The biophysics and biochemistry of smooth muscle contraction

Sinomenine in Cardio-Cerebrovascular Diseases: Potential Therapeutic Effects and Pharmacological Evidences

Cardio-cerebrovascular diseases, as a major cause of health loss all over the world, contribute to an important part of the global burden of disease. A large number of traditional Chinese medicines have been proved effective both clinically and in pharmacological investigations, with the acceleration of the modernization of Chinese medicine. Sinomenine is the main active constituent of sinomenium acutum and has been generally used in therapies of rheumatoid arthritis and neuralgia. Varieties of pharmacological effects of sinomenine in cardio-cerebrovascular system have been discovered recently, suggesting an inspiring application prospect of sinomenine in cardio-cerebrovascular diseases. Sinomenine may retard the progression of atherosclerosis by attenuating endothelial inflammation, regulating immune cells function, and inhibiting the proliferation of vascular smooth muscle cells. Sinomenine also alleviates chronic cardiac allograft rejection relying on its anti-inflammatory and anti-hyperplastic activities and suppresses autoimmune myocarditis by immunosuppression. Prevention of myocardial or cerebral ischemia-reperfusion injury by sinomenine is associated with its modulation of cardiomyocyte death, inflammation, calcium overload, and oxidative stress. The regulatory effects on vasodilation and electrophysiology make sinomenine a promising drug to treat hypertension and arrhythmia. Here, in this review, we will illustrate the pharmacological activities of sinomenine in cardio-cerebrovascular system and elaborate the underlying mechanisms, as well as give an overview of the potential therapeutic roles of sinomenine in cardio-cerebrovascular diseases, trying to provide clues and bases for its clinical usage.

Sensitized airway smooth muscle plasticity and hyperreactivity: a review

To help elucidate the mechanisms underlying asthmatic bronchospasm, the goal of our research has been to determine whether airway smooth muscle (ASM) hyperreactivity was the responsible factor. We reported that in a canine model of asthma, the shortening capacity (DeltaLmax) and velocity (Vo) of in vitro sensitized muscle were significantly increased. This increase was of sufficient magnitude to account for 75% narrowing of the in vivo airway, but maximal isometric force was unchanged. This last feature has been reported by others. Under lightly loaded conditions, ASM completes 75% of its isotonic shortening within the first 2 s. Furthermore, 90% of the increased shortening of ragweed pollen-sensitized ASM (SASM), compared with control (CASM), is complete within the first 2 s. The study of shortening beyond this period will apparently not yield much useful information, and studies of isotonic shortening should be focused on this interval. Although both CASM and SASM showed plasticity and adaptation with respect to isometric force, neither muscle type showed a difference in the force developed in these phases. During isotonic shortening, no evidence of plasticity was seen, but the equilibrated SASM showed increased DeltaLmax and Vo of shortening. Molecular mechanisms of changes in Vo could result from changes in the kinetics of the myosin heavy chain ATPase. Motility assay, however, showed no changes between CASM and SASM in the ability of the purified myosin molecule (SF1) to translocate a marker actin filament. On the other hand, we found that the state of activation of the ATPase by phosphorylation of smooth muscle myosin light chain (molecular mass 20,000 Da) was greater in the SASM. This would account for the increased Vo. Investigating the signalling pathway, we found that whereas [Ca2+]i increased in both isometric and isotonic contraction, there was no significant difference between CASM and SASM. The content and activity of calmodulin were also not different between the 2 muscles. Nevertheless, we did find that content and total activity of smooth muscle myosin light chain kinase (smMLCK) and the abundance of its message were greater; this would explain the increased MLC20 phosphorylation. The binding affinity between Ca2+ and calmodulin and between 4 Ca2+ calmodulin and smMLCK remains to be studied. We conclude that SASM shows increased isotonic shortening capacity and velocity. It also shows increased content and total activity of smMLCK, which is consistent with the increased shortening. Plasticity produced by oscillation is not seen in the shortening muscle, although it is seen with respect to force development. It did not modulate the behaviour of the sensitized muscle.

