Effects of arterial wall stress on vasomotion

Smooth muscle and endothelial cells in the arterial wall are exposed to mechanical stress. Indeed blood flow induces intraluminal pressure variations and shear stress. An increase in pressure may induce a vessel contraction, a phenomenon known as the myogenic response. Many muscular vessels present vasomotion, i.e., rhythmic diameter oscillations caused by synchronous cytosolic calcium oscillations of the smooth muscle cells. Vasomotion has been shown to be modulated by pressure changes. To get a better understanding of the effect of stress and in particular pressure on vasomotion, we propose a model of a blood vessel describing the calcium dynamics in a coupled population of smooth muscle cells and endothelial cells and the consequent vessel diameter variations. We show that a rise in pressure increases the calcium concentration. This may either induce or abolish vasomotion, or increase its frequency depending on the initial conditions. In our model the myogenic response is less pronounced for large arteries than for small arteries and occurs at higher values of pressure if the wall thickness is increased. Our results are in agreement with experimental observations concerning a broad range of vessels.     

Mechanics of caudal artery relaxation in control and hypertensive rats

Alterations of smooth muscle function can just as easily stem from mechanical alterations in its ability to relax as from alteration in contraction. Since a failure of arterial smooth muscle to relax may contribute to the development of hypertension, we felt it necessary to study the relaxation process in greater depth. The effect of load on the time course of relaxation of rat caudal artery smooth muscle was analyzed either by comparing afterloaded contractions against various loads or by imposing abrupt alterations in load. Unlike mammalian striated muscles in which relaxation was reported sensitive to loading conditions, relaxation in the smooth muscle of the rat caudal artery (n = 17) was found to be largely independent of loading conditions. This type of relaxation has been termed "inactivation-dependent" relaxation; it is typical of muscle tissue in which the calcium sequestering apparatus is poorly developed. Our results suggest that calcium resequestration, or some biochemical process downstream to it, is the rate-limiting step during relaxation in arterial smooth muscle and that this is not qualitatively different for hypertensive arterial smooth muscle. These analytic techniques were used in the study of relaxation of hypertensive vessels. Quantitative analysis of the relaxation curves showed that both isometric and isotonic relaxation time was prolonged in hypertensive arterial smooth muscle. Prolonged isotonic relaxation indicates that hypertensive arteries remain narrowed for prolonged periods compared with normotensive vessels. Such narrowed vessels may be a factor in the increased total peripheral resistance seen in genetic hypertension.     

Energy state, pH, and vasomotor tone during hypoxia in precontracted pulmonary and femoral arteries

To assess effects of smooth muscle energy state and intracellular pH (pH(i)) on pulmonary arterial tone during hypoxia, we measured ATP, phosphocreatine, P(i), and pH(i) by (31)P-NMR spectroscopy and isometric tension in phenylephrine-contracted rings of porcine proximal intrapulmonary arteries. Hypoxia caused early transient contraction followed by relaxation and late sustained contraction. Energy state and pH(i) decreased during relaxation and recovered toward control values during late contraction. Femoral arterial rings had higher energy state and lower pH(i) under baseline conditions and did not exhibit late contraction or recovery of energy state and pH(i) during hypoxia. In pulmonary arteries, glucose-free conditions abolished late hypoxic contraction and recovery of energy state and pH(i), but endothelial denudation abolished only late hypoxic contraction. NaCN had little effect at 0. 1 and 1.0 mM but caused marked vasorelaxation and decreases in energy state and pH(i) at 10 mM. These results suggest that 1) regulation of tone, energy state, and pH(i) differed markedly in pulmonary and femoral arterial smooth muscle, 2) hypoxic relaxation was mediated by decreased energy state or pH(i) due to hypoxic inhibition of oxidative phosphorylation, 3) recovery of energy state and pH(i) in hypoxic pulmonary arteries was due to accelerated glycolysis mediated by mechanisms intrinsic to smooth muscle, and 4) late hypoxic contraction in pulmonary arteries was mediated by endothelial factors that required hypoxic recovery of energy state and pH(i) for transduction in smooth muscle or extracellular glucose for production and release by endothelium.     

