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J Appl Physiol 104: 1014-1020, 2008. First published January 24, 2008; doi:10.1152/japplphysiol.01203.2007
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Adaptive response of pulmonary arterial smooth muscle to length change

Harley Syyong,1 Christine Cheung,2 Dennis Solomon,3 Chun Y. Seow,2,3 and Kuo H. Kuo1

1Department of Anesthesiology, Pharmacology, and Therapeutics, 2The James Hogg iCAPTURE Centre, and 3Department of Pathology and Laboratory Medicine, University of British Columbia, Vancouver, British Columbia, Canada

Submitted 11 November 2007 ; accepted in final form 19 January 2008


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Hypervasoconstriction is associated with pulmonary hypertension and dysfunction of the pulmonary arterial smooth muscle (PASM) is implicated. However, relatively little is known about the mechanical properties of PASM. Recent advances in our understanding of plastic adaptation in smooth muscle may shed light on the disease mechanism. In this study, we determined whether PASM is capable of adapting to length changes (especially shortening) and regain its contractile force. We examined the time course of length adaptation in PASM in response to step changes in length and to length oscillations mimicking the periodic stretches due to pulsatile arterial pressure. Rings from sheep pulmonary artery were mounted on myograph and stimulated using electrical field stimulation (12–16 s, 20 V, 60 Hz). The length-force relationship was determined at Lref to 0.6 Lref, where Lref was a reference length close to the in situ length of PASM. The response to length oscillations was determined at Lref, after the muscle was subjected to length oscillation of various amplitudes for 200 s at 1.5 Hz. Release (or stretch) of resting PASM from Lref to 0.6 (and vice versa) was followed by a significant force recovery (73 and 63%, respectively), characteristic of length adaptation. All recoveries of force followed a monoexponential time course. Length oscillations with amplitudes ranging from 5 to 20% Lref caused no significant change in force generation in subsequent contractions. It is concluded that, like many smooth muscles, PASM possesses substantial capability to adapt to changes in length. Under pathological conditions, this could contribute to hypervasoconstriction in pulmonary hypertension.

blood vessel; mechanics; length-force relationship

EXCESSIVE VASOCONSTRICTION is a hallmark of pulmonary hypertension, and abnormality in PASM has been implicated (5–8, 16, 17, 24, 25, 29, 30, 34, 44). Contributing factors to the smooth muscle abnormality include the proinflammatory mediator-induced cell proliferation and remodeling (9, 14, 15, 18, 23, 44), and the disturbances in the signaling pathways (5, 6, 16), as well as the defects in potassium channels (7, 8, 30). The elastic modulus of the artery appears to be unchanged in the disease state (10, 11, 28). The plastic properties of the arteries from hypertensive humans or animal models, however, have never been compared with those from normals.

It is interesting to note the parallels between pulmonary hypertension and another smooth muscle-related lung disease, asthma, in terms of the potential factors contributing to their pathogeneses. Remodeling of the airways (12, 14, 39), including airway smooth muscle (ASM) proliferation (13, 21, 23, 32, 41), is thought to be associated with asthma, and many signaling pathways regulating smooth muscle function have been identified as potential targets for asthma therapy (22). One hypothesis regarding the disease mechanism for asthma is based on the finding that ASM is adaptable to changes in length in that the muscle is able to generate maximal or near-maximal force at a wide range of lengths (or airway diameters) (4). It is possible that an inflammatory airway environment may allow the muscle to adapt to an abnormally short length, leading to excessively narrowed airways. Could this scenario be applied to the hypervasoconstriction seen in pulmonary hypertension? To address this question, we first need to find out whether the PASM behaves similarly as ASM in terms of its ability to adapt to changes in length. Although there is evidence for altered mechanical/pharmacological properties of the arterial smooth muscle in the disease state of hypertension, the compounding effect of length adaptation of smooth muscle in producing excessively narrowed arteries is not clear. There are only a few studies done examining the mechanical properties of PASM (5, 9, 31); none of them address the adaptive behavior of the muscle. The present study is the first to examine the time-dependent process of length adaptation in PASM in response to step changes in length, as well as to length oscillations that mimic the periodic stretches on the muscle due to the pulsatile pressure in the artery in vivo. The aim of this work is to study the normal behavior of the muscle from a healthy lung to establish a basic knowledge of the smooth muscle adaptive response in pulmonary artery. This will be needed in future studies to understand abnormalities that may exist in arterial smooth muscle in pulmonary hypertension.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Muscle preparation.   Fresh sheep lungs were obtained from a local abattoir and transported to the laboratory in ice-cold physiological saline solution (composition in mM: 118 NaCl, 5 KCl, 1.2 NaH2PO4, 22.5 NaHCO3, 2 MgSO4, and 2 CaCl2, and 2 g/l dextrose). The use of the tissue for this study was approved by the Committee on Animal Care of the University of British Columbia. A segment of the major pulmonary artery was dissected from the lung, and the loose connective tissue associated with the adventitia was removed. From the segment, a ring (1–2 mm in width) of the artery was cut, and the endothelial layer removed by rubbing the lumen side of the artery gently with a pair of forceps. The ring was then used for the experiment.

