Journal of Applied Physiology Watch the video to see how APS reaches out to developing nations.
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

J Appl Physiol 97: 431-438, 2004. First published February 20, 2004; doi:10.1152/japplphysiol.01122.2003
8750-7587/04 $5.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF) Free
Right arrow All Versions of this Article:
97/1/431    most recent
01122.2003v1
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Setty, S.
Right arrow Articles by Tune, J. D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Setty, S.
Right arrow Articles by Tune, J. D.

HIGHLIGHTED TOPICS
Skeletal and Cardiac Muscle Blood Flow

{alpha}-Adrenoceptor-mediated coronary vasoconstriction is augmented during exercise in experimental diabetes mellitus

Srinath Setty, Wei Sun, Rodolfo Martinez, H. Fred Downey, and Johnathan D. Tune

Department of Integrative Physiology, University of North Texas Health Science Center, Fort Worth, Texas 76107-2699

Submitted 16 October 2003 ; accepted in final form 18 February 2004


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
This study tested whether {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented during exercise in diabetes mellitus. Experiments were conducted in dogs instrumented with catheters in the aorta and coronary sinus and with a flow transducer around the circumflex coronary artery. Diabetes was induced with alloxan monohydrate (n = 8, 40 mg/kg iv). Arterial plasma glucose concentration increased from 4.7 ± 0.2 mM in nondiabetic, control dogs (n = 8) to 21.4 ± 1.9 mM 1 wk after alloxan injection. Coronary blood flow, myocardial oxygen consumption (MO2), aortic pressure, and heart rate were measured at rest and during graded treadmill exercise before and after infusion of the {alpha}-adrenoceptor antagonist phentolamine (1 mg/kg iv). In untreated diabetic dogs, exercise increased MO2 2.7-fold, coronary blood flow 2.2-fold, and heart rate 2.3-fold. Coronary venous PO2 fell as MO2 increased during exercise. After {alpha}-adrenoceptor blockade, exercise increased MO2 3.1-fold, coronary blood flow 2.7-fold, and heart rate 2.1-fold. Relative to untreated diabetic dogs, {alpha}-adrenoceptor blockade significantly decreased the slope of the relationship between coronary venous PO2 and MO2. The difference between the untreated and phentolamine-treated slopes was greater in the diabetic dogs than in the nondiabetic dogs. In addition, the decrease in coronary blood flow to intracoronary norepinephrine infusion was significantly augmented in anesthetized, open-chest, {beta}-adrenoceptor-blocked diabetic dogs compared with the nondiabetic dogs. These findings demonstrate that {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented in alloxan-induced diabetic dogs during physiological increases in MO2.

coronary blood flow; myocardial oxygen consumption

EARLIER STUDIES HAVE SHOWN that Type 1 diabetes mellitus increases myocardial {alpha}-adrenoceptor sensitivity but decreases the overall number of {alpha}-adrenoceptor binding sites (27, 32, 48). These changes correspond with enhancement of {alpha}-adrenoceptor-mediated increases in cardiac function, i.e., maximal change in pressure over time and developed tension, after administration of a selective {alpha}-agonist to acute diabetic animals (2, 14, 24). Diabetes may also alter {alpha}-adrenoceptor-mediated coronary vasoconstriction because stimulation of the cardiac plexus induces vasoconstriction in alloxan-induced diabetic dogs but vasodilation in normal, control dogs (31). In addition, studies from isolated coronary microvessels found that arterioles from diabetic rats constrict to norepinephrine superfusion, whereas arterioles from nondiabetic rats dilate (40). These findings suggest that diabetes may significantly augment {alpha}-adrenoceptor-mediated coronary vasoconstriction; however, they do not address whether this alteration contributes to the physiological impairment of the balance between coronary blood flow and myocardial metabolism induced by diabetes (47). Characterizing the mechanisms of diabetic coronary dysfunction is important because it could help explain the increased incidence of myocardial ischemia, infarction, and sudden cardiac death among patients with diabetes (18, 22).

The present investigation was designed to test the hypothesis that {alpha}-adrenoceptor-mediated coronary vasoconstriction is elevated in diabetic animals during increases in myocardial oxygen demand. Experiments were conducted in chronically instrumented dogs with and without {alpha}-adrenoceptor blockade with phentolamine (1 mg/kg iv). These experiments were performed in both nondiabetic and diabetic dogs at rest and during graded treadmill exercise. Exercise was used because it is an excellent physiological stimulus that dramatically increases cardiac work and myocardial metabolism. Diabetes was induced with alloxan monohydrate (40 mg/kg iv) (46, 47). Additional experiments were also conducted in anesthetized, open-chest, {beta}-adrenoceptor-blocked (propranolol 2 mg/kg iv) dogs to determine whether norepinephrine-mediated coronary vasoconstriction is augmented in alloxan-induced diabetic dogs.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Studies Conducted in Chronically Instrumented Dogs