Vasoconstriction of carotid artery induced by hydroperoxides

The influence of hydroperoxides (hydrogen peroxide, t-butylhydroperoxide) on tone of arterial smooth muscle was studied. The results of the experiments which were performed with segments of rabbit carotid arteries under isometric conditions show that peroxide concentrations higher than 10(-4) M induced vasoconstriction. These contractions were reversible when glucose was present in the superfused solution. In the absence of glucose a long-lasting increase in tone was found. The contraction response persisted even in Ca2+-free solution. These results indicate a stimulatory effect of hydroperoxides on vascular smooth muscle probably related to a liberation of intracellularly bound calcium ions.     

Studies of arterial smooth muscle relaxation in younger (16-18 week) and older (28-31 week) spontaneously hypertensive rats

Both isometric and isotonic relaxation rates have previously been reported to be decreased in caudal arterial and mesenteric resistance arterial smooth muscle from 16- to 21-week-old spontaneously hypertensive rats (SHR) compared with muscle from age-matched normotensive Wistar-Kyoto rats (WKY). An increased maximum velocity of shortening (Vmax) and an increased shortening ability (delta Lmax) have also been reported for arterial smooth muscle from 16- to 21-week-old SHR. It has been suggested that both increased narrowing and prolonged narrowing of arteries contribute to the development of hypertension. However, SHR Vmax is not different from WKY Vmax when studying arterial muscle from older (28- to 31-week-old) rats. Thus increased arterial narrowing ability cannot be a contributing factor to the maintenance of hypertension. In this study the role of relaxation rate in the maintenance of hypertension was examined by comparing the relaxation rates of isometric and isotonic contractions of caudal arterial strips from 16- to 21-week-old SHR (n = 9) and WKY (n = 8) and from 28- to 31-week-old SHR (n = 7) and WKY (n = 5). While relaxation rates were lower for 16- to 21-week-old SHR compared with age-matched WKY preparations for both isometric and isotonic contractions, only isometric relaxation rates were found to be different in 28- to 31-week-old SHR compared with 28- to 31-week-old caudal arterial muscle (p less than 0.05). Vmax tended to normalize from a once-elevated velocity, while isometric relaxation rate remained decreased in SHR with ageing and (or) with progression of the hypertensive condition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calcium sensitivity of vasospastic basilar artery after experimental subarachnoid hemorrhage

Arteries that develop vasospasm after subarachnoid hemorrhage (SAH) may have altered contractility and compliance. Whether these changes are due to alterations in the smooth muscle cells or the arterial wall extracellular matrix is unknown. This study elucidated the location of such changes and determined the calcium sensitivity of vasospastic arteries. Dogs were placed under general anesthesia and underwent creation of SAH using the double-hemorrhage model. Vasospasm was assessed by angiography performed before and 4, 7, or 21 days after SAH. Basilar arteries were excised from SAH or control dogs (n = 8-52 arterial rings from 2-9 dogs per measurement) and studied under isometric tension in vitro before and after permeabilization of smooth muscle with alpha-toxin. Endothelium was removed from all arteries. Vasospastic arteries demonstrated significantly reduced contractility to KCl with a shift in the EC(50) toward reduced sensitivity to KCl 4 and 7 days after SAH (P < 0.05, ANOVA). There was reduced compliance that persisted after permeabilization (P < 0.05, ANOVA). Calcium sensitivity was decreased during vasospasm 4 and 7 days after SAH, as assessed in permeabilized arteries and in those contracted with BAY K 8644 in the presence of different concentrations of extracellular calcium (P < 0.05, ANOVA). Depolymerization of actin with cytochalasin D abolished contractions to KCl but failed to alter arterial compliance. In conclusion, it is shown for the first time that calcium sensitivity is decreased during vasospasm after SAH in dogs, suggesting that other mechanisms are involved in maintaining the contraction. Reduced compliance seems to be due to an alteration in the arterial wall extracellullar matrix rather than the smooth muscle cells themselves because it cannot be alleviated by depolymerization of smooth muscle actin.     

