygenase inhibition.

A third possible mediator released by the endothelium is the EDHF(s), the identity of which is not yet known. The effects of EDHF on vascular diameters can be examined by inhibiting its effect with 20 mM KCl (1). The presence of 20 mM KCl attenuated acetylcholine-induced dilation in feed arteries, indicating that part of the response to acetylcholine is dependent on EDHF (Fig. 9). This attenuation was not different between arteries from sedentary control and exercise-trained rats. Thus exercise training did not alter the dependence of the response on EDHF.


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  		Fig. 9.   Dose-response relationship of soleus feed arteries to acetylcholine before and after incubation in 20 mM KCl. Values are means ± SE. Beginning diameters: Sed, 116 ± 14 µm; ExTr, 101 ± 16 µm; Sed w/ KCl, 129 ± 13 µm; ExTr w/ KCl, 113 ± 17 µm. KCl significantly attenuated responses to acetylcholine in both Sed and ExTr groups. There were no significant differences between groups before or after KCl.

To confirm that acetylcholine-induced dilation was endothelium dependent, the acetylcholine dose response was repeated in two arteries after 5 ml of air were passed through the lumen. After perfusion with air, acetylcholine (10-9 to 10-4 M) did not elicit any change in diameter, but both arteries dilated markedly to the endothelium-independent dilator sodium nitroprusside (10-4 M). These responses confirm the endothelial dependence of acetylcholine dilator responses.


    DISCUSSION
Top
Abstract
Introduction
Methods
Results
Discussion
References

The results of this study provide new information about the vasomotor responsiveness of soleus feed arteries of the rat. First, the results demonstrate that soleus feed arteries respond in a dose-dependent fashion to a number of vasoactive stimuli. These results are consistent with the notion that feed arteries are active sites in the control of vascular resistance to the soleus muscle. Second, there are some vasoactive stimuli to which other vascular beds respond but to which soleus feed arteries do not respond (adenosine, bradykinin, clonidine, substance P). Third, contrary to our hypothesis, the results indicate that exercise training does not alter the vasomotor responsiveness of soleus feed arteries.

Vasomotor responses of soleus feed arteries. The vasoconstrictor responses of soleus feed arteries were examined by using norepinephrine, KCl, and changes in intraluminal pressure (myogenic reactivity). Respectively, this provided examination of receptor-mediated responses, receptor-independent responses, and responses to a mechanical stimulus. Soleus feed arteries constricted markedly to norepinephrine, with 10-4 M norepinephrine causing diameter to decrease to 30% of the beginning diameter (Fig. 1). Constrictions to KCl were also strong, although they were less vigorous than norepinephrine-induced constrictions (Fig. 2). Thus soleus feed arteries respond to both receptor-mediated and receptor-independent vasoconstrictor stimuli.

Feed arteries also exhibited myogenic reactivity in that steady-state diameter was maintained constant over a large range of intraluminal pressures (Fig. 3). In the presence of extracellular calcium, steady-state diameter did not change as intraluminal pressure was incrementally increased to 135 cmH2O or as it was decreased to 30 cmH2O (Fig. 3: active curves). Only at <= 15 cmH2O did diameter decrease from basal levels. Thus these arteries display a remarkable ability to maintain diameter as pressure changes. In the absence of calcium, feed arteries exhibited a typical passive relationship between pressure and diameter over the pressure range of 0-45 cmH2O. At higher pressures, the steady-state diameters were considerably larger in calcium-free PSS than with calcium present, but diameter did not change over a large range of pressures (Fig. 3: passive curves). In one feed artery, pressure was increased all the way to 200 mmHg (equivalent to ~275 cmH2O) during the calcium-free curve and diameter still did not change (data not presented). These passive responses to changing pressure differ from those reported previously in porcine coronary arterioles (5, 20, 24), in which diameter continued to increase gradually as pressure was raised under passive conditions. Rat cremaster (31) and gracilis arterioles (30) appeared to reach a plateau in diameter with increases in pressure, but the plateau did not occur until intraluminal pressures of 60 to 100 mmHg or more were reached (equivalent to ~82-122 cmH2O) (29). These data suggest that there is a structural constraint present in soleus feed arteries that enables maintenance of diameter during changing pressures above 45 cmH2O. These constraints may be beneficial in vivo because, while increases in diameter in arterioles are limited by the muscle tissue in which they reside, feed arteries lie external to the muscle and do not have these external constraints limiting passive stretch of the vessel.