Surgical preparation.   This investigation was approved by the Institutional Animal Care and Use Committee and was conducted in accordance with the Guide for the Care and Use of Laboratory Animals (NIH publication no. 85-23, revised 1996). Experiments were performed on 13 adult mongrel dogs of either sex (weighing 16–37 kg) taught to run on a motorized treadmill. Eight of these dogs served in two or more of the experimental groups. Preanesthesia (acepromazine 0.03 mg/kg im) was administered 30 min before induction of anesthesia with thiopental sodium (5 mg/kg iv). After endotracheal intubation, a surgical plane of anesthesia was maintained by mechanical ventilation with 1–3% isoflurane gas plus supplemental oxygen. With the use of sterile technique, a left lateral thoractomy was performed in the fifth intercostal space. A custom-made, coextruded polyurethane catheter (Putnam Plastics) was implanted in the descending thoracic aorta to measure aortic blood pressure and to obtain arterial blood samples (44–46). A second polyurethane catheter was placed in the coronary sinus via a purse-string suture in the right atrial appendage for coronary venous blood sampling. The circumflex coronary artery was dissected free, and a flow transducer (see Pressure and flow measurements) was placed around the artery. A chest tube was placed to evacuate the pneumothorax, and the chest was closed in layers. The catheters and the flow transducer wire were tunneled subcutaneously and exteriorized between the scapulae. The incision was infiltrated with 2.5% bupivacaine, and Buprenex (0.3 mg im) was administered to minimize postoperative pain. Clavamox (6.25 mg/lb) and aspirin (81 mg) were administered twice daily for 10 days after surgery. Nylon jackets (Alice King Chatham) were placed on the animals to protect the catheters and the flow transducer wire. A elastomeric balloon pump (Access Technologies) was connected to the coronary sinus catheter so heparinized saline (5 U/ml) could be continuously infused at 5 ml/h. The aortic catheter was flushed daily and filled with heparinized saline (1,000 U/ml). The animals were allowed at least 7 days for recovery before the experiments were conducted.

Pressure and flow measurements.   A coextruded polyurethane catheter was implanted in the aorta, so that a high-fidelity Mikro-tip catheter-tipped pressure transducer (3F, Millar Instruments) could be inserted through it at the time of the experiment to measure aortic blood pressure (44–46). This pressure transducer was introduced into the polyurethane aortic catheter through a Tuohy-Borst hemostatic control valve (Mallinckrodt Medical), which allowed arterial blood samples to be withdrawn while maintaining a fluid tight seal around the Millar catheter.

Phasic and mean coronary blood flows were continuously measured throughout the experimental protocol (see Experimental protocols) with an ultrasonic, perivascular flow transducer (Transonic Systems). After experiments were completed, the animals were euthanized with pentobarbital sodium, and the circumflex perfusion territory dyed with Evans blue dye. The weight of the dyed tissue was used to calculate coronary blood flow per gram of perfused myocardium.

Blood sampling.   Arterial and coronary venous blood samples were collected simultaneously in heparinized syringes that were immediately sealed and placed on ice. The samples were analyzed in duplicate for pH, PCO2, PO2, hematocrit, and oxygen content with an Instrumentation Laboratories automatic blood-gas analyzer (Synthesis 30) and CO-oximeter (682) system. Plasma glucose and lactate concentrations were measured with a Radiometer model EML105 analyzer. Myocardial oxygen consumption (MO2; µl O2·min–1·g–1) was calculated by multiplying coronary blood flow per gram of perfused tissue by the coronary arterial-venous difference in oxygen content. Lactate uptake (µmol·min–1·g–1) was calculated by multiplying coronary plasma flow per gram of perfused tissue by the coronary arterial-venous difference in plasma lactate concentration.

Experimental protocols.   The hypothesis that {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented in diabetic subjects was tested in nondiabetic, control, and alloxan-induced diabetic dogs with and without {alpha}-adrenoceptor blockade with phentolamine (1 mg/kg iv). Coronary blood flow, aortic pressure, and heart rate were continuously measured while the dogs were resting in a sling and then during three levels of treadmill exercise: 1) 2 mph, 0% grade; 2) 3 mph, 5% grade; and 3) 4 mph, 10% grade. The animals were exercised at similar levels (i.e., speed and percent grade) before and after alloxan and/or phentolamine treatment. Arterial and coronary venous blood samples were collected when hemodynamic variables were stable at each exercise level. Each exercise period was ~2 min in duration, and the animals were allowed to rest sufficiently between each level for hemodynamic variables to return to baseline.

After the nondiabetic experiments, alloxan monohydrate (40 mg/kg) was administered intravenously over 1 min to induce diabetes mellitus, as previously described (46, 47, 49). Alloxan was prepared as a 5% solution in citrate buffer (pH 4.0–4.5). The dogs were fasted at least 12 h before the injection of alloxan and were then fed immediately. Dogs were considered diabetic if the plasma glucose concentration was >11 mM after a 12-h fast. The experimental protocol described above was repeated (control and/or phentolamine) in the diabetic animals at least 1 wk (range 7–17 days) after the alloxan injection. Earlier studies from our laboratory have demonstrated that this dose of alloxan does not induce significant changes in renal function (46, 47).

Studies Conducted in Anesthetized Dogs

Additional experiments were also performed to determine whether norepinephrine-mediated coronary vasoconstriction is augmented in diabetic animals. Nondiabetic control (n = 5) and alloxan-induced diabetic dogs (n = 5) were sedated with morphine (3 mg/kg sc) and anesthetized with {alpha}-chloralose (100 mg/kg iv). The animals were then intubated and ventilated (Harvard respirator) with room air and supplemental oxygen. Catheters were placed in the right femoral artery to withdraw blood for the pressure controlled extracorporeal coronary arterial perfusion system and right femoral vein for intravenous administration of supplemental anesthetic, sodium bicarbonate, and propranolol. A catheter was also placed in the left femoral artery to measure arterial blood pressure.