K(Ca) channel blockers reveal hyperpolarization and relaxation to K+ in rat isolated mesenteric artery

Raising extracellular K+ concentration ([K+](o)) around mesenteric resistance arteries reverses depolarization and contraction to phenylephrine. As smooth muscle depolarizes and intracellular Ca(2+) and tension increase, this effect of K+ is suppressed, whereas efflux of cellular K+ through Ca(2+)-activated K+ (K(Ca)) channels is increased. We investigated whether K+ efflux through K(Ca) suppresses the action of exogenous K+ and whether it prestimulates smooth muscle Na(+)-K(+)-ATPase. Under isometric conditions, 10.8 mM [K+](o) had no effect on arteries contracted >10 mN, unless 100 nM iberiotoxin (IbTX), 100 nM charybdotoxin (ChTX), and/or 50 nM apamin were present. Simultaneous measurements of membrane potential and tension showed that phenylephrine depolarized and contracted arteries to -32.2 +/- 2.3 mV and 13.8 +/- 1.6 mN (n = 5) after blockade of K(Ca), but 10.8 mM K+ reversed fully the responses (107.6 +/- 8.6 and 98.8 +/- 0.6%, respectively). Under isobaric conditions and preconstriction with phenylephrine, 10.7 mM [K+](o) reversed contraction at both 50 mmHg (77.0 +/- 8.5%, n = 9) and 80 mmHg (83.7 +/- 5.5%, n = 5). However, in four additional vessels at 80 mmHg, raising K+ failed to reverse contraction unless ChTX was present. Increases in isometric and decreases in isobaric tension with phenylephrine were augmented by either ChTX or ouabain (100 microM), whereas neither inhibitor altered tension under resting conditions. Inhibition of cellular K+ efflux facilitates hyperpolarization and relaxation to exogenous K+, possibly by indirectly reducing the background activation of Na(+)-K(+)-ATPase.     

Modulation of lymphatic muscle contractility by the neuropeptide substance P

Substance P (SP) is a neuropeptide associated with sensory innervation of lymphoid tissue and a suspected modulator of lymphatic function in inflammation. Only a few studies have examined the effects of SP on lymphatic contraction, and it is not clear to what extent SP acts directly on the lymphatic muscle and/or endothelium or indirectly through changes in intraluminal filling pressure secondary to increases in capillary permeability/filtration. We tested the effects of SP on the spontaneous contractions of rat isolated mesenteric lymphatic vessels under isometric and isobaric conditions, hypothesizing that low concentrations would stimulate lymphatic pumping by enhancing lymphatic muscle contraction in a manner complementary to the effect of increased preload. Under isometric conditions, SP (10 nM) dramatically enhanced lymphatic chronotropy and inotropy. Unlike guinea pig lymphatics, SP actions were not blocked by cyclooxygenase or PLA(2) inhibition. In the absence of SP, ramp increases in isometric preload resulted in x approximately 1.6 increases in contraction amplitude (Amp) and x approximately 1.7 increases in frequency (Freq). SP increased Freq by x approximately 2.4, Amp by x approximately 1.9, and the Amp-Freq product (AFP) by x approximately 3.5. Under isobaric conditions, the pressure elevation from 0.5 to 10 cmH(2)O in the absence of SP decreased Amp by x approximately 0.6 and increased Freq by x approximately 1.8. SP caused a modest increase in Amp, a robust increase in Freq at all pressures, and shifted the AFP-pressure relationship upward and leftward. Therefore, SP has substantial positive inotropic and chronotropic effects on rat lymphatic muscle, improving pump efficiency independent of the effects of preload and broadening of the working range of the lymphatic pump.     

