Ca2+, cAMP, and changes in myosin phosphorylation during contraction of smooth muscle

Rho-associated kinase (ROCK) function is essential for cell cycle progression, senescence and tumorigenesis

Rho-associated kinases 1 and 2 (ROCK1/2) are Rho-GTPase effectors that control key aspects of the actin cytoskeleton, but their role in proliferation and cancer initiation or progression is not known. Here, we provide evidence that ROCK1 and ROCK2 act redundantly to maintain actomyosin contractility and cell proliferation and that their loss leads to cell-cycle arrest and cellular senescence. This phenotype arises from down-regulation of the essential cell-cycle proteins CyclinA, CKS1 and CDK1. Accordingly, while the loss of either Rock1 or Rock2 had no negative impact on tumorigenesis in mouse models of non-small cell lung cancer and melanoma, loss of both blocked tumor formation, as no tumors arise in which both Rock1 and Rock2 have been genetically deleted. Our results reveal an indispensable role for ROCK, yet redundant role for isoforms 1 and 2, in cell cycle progression and tumorigenesis, possibly through the maintenance of cellular contractility.

Emodin inhibits tonic tension through suppressing PKCδ-mediated inhibition of myosin phosphatase in rat isolated thoracic aorta

BACKGROUND AND PURPOSE

Dysregulated tonic tension and calcium sensitization in blood vessels has frequently been observed in many cardiovascular diseases. Despite a huge therapeutic potential, little is known about natural products targeting tonic tension and calcium sensitization.

EXPERIMENTAL APPROACH

We screened natural products for inhibitory effects on vasoconstriction using the rat isolated thoracic aorta and found that an anthraquinone derivative, emodin, attenuated tonic tension. Organ bath system, primary vascular smooth muscle cells, confocal microscopy and Western blot analysis were employed to demonstrate the suppressive effects of emodin on PKCδ-mediated myosin phosphatase inhibition.

KEY RESULTS

Emodin, an active ingredient of Polygonum multiflorum extract, inhibited phenylephrine-induced vasoconstriction in rat isolated thoracic aorta, and inhibited vasoconstriction induced by 5-HT and endothelin-1. It also generally suppressed vasoconstrictions mediated by voltage-operated, store-operated calcium channels and intracellular calcium store. However, emodin did not affect agonist-induced calcium increases in primary smooth muscle cells. In contrast, post-treatment with emodin following phenylephrine stimulation potently suppressed tonic tension in rat aortic rings. Western blot analysis revealed that emodin inhibited phenylephrine-induced phospho-myosin light chain (pMLC) and the phosphorylation of myosin-targeting subunit and C-kinase-activated protein phosphatase-1 inhibitor (CPI-17). This was mediated by selective inhibition of PKCδ, whereas PKCα was not involved.

CONCLUSION AND IMPLICATIONS

Emodin attenuates tonic tension through the blockade of PKCδ and CPI-17-mediated MLC-phosphatase inhibition. This new mode of action for the suppression of tonic tension and structural insights into PKCδ inhibition revealed by emodin may provide new information for the development of modulators of tonic tension and for the treatment of hypertension.

Signaling processes for initiating smooth muscle contraction upon neural stimulation

Relationships among biochemical signaling processes involved in Ca2+/calmodulin (CaM)-dependent phosphorylation of smooth muscle myosin regulatory light chain (RLC) by myosin light chain kinase (MLCK) were determined. A genetically-encoded biosensor MLCK for measuring Ca(2+)-dependent CaM binding and activation was expressed in smooth muscles of transgenic mice. We performed real-time evaluations of the relationships among [Ca2+](i), MLCK activation, and contraction in urinary bladder smooth muscle strips neurally stimulated for 3 s. Latencies for the onset of [Ca2+](i) and kinase activation were 55 +/- 8 and 65 +/- 6 ms, respectively. Both increased with RLC phosphorylation at 100 ms, whereas force latency was 109 +/- 3 ms. [Ca2+](i), kinase activation, and RLC phosphorylation responses were maximal by 1.2 s, whereas force increased more slowly to a maximal value at 3 s. A delayed temporal response between RLC phosphorylation and force is probably due to mechanical effects associated with elastic elements in the tissue. MLCK activation partially declined at 3 s of stimulation with no change in [Ca2+](i) and also declined more rapidly than [Ca2+](i) during relaxation. The apparent desensitization of MLCK to Ca2+ activation appears to be due to phosphorylation in its calmodulin binding segment. Phosphorylation of two myosin light chain phosphatase regulatory proteins (MYPT1 and CPI-17) or a protein implicated in strengthening membrane adhesion complexes for force transmission (paxillin) did not change during force development. Thus, neural stimulation leads to rapid increases in [Ca2+](i), MLCK activation, and RLC phosphorylation in phasic smooth muscle, showing a tightly coupled Ca2+ signaling complex as an elementary mechanism initiating contraction.