Experiment protocols.   The artery ring was mounted on a length-force duo-mode transducer vertically, stretched between two L-shaped hooks. The bottom hook was fixed near the bottom of the muscle bath; the other hook was connected to the lever arm of the length-force transducer. The muscle preparation was immersed in the muscle bath with physiological saline solution (37°C) aerated with a gas mixture of 95% O2-5% CO2 that maintained the pH value at 7.4. Each preparation was preconditioned for ~1 h, during which it was stimulated to produce tetani of 12- to 16-s duration at 5-min intervals. (For muscle preparations from different sheep, the stimulation time required for the tetanic contraction to reach plateau varied from 12 to 16 s; stimulation was turned off once the plateau was reached). During the preconditioning period, the circumferential length of the artery ring associated with an arterial wall tension corresponding to that caused by a luminal pressure of 15 mmHg (2 kPa) was determined, a value corresponding to the mean arterial pressure of the pulmonary circulation in sheep (35). More specifically, determination of resting force that corresponded to the mean blood pressure was done in an iterative manner. First, we stretched the arterial ring preparation and measured both the circumferential length, ring segment width, and the associated resting force. Then we used the Laplace law to determine whether the resting force would give the correct pressure (with the constraints of circumferential length and segment width). We kept varying the amount of stretch until the Laplace law was satisfied and at the same time the arterial wall tension associated with the mean blood pressure was obtained. The Laplace law used was in the form of tension (mN/mm) = pressure (kPa) x radius (mm). This circumferential length corresponding to the mean blood pressure was used as a reference length (Lref).

For determination of the length-force relationship, the muscle preparation was first preconditioned at one of the three predetermined lengths: Lref, 0.8 Lref, and 0.6 Lref. After maximal, stable force was obtained at the chosen length, the muscle was then released or stretched to one of the other predetermined lengths. Changes in both the actively developed (electrically stimulated) and the resting forces were recorded at 5-min intervals until the forces reached a steady state, which usually took three to six contractions over a period of 15–30 min. After each length change, the muscle was readapted fully at Lref before the next length change. We did not measure force at lengths greater than Lref, because we believe that understanding the behavior of the muscle at lengths less than Lref is more important in terms of disease relevance. Another reason for not going over Lref was our fear of irreversible damage of the tissue due to the high passive tension associated with a large stretch. Yet another reason was the limitation of our muscle lever system that had a maximal capacity for force measurement, and that capacity would likely be exceeded during a stretch beyond Lref, especially after the muscle had been adapted at a length much short than Lref.

For determination of the response to length oscillation, we first preconditioned the muscle preparation at Lref, sinusoidal length oscillation with various preset amplitudes at a frequency of 1.5 Hz was then applied to the muscle for 200 s. The frequency used corresponds to the heart rate in sheep (15). Three seconds after the cessation of length oscillation (200 s in duration) and 5 min after the last stimulation, the muscle was stimulated to produce a tetanic isometric contraction, from which the resting force (average of recorded force values over 1-s period just before stimulation) and active plateau force were measured (12–16 s after stimulation).