Prolonged isobaric relaxation time in small mesenteric arteries of the spontaneously hypertensive rat

Prolonged isometric relaxation in hypertensive aortic and caudal arterial smooth muscle has been demonstrated; however, isobaric relaxation in resistance arteries is more pertinent to studies in hypertension. A comparative study of mesenteric arterial isobaric relaxation times was made using spontaneously hypertensive rats (SHR), normotensive Wistar-Kyoto rats (WKY), and MK-421 treated SHR (treatment commenced at 8 weeks of age and was maintained until sacrifice). Relaxation rates of vessels constricting against a range of pressures and achieving different degrees of narrowing or changes in circumference were analyzed. Comparisons were made between SHR, WKY, and MK-421 treated SHR arteries that had constricted from the same initial circumference and against the same magnitude of pressure. The SHR mesenteric arteries relaxed at a slower rate than did the WKY vessels. The normotensive MK-421 treated SHR showed the same prolonged relaxation rate as did the untreated SHR preparations. Thus the slower rate of relaxation in SHR arteries does not appear to be a consequence of the hypertension. Such prolonged time for narrowing would function to increase the average peripheral resistance and thus may contribute to the initiation and maintenance of increased blood pressure.     

Levobupivacaine-induced contraction of isolated rat aorta is calcium dependent

Levobupivacaine is a long-acting local anesthetic that intrinsically produces vasoconstriction in isolated vessels. The goals of this study were to investigate the calcium-dependent mechanism underlying levobupivacaine-induced contraction of isolated rat aorta in vitro and to elucidate the pathway responsible for the endothelium-dependent attenuation of levobupivacaine-induced contraction. Isolated rat aortic rings were suspended to record isometric tension. Cumulative levobupivacaine concentration-response curves were generated in either the presence or absence of the antagonists verapamil, nifedipine, SKF-96365, 2-aminoethoxydiphenylborate, Gd(3+), N(W)-nitro-l-arginine methyl ester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), and methylene blue, either alone or in combination. Verapamil, nifedipine, SKF-96365, 2-aminoethoxydiphenylborate, low calcium concentrations, and calcium-free Krebs solution attenuated levobupivacaine-induced contraction. Gd(3+) had no effect on levobupivacaine-induced contraction. Levobupivacaine increased intracellular calcium levels in vascular smooth muscle cells. L-NAME, ODQ, and methylene blue increased levobupivacaine-induced contraction in endothelium-intact aorta. SKF-96365 attenuated calcium-induced contraction in a previously calcium-free isotonic depolarizing solution containing 100?mmol/L KCl. Levobupivacaine-induced contraction of rat aortic smooth muscle is mediated primarily by calcium influx from the extracellular space mainly via voltage-operated calcium channels and, in part, by inositol 1,4,5-trisphosphate receptor-mediated release of calcium from the sarcoplasmic reticulum. The nitric oxide - cyclic guanosine monophosphate pathway is involved in the endothelium-dependent attenuation of levobupivacaine-induced contraction.     

Mechanical properties of porcine intralobar pulmonary arteries

The isobaric and isovolumetric properties of intrapulmonary arteries were evaluated by placing a highly compliant balloon inside arterial segments. The passive pressure-volume (P-V) curve was obtained by changing volume (0.004 ml/s) and measuring pressure. The isobaric active volume change (delta V) or isovolumetric active pressure change (delta P) generated by submaximal histamine was measured at four different transmural pressures (Ptm's) reached by balloon inflation. The maximal delta P = 11.2 +/- 0.6 cmH2O (mean +/- SE) was achieved at 30.8 +/- 1.2 cmH2O Ptm and maximal delta V = 0.20 +/- 0.02 ml at 16.7 +/- 1.7 cmH2O Ptm. The P-V relationships were similar when volume was increased after either isobaric or isovolumetric contraction. The calculated length-tension (L-T) relationship showed that the active tension curve was relatively flat and that the passive tension at the optimal length was 149 +/- 11% of maximal active tension. These data show that 1) a large elastic component operates in parallel with the smooth muscle in intralobar pulmonary arteries, and 2) the change in resistance associated with vascular expansion of the proximal arteries is independent of the type of contraction that occurs in the more distal arterial segments.     