Endothelium-independent vasodilator responses were examined by using the nitro vasodilator sodium nitroprusside and the metabolic vasodilator adenosine. Soleus feed arteries demonstrated dose-related dilation to sodium nitroprusside (Fig. 4) but did not dilate to adenosine. Furthermore, feed arteries also failed to dilate to the nonhydrolyzable adenosine analog 2-chloro-adenosine. These data indicate that the lack of response of feed arteries to adenosine does not result from an increased adenosine hydrolysis in feed arteries. Skeletal muscle arterial vessels generally respond to adenosine. For example, adenosine has been reported to dilate rat cremaster (10) and gracilis muscle arterioles (28). The absence of a response of soleus feed arteries to adenosine may result from a difference in either arterial branch order or in the skeletal muscle studied. Teleologically, it is possible that it is unnecessary for feed arteries to respond to adenosine because this metabolite is produced in skeletal muscle, and vessels (such as feed arteries) that lie proximal to skeletal muscle would be exposed to little adenosine under normal circumstances in vivo.

Endothelium-dependent dilation was studied by using the receptor-dependent agent acetylcholine and the mechanical stimulus, changes in flow. Additional putative receptor-dependent dilators were examined (bradykinin, substance P, and clonidine), but these agents failed to have any effect on feed artery diameter. Feed arteries were very sensitive to changes in intraluminal flow, with a large dilation occurring at very low levels of flow and very little further dilation occurring at higher levels of flow (Fig. 6). Acetylcholine caused dose-dependent dilation of soleus feed arteries (Fig. 5), similar to that found in arterioles of the gracilis (28) and feed arteries and arterioles of the spinotrapezius (15). This dilation was unaffected by indomethacin (Fig. 8) but was slightly inhibited by 20 mM KCl (Fig. 9) and inhibited to a greater degree by arginine analogs (Fig. 7). Collectively, these data suggest that prostaglandins are not involved in endothelium-mediated dilations to acetylcholine, but that this dilation is mediated largely by nitric oxide with a lesser contribution from an EDHF.

A number of studies over recent years have demonstrated that a significant proportion of vascular resistance resides in the feed arteries and that dilation of feed arteries is necessary to achieve the magnitude of hyperemia measured during exercise in skeletal muscle (7, 26). Furthermore, feed arteries of the soleus (33) and spinotrapezius (12) muscles have been shown to dilate during muscle contraction in anesthetized rats. The data presented in this study demonstrate that feed arteries respond to a number of vasoactive stimuli and are consistent with the notion that feed arteries are important sites in the control of blood flow to skeletal muscle (7, 12, 26, 33). The location of these arteries just proximal to the circulation within the muscle puts feed arteries in a strategic location for controlling blood flow to the whole muscle. Thus the responsiveness of feed arteries to a number of vasoactive stimuli has important implications for the control of blood flow to skeletal muscle.