Myosin cross-bridge kinetics and the mechanism of catch

Catch force in molluscan smooth muscle requires little, if any, energy input and is controlled by the phosphorylation state of the thick filament-associated mini-titin, twitchin. The kinetic parameters of myosin cross-bridge turnover in permeabilized catch muscle, and how they are potentially modified by the catch mechanism, were determined by single turnover measurements on myosin-bound ADP. Under isometric conditions, there are fast and slow components of cross-bridge turnover that probably result from kinetic separation of calcium-bound and calcium-free cross-bridge pools. The structure responsible for catch force maintenance at intermediate [Ca+2] does not alter the processes responsible for the fast and slow components under isometric conditions. Also, there is no measurable turnover of myosin-bound ADP during relaxation of catch force by phosphorylation of twitchin at pCa > 8. The only effects of the catch link on myosin-bound ADP turnover are 1), a small, very slow extra turnover when catch force is maintained at very low [Ca+2] (pCa > 8); and 2), attenuation of the shortening-induced increase in turnover at subsaturating [Ca(+2)]. These limited interactions between the catch link and myosin cross-bridge turnover are consistent with the idea that catch force is maintained by a thick and thin filament linkage other than the myosin cross-bridge.

Inhibition of myosin light-chain phosphorylation inverts the birefringence response of porcine airway smooth muscle

Muscle birefringence, caused mainly by parallel thick filaments, increases in smooth muscle during stimulation, signalling thick filament formation upon activation. The reverse occurs in skeletal muscle, where a decrease in birefringence has been correlated with crossbridge movement away from the thick filaments. When force generation by trachealis muscle was inhibited with wortmannin, which inhibits myosin light-chain phosphorylation and thick-filament formation, but not the calcium increase caused by stimulation, the birefringence response inverted, suggesting crossbridge movement similar to that of skeletal muscle. Resistance to quick stretches was much greater in stimulated muscle than in unstimulated muscle before wortmannin treatment and no different in stimulated and unstimulated muscle after force inhibition by wortmannin. Before wortmannin treatment, stimulation reduced thick-filament cross-sectional areas in electron micrographs by 44%. After force inhibition by wortmannin, filament areas were not significantly different in stimulated and unstimulated muscle and not significantly different from those of relaxed muscle without wortmannin treatment. These results suggest that myofibrillar-space calcium causes crossbridges to move away from the thick filaments without firmly attaching to thin filaments.

Stretch-induced triphosphorylation of myosin light chain and myogenic tone in canine basilar artery

The relationship between phosphorylation of 20,000 Da myosin light chain (MLC20) and contraction in response to mechanical stretch was investigated in the canine basilar artery. A slow stretch (at a rate of 1 mm/s and a stimulus period for 15 min) increased triphosphorylated MLC20 despite lowered intracellular calcium concentration and mechanical activities, such as myogenic tone, shortening velocity and stiffness of the artery. Nicardipine, a Ca2+ channel blocker, and ML-9, a myosin light chain kinase (MLCK) inhibitor, partially inhibited the stretch-induced MLC20 phosphorylation. The remained phosphorylation was further reduced by calphostin C, a protein kinase C (PKC) inhibitor. Y-27632, a Rho-kinase inhibitor, inhibited phosphorylation of myosin light chain phosphatase and attenuated MLC20 phosphorylation. These results suggest that slow stretch induces triphosphorylation of MLC20, which is mediated by MLCK, PKC, and Rho-kinase, and that the triphosphorylation of MLC20 does not result in myogenic contraction, rather seems to counteract it.