Intramuscular pressure, force and blood flow in rabbit tibialis anterior muscles during single and repetitive contractions

The elevated intramuscular pressure (IMP) associated with sustained muscle contraction can affect blood flow, and could influence the long-term viability of functional skeletal muscle grafts. We therefore examined the relationship between force, peak IMP and blood flow in the tibialis anterior muscle of the anaesthetized rabbit. During isometric contractions, IMP was related linearly to force, and only the slope of the relationship varied between animals. During isotonic contractions, however, the highest values of IMP were found at the lowest force levels, and IMP appeared to be related to the amount and speed of shortening. During repeated isometric contractions, the ratio of IMP to force varied with time, stimulation pattern and subject. Mean blood flow did not differ appreciably between␣repetitive isometric contractions at duty cycles of 10–40%, and was unrelated to integrated pressure, integrated force, or depth from the surface. We conclude: (1) that IMP is unlikely to affect mean blood flow during cyclic activity that has a duty cycle less than 40%; and (2) that the clinical use of IMP as a predictor of muscle force appears to be justified only for single isometric contractions, and needs to be interpreted cautiously when contractions involve shortening or fatigue. Accepted: 17 November 1997     

A dynamic model of smooth muscle contraction

A dynamic model of smooth muscle contraction is presented and is compared with the mechanical properties of vascular smooth muscle in the rat portal vein. The model is based on the sliding filament theory and the assumption that force is produced by cross-bridges extending from the myosin to the actin filaments. Thus, the fundamental aspects of the model are also potentially applicable to skeletal muscle. The main concept of the model is that the transfer of energy via the cross-bridges can be described as a 'friction clutch' mechanism. It is shown that a mathematical formulation of this concept gives rise to a model that agrees well with experimental observations on smooth muscle mechanics under isotonic as well as isometric conditions. It is noted that the model, without any ad hoc assumptions, displays a nonhyperbolic force-velocity relationship in its high-force portion and that it is able to maintain isometric force in conditions of reduced maximum contraction velocity. Both these findings are consistent with new experimental observations on smooth muscle mechanics cannot be accounted for by the classical Hill model.     

A phenomenological model for estimating metabolic energy consumption in muscle contraction

A phenomenological model for muscle energy consumption was developed and used in conjunction with a simple Hill-type model for muscle contraction. The model was used to address two questions. First, can an empirical model of muscle energetics accurately represent the total energetic behavior of frog muscle in isometric, isotonic, and isokinetic contractions? And second, how does such a model perform in a large-scale, multiple-muscle model of human walking? Four simulations were conducted with frog sartorius muscle under full excitation: an isometric contraction, a set of isotonic contractions with the muscle shortening a constant distance under various applied loads, a set of isotonic contractions with the muscle shortening over various distances under a constant load, and an isokinetic contraction in lengthening. The model calculations were evaluated against results of similar thermal in vitro experiments performed on frog sartorius muscle. The energetics model was then incorporated into a large-scale, multiple-muscle model of the human body for the purpose of predicting energy consumption during normal walking. The total energy estimated by the model accurately reflected the observed experimental behavior of frog muscle for an isometric contraction. The model also accurately reproduced the experimental behavior of frog muscle heat production under isotonic shortening and isokinetic lengthening conditions. The estimated rate of metabolic energy consumption for walking was 29% higher than the value typically obtained from gait measurements.     