A left lateral thoractomy was performed in the fifth intercostal space, and the heart was suspended in a pericardial cradle. The left anterior descending coronary artery (LAD) was isolated distal to its first major branch. Propranolol (2 mg/kg iv) was then infused to inhibit norepinephrine-mediated activation of {beta}-adrenoceptors. After heparinization (500 U/kg iv), the LAD was cannulated with a stainless steel cannula (3.0 mm OD, 2.2 mm ID) connected to the extracorporeal perfusion system. Coronary perfusion pressure was measured through a saline-filled catheter advanced to the orifice of the LAD cannula. The coronary perfusion pressure was maintained at 100 mmHg throughout the entire protocol. After 15 min recovery, norepinephrine was infused into the LAD perfusion line (0.05–0.75 µg·kg–1·min–1). Each norepinephrine infusion rate was maintained for ~3 min, and data were recorded when coronary blood flow was stable at each level.

Statistical Analyses

Data are presented as means ± SE. Hemodynamic variables were recorded with Powerlab 3.1.4 data-acquisition software (ADInstruments). A three-way ANOVA was used to compare the effects of diabetes, {alpha}-adrenoceptor blockade, and exercise on coronary and systemic hemodynamics at rest and during exercise in the diabetic dogs. A two-way ANOVA was used to test for differences in norepinephrine-mediated coronary vasoconstriction between the normal control and diabetic dogs. When significance was found with ANOVA (P < 0.05), a Student-Newman-Keuls multiple-comparison test was performed. Linear regression analysis was used to compare the slope of the relationship between coronary venous PO2 vs. MO2. Regression analyses were performed on mean values. If the slopes of the regression lines were not significantly different (P > 0.05), an analysis of covariance was employed to test for differences in elevation (intercept).


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Hemodynamic, blood gas, and metabolic variables at rest and during exercise are given in Table 1. Body weight was reduced from 26.3 ± 1.7 to 24.5 ± 1.6 kg after alloxan administration. In nondiabetic control dogs, exercise increased coronary conductance 2.1-fold, coronary blood flow 2.5-fold, MO2 3.1-fold, and heart rate 2.5-fold (Table 1). In diabetic dogs, exercise increased coronary conductance 1.8-fold, coronary blood flow 2.2-fold, MO2 2.7-fold, and heart rate 2.3-fold. Under baseline, resting conditions, {alpha}-adrenoceptor blockade with phentolamine did not significantly increase coronary blood flow or conductance in the nondiabetic or diabetic animals (Table 1). Although these variables tended to be higher after {alpha}-blockade, this finding indicates that augmented {alpha}-vasoconstriction is not the mechanism by which diabetes impairs the balance between coronary blood flow and myocardial metabolism under baseline conditions.


View this table:
[in this window]
[in a new window]
  		Table 1. Hemodynamic variables at rest and during graded treadmill exercise

 
A sensitive index of the relationship between coronary blood flow and myocardial metabolism is shown in the plot of coronary venous PO2 vs. MO2 (Fig. 1). MO2 is computed from the product of coronary blood flow and the arterial-coronary venous difference in oxygen content. Thus the illustrated variables are not completely independent. However, such a plot is crucial to coronary flow studies because MO2 is the primary determinant of coronary blood flow. Diabetes lowered coronary venous PO2 at a given MO2 (parallel shift, P = 0.02), indicating that diabetes impairs the balance between coronary blood flow and myocardial metabolism equally at rest and during exercise. This finding is consistent with earlier reports from our laboratory (47).



View larger version (18K):
[in this window]
[in a new window]
  		Fig. 1. Relationship between coronary venous PO2 and myocardial oxygen consumption at rest and during graded treadmill exercise in control (nondiabetic) and alloxan-induced diabetic dogs. Diabetes significantly reduced coronary venous PO2 at a given level of myocardial oxygen consumption (parallel shift downward), indicating that diabetes impairs the balance between coronary blood flow and myocardial metabolism equally at rest and during exercise.

 
The percent increase in coronary blood flow (Fig. 2A) and coronary conductance (Fig. 2B) at rest and during exercise in nondiabetic and diabetic {alpha}-adrenoceptor blocked dogs is shown in Fig. 2. The increase in both coronary blood flow and conductance to exercise was greater in the diabetic dogs relative to nondiabetic dogs, indicating that diabetes increases {alpha}-adrenoceptor-mediated coronary vasoconstriction during increases in MO2. Figure 3 shows the relationship between coronary venous PO2 vs. MO2 in nondiabetic control dogs (A) and diabetic dogs (B) both before and after {alpha}-adrenoceptor blockade with phentolamine. Phentolamine decreased the slope of this relationship, i.e., made the slope less negative, in both experimental groups, indicating that {alpha}-adrenoceptor-mediated vasoconstriction limits coronary dilation during exercise in both nondiabetic and diabetic dogs. The difference between the untreated slope and the phentolamine-treated slope was greater in the diabetic dogs (control = –0.02 Torr/µl·min–1·g–1 vs. phentolamine = 0.0009 Torr/µl·min–1·g–1) than in the nondiabetic control dogs (control = –0.02 Torr·µl–1· min–1·g–1 vs. phentolamine = –0.008 Torr·µl–1·min–1·g–1). In addition, the slope of the coronary venous PO2 vs. MO2 relationship after phentolamine treatment was significantly lower in the diabetic dogs. This difference in slope is supported by data presented in Fig. 4 that demonstrate that the change in coronary venous PO2, relative to the resting-baseline condition (%baseline), is significantly lower when MO2 is elevated during exercise in the phentolamine-treated diabetic dogs than in the nondiabetic control dogs treated with phentolamine. This finding indicates that blockade of {alpha}-adrenoceptors in diabetic dogs results in a more improved balance between coronary flow and metabolism when myocardial oxygen demand is elevated. Taken together, these results indicate that {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented in alloxan-induced diabetic dogs during exercise-induced increases in MO2.