Exercise training. The second aim of this study was to determine whether soleus feed artery responsiveness is altered by exercise training. Because of the role of feed arteries in controlling blood flow to skeletal muscle, we hypothesized that altered vasomotor responsiveness of these arteries may contribute to previously reported training-induced alterations in muscle blood flow (2). Previous studies examining the effect of training on the vasomotor responsiveness of skeletal muscle resistance vessels have reported altered responses to norepinephrine in rat cremaster feed arteries (31) and to muscle stimulation, acetylcholine, and sodium nitroprusside, norepinephrine, and epinephrine in rat spinotrapezius arterioles and feed arteries (13-15). Additionally, short-term daily exercise has been reported to enhance responses of rat gracilis arterioles to acetylcholine (28) and intraluminal flow (9) These previous studies of the effects of exercise training have examined muscles that would not be expected to directly contribute to locomotory activity. Blood flow does not increase during exercise in either the spinotrapezius (22) or the gracilis muscle (2, 16). The gracilis muscle does not develop training-induced changes in blood flow either at rest or during exercise (2), and the spinotrapezius muscle does not demonstrate increases in oxidative enzyme capacity with exercise training (13-15). Thus previous studies have not examined muscles that are important in accomplishing the task of locomotion. In addition, the studies examining gracilis arterioles used exercise programs of very short duration and low intensity. A major strength of the present study is that we examined arteries from the soleus muscle, an ankle extensor muscle that has increased blood flow during exercise (16) and that has training-induced alterations in blood flow (2). In addition, we selected an exercise training program that effectively increased citrate synthase activity in the soleus muscle (Table 1). Thus the soleus muscle shows training adaptations in both the control of blood flow and in oxidative enzyme capacity.

In the present study, soleus feed arteries from sedentary and exercise-trained rats exhibited similar responses to vasoconstrictor agents and exhibited similar myogenic responses. In addition, soleus feed arteries from sedentary and exercise-trained rats exhibited similar responses to endothelium-independent and endothelium-dependent vasodilators. Finally, our results indicate that exercise training did not alter the relative importance of the known endothelium-derived dilator substances. Thus training did not alter vasomotor properties of soleus feed arteries. These results were surprising in light of past studies that have demonstrated training-induced alterations in vasomotor responses in nonlocomotory muscles.

Compared with previously published results, the present data bring up an interesting paradox. Arteries from the soleus, which has increased contractile activity and blood flow during exercise, do not have training-induced alterations, whereas arteries and arterioles from spinotrapezius and gracilis, which do not have increased activity or blood flow during exercise, do exhibit alterations in vascular control (9, 13-15, 28). The cause of this paradox and the purpose of these presently inexplicable changes in nonlocomotory muscle are unknown.

Despite the absence of altered vasomotor responsiveness after training in soleus feed arteries, soleus muscle blood flow is altered by training. Two potential mechanisms may be responsible for these changes in blood flow. First, although vascular control mechanisms in feed arteries of the soleus are not altered, vasomotor responsiveness of arterioles within the muscle may be altered and may contribute to alterations in soleus blood flow. This hypothesis is untested. Second, training may elicit structural changes in the soleus vasculature that alter soleus blood flow. Capillary density in the soleus (6) and the number and passive diameters of soleus feed arteries (Table 1) are not altered by endurance exercise training. Therefore, if structural adaptations are responsible for training-induced alterations in soleus blood flow, the adaptations must occur in the arteriolar network. The possibility that soleus arteriolar density is altered by training has not yet been examined.

In summary, we found that soleus feed arteries in vitro responded to a number of vasoconstrictor, endothelium-independent vasodilator, and endothelium-dependent vasodilator stimuli. These responses are consistent with the idea that feed arteries are important sites in the regulation of skeletal muscle vascular resistance. In addition, contrary to our hypothesis, soleus feed arteries did not exhibit exercise training-induced alterations in vascular control mechanisms.


    ACKNOWLEDGEMENTS

The authors gratefully acknowledge Pamela Thorne and Tammy Strawn for technical contributions to this study.


    FOOTNOTES

This research was supported by National Heart, Lung, and Blood Institute Grants HL-36088 and HL-52490.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests: M. H. Laughlin, E102, Veterinary Biomedical Sciences, Univ. of Missouri, Columbia, MO 65211 (E-mail: LaughlinM{at}missouri.edu).

Received 4 August 1998; accepted in final form 15 October 1998.


    REFERENCES
Top
Abstract
Introduction
Methods
Results
Discussion
References

1.   Adeagbo, A. S. O., and C. R. Triggle. Varying extracellular [K-]: a functional approach to separating EDHF- and EDNO-related mechanisms in perfused rat mesenteric arterial bed. J. Cardiovasc. Pharmacol. 21: 423-429, 1993[Medline].