Pressure dependence of baroreceptor-mediated vasoconstriction in rat skeletal muscle

To determine whether microvessels in resting or contracting skeletal muscle constrict during baroreceptor activation, vascular diameters were measured in the spinotrapezius muscle of adult rats (n = 12) during occlusion of the common carotid arteries. Neural and myogenic components were distinguished using two types of occlusion: 1) "normal" (arterial pressure was allowed to increase with baroreceptor activation) and 2) "isobaric" (arterial pressure was maintained constant by decreasing blood volume). During normal occlusions, intermediate and small arteriolar diameters decreased in resting and contracting muscle (10-15% and 25-30%, respectively). Large arterioles and all-sized venules distended slightly (approximately 5%) in resting muscle, but diameters were maintained or decreased in contracting muscle. When arterial pressure was maintained constant (isobaric), the microvascular responses to baroreceptor activation in both resting and contracting muscle were essentially eliminated. We conclude that nearly all the arteriolar constriction observed in the spinotrapezius muscle during normal carotid artery occlusion is myogenic in origin, secondary to increased arterial pressure. This pressure-dependent constriction is augmented during skeletal muscle contraction and functional vasodilation.     

Vasomotion dynamics following calcium spiking depend on both cell signalling and limited constriction velocity in rat mesenteric small arteries

Vascular smooth muscle cell contraction depends on intracellular calcium. However, calcium-contraction coupling involves a complex array of intracellular processes. Quantitating the dynamical relation between calcium perturbations and resulting changes in tone may help identifying these processes. We hypothesized that in small arteries accurate quantitation can be achieved during rhythmic vasomotion, and questioned whether these dynamics depend on intracellular signalling or physical vasoconstriction. We studied calcium-constriction dynamics in cannulated and pressurized rat mesenteric small arteries ( approximately 300 microm in diameter). Combined application of tetra-ethyl ammonium (TEA) and BayK8644 induced rhythmicity, consisting of regular and irregular calcium spiking and superposition of spikes. Calcium spikes induced delayed vasomotion cycles. Their dynamic relation could be fitted by a linear second-order model. The dirac impulse response of this model had an amplitude that was strongly reduced with increasing perfusion pressure between 17 and 98 mmHg, while time to peak and relaxation time were the largest at an intermediate pressure (57 mmHg: respectively 0.9 and 2.3 sec). To address to what extent these dynamics reside in intracellular signalling or vasoconstriction, we applied rhythmic increases in pressure counteracting the vasoconstriction. This revealed that calcium-activation coupling became faster when vasoconstriction was counteracted. During such compensation, a calcium impulse response remained that lasted 0.5 sec to peak activation, followed by a 1.0 sec relaxation time, attributable to signalling dynamics. In conclusion, this study demonstrates the feasibility of quantitating calcium-activation dynamics in vasomoting small arteries. These dynamics relate to both intracellular signalling and actual vasoconstriction. Performing such analyses during pharmacological intervention and in genetic models provides a tool for unravelling calcium-contraction coupling in small arteries.     

Recruitment of smooth muscle cells and arterial vasomotion

Investigating the recruitment and synchronization of smooth muscle cells (SMCs) is the key to understanding the physical mechanisms leading to contraction and spontaneous diameter oscillations of arteries, called vasomotion. We improved a method that allows the correlation of calcium oscillations (flashing) of individual SMCs with mean calcium variations and arterial contraction using confocal microscopy. Endothelium-stripped rat mesenteric arteries were cut open, loaded with dual calcium fluorescence probes, and stimulated by increasing concentrations of the vasoconstrictors phenylephrine (PE) and KCl. We found that the number and synchronization of flashing cells depends on vasoconstrictor concentration. At low vasoconstrictor concentration, few cells flash asynchronously and no local contraction is detected. At medium concentration, recruitment of cells is complete and synchronous, leading to strip contraction after KCl stimulation and to vasomotion after PE stimulation. High concentration of PE leads to synchronous calcium oscillations and fully contracted vessels, whereas high concentration of KCl leads to a sustained nonoscillating increase of calcium and to fully contracted vessels. We conclude that the number of simultaneously recruited cells is an important factor in controlling rat mesenteric artery contraction and vasomotion.     