View larger version (21K):
[in this window]
[in a new window]
  		Fig. 2. Percent increase in coronary blood flow (A) and conductance (B) at rest and at 3 levels of exercise in nondiabetic and diabetic dogs treated with phentolamine. The increase in coronary blood flow and conductance to exercise was greater in the diabetic dogs, indicating that diabetes increases {alpha}-adrenoceptor-mediated coronary vasoconstriction during increases in myocardial oxygen demand.

 


View larger version (21K):
[in this window]
[in a new window]
  		Fig. 3. Relationship between coronary venous PO2 and myocardial oxygen consumption at rest and during graded treadmill exercise in control (nondiabetic) (A) and alloxan-induced diabetic dogs (B) with and without {alpha}-adrenoceptor blockade with phentolamine. Blockade of {alpha}-adrenoceptors decreased the slope of this relationship in both experimental groups. However, the difference between the untreated slope and the phentolamine-treated slope was greater in the diabetic dogs than in the nondiabetic dogs. The slope of this relationship was also significantly lower after phentolamine treatment in the diabetic dogs. These findings indicate that {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented in diabetic dogs during exercise. Arrow and * denote statistical comparison between the 2 groups presented in each panel.

 


View larger version (18K):
[in this window]
[in a new window]
  		Fig. 4. Relationship between coronary venous PO2 (%baseline) and myocardial oxygen consumption at rest and during graded treadmill exercise in control (nondiabetic) and alloxan-induced diabetic dogs treated with phentolamine. The decrease in coronary venous PO2 was significantly diminished in the diabetic dogs, suggesting that {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented during exercise relative to nondiabetic controls.

 
Figure 5 shows the effects of graded norepinephrine infusion on coronary blood flow (% baseline) in nondiabetic control and diabetic dogs. Propranolol (2 mg/kg iv) was administered to these animals to inhibit the {beta}-adrenoceptor effects of norepinephrine, i.e., positive inotropic, chronotropic, and "feedforward" vasodilation (16, 20, 35). Coronary perfusion pressure was maintained at 100 mmHg throughout the experimental protocol so that alterations in perfusion pressure would not influence the coronary flow response to norepinephrine. We found that the decrease in coronary blood flow to norepinephrine was significantly augmented at the lower doses of norepinephrine in the diabetic dogs compared with the nondiabetic dogs.



View larger version (18K):
[in this window]
[in a new window]
  		Fig. 5. Relationship between coronary blood flow (%baseline) and intracoronary norepinephrine dose in {beta}-adrenoceptor-blocked, open-chest nondiabetic and diabetic dogs. The reduction in coronary blood flow to norepinephrine was significantly increased in the diabetic dogs compared with the nondiabetic control dogs. This finding indicates that diabetes increases {alpha}-adrenoceptor-mediated coronary vasoconstriction.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
{alpha}-Adrenoceptor Coronary Vasoconstriction in Diabetes

The purpose of the present investigation was to examine the hypothesis that {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented during physiological increases in myocardial oxygen demand in experimental type I diabetes. We found that the increase in coronary blood flow and conductance to exercise was greater after {alpha}-adrenoceptor blockade in the diabetic dogs (Fig. 2) and that the difference in the slope of the coronary venous PO2 vs. MO2 relationship between the untreated control and phentolamine-treated groups was greater in the diabetic dogs than in the normal, control dogs (Fig. 3). In addition, the slope of the coronary venous PO2 vs. MO2 relationship after phentolamine treatment was significantly lower in the diabetic dogs compared with the nondiabetic control dogs; i.e., the decrease in coronary venous PO2 as a function of MO2 was diminished in the diabetic dogs (Fig. 4). These results indicate that {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented in alloxan-induced diabetic dogs during exercise-induced increases in MO2. There does not appear to be a significant increase in resting {alpha}-adrenoceptor tone in the diabetic animals because the increase in blood flow after phentolamine administration tended to be less in the diabetic dogs (Table 1).

To further test the hypothesis that {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented in diabetic subjects, additional experiments were performed in anesthetized, open-chest, {beta}-adrenoceptor blocked dogs. In these studies, we found that coronary vasoconstriction to the lower, more physiological doses of norepinephrine was significantly increased in the diabetic dogs (Fig. 5). This supports our findings from the conscious, exercise studies and indicates that experimental type I diabetes augments {alpha}-adrenoceptor-mediated coronary vasoconstriction. Earlier studies have reported that diabetes increases myocardial {alpha}-adrenoceptor sensitivity but decreases the overall number of {alpha}-adrenoceptor binding sites (27, 32, 48). However, whether {alpha}-adrenoceptor density or sensitivity of coronary vascular smooth muscle cells is altered by diabetes is presently unknown.