Human atrial myosin light chain 1 expression attenuates heart failure

Most patients with hypertrophic cardiomyopathy and congenital heart diseases express the atrial essential myosin light chains (ALC-1) in their ventricles, replacing the ventricular essential light chains (VLC-1). VLC-1/ALC-1 isoform shift is correlated with increases in cardiac contractile parameters of a transgenic rat model overexpressing hALC-1 in the heart (TGR/hALC-1) compared to normal WKY rats. To investigate, whether the benefical effects of the hALC-1 on cardiac contractility could attenuate contractile failure of the overloaded heart, aortocaval shunt operations of 9-10 weeks old WKY and TGR/hALC-1 were performed. 5 weeks later, both animals groups were sacrificed for analysis of cardiac contraction and transgene expression. Control animals were operated but remained normal body and heart weights. The whole heart contractility parameters were evaluated using the Langendorff heart preparation. Shunt-operated TGR/hALC-1 and WKY rats developed comparable levels of cardiac hypertrophy which was associated with significant reduction of contractile parameters of the Langendorff hearts. However, the decline of cardiac contractility was less pronounced in shunt-operated TGR/hALC-1 compared to shunt-operated WKY. In fact, developed left ventricular pressure as well as maximal velocity of pressure development and relaxation were significantly higher in shunt-operated TGR/hALC-1 as compared to shunt-operated WKY. Expression of hALC-1 was 17 microg/mg whole SDS-protein in control (sham-operated) controls and declined significantly to 14 microg/mg whole SDS-protein in hypertrophied TGR/hALC-1. These results demonstrate that the expression of hALC-1 could have a beneficial effect on the overloaded hypertrophied heart.

Heat shock protein 20-mediated force suppression in forskolin-relaxed swine carotid artery

Increases in cyclic nucleotide levels induce smooth muscle relaxation by deactivation [reductions in myosin regulatory light chain (MRLC) phosphorylation (e.g., by reduced [Ca(2+)])] or force suppression (reduction in force without reduction in MRLC phosphorylation). Ser(16)-heat shock protein 20 (HSP20) phosphorylation is the proposed mediator of force suppression. We evaluated three potential hypotheses whereby Ser(16)-HSP20 phosphorylation could regulate smooth muscle force: 1) a threshold level of HSP20 phosphorylation could inactivate a thin filament as a whole, 2) phosphorylation of a single HSP20 could fully inactivate a small region of a thin filament, or 3) HSP20 phosphorylation could weakly inhibit myosin binding at either the thin- or thick-filament level. We tested these hypotheses by analyzing the dependence of force on Ser(16)-HSP20 phosphorylation in swine carotid media. First, we determined that swine HSP20 has a second phosphorylation site at Ser(157). Ser(157)-HSP20 phosphorylation values were high and did not change during contractile activation or forskolin-induced relaxation. Forskolin significantly increased Ser(16)-HSP20 phosphorylation. The relationship between Ser(16)-HSP20 phosphorylation and force remained linear and was shifted downward in partially activated muscles relaxed with forskolin. Neither forskolin nor nitroglycerin induced actin depolymerization as detected using the F/G-actin ratio method in smooth muscle homogenates. These results suggest that force suppression does not occur in accordance with the first hypothesis (inactivation of a thin filament as a whole). Our data are more consistent with the second and third hypotheses that force suppression is mediated by full or partial inhibition of local myosin binding at the thin- or thick-filament level.