Contribution of the remodeling response to cerebral vasospasm

The authors review the remodeling response of blood vessels that occurs after various injuries to arteries. The role of this response in vasospasm after subarachnoid hemorrhage (SAH) is reviewed. There is some evidence that cerebral arteries remodel after SAH in that they are less compliant and contractile than normal. Evidence for other features, such as alteration of smooth muscle phenotype, proliferation of cells and synthesis of extracellular matrix, is conflicting and requires a further study. A remodeling response probably contributes to vasospasm but the magnitude of its importance, in relation to smooth muscle contraction, which also occurs, also needs to be further defined.

Pkd2+/- vascular smooth muscles develop exaggerated vasocontraction in response to phenylephrine stimulation

Vascular complications are the leading cause of morbidity and mortality in autosomal dominant polycystic kidney disease. Although evidence suggests an abnormal vascular reactivity, contractile function in Pkd mutant vessels has not been studied previously. Contractile response to phenylephrine (PE; 10(-10) to 10(-4)M), an alpha1-adrenergic receptor agonist, was examined. De-endothelialized Pkd2(+/-) aortic rings generated a higher maximum force (F(max)) than that in wild-type (wt; 5.78 +/- 0.73 versus 2.69 +/- 0.43 mN; P < 0.001) and a significant left shift in PE dosage-response curve. On simultaneous recordings, Pkd2(+/-) aortic helical strips also responded to PE with a greater F(max) but a lesser [Ca(2+)](i) rise, resulting in a greatly enhanced Deltaforce/DeltaCa(2+) ratio than that in wt. At F(max), a higher elevation in the phosphorylated regulatory myosin light chain was observed in Pkd2(+/-) strips. Ca(2+)-dependent calmodulin/myosin light-chain kinase-mediated contraction was examined by direct Ca(2+) (pCa8-5) stimulation to beta-escin permeabilized aortic strips; the pCa-force curve in Pkd2(+/-) strips was not shifted, thereby indicating that PE induced dosage-response alteration that resulted from Ca(2+)-independent mechanisms. Quantitative analyses of contractile proteins demonstrated elevated expressions in smooth muscle alpha-actin and myosin heavy chain in Pkd2(+/-) arteries, changes that likely contribute to the higher F(max). Similar to those in aortas, de-endothelialized Pkd2(+/-) resistance (fourth-order mesenteric) arteries responded to PE with a stronger contraction but a lesser [Ca(2+)](i) rise than in wt. Taken together, the arterial vasculature in Pkd2(+/-) mice exhibits an exaggerated contractile response and increased sensitivity to PE. An enhanced Ca(2+)-independent force generation and elevated contractile protein expression likely contribute to these abnormalities.

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 α-toxin. Endothelium was removed from all arteries. Vasospastic arteries demonstrated significantly reduced contractility to KCl with a shift in the EC50toward 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.

Mechanical properties of (urinary bladder) smooth muscle

This short overview of the mechanical properties of smooth muscle focusses on the force-velocity relation of (mainly pig urinary bladder) smooth muscle, and its dependence on the length of the muscle and its degree of activation. Also the response of the muscle to length and force changes at a rate beyond the physiological range is discussed. The force-velocity relation of this type of muscle can be approximated by the hyperbolic Hill equation, with a normalised maximum shortening velocity in the order of 0.25 muscle lengths/s. As in striated muscle, the maximum isometric force depends on the stretched muscle length and shows a maximum at a certain length. Interestingly, smooth muscle does not normally seem to operate at this length, but far below it. Both the isometric force and the unloaded shortening velocity depend on the degree of activation of the muscle, and so does the 'curvature' of the Hill equation. The series elasticity of the muscle, which can be measured by applying length changes at a rate beyond the physiological shortening velocity, is found partly in the cross-bridges, and partly external to these. An isometric quick release of 4-10% of the muscle length is necessary to remove all tension, depending on the total force exerted by the muscle. Force recovery after such a release is biexponential in a 700 ms window. The slowest component of this recovery, with a time constant in the order of 0.45 s is mainly associated with cycling of the cross-bridges, the fastest with the external series (visco)elasticity. Isometric force development has a time constant in the order of 3 s. indicating that excitation-contraction coupling rather than cross-bridge cycling is rate limiting in this process.