Although {alpha}-adrenoceptor-mediated coronary vasoconstriction restricts increases in coronary blood flow, it has been shown to be important in maintaining blood flow to the vulnerable subendocardium when heart rate, contractility, and MO2 are elevated during exercise (16, 29, 45). The postulated mechanism is that increased release of norepinephrine during exercise stimulates {alpha}-adrenoceptor vasoconstriction of medium-sized intramyocardial vessels (diameter > 100 µm), which decreases intramyocardial vascular capacitance and wasteful antegrade-retrograde flow oscillations during the cardiac cycle (36). This hypothesis is supported by findings indicating that {alpha}1-adrenoceptor-mediated vasoconstriction occurs predominantly in larger coronary vessels (diameter > 100 µm) (7, 8, 26) and that the limitation of exercise-induced increases in coronary blood flow is primarily due to {alpha}1-adrenoceptor vasoconstriction (3, 4, 10, 12, 23). However, it should be pointed out that other investigations have shown that {alpha}-adrenoceptor-mediated coronary vasoconstriction does not affect transmural blood flow distribution (5, 12). The reason for this discrepancy is unclear, but it may be related to the use of different {alpha}-adrenoceptor antagonists, i.e., phentolamine vs. phenoxybenzamine. In addition, studies indicate that {alpha}2-adrenoceptors contribute to coronary vasoconstriction during sympathetic stimulation (26) and during ischemia (25, 30, 39). Whether the augmentation of {alpha}-adrenoceptor coronary vasoconstriction in diabetes is mediated by {alpha}1 and/or {alpha}2-adrenoceptors merits future investigation. Additional studies are also needed to determine whether this increase in {alpha}-vasoconstriction alters transmural blood flow distribution during increases in MO2.

Confounding Effects of {alpha}-Adrenoceptor Blockade

Although the findings of this investigation indicate that {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented in alloxan-induced diabetic dogs during exercise, it must be acknowledged that mechanisms other than abolition of {alpha}-adrenoceptor-mediated vasoconstrictor tone could be activated to cause coronary vasodilation after phentolamine administration. Systemic administration of phentolamine, as performed in the present investigation, increases circulating and cardiac norepinephrine concentrations, which may increase feedforward {beta}-adrenoceptor vasodilation (16, 20, 21, 35). Administering a {beta}-adrenoceptor antagonist during exercise would not quantify this confounding effect of phentolamine because any decrease in the balance between coronary blood flow and myocardial metabolism (i.e., decrease in slope of coronary venous PO2 vs. MO2) after {beta}-blockade could be due to either the loss of normal {beta}-adrenoceptor vasodilation, loss of augmented {beta}-adrenoceptor vasodilation, or a combination of the two. Because these experiments cannot differentiate between normal and augmented {beta}-adrenoceptor-mediated coronary vasodilation, they would not help quantify the augmentation effect. In addition, a similar problem exists if {beta}-adrenoceptor blockade alone is used because elevated catecholamine release would exaggerate the unopposed {alpha}-adrenoceptor vasoconstriction (41). Therefore, quantifying the degree of augmented {beta}-adrenoceptor-mediated vasodilation after {alpha}-adrenoceptor blockade during exercise is not possible.

To address this question, we conducted experiments in anesthetized, open-chest dogs in which the LAD was cannulated and perfused by a extracorporeal perfusion system so that graded doses of norepinephrine could be infused into the coronary circulation while coronary perfusion pressure was maintained constant. These experiments were conducted in nondiabetic and alloxan-induced diabetic dogs in the presence of propranolol to inhibit {beta}-adrenoceptor-mediated increases in heart rate, contractility, and {beta}-feedforward coronary vasodilation. We found that the decrease in coronary blood flow to lower doses norepinephrine was greater in the diabetic dogs relative to the nondiabetic control dogs (Fig. 5). This finding further supports the hypothesis that {alpha}-adrenoceptor-mediated coronary vasoconstriction is increased in experimental diabetes and argues against a significant augmentation of {beta}-adrenoceptor-mediated vasodilation after phentolamine in the exercise studies. In other words, if the present findings were due entirely to enhanced {beta}-adrenoceptor-mediated effects then there would have been no change in norepinephrine-induced coronary vasoconstriction in the diabetic dogs. A further argument against an increase in {beta}-adrenoceptor vasodilation is that {beta}-adrenoceptor density (27, 32) is significantly reduced in streptozotocin-induced diabetic rats with little or no change in receptor affinity (2, 27).

A recent study by Takamura et al. (42) found that nitric oxide-mediated vasodilation was enhanced after {alpha}-adrenoceptor inhibition in exercising dogs. This finding is intriguing in that it suggests that after {alpha}-adrenoceptor inhibition there is a significant increase in to-and-fro flow oscillation, i.e., shear stress (36), which stimulates an increase in nitric oxide production. Such a mechanism could have contributed to the greater increases in coronary blood flow and conductance in the diabetic animals, if nitric oxide release in the diabetic dogs was greater than that of the nondiabetic control dogs. Although several studies have reported that endothelium-dependent vasodilation is significantly impaired in experimental diabetic animals (17, 19, 34, 37, 49), others have reported an increase in endothelium-dependent relaxation (9, 38). Thus, whether nitric oxide-mediated vasodilation is enhanced after {alpha}-adrenoceptor inhibition in diabetic dogs merits further investigation.