Gene delivery of l-caldesmon protects cytoskeletal cell membrane integrity against adenovirus infection independently of myosin ATPase and actin assembly

The cytoskeleton is critical to the viral life cycle. Agents like cytochalasin inhibit viral infections but cannot be used for antiviral therapy because of their toxicity. We report the efficacy, safety, and mechanisms by which gene delivery of human wild-type low-molecular-weight caldesmon (l-CaD) protects cell membrane integrity from adenovirus infection in a DF-1 cell line, an immortalized avian fibroblast that is null for l-CaD. Transfection with an adenovirus (Ad)-controlled construct mediated a dose-dependent decline in transcellular resistance. In accordance with a computational model of cytoskeletal membrane properties, Ad disturbed cell-cell and cell-matrix adhesion and membrane capacitance. Transfection with the Ad-l-CaD construct attenuated adenovirus-mediated loss in transcellular resistance. Quantitation of vinculin-stained plaques revealed an increase in total focal contact mass in monolayers transfected with the Ad-l-CaD construct. Expression of l-CaD protected transcellular resistance through primary effects on membrane capacitance and independently of actin solubility and effects on prestress, as measured by the decline in isometric tension in response to cytochalasin D. Expression of l-CaD exhibited less Trypan blue cell toxicity than cytochalasin, and, unlike cytochalasin, it did not interfere with wound closure or adversely effect transcellular resistance. These findings demonstrate the gene delivery of wild-type human l-CaD as a potentially efficacious and safe agent that inhibits some of the cytopathic effects of adenovirus.

The myogenic response in isolated rat cerebrovascular arteries: smooth muscle cell model

Previous models of the cerebrovascular smooth muscle cell have not addressed the interaction between the electrical, chemical, and mechanical components of cell function during the development of active tension. These models are primarily electrical, biochemical or mechanical in their orientation, and do not permit a full exploration of how the smooth muscle responds to electrical or mechanical forcing. To address this issue, we have developed a new model that consists of two major components: electrochemical and chemomechanical subsystem models of the smooth muscle cell. Included in the electrochemical model are models of the electrophysiological behavior of the cell membrane, fluid compartments, Ca2+ release and uptake by the sarcoplasmic reticulum (SR), and cytosolic Ca2+ buffering, particularly by calmodulin (CM). With this subsystem model, we can study the mechanics of the production of intracellular Ca2+ transient in response to membrane voltage clamp pulses. The chemomechanical model includes models of: (a) the chemical kinetics of myosin phosphorylation, and the formation of phosphorylated (cycling) myosin cross-bridges with actin, as well as attached (non-cycling) latch-type cross-bridges; and (b) a model of force generation and mechanical coupling to the contractile filaments and their attachments to protein structures and the skeletal framework of the cell. The two subsystem models are tested independently and compared with data. Likewise, the complete (combined) cell model responses to voltage pulse stimulation under isometric and isotonic conditions are calculated and compared with measured single cell length-force and force-velocity data obtained from literature. This integrated cell model provides biophysically based explanations of electrical, chemical, and mechanical phenomena in cerebrovascular smooth muscle, and has considerable utility as an adjunct to laboratory research and experimental design.

cGMP-mediated phosphorylation of heat shock protein 20 may cause smooth muscle relaxation without myosin light chain dephosphorylation in swine carotid artery

Nitrovasodilators such as nitroglycerine, via production of nitric oxide and an increase in [cGMP], can induce arterial smooth muscle relaxation without proportional reduction in myosin light chain (MLC) phosphorylation or myoplasmic [Ca2+]. These findings suggest that regulatory systems, other than MLC phosphorylation and Ca2+, partially mediate nitroglycerine-induced relaxation. In swine carotid artery, we found that a membrane-permeant cGMP analogue induced relaxation without MLC dephosphorylation, suggesting that cGMP mediated the relaxation. Nitroglycerine-induced relaxation was associated with a reduction in O2 consumption, suggesting that the interaction between phosphorylated myosin and the thin filament was inhibited. Nitroglycerine-induced relaxation was associated with a 10-fold increase in the phosphorylation of a protein on Ser16. We identified this protein as heat shock protein 20 (HSP20), a member of a family of proteins known to bind to thin filaments. When homogenates of nitroglycerine-relaxed tissues were centrifuged at 6000 g, phosphorylated HSP20 preferentially sedimented in the pellet, suggesting that phosphorylation of HSP20 may increase its affinity for the thin filament. We noted that a domain of HSP20 is partially homologous to the 'minimum inhibitory sequence' of skeletal troponin I. The peptide HSP20110-121, which contains this domain, bound to actin-containing filaments only in the presence of tropomyosin, a characteristic of troponin I. High concentrations of HSP20110-121 abolished Ca2+-activated force in skinned swine carotid artery. HSP20110-121 also partially decreased actin-activated myosin S1 ATPase activity. These data suggest that cGMP-mediated phosphorylation of HSP20 on Ser16 may have a role in smooth muscle relaxation without MLC dephosphorylation. HSP20 contains an actin-binding sequence at amino acid residues 110-121 that inhibited force production in skinned carotid artery. We hypothesize that phosphorylation of HSP20 regulates force independent of MLC phosphorylation via binding of HSP20 to thin filaments and inhibition of cross-bridge cycling.