2.   Armstrong, R. B., and M. H. Laughlin. Exercise blood flow patterns within and among rat muscles after training. Am. J. Physiol. 246 (Heart Circ. Physiol. 15): H59-H68, 1984[Abstract/Free Full Text].

3.   Delp, M. D., R. M. McAllister, and M. H. Laughlin. Exercise training alters endothelium-dependent vasoreactivity of rat abdominal aorta. J. Appl. Physiol. 75: 1354-1363, 1993[Abstract/Free Full Text].

4.   Dudley, G. A., W. M. Abraham, and R. L. Terjung. Influence of exercise intensity and duration on biochemical adaptations in skeletal muscle. J. Appl. Physiol. 53: 844-850, 1982[Abstract/Free Full Text].

5.   Giezeman, M. J. M. M., E. VanBavel, C. A. Grimbergen, and J. A. E. Spaan. Compliance of isolated porcine coronary small arteries and coronary pressure-flow relations. Am. J. Physiol. 267 (Heart Circ. Physiol. 36): H1190-H1198, 1994[Abstract/Free Full Text].

6.   Gute, D. C., M. H. Laughlin, and J. F. Amann. Regional changes in capillary supply in skeletal muscle of interval-sprint and low-intensity, endurance training. Microcirculation 1: 183-193, 1994[Medline].

7.   Hester, R. L., and B. R. Duling. Red cell velocity during functional hyperemia: implications for rheology and oxygen transport. Am. J. Physiol. 255 (Heart Circ. Physiol. 24): H236-H244, 1988[Abstract/Free Full Text].

8.   Jasperse, J. L., and M. H. Laughlin. Flow-induced dilation of rat soleus feed arteries. Am. J. Physiol. 273 (Heart Circ. Physiol. 42): H2423-H2427, 1997[Abstract/Free Full Text].

9.   Koller, A., A. Huang, D. Sun, and G. Kaley. Exercise training augments flow-dependent dilation in rat skeletal muscle arterioles: role of endothelial nitric oxide and prostaglandins. Circ. Res. 76: 544-550, 1995[Abstract/Free Full Text].

10.   Koller, A., E. J. Messina, M. S. Wolin, and G. Kaley. Endothelial impairment inhibits prostaglandin and EDRF-mediated arteriolar dilation in vivo. Am. J. Physiol. 257 (Heart Circ. Physiol. 26): H1966-H1970, 1989[Abstract/Free Full Text].

11.   Kuo, L., M. J. Davis, and W. M. Chilian. Endothelium-dependent, flow-induced dilation of isolated coronary arterioles. Am. J. Physiol. 259 (Heart Circ. Physiol. 28): H1063-H1070, 1990[Abstract/Free Full Text].

12.   Lash, J. M. Contribution of arterial feed vessels to skeletal muscle functional hyperemia. J. Appl. Physiol. 76: 1512-1519, 1994[Abstract/Free Full Text].

13.   Lash, J. M. Exercise training enhances adrenergic constriction and dilation in the rat spinotrapezius muscle. J. Appl. Physiol. 85: 168-174, 1998[Abstract/Free Full Text].

14.   Lash, J. M., and H. G. Bohlen. Functional adaptations of rat skeletal muscle arterioles to aerobic exercise training. J. Appl. Physiol. 72: 2052-2062, 1992[Abstract/Free Full Text].

15.   Lash, J. M., and H. G. Bohlen. Time- and order-dependent changes in functional and NO-mediated dilation during exercise training. J. Appl. Physiol. 82: 460-468, 1997[Abstract/Free Full Text].

16.   Laughlin, M. H., and R. B. Armstrong. Rat muscle blood flows as a function of time during prolonged slow treadmill exercise. Am. J. Physiol. 244 (Heart Circ. Physiol. 13): H814-H824, 1983[Abstract/Free Full Text].