Endothelin and {alpha}-Adrenoceptor Coronary Vasoconstriction

Although endothelium-dependent vasodilation may be depressed in diabetics (17, 19, 34, 37, 49), it is doubtful that a decrease in nitric oxide dilation is solely responsible for the coronary dysfunction induced by diabetes because nitric oxide is not required for balancing coronary blood flow with myocardial metabolism during increases in MO2 (1, 6, 15, 44). However, endothelial dysfunction is characterized not only by decreased nitric oxide production and/or vasodilation but also by increases in vasoconstricting factors such as endothelin (33). Recent studies have reported increases in circulating endothelin levels in diabetic rats (28) and humans (13). Interestingly, increased circulating endothelin levels in diabetic subjects could be linked with elevated {alpha}-adrenoceptor activation because {alpha}-adrenergic stimulation of cardiac myocytes results in endothelin-dependent coronary vasoconstriction both in vitro (43) and in vivo (11). Whether augmented {alpha}-adrenoceptor stimulation increases endothelin-induced coronary vasoconstriction in diabetic subjects has not been investigated.

Limitations of the Study

A major limitation of the present investigation is the acute diabetic state and loss of body weight (~7%) in the alloxan-treated dogs. This model does not represent the current clinical course of long-term diabetes that elicits coronary disease. Thus future experiments will require study of augmented {alpha}-adrenoceptor-mediated coronary vasoconstriction in stable, chronically diabetic animals.

In conclusion, this study is the first to document that {alpha}-adrenoceptor-mediated coronary vasoconstriction is augmented in alloxan-induced diabetic dogs during physiological increases in MO2. This increase in sympathetic vasoconstriction is one mechanism by which diabetes decreases myocardial oxygen delivery and impairs the balance between coronary blood flow and myocardial metabolism during exercise (47).


   GRANTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
ACKNOWLEDGMENTS
 REFERENCES
 
This work was supported by a Career Development Award to J. D. Tune from the American Diabetes Association.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
REFERENCES
 
The authors thank Arthur G. Williams, Jr. for expert technical assistance in all phases of this study.


   FOOTNOTES
 

Address for reprint requests and other correspondence: J. D. Tune, Dept. of Physiology, Louisiana State University Health Sciences Center, 1901 Perdido St., New Orleans, Louisiana 70112-1393 (E-mail: jtune{at}lsuhsc.edu).

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. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 