Effect of Mg2+ on stress, myosin phosphorylation, and ATPase activity in detergent-skinned swine carotid media

Smooth muscle contraction has a relatively high requirement for free magnesium (Mg2+). In this study we examined the effect of Mg2+ concentration ([Mg2+]) on Ca2+-dependent stress development and stress maintenance, myosin ATPase activity, and myosin light chain (MLC) phosphorylation levels in Triton X-100 detergent-skinned fibers of the swine carotid media. Increasing [Mg2+] in a stepwise fashion from 0.1 to 6 mM 1) decreased the magnitude and Ca2+ sensitivity of stress development but augmented the amount of stress maintained without proportional MLC phosphorylation, 2) produced a greater decrease in the Ca2+ sensitivity of MLC phosphorylation than that of stress development, and 3) decreased myosin ATPase activity. These findings demonstrate that Mg2+ differentially modulates the MLC phosphorylation-dependent development of stress and the MLC phosphorylation-independent maintenance of stress. We suggest that increases in [Mg2+] enhance stress maintenance by increasing [MgADP], thus increasing the number of cross bridges in a force-generating state, and by a direct effect on the pathway responsible for Ca2+-dependent, MLC phosphorylation-independent contractions.

Myosin light chain phosphorylation and contractile proteins in a canine two-hemorrhage model of subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Subarachnoid hemorrhage (SAH) impairs both contraction and relaxation response in cerebral arteries. We tested the hypothesis that cerebral vasospasm might be ATP-independent contraction, such as latch state, and protein synthesis might be substantially downregulated due to ATP consumption after long-lasting contraction.

METHODS

Chronic cerebral vasospasm was induced in the canine 2-hemorrhage model of SAH. The normal and spastic basilar arteries were stabilized in Krebs-Henseleit solution, and contraction was induced by 30 micromol/L prostaglandin F2alpha (PGF2alpha) in vitro and in vivo. Before and at 15 minutes and 1 hour after the treatment with PGF2alpha, the levels of phosphorylated 20-kDa myosin light chain (MLC20) were measured. The time course of expression of contraction proteins actin and MLC20, and contraction-inhibiting proteins h-caldesmon and calponin was determined by immunoblotting techniques.

RESULTS

A significant vasospasm occurred in the basilar artery during days 4 to 21, most prominently on days 7 and 14. There were no significant differences in the baseline levels of phosphorylated MLC20 between normal and spastic basilar arteries. The increase in MLC20 phosphorylation by PGF2alpha was significantly attenuated in the spastic basilar artery in vitro and in vivo (P<0.05). The immunoreactivity for actin, h-caldesmon, and calponin in the spastic basilar arteries was progressively decreased until day 14 and returned to the normal level on day 21. In contrast, protein levels of MLC20 did not significantly change during days 0 to 21.

CONCLUSIONS

Chronic cerebral vasospasm closely resembles the latch state, and temporary deficiencies of contractile proteins may result from increased destruction and inhibition of protein synthesis.