Apparatus.   The servo-controlled force-length lever system has a force resolution of 10 µN and a length resolution of 1 µm. It is capable of measuring both force and length of the muscle simultaneously. It is also able to apply length or force oscillation to the muscle at rest or while the muscle is being stimulated. The electrical field stimulation (EFS) was provided by a 60-Hz alternating-current stimulator with platinum electrodes and a voltage of 20 V that produced stable, maximal response from the muscle. The analog signals (length and force) from the lever system were converted to digital signals by a National Instrument analog-to-digital converter and then recorded by a computer that also controlled the onset and duration of the EFS.

Morphometry.   Trichrome staining of the artery preparation was carried out to enable us to examine the tissue composition of the artery ring, to confirm the removal of endothelial layer without significant damage to the muscle layer, and to measure the dimensions of the muscle preparation. At the end of the experiment, while the ring preparation was still attached to the myograph and being maintained at its in situ length, it was fixed briefly (15 min) with a solution mixture of 2% gluteraldehyde, 2% formaldehyde and 0.1 M Na-cacodylate buffer at pH 7.3. The tissue was then removed from the apparatus and placed in 10% formalin for 24 h. After the overnight fixation, the tissue was processed and embedded in paraffin wax. The tissue was later sectioned on a Leica RM 2145 microtome at ~3- to 5-µm thickness and mounted on glass slides. The tissue was then stained with Masson's trichrome that stained the muscle red and the collagen-elastin extracellular matrix bluish green. In all preparations, we observed complete removal of the endothelial layer and intact muscle layer. Thickness and width of the ring preparation were measured from the micrographs; these measurements were later used for normalization of force.

Statistical analysis.   Values are all expressed as means ± SE. Means were compared using the most robust test appropriate to each experimental design. Force development after oscillations were compared by one-way ANOVA, with pairwise comparisons made by Bonferroni post hoc tests. Two-way ANOVA was used to determine differences in the recovery of force following length changes to and from Lref, 0.8 Lref, and 0.6 Lref. Number Cruncher Statistical System statistical software was used to perform statistical tests. All data were compiled and analyzed in GraphPad Prism 4.0, in coordination with Microsoft Excel. Statistical significance was assumed at P < 0.05.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Adaptation of muscle at different lengths.   For clarity, all results were expressed with reference to the maximal isometric forces produced at 0.6, 0.8, or 1.0 Lref. The mean and SE of absolute value for Lref was 31 ± 2.1 mm. The maximal isometric forces (after adaptation) at 0.6, 0.8, and 1.0 Lref were 17.5 ± 2.1, 26.7 ± 3.8, and 31.9 ± 5.4 kPa, respectively.

Release of a resting muscle, which was fully adapted at Lref, to 0.8 and 0.6 Lref resulted in significantly reduced force production in the immediate, subsequent contraction (Fig. 1A, filled circles). In the following 30-min adaptation period at the shortened lengths (during which the muscle was stimulated electrically once every 5 min), actively developed force increased monotonically to reach a plateau. The plateau force for the larger release (from Lref to 0.6 Lref) was significantly different from the initial force measured right after the length release (P < 0.01); for the smaller release (from Lref to 0.8 Lref) force recovery was also observed, but the plateau force was not significantly different from the initial force. This was also true for the small length change in the reversed direction (from 0.8 Lref to Lref) (Fig. 1B). Although active force recovered somewhat after the stretch, the plateau value was not significantly different from the initial one. For the larger stretch (from 0.6 Lref to Lref), the difference between the plateau force and the initial force was significant (P < 0.01) (Fig. 1C). It should be pointed out that before being stretched from a shorter length to Lref, the muscle was fully adapted at the shorter length.