Invited review: significance of spatial and temporal heterogeneity of calcium transients in smooth muscle

The multiplicity of mechanisms involved in regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in smooth muscle results in both intra- and intercellular heterogeneities in [Ca(2+)](i). Heterogeneity in [Ca(2+)](i) regulation is reflected by the presence of spontaneous, localized [Ca(2+)](i) transients (Ca(2+) sparks) representing Ca(2+) release through ryanodine receptor (RyR) channels. Ca(2+) sparks display variable spatial Ca(2+) distributions with every occurrence within and across cellular regions. Individual sparks are often grouped, and fusion of sparks produces large local elevations in [Ca(2+)](i) that occasionally trigger propagating [Ca(2+)](i) waves. Ca(2+) sparks may modulate membrane potential and thus smooth muscle contractility. Sparks may also be the target of other regulatory factors in smooth muscle. Agonists induce propagating [Ca(2+)](i) oscillations that originate from foci with high spark incidence and also represent Ca(2+) release through RyR channels. With increasing agonist concentration, the peak of regional [Ca(2+)](i) oscillations remains relatively constant, whereas both frequency and propagation velocity increase. In contrast, the global cellular response appears as a concentration-dependent increase in peak as well as mean cellular [Ca(2+)](i), representing a spatial and temporal integration of the oscillations. The significance of agonist-induced [Ca(2+)](i) oscillations lies in the establishment of a global [Ca(2+)](i) level for slower Ca(2+)-dependent physiological processes.

Mutagenesis analysis of human SM22: characterization of actin binding

SM22 is a 201-amino acid actin-binding protein expressed at high levels in smooth muscle cells. It has structural homology to calponin, but how SM22 binds to actin remains unknown. We performed site-directed mutagenesis to generate a series of NH(2)-terminal histidine (His)-tagged mutants of human SM22 in Escherichia coli and used these to analyze the functional importance of potential actin binding domains. Purified full-length recombinant SM22 bound to actin in vitro, as demonstrated by cosedimentation assay. Binding did not vary with calcium concentration. The COOH-terminal domain of SM22 is required for actin affinity, because COOH terminally truncated mutants [SM22-(1-186) and SM22-(1-166)] exhibited markedly reduced cosedimentation with actin, and no actin binding of SM22-(1-151) could be detected. Internal deletion of a putative actin binding site (154-KKAQEHKR-161) partially prevented actin binding, as did point mutation to neutralize either or both pairs of positively charged residues at the ends of this region (KK154LL and/or KR160LL). Internal deletion of amino acids 170-180 or 170-186 also partially or almost completely inhibited actin cosedimentation, respectively. Of the three consensus protein kinase C or casein kinase II phosphorylation sites in SM22, only Ser-181 was readily phosphorylated by protein kinase C in vitro, and such phosphorylation greatly decreased actin binding. Substitution of Ser-181 to aspartic acid (to mimic serine phosphorylation) also reduced actin binding. Immunostains of transiently transfected airway myocytes revealed that full-length NH(2)-terminal FLAG-tagged SM22 colocalizes with actin filaments, whereas FLAG-SM22-(1-151) does not. These data confirm that SM22 binds to actin in vitro and in vivo and, for the first time, demonstrate that multiple regions within the COOH-terminal domain are required for full actin affinity.

The length dependency of calcium activated contractions in the femoral artery smooth muscle studied with different methods of skinning

The relationship between the calcium concentration and the isometric tension obtained with different techniques of skinning provides information on the biochemical events of contraction in vascular smooth muscle. Muscle preparations of the rabbit femoral artery were skinned with triton X-100, saponin, beta-escin and alpha-toxin and the relationship between the calcium concentration and isometric tension was determined at different preparation lengths. We determined the calcium sensitivity as a function of muscle length with different techniques of skinning. At a pCa of 6.0, triton X-100 skinned smooth muscle of the femoral artery generated 50% of the maximal tension. In alpha-toxin skinned preparations, this calcium sensitivity was shifted to a pCa of 5.6. The sensitivity of the saponin and 3-escin skinned preparations were in between those of the triton X-100 and the alpha-toxin skinned preparations. The cooperativity of the regulation of contraction varied among the differently skinned preparations between 3 (alpha-toxin) and 6 (triton X-100). The relationships between the calcium concentration and the isometric tension of the differently skinned preparations up to the optimal length for tension generation did not exhibit any length dependency. The length tension relationship, obtained from the maximal response at the highest calcium concentration is in line with that from other studies. The presence of intracellular proteins and membranes affects the regulation of contraction in smooth muscle of the femoral artery.