  1. Altman JD, Kinn J, Duncker DJ, and Bache RJ. Effect of inhibition of nitric oxide formation on coronary blood flow during exercise in the dog. Cardiovasc Res 28: 119–124, 1994.[Abstract/Free Full Text]
  2. Austin CE and Chess-Williams R. Transient elevation of cardiac {beta}-adrenoceptor responsiveness and receptor number in the streptozotocin-diabetic rat. J Auton Pharmacol 12: 205–214, 1992.[Web of Science][Medline]
  3. Bache RJ, Dai XZ, Herzog CA, and Schwartz JS. Effects of nonselective {alpha}1-adrenergic blockade on coronary blood flow during exercise. Circ Res 61: II36–II41, 1987.[Medline]
  4. Baran KW, Bache RJ, Dai XZ, and Schwartz JS. Effect of {alpha}-adrenergic blockade with prazosin on large coronary diameter during exercise. Circulation 85: 1139–1145, 1992.[Abstract/Free Full Text]
  5. Baumgart D, Ehring T, Kowallik P, Guth BD, Krajcar M, and Heusch G. Impact of alpha-adrenergic coronary vasoconstriction on the transmural myocardial blood flow distribution during humoral and neuronal adrenergic activation. Circ Res 73: 869–886, 1993.[Abstract/Free Full Text]
  6. Bernstein RD, Ochoa FY, Xu X, Forfia P, Shen W, Thompson CI, Hintze TH. Function and production of nitric oxide in the coronary circulation of the conscious dog during exercise. Circ Res 79: 840–848, 1996.[Abstract/Free Full Text]
  7. Chilian WM. Functional distribution of {alpha}1- and {alpha}2-adrenergic receptors in the coronary microcirculation. Circulation 84: 2108–2122, 1991.[Abstract/Free Full Text]
  8. Chilian WM, Layne SM, Easham CL, and Marcus ML. Heterogeneous microvascular coronary {alpha}-adrenergic vasoconstriction. Circ Res 64: 376–388, 1989.[Abstract/Free Full Text]
  9. Corbett JA, Tilton RG, Chang K, Hasan KS, Ido Y, Wang JL, Sweetland MA, Lancaster JR Jr, Williamson JR, and McDaniel ML. Aminoguanidine, a novel inhibitor of nitric oxide formation, prevents diabetic vascular dysfunction. Diabetes 41: 552–556, 1992.[Abstract]
  10. Dai XZ, Sublett E, Lindstrom P, Schwartz JS, Homans DC, and Bache RJ. Coronary flow during exercise after selective {alpha}1- and {alpha}2-adrenergic blockade. Am J Physiol Heart Circ Physiol 256: H1148–H1155, 1989.[Abstract/Free Full Text]
  11. DeFily DV, Nishikawa Y, and Chilian WM. Endothelin antagonists block {alpha}1-adrenergic constriction of coronary arterioles. Am J Physiol Heart Circ Physiol 276: H1028–H1034, 1999.[Abstract/Free Full Text]
  12. Dodd-o JM and Gwirtz PA. Coronary {alpha}1-adrenergic constrictor tone varies with intensity of exercise. Med Sci Sports Exerc 28: 62–71, 1996.[Web of Science][Medline]
  13. Donatelli M, Hoffmann E, Colletti I, Andolina G, Russo V, Bucalo ML, Valenti TM, Compagno V, Cataldo MG, and Morici ML. Circulating endothelin-1 levels in type 2 diabetic patients with ischemic heart disease. Acta Diabetol 33: 246–248, 1996.[Web of Science][Medline]
  14. Downing SE, Lee JC, and Fripp RR. Enhanced sensitivity of diabetic hearts to {alpha}-adrenoceptor stimulation. Am J Physiol Heart Circ Physiol 245: H808–H813, 1983.[Abstract/Free Full Text]
  15. Duncker DJ and Bache RJ. Inhibition of nitric oxide production aggravates myocardial hypoperfusion during exercise in the presence of a coronary artery stenosis. Circ Res 74: 629–640, 1994.[Abstract/Free Full Text]
  16. Feigl EO. Neural control of coronary blood flow. J Vasc Res 35: 85–92, 1998.[CrossRef][Web of Science][Medline]
  17. Frisbee JC and Stepp DW. Impaired NO-dependent dilation of skeletal muscle arterioles in hypertensive diabetic obese Zucker rats. Am J Physiol Heart Circ Physiol 281: H1304–H1311, 2001.[Abstract/Free Full Text]
  18. Gaba MK, Gaba S, and Clark LT. Cardiovascular disease in patients with diabetes: clinical considerations. J Assoc Acad Minor Phys 10: 15–22, 1999.[Medline]
  19. Gebremedhin D, Koltai MZ, Pogatsa G, Magyar K, and Hadhazy P. Influence of experimental diabetes on the mechanical responses of canine coronary arteries: role of endothelium. Cardiovasc Res 22: 537–544, 1988.[Web of Science][Medline]
  20. Gorman MW, Tune JD, Richmond KN, and Feigl EO. Feedforward sympathetic coronary vasodilation in exercising dogs. J Appl Physiol 89: 1892–1902, 2000.[Abstract/Free Full Text]
  21. Gorman MW, Tune JD, Richmond KN, and Feigl EO. Quantitative analysis of feedforward sympathetic coronary vasodilation in exercising dogs. J Appl Physiol 89: 1903–1911, 2000.[Abstract/Free Full Text]
  22. Grundy SM, Benjamin IJ, Burke GL, Chait A, Eckel RH, Howard BV, Mitch W, Smith SC, and Sowers JR. Diabetes and cardiovascular disease: a statement for healthcare professions from the American Heart Association. Circulation 200: 1134–1145, 1999.
  23. Gwirtz PA, Overn SP, Mass HJ, and Jones CE. {alpha}1-Adrenergic constriction limits coronary flow and cardiac function in running dogs. Am J Physiol Heart Circ Physiol 250: H1117–H1126, 1986.[Abstract/Free Full Text]
  24. Heijnis JB and Zwieten PA. Enhanced inotropic responsiveness to alpha 1-adrenoceptor stimulation in isolated working hearts from diabetic rats. J Cardiovasc Pharmacol 20: 559–562, 1992.[Web of Science][Medline]
  25. Heusch G and Deussen A. The effects of cardiac sympathetic nerve stimulation on perfusion of stenotic coronary arteries in the dog. Circ Res 53: 8–15, 1983.[Free Full Text]
  26. Heusch G, Deussen A, Schipke J, and Thamer V. {alpha}1- And {alpha}2-adrenoceptor-mediated vasoconstriction of large and small canine coronary arteries in vivo. J Cardiovasc Pharmacol 6: 961–968, 1984.[Web of Science][Medline]