Figure 1
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  		Fig. 1. Adaptation of active force after changes in length. A: the muscle was fully adapted at reference length (Lref), the resting (nonstimulated) muscle was then quickly (within 1 s) released to either 0.8 Lref or 0.6 Lref and allowed to adapt at the shortened lengths through regular electrical field stimulation (once every 5 min) until force reached a plateau. bullet, Initial force measured immediately after the length change; {circ}, plateau force, after the muscle was fully adapted. Solid lines (with arrows) connecting circles indicate direction of length change. Vertical arrow indicates direction of force recovery during the adaptation process. Fmax, maximal isometric force. **Statistical significance at P < 0.01. B: muscle was fully adapted at 0.8 Lref before it was quickly stretched (at resting state) to Lref. It was then allowed to recover through regular stimulation at Lref until it was fully adapted to the stretched length. C: muscle was fully adapted at 0.6 Lref before it was quickly stretched (at resting state) to Lref. It was then allowed to recover through regular stimulation at Lref until it was fully adapted, which was not different from the active force found at Lref before quick release (A). Vertical arrow indicates the direction of force recovery. **Statistical significance at P < 0.01.

 
Changes in resting force (measured just before stimulation) and changes in active force (shown in Fig. 1) are shown in Fig. 2. Again, significant force changes were only observed in the large release (from Lref to 0.6 Lref) or stretch (from 0.6 Lref to Lref).


Figure 2
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  		Fig. 2. Adaptation of resting force after changes in length. These measurements were made just before electrical field stimulation that produced the active force plotted in Fig. 1. A: release from Lref to 0.8 and 0.6 Lref. B: stretch from 0.8 Lref to Lref. C: stretch from 0.6 Lref to Lref. *Statistical significance at P < 0.05.

 
Time course of force change after length change.   Force increase or decrease after a length change occurred gradually; depending on the step size and direction of length change, the rates of force change were different (Fig. 3). Active force recovery after a release was slower than that after a stretch (Fig. 3, A and B). Figure 3A depicts the time course of force recovery after a release from 1.0 to 0.6 Lref (indicated by the vertical arrow shown in Fig. 1A); Fig. 3B depicts the time course of force recovery after a stretch from 0.6 to 1.0 Lref (indicated by the vertical arrow shown in Fig. 1C). Although a monoexponential equation was used to fit the data in Fig. 3B, there were insufficient data points to be sure that the recovery was indeed monoexponential. Because we could not stimulate the muscle more frequently than once every 5 min, we can only conclude that most of the recovery occurred within 5 min after the length change. The time courses of force recovery after a release from 1.0 to 0.8 Lref and after a stretch from 0.8 to 1.0 Lref were similar to the time courses depicted in Fig. 3, A and B, respectively (data not shown) in terms of their rates of recovery. However, the extent of recovery was not significant in both cases.


Figure 3
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  		Fig. 3. Time course of force change after changes in length. The monoexponential equations and the constants used for the curve fitting are listed in Table 1. A: active force recovery after a quick release from Lref to 0.6 Lref. Force values were normalized to the maximal recovery found during the adaptation process. Initial and final forces were plotted in Fig. 1A. B: active force recovery after a quick stretch from 0.6 Lref to Lref. Initial and final forces were plotted in Fig. 1C. C: resting force recovery after a quick release from Lref to 0.6 Lref. Initial and final forces were plotted in Fig. 2A. D: resting force decrease after a quick stretch from 0.6 Lref to Lref. Initial and final forces were plotted in Fig. 2C.

 
Changes in resting force after a release or stretch could also be described well by monoexponential equations (Fig. 3, C and D). The rate constants are listed in Table 1. Figure 3C depicts the time course of force recovery after a large release (from Lref to 0.6 Lref) shown in Fig. 2A. Although the absolute amount of force recovery was small, the recovered force was statistically significantly higher than the initial force. Figure 3D depicts the time course of force decrease after a large stretch (from 0.6 Lref to Lref) shown in Fig. 2C.


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  		Table 1. Constants for the monoexponential force recovery shown in Fig. 3

 
The fact that most of the time courses of force change examined above could be described well with a monoexponential equation (with different constants a and b) simplified comparison of rates of change in the processes. The rate constant b could be interpreted as a reflection of the speed at which the muscle was adapting to a length change.

Force changes after length oscillation at different amplitudes.   To examine the ability of PASM to generate force in the pulsatile in vivo environment, the muscle preparation was subjected to oscillations at physiological frequency and in a range of oscillation amplitudes that included the physiological amplitude. As shown in Fig. 4, without length oscillation, the muscle generated an active force that was comparable to the resting force (calculated from the mean blood pressure of pulmonary circulation in sheep). As the oscillation amplitude increased from 5 to 20% Lref, it appeared that the total force (active plus resting) remained constant while active force contributed more, and resting force contributed less, to the total force. However, these changes were not statistically significant. Mechanical properties of the artery in terms of its ability to generate active force and maintain total tension in the blood vessel wall therefore were not affected by the oscillation amplitudes in the range examined.