Simultaneous measures of contraction and intracellular calcium in single, cultured smooth muscle cells

Simple methods are presented for quantitating contraction and intracellular calcium simultaneously in single, cultured smooth muscle cells. These methods are the first to demonstrate that reliable velocities of cell shortening can be measured in cultured smooth muscle cells and that cells in vitro exhibit shortening velocities comparable to those measured in the fastest phasic muscles in situ. Temporal relationships between changes in intracellular calcium and shortening within single cells were determined with a resolution of 100 ms and were consistent with measures in more "classical" preparations. Intracellular calcium rose quickly and transiently 10-fold above the basal level of 80-90 nM in response to the muscarinic agonist, carbachol. Shortening of the cells occurred 200 ms after intracellular calcium began to rise. The sensitivity and reliability of these methods allowed the effects of different stimuli to be easily resolved. The present report demonstrates that genuine contractility need not be ignored in cultured smooth muscle cells and that the temporal relations between shortening and intracellular calcium mobilization can be quantitatively assessed in controlled in vitro environments.

The effect of length on the sensitivity to phenylephrine and calcium in intact and skinned vascular smooth muscle

The length dependency of the sensitivity to activators of the smooth muscle of different blood vessels is not yet fully understood. Muscle preparations of the aorta, the femoral artery and the portal vein of the rabbit were investigated for the length dependency of the sensitivity to phenylephrine and calcium in both intact and triton X-100 skinned preparations. For intact smooth muscles we found that at increased preparation length, the sensitivity of contraction was increased. The femoral artery showed the largest effect and the portal vein the smallest. In the skinned preparations of the three preparations the calcium sensitivity was not dependent on the preparation length. We conclude that the changes of the sensitivity in intact preparations are not caused by changes of the calcium sensitivity of the contractile proteins.

Dissociation between hysteresivity and tension in constricted tracheal and parenchymal strips

The object of this study was to investigate how changes in the contractile state of smooth muscle would modify oscillatory mechanics of tracheal muscle and lung parenchyma during agonist challenge. Guinea pig tracheal and parenchymal lung strips were suspended in an organ bath. Measurements of length (L) and tension (T) were recorded during sinusoidal oscillations under baseline conditions and after challenge with 1 mM ACh. Measurements were also obtained in strips pretreated with the calmodulin inhibitor calmidazolium (Cmz) or staurosporine (Stauro), a protein kinase C inhibitor. Elastance (E) and resistance (R) were calculated by fitting changes in T, L, and DeltaL/Deltat to the equation of motion. Hysteresivity (eta) was obtained from the following equation: eta = (R/E)2pif, where f is frequency. Finally, maximal unloaded shortening velocity during electrical field stimulation was measured in Cmz-pretreated and control tracheal strips. In tracheal strips, pretreatment with Cmz caused a significant decrease in the eta response to ACh challenge and in maximal unloaded shortening velocity measured during electrical field stimulation; Stauro decreased the T, E, and R response to ACh. In parenchymal strips, Cmz decreased the eta response, whereas Stauro had no effect. These results suggest that modifications in the contractile state of the smooth muscle are reflected in changes in the hysteretic behavior and that T and eta may be controlled independently. Second, inasmuch as changes in eta were similar in parenchymal and tracheal strips, the contractile element is implicated as the structure responsible for constriction-induced changes in the mechanical behavior of the lung periphery.

Proximal and distal esophageal contractions have similar manometric features

The human esophagus is composed of striated muscle proximally and of smooth muscle distally with a transition zone between the two. Striated muscle contracts much faster than smooth muscle. The change in pressure over time (dP/dt) of the contraction amplitude should therefore be higher in proximal than in distal esophagus, reflecting the presence of striated muscle proximally. There were 34 normal esophageal manometries of patients analyzed for swallow amplitude and dP/dt in the pharynx and esophagus. An additional 11 healthy controls were similarly studied. Amplitudes in pharynx and proximal and distal esophagus were not different. The mid-esophagus had a pressure trough (P < 0.001). The dP/dt in the pharynx was much higher than that in the esophagus (P < 0.001). The dP/dt of proximal and distal esophagus were of the same order of magnitude. The manometric behavior of the striated muscle portion of the proximal esophagus differs from that seen in the pharynx and shows similar characteristics to distal esophageal smooth muscle.