17.   Laughlin, M. H., R. J. Korthuis, D. J. Duncker, and R. J. Bache. Control of blood flow to cardiac and skeletal muscle during exercise. In: Handbook of Physiology. Exercise: Regulation and Integration of Multiple Systems. Bethesda, MD: Am. Physiol. Soc., 1996, sect. 12, chapt. 15, p. 705-769.

18.   Laughlin, M. H., and J. Ripperger. Vascular transport capacity of hindlimb muscles of exercise-trained rats. J. Appl. Physiol. 62: 438-443, 1987[Abstract/Free Full Text].

19.   McAllister, R. M., J. K. Kimani, J. L. Webster, J. L. Parker, and M. H. Laughlin. Effects of exercise training on responses of peripheral and visceral arteries in swine. J. Appl. Physiol. 80: 216-225, 1996[Abstract/Free Full Text].

20.   Muller, J. M., P. R. Myers, and M. H. Laughlin. Exercise training alters myogenic responses in porcine coronary resistance arteries. J. Appl. Physiol. 75: 2677-2682, 1993[Abstract/Free Full Text].

21.   Muller, J. M., P. R. Myers, and M. H. Laughlin. Vasodilator responses of coronary resistance vessels of exercise-trained pigs. Circulation 89: 2308-2314, 1994[Medline].

22.   Musch, T. I., and D. C. Poole. Blood flow response to treadmill running in the rat spinotrapezius muscle. Am. J. Physiol. 271 (Heart Circ. Physiol. 40): H2730-H2734, 1996[Abstract/Free Full Text].

23.   Oltman, C. L., J. L. Parker, and M. H. Laughlin. Endothelium-dependent vasodilation of proximal coronary arteries from exercise-trained pigs. J. Appl. Physiol. 79: 33-40, 1995[Abstract/Free Full Text].

24.   Rajagopalan, S., S. Dube, and J. M. Canty, Jr. Regulation of coronary diameter by myogenic mechanisms in arterial microvessels greater than 100 µm in diameter. Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H788-H793, 1995[Abstract/Free Full Text].

25.   Segal, S. S. Communication among endothelial and smooth muscle cells coordinates blood flow control during exercise. News Physiol. Sci. 7: 152-156, 1992.[Abstract/Free Full Text]

26.   Segal, S. S., and B. R. Duling. Communication between feed arteries and microvessels in hamster striated muscle: segmental vascular responses are functionally coordinated. Circ. Res. 59: 283-290, 1986[Abstract].

27.   Srere, P. A. Citrate synthase. Methods Enzymol. 13: 3-5, 1969.

28.   Sun, D., A. Huang, A. Koller, and G. Kaley. Short-term daily exercise activity enhances endothelial NO synthesis in skeletal muscle arterioles of rats. J. Appl. Physiol. 76: 2241-2247, 1994[Abstract/Free Full Text].

29.   Sun, D., G. Kaley, and A. Koller. Role of endothelium in function of isolated arterioles of rat mesentery and gracilis muscle. Endothelium 1: 115-122, 1993.

30.   Sun, D., G. Kaley, and A. Koller. Characteristics and origin of myogenic response in isolated gracilis muscle arterioles. Am. J. Physiol. 266 (Heart Circ. Physiol. 35): H1177-H1183, 1994[Abstract/Free Full Text].

31.   Wiegman, D. L., P. D. Harris, I. G. Joshua, and F. N. Miller. Decreased vascular sensitivity to norepinephrine following exercise training. J. Appl. Physiol. 51: 282-287, 1981[Abstract/Free Full Text].

32.   Williams, D. A., and S. S. Segal. Microvascular architecture in rat soleus and extensor digitorum longus muscles. Microvasc. Res. 43: 192-204, 1992[Medline].

33.   Williams, D. A., and S. S. Segal. Feed artery role in blood flow control to rat hindlimb skeletal muscles. J. Physiol. (Colch.) 463: 631-646, 1993.

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