Figure 4
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  		Fig. 4. Changes in active, resting, and total forces after length oscillation at different amplitudes. Oscillation frequency and duration for all amplitudes were 1.5 Hz and 200 s, respectively. Total force is the sum of active plus resting forces.

 
Morphology of the sheep pulmonary artery.   There were multiple layers of muscle cells separated by layers of extracellular connective tissue mainly composed of collagen and elastin in all the pulmonary arteries examined. Presumably these collagen-elastin layers are responsible for maintaining passive tension, and the muscle cells are relaxed. The much more extensive extracellular matrix in the medial layer of pulmonary artery (with endothelial layer and adventitia removed) compared with the ASM bundle (26) probably explains the higher passive force seen in this arterial preparation. It was noted that the general appearance of the wall structure of pulmonary artery (i.e., the interlaced smooth muscle layers with collagen-elastin layers) was very similar to that of systemic arteries such as the carotid artery.


   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Increasingly, it has been recognized that smooth muscle is extremely adaptable to changes in its environment (42), including changes that reset the resting length of the muscle (38) that could have important implications in its physiological function (36, 40, 42) and the pathogeneses of smooth muscle-related diseases (33). The major finding of the present study is that, like many other types of smooth muscle, PASM is also adaptable to changes in cell length. The present study is the first to characterize this dynamic mechanical property of PASM in detail. A dynamic length-force relationship is defined as one that varies with time. Unlike the static length-force relationship found in maximally activated striated muscle (20) where the muscle force is strictly a function of the sarcomere length alone, the dynamic length-force relationship of smooth muscle changes with time, possibly due to time-dependent adaptive processes that optimize the overlap of the contractile filaments (37).

Length adaptation and changes in force after a step change in length.   After a quick release in length under resting condition, the ability of PASM to generate force was found to be reduced. This probably was largely due to a reduced contractile filament overlap as a result of acute shortening, similar to what our laboratory has observed in ASM (3). Substantial recovery of the ability to generate force was observed in the subsequent contractions. For the larger release (from Lref to 0.6 Lref), the force approximately doubled over a 30-min period (Figs. 1A and 3A). Although the mechanism is not entirely clear, rearrangement of the contractile filaments and optimization of the overlap between the thin (actin) and thick (myosin) filaments may be responsible for the force recovery. The acute force change was highly dependent on the direction of length change and was highly asymmetric. A quick stretch applied to a relaxed muscle did not cause a decrease in the ability of the muscle to generate force, but substantial force recovery occurred when the muscle was allowed to adapt at the stretched length (Fig. 1, B and C). The asymmetry in the acute force response to bidirectional length change is again similar to what our laboratory has observed in ASM (3). Our interpretation of the airway muscle data was based on a model of contractile unit (Fig. 6 in Ref. 3) where the thin filaments were longer than the thick filaments, so that during a stretch the amount of thin-thick filament overlap was less affected than it was during a release. This may also explain why the rate of force recovery was faster after a stretch than that after a release (Fig. 3, A and B). But, unlike our laboratory's previous study, ultrastructure of the muscle was not examined in the present study, and therefore, applicability of the explanation to the present finding needs to be tested in future studies that include ultrastructural examination.

The substantial force recovery after a shortening found in the present study (Fig. 1) perhaps has more pathological than physiological implications. As the blood vessel narrows, the wall tension required to maintain a constant intramural pressure is reduced, as dictated by the law of Laplace. An increase in wall tension at short muscle length as a result of length adaptation could lead to further narrowing of the blood vessel, which could result in a vicious cycle of ever-increasing resistance to blood flow. For smooth muscles lining the wall of hollow organs that regularly undergo large volumes changes (e.g., urinary bladder), plasticity is needed for the muscle to generate force even at very short lengths. In arterial smooth muscle, this aspect of plasticity appears to impart a precarious property to the blood vessel.