Regulation of intracellular calcium oscillations in porcine tracheal smooth muscle cells

Using real-time confocal microscopy, we examined the dynamic intracellular Ca2+ concentration ([Ca2+]i) response of porcine tracheal smooth muscle (TSM) cells to acetylcholine (ACh). Exposure to ACh caused regenerative, propagating [Ca2+]i oscillations. The amplitude and fall time of the [Ca2+]i oscillations were inversely correlated to basal [Ca2+]i, whereas the frequency and rise time were directly correlated to basal [Ca2+]i. ACh-induced [Ca2+]i oscillations were initiated in the absence of extracellular Ca2+ and after membrane depolarization with KCl, suggesting that 1) [Ca2+]i oscillations primarily arise by release from internal stores such as the sarcoplasmic reticulum (SR), and 2) Ca2+ influx is necessary for maintenance of oscillations. Exposure to both caffeine and ryanodine inhibited ongoing ACh-induced [Ca2+]i oscillations, suggesting a role for caffeine-sensitive ryanodine receptor (RyR) SR Ca2+ channels. Inhibition of SR Ca2+ reuptake by thapsigargin increased basal [Ca2+]i and decreased [Ca2+]i oscillation amplitude, suggesting that Ca2+ reuptake is also essential. The present results suggest that [Ca2+]i oscillations in porcine TSM cells involve repetitive Ca2+ release and reuptake from RyR channels, perhaps through a Ca2+ -induced Ca2+ release mechanism.

The time course of myosin light-chain phosphorylation in blood-induced vasospasm

The phosphorylation of an M(r) 20,000 myosin light chain (MLC20) promotes the generation of contractile force through actin-myosin adenosine triphosphatase in most agonist-mediated vascular smooth muscle cell contraction. However, the role of calcium-mediated contractile processes in sustained arterial narrowing after subarachnoid hemorrhage remains unknown. In a femoral artery model of vasospasm, whole blood was applied to arteries in 54 rats for periods of 2 to 10 days; the contralateral artery treated with platelet-rich plasma served as matched control. During the early stage of vasospasm (Days 2-5), in the media of arteries exposed to blood, MLC20 phosphorylation (including diphosphorylated forms) increased significantly (30-38%; P < 0.05); total medial MLC20 during this interval was comparable to that in controls. After 5 days, however, total MLC20 decreased markedly (> 90%; P < 0.01) compared with controls; phosphorylated MLC20 was undetectable during this interval. MLC20-mediated contractile processes may be prominent in the early stages of arterial narrowing after subarachnoid hemorrhage; later stages are associated with the loss of MLC20 and the possible persistence of arterial narrowing by other mechanisms.

Calcium and smooth muscle contraction

The fact that smooth muscle exists in almost every hollow organ and is involved in a large number of disease states has led to a vast increase in smooth muscle research, covering areas from testing response to antagonists and agonists to measuring the molecular force generated by a single actin filament. Yet, the exact mechanisms regulating contractile response of smooth muscle remain unsolved. Calcium has been a central player in mediating smooth muscle contraction through binding with calmodulin, although there is evidence showing that under special circumstances smooth muscle can contract without change in intracellular Ca2+. In addition to the major regulatory pathway of Ca(2+)-calmodulin-myosin light chain kinase, there are other thin filament linked regulatory mechanisms in which Ca(2+)-calmodulin dependent phosphorylation of calponin and caldesmon may be involved. Ca2+ sensitivity of smooth muscle contraction may vary under different situations and this has recently been recognized as an important regulatory mechanism. Examples are protein kinase C (PKC) dependent phosphorylation of myosin light chain kinase which results in partial inhibition of contraction, and activation of myosin light chain phosphatase. There is new evidence showing that not only does Ca2+ regulate contraction by regulating the interaction of contractile proteins in smooth muscle, but also that shortening of smooth muscle itself reduces intracellular Ca2+ concentration, via a negative feedback.