  27. Heylinger CE, Pierce GN, Singal PK, Beamish RE, and Dhalla NS. Cardiac alpha- and beta adrenergic receptor alterations in diabetic cardiomyopathy. Basic Res Cardiol 77: 610–618, 1982.[CrossRef][Web of Science][Medline]
  28. Hopfer RL, McNeill R, and Gopalakrishnan V. Plasma endothelin levels and vascular responses at different temporal stages of streptozotocin diabetes. Eur J Pharmacol 374: 221–227, 1999.[CrossRef][Web of Science][Medline]
  29. Huang AH and Feigl EO. Adrenergic coronary vasoconstriction helps maintain uniform transmural blood flow distribution during exercise. Circ Res 62: 286–298, 1988.[Abstract/Free Full Text]
  30. Ishibashi Y, Duncker DJ, and Bache RJ. Endogenous nitric oxide masks {alpha}2-adrenergic coronary vasoconstriction during exercise in the ischemic heart. Circ Res 80: 196–207, 1997.[Abstract/Free Full Text]
  31. Koltai MZ, Jermendy G, Kiss V, Wagner M, and Pogatsa G. The effects of sympathetic stimulation and adenosine on coronary circulation and heart function in diabetes mellitus. Acta Physiol Hung 63: 119–125, 1984.[Web of Science][Medline]
  32. Latifpour J and McNeill JH. Cardiac autonomic receptors: effect of long-term experimental diabetes. J Pharmacol Exp Ther 230: 242–249, 1984.[Abstract/Free Full Text]
  33. Lerman A and Burnett JC. Intact and altered endothelium in regulation of vasomotion. Circulation 86: III12–III19, 1992.[Medline]
  34. Matsunaga T, Okumura K, Ishizaka H, Tsunoda R, Tayama S, Tabuchi T, and Yasue H. Impairment of coronary blood flow regulation by endothelium-derived nitric oxide in dogs with alloxan-induced diabetes. J Cardiovasc Pharmacol 28: 60–67, 1996.[CrossRef][Web of Science][Medline]
  35. Miyashiro JK and Feigl EO. Feedforward control of coronary blood flow via coronary {beta}-receptor stimulation. Circ Res 73: 252–263, 1993.[Abstract/Free Full Text]
  36. Morita K, Mori H, Tsujioka K, Kimura A, Ogasawara Y, Goto M, Hiramatsu O, Kajiya F, and Feigl EO. {alpha}-Adrenergic vasoconstriction reduces systolic retrograde coronary blood flow. Am J Physiol Heart Circ Physiol 273: H2746–H2755, 1997.[Abstract/Free Full Text]
  37. Pieper GM. Review of alterations in endothelial nitric oxide production in diabetes; protective role of arginine on endothelial dysfunction. Hypertension 31: 1047–1060, 1998.[Free Full Text]
  38. Pieper GM. Enhanced, unaltered and impaired nitric oxide-mediated endothelium-dependent relaxation in experimental diabetes mellitus: importance of disease duration. Diabetologia 42: 204–213, 1999.[CrossRef][Web of Science][Medline]
  39. Seitelberger R, Guth BD, Heusch G, Lee JD, Kayayama K, and Ross J. Intracoronary alpha 2-adrenergic receptor blockade attenuates ischemia in conscious dogs during exercise. Circ Res 62: 436–442, 1988.[Abstract/Free Full Text]
  40. Sjogren A and Edvinsson L. Vasomotor changes in isolated coronary arteries from diabetic rats. Acta Physiol Scand 134: 429–436, 1988.[Web of Science][Medline]
  41. Staib AH, Appel E, Starey F, Lindner E, Grotsch H, Palm D, and Grobecker H. Exercise induced changes of catecholamines and potassium in plasma of dogs after treatment with propranolol. Arsneimittelforschung 30: 1514–1517, 1980.
  42. Takamura M, Parent R, and Lavallee M. Enhanced contribution of NO to exercise-induced coronary responses after {alpha}-adrenergic receptor blockade. Am J Physiol Heart Circ Physiol 282: H508–H515, 2002.[Abstract/Free Full Text]
  43. Tiefenbacher CP, DeFily DV, and Chilian WM. Requisite role cardiac myocytes in coronary {alpha}1-adrenergic constriction. Circulation 98: 9–12, 1998.[Abstract/Free Full Text]
  44. Tune JD, Richmond KN, Gorman MW, and Feigl EO. Role of nitric oxide and adenosine in control of coronary blood flow in exercising dogs. Circulation 101: 2942–2948, 2000.[Abstract/Free Full Text]
  45. Tune JD, Richmond KN, Gorman MW, and Feigl EO. Control of coronary blood flow during exercise. Exp Biol Med 227: 238–250, 2002.[Abstract/Free Full Text]
  46. Tune D, Yeh C, Setty S, and Downey HF. ATP-Dependent K+ Channels contribute to local metabolic coronary vasodilation in experimental diabetes. Diabetes 51: 1201–1207, 2002.[Abstract/Free Full Text]
  47. Tune JD, Yeh C, Setty S, Zong P, and Downey HF. Coronary blood flow control is impaired at rest and during exercise in conscious diabetic dogs. Basic Res Cardiol 97: 248–257, 2002.[CrossRef][Web of Science][Medline]
  48. Wald M, Borda ES, and Sterin-Borda L. Alpha-adrenergic supersensitivity and decreased number of alpha-adrenoceptors in heart from acute diabetic rats. Can J Physiol Pharmacol 66: 1154–1160, 1988.[Web of Science][Medline]
  49. Zhao G, Zhang X, Smith CJ, Xu X, Ochoa M, Greenhouse D, Vogel T, Curran C, and Hintze T. Reduced coronary NO production in conscious dogs after the development of alloxan-induced diabetes. Am J Physiol Heart Circ Physiol 277: H268–H278, 1999.[Abstract/Free Full Text]



This article has been cited by other articles:


Home page
 Exp. Biol. Med.Home page
M.-G. Ryou, J. Sun, K. N. Oguayo, E. B. Manukhina, H. F. Downey, and R. T. Mallet
Hypoxic Conditioning Suppresses Nitric Oxide Production upon Myocardial Reperfusion
Experimental Biology and Medicine, June 1, 2008; 233(6): 766 - 774.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF) Free
Right arrow All Versions of this Article:
97/1/431    most recent
01122.2003v1
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Setty, S.
Right arrow Articles by Tune, J. D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Setty, S.
Right arrow Articles by Tune, J. D.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online
Copyright © 2004 by the American Physiological Society.