Force recovery after a stretch seems to have an auto-regulatory function. When the diameter of a blood vessel is increased, wall tension has to be increased even when the transmural pressure remains the same, as required by the law of Laplace. The increased ability of arterial smooth muscle to generate force after adaptation to a longer length (Fig. 1) will reduce the load on the collagen-elastin layers of the blood vessel and also allow the vessel constriction to occur if necessary.

Changes in resting force after step changes in length were less striking than those in active force, but for the changes after the larger length steps (from Lref to 0.6 Lref and vice versa) there were significant force recoveries (Fig. 2). The force recovery may reflect passive stress relaxation in the tissue, but active muscle tone or even myogenic tone could also make up a portion of it. Morphological examination revealed that the large pulmonary artery from sheep contained multiple layers of collagen-elastin fibers laid side by side with the muscle layers. This type of arrangement is likely responsible for the high passive force (with stretch) observed in the preparation (Fig. 2).

The time course of active force recovery after a release (Fig. 3A) is similar to that observed in the carotid artery of the rabbits; the rate constants for the recovery are 0.138 ± 0.048 min–1 for carotid artery (36) and 0.145 ± 0.018 min–1 for pulmonary artery (Table 1) for a comparable step size of release. However, for the comparable step size of release, the initial decrease in force and the subsequent force recovery are much greater in PASM compared with the carotid arterial smooth muscle (36). The discrepancy appears to be species and/or tissue specific, because the methods used for the measurements in the two studies are virtually the same. After a stretch, the rate of active force recovery (Fig. 3B, Table 1) is also similar to that observed in the carotid artery (36). The amount of change in passive force is small but significant (Fig. 2), and the time courses can be described by monoexponential equations (Fig. 3, C and D, Table 1).

The above-described changes in force due to changes in length are all reversible. Reversible adaptation is important if this is to happen in vivo on a regular basis. Nonreversible adaptation in either direction cannot be sustained indefinitely without running into pathological problems. The mechanism for reversible changes in passive force is likely different from that for active force, elucidating these mechanisms will likely lead to a totally new area of smooth muscle research.

In a study by Zulliger et al. (45), mechanical adaptation to different distending pressures was examined in segments of explanted porcine carotid artery over a 3-day period. At high pressure, a significant increase in vessel diameter was observed after 3 days. This is similar to the change in passive force after a stretch observed in the present study. Reversibility of the changes observed by Zulliger et al. is, however, not known. In the same study, the maximal ability to constrict the vessel under intrinsic smooth muscle tone as a function of intramural pressure was found to be shifted to a higher pressure in vessels subjected to high distending pressures after 3 days (45); this may indicate that some adaptation has occurred in the smooth muscle. However, when they activated the muscle with norepinephrine, the shifts (or adaptation) disappeared. The last observation appeared to be contradictory to the results of the present study. The source of this discrepancy is not clear. It should be pointed out that the type of length adaptation we studied is reversible and happens within a matter of minutes, much shorter than that studied by Zulliger et al. Also in the study of Zulliger et al., repeated contraction and relaxation was not part of their adaptation protocol.

In the present study, we have purposely removed the endothelial layer from our PASM preparation. This also removed the endothelium-mediated tone that is likely present in vivo. Results of the present study therefore reflect the muscle behavior without the influence of tone. To gain insights into the more complex behavior of PASM in vivo, we need to consider the influence of muscle tone, among other factors associated with the in vivo environment, such as the perpetual pulsatile pressure fluctuation.

Changes in force after length oscillation.   Force produced by PASM appears to be unaffected by length oscillation applied to the resting muscle with amplitude and frequency comparable to those experience by the muscle in vivo (Fig. 4). This is different from the response of rabbit carotid arterial smooth muscle to length oscillation (36). Length oscillation at a physiological amplitude (~12% stretch) induced a significant potentiation of active force generation in rabbit carotid ring preparation (36); in sheep PASM, length oscillation at amplitudes from 5 to 20% of in situ length caused no change in active force production (Fig. 4). Although there is a trend of increase in active force and a similar trend of decrease in passive force, these changes were not statistically significant, and the total force remained unchanged in the amplitude range. The different response of pulmonary artery to length oscillation compared with that of carotid artery (36) is likely a tissue-specific difference, and may be due to the fact that pulmonary artery is normally subjected to a greater length perturbation due to lung-volume changes associated with breathing, besides being under the pulsatile blood pressure. Beyond the physiological amplitude range (>20% stretch) there might be a decrease in the capacity to generate active force, like that observed in rabbit carotid arterial smooth muscle; however, this was not examined in this study.