Biomechanical adaptation of porcine carotid vascular smooth muscle to hypo and hypertension in vitro

Previous research in arterial remodeling in response to changes in blood pressure seldom included both hyper- and hypotension. To compare the effects of low and high pressure on arterial remodeling and vascular smooth muscle tone and performance, we have utilized an in vitro model. Porcine carotid arteries were cultured for 3 days at 30 and 170mmHg and compared to controls cultured at 100mmHg for 1 and 3 days. On the first and last day of culture, pressure-diameter and pressure-wall thickness curves were measured under normal smooth muscle tone using a high-resolution ultrasonic device. Last-day experiments included measurements where vascular smooth muscle was contracted or totally relaxed. From the data wall cross-sectional area, Hudetz elastic modulus and a contraction index related to the diameter reduction under normal smooth muscle tone were calculated. We found that although wall cross-sectional area (indicating wall mass) did not change much, Hudetz elastic modulus was significantly reduced in the 3-day hypotension group. Inspection of the wall contraction index suggests that this is due to a reduction in the vascular smooth muscle tone. Further, the peak of contraction index was found to be shifted to higher pressures in the 3-day 170mmHg group. We conclude that vascular smooth muscle performance adapts to both hypo- and hypertension at short time scales and can alter the biomechanics of the vascular wall in vitro.     

Activation of Rho-associated kinase during augmented contraction of the basilar artery to serotonin after subarachnoid hemorrhage

Delayed cerebral vasospasm after subarachnoid hemorrhage (SAH) may be due, in part, to altered regulation of arterial smooth muscle contraction. Contraction of cerebral arteries to serotonin is augmented after experimental SAH. We hypothesized that activation of Rho-associated kinase (Rho kinase) contributes to augmented contraction of cerebral arteries to serotonin after SAH. Autologous arterial blood (SAH) or artificial cerebrospinal fluid (control) was injected into the cisterna magna of anesthetized rabbits. At 2 days after injection, the basilar artery was excised and isometric contraction of arterial rings was recorded. Maximum contraction of the basilar artery to serotonin was augmented about fourfold in SAH compared with control rabbits (P < 0.01). Contraction to histamine was similar in the two groups. Fasudil hydrochloride (3 mumol/l), an inhibitor of Rho kinase, markedly attenuated serotonin-induced contraction. Fasudil had little effect on contractions induced by histamine or phorbol 12,13-dibutyrate. In addition, phosphorylation of myosin phosphatase, a major target of Rho kinase in regulation of smooth muscle contraction, in the basilar artery was examined by Western blotting. In basilar arteries of SAH, but not control, rabbits, serotonin increased phosphorylation of myosin phosphatase about twofold at Thr(853) of the myosin-targeting subunit. These results suggest that enhanced activation of Rho kinase contributes to augmented contraction of the basilar artery to serotonin after SAH.     

Adenosine 5''-triphosphate consumption by smooth muscle as predicted by the coupled four-state crossbridge model.

We have proposed a four-state crossbridge model to explain contraction and the latch state in arterial smooth muscle. Ca(2+)-dependent crossbridge phosphorylation was the only postulated regulatory mechanism and the latchbridge (a dephosphorylated, attached crossbridge) was the only novel element in the model. In this study, we used the model to predict rates of ATP consumption by crossbridge phosphorylation (JPhos) and cycling (JCycle) during isometric and isotonic contractions in arterial smooth muscle; then we compared model predictions with experimental data. The model predicted that JPhos and JCycle were similar in magnitude in isometric contractions, and both increased almost linearly with myosin phosphorylation. The predicted relationship between isometric stress and ATP consumption was quasihyperbolic, but approximately linear when myosin phosphorylation was below 35%, in agreement with most of the available data. Muscle shortening increased the predicted values of JCycle up to 3.7-fold depending on shortening velocity and the level of myosin phosphorylation. The predicted maximum work output per ATP was 7.4-7.8 kJ/mol ATP and was relatively insensitive to changes in myosin phosphorylation. The predicted increase in JCycle with shortening was in agreement with available data, but the model prediction that work output per ATP was insensitive to changes in myosin phosphorylation was unexpected and remains to be tested in future experiments.