Compared with ASM, there are some interesting differences. Length oscillation applied to resting ASM caused a significant decrease in the ability of the muscle to generate force in the subsequent contractions, and the amplitude of oscillation is linearly related to the decrease in the force-generating capacity (43). In ASM, the decrease in force was shown to match the decrease in the density of myosin filaments in the cell cross section (27). This was interpreted as evidence for lability of the myosin filaments in ASM cells. Myosin filament lability is probably only one aspect of the malleable structure of the contractile apparatus of ASM (37). A malleable structure in our hypothetical scheme is one that allows actin filament lattice to be reconfigured at different muscle cell lengths and also allows myosin filaments to disassemble and then reassemble in different actin filament lattices during the process of length adaptation, and therefore it preserves optimal overlap between myosin and actin filaments and hence a broader plateau in the length-force relationship. The present finding (Fig. 4) suggests that myosin filaments in PASM may be more stable (less labile) than those in ASM, and therefore PASM may be less adaptable than ASM. This could explain the much broader plateau seen in the length-force relationship from fully adapted ASM, compared with that from the fully adapted PASM.

Relevance to pulmonary hypertension and future studies.   Although the extent of length adaptation is less compared with other smooth muscles (27, 40, 42, 43), it is clear that the length-force relationship in PASM is not static. The mutable length-force relationship suggests that the structure of the contractile apparatus in PASM may be malleable (plastic) and able to adapt to externally imposed stress and strain. Several studies of human and animal models of hypertension have revealed that the arterial wall hypertrophy associated with hypertension is not accompanied by an increase in the elastic modulus of the arterial wall material (10, 11, 28). However, plastic adaptation of arterial smooth muscle to the sustained essential hypertension was not examined in these studies. It is possible that remodeling of the arterial wall during the disease process, such as the increased cell-matrix connection reported by Bézie et al. (10, 11), affects plastic behavior of the arterial wall and not its elastic properties. This hypothesis needs to be tested in future experiments.

An interesting and recent study suggests that protein kinase C is involved in endothelin-1 (ET-1) induced hyper-contractile response in PASM from hypertensive rats (5). An even more interesting finding from the same study is that ET-1 increased RhoA expression in PASM, especially in the hypertensive rats (5). Because Rho kinase is involved in the remodeling of cytoskeleton of smooth muscle cells (19), and since cytoskeletal remodeling is likely an important mechanism underlying the phenomenon of length adaptation (37) such as that described in the present study, pulmonary hypertension may therefore be linked to length adaptation of PASM through the Rho kinase-mediated pathway. It is interesting to note that treatment with Rho kinase inhibitor has been demonstrated to have beneficial effects in animal models of pulmonary hypertension (1, 2, 18). In this context, it is important to find out in future experiments whether length adaptability of PASM is altered in the disease state of pulmonary hypertension and whether Rho kinase inhibitors can reverse the altered length adaptability in PASM.


   GRANTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
ACKNOWLEDGMENTS
 REFERENCES
 
This study was supported Canadian Institutes of Health Research Grant MOP-13271 (to C. Y. Seow) and by Natural Sciences and Engineering Research Council of Canada Operating Grant 342193-07 (to K. H. Kuo).


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
REFERENCES
 
Special thanks to Pitt Meadows Meats Limited (Pitt Meadows, BC, Canada) for the supply of fresh sheep lungs in kind support for this research project.


   FOOTNOTES
 

Address for reprint requests and other correspondence: C. Y. Seow, The iCAPTURE Centre/St. Paul's Hospital, Univ. of British Columbia 1081 Burrard St., Rm. 166, Vancouver, BC, Canada V6Z 1Y6 (e-mail: cseow{at}mrl.ubc.ca)

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
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 

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