Caldesmon inhibits active crossbridges in unstimulated vascular smooth muscle: an antisense oligodeoxynucleotide approach

Caldesmon is a thin-filament-associated protein believed to be important in the regulation of smooth muscle contraction, although the precise mechanism is unknown. We used antisense oligodeoxynucleotides to produce intact swine carotid smooth muscle tissue deficient in h-caldesmon. Caldesmon content was decreased by 78% after 7 days in culture with antisense oligodeoxynucleotides but was unchanged in tissues in the presence of sense oligodeoxynucleotides or vehicle. Antisense oligodeoxynucleotides produced a significant decrease in the caldesmon/actin ratio, but no change was measured in the calponin/actin ratio, suggesting that the effect was specific to caldesmon and not other thin-filament-associated proteins. Basal and KCl-stimulated levels of myosin light chain phosphorylation were not different among tissues from all 3 groups. In contrast, h-caldesmon-deficient tissues produced 62% less KCl-induced force than controls. Unstimulated h-caldesmon-deficient smooth muscle tissues stretched and then released, redeveloped force, demonstrating active crossbridge cycling; strips containing normal h-caldesmon content did not redevelop force on release. We suggest that in resting vascular smooth muscle, active crossbridges are inhibited by caldesmon. Therefore, regulation of smooth muscle includes a thin-filament-based disinhibition component.

Studies on vasorelaxation by tetrapentylammonium ions in rat aortic rings

Tetrapentylammonium ions (TPA+) relaxed the isolated rat aortic rings precontracted with phenylephrine and high extracellular K+ in a concentration-dependent manner with respective IC50 values of 38.9 +/- 3.9 microM and 40.2 +/- 2.9 microM. Other quaternary ammonium ions with a carbon side chain of varying length did not induce relaxation. The relaxant effect of TPA+ was independent of the presence of the endothelium, and was unaffected by various putative blockers of K+ channels such as iberiotoxin (100 nM), glibenclamide (3 microM) and 4-aminopyridine (1 mM). In addition, tetrodotoxin (3 microM), indomethacin (10 microM) and methylene blue (10 microM) had no effect on the TPA+-induced relaxation. TPA+ (50 microM) and procaine (10 mM) completely abolished the phasic contractile response to caffeine in Ca2+-free solution. In the absence of extracellular Ca2+, phorbol 12,13-diacetate (PDA) evoked a sustained tension and TPA+ concentration-dependently reduced the contraction with IC50 of 30.7 +/- 3.1 microM. TPA+ reduced the sustained tension of the similar magnitude induced by phenylephrine, 60 mM K+ and active phorbol ester with similar potencies. These results indicate that TPA+ could act as a non-selective relaxant in arterial smooth muscle. This vasorelaxant effect is unique for TPA+ since other quaternary ammonium ions did not show the similar action in the rat aorta.

Both N-terminal myosin-binding and C-terminal actin-binding sites on smooth muscle caldesmon are required for caldesmon-mediated inhibition of actin filament velocity

It has been suggested that the tethering caused by binding of the N-terminal region of smooth muscle caldesmon (CaD) to myosin and its C-terminal region to actin contributes to the inhibition of actin-filament movement over myosin heads in an in vitro motility assay. However, direct evidence for this assumption has been lacking. In this study, analysis of baculovirus-generated N-terminal and C-terminal deletion mutants of chicken-gizzard CaD revealed that the major myosin-binding site on the CaD molecule resides in a 30-amino acid stretch between residues 24 and 53, based on the very low level of binding of CaDDelta24-53 lacking the residues 24-53 to myosin compared with the level of binding of CaDDelta54-85 missing the adjacent residues 54-85 or of the full-length CaD. As expected, deletion of the region between residues 24 and 53 or between residues 54 and 85 had no effect on either actin-binding or inhibition of actomyosin ATPase activity. Deletion of residues 24-53 nearly abolished the ability of CaD to inhibit actin filament velocity in the in vitro motility experiments, whereas CaDDelta54-85 strongly inhibited actin filament velocity in a manner similar to that of full-length CaD. Moreover, CaD1-597, which lacks the major actin-binding site(s), did not inhibit actin-filament velocity despite the presence of the major myosin-binding site. These data provide direct evidence for the inhibition of actin filament velocity in the in vitro motility assay caused by the tethering of myosin to actin through binding of both the CaD N-terminal region to myosin and the C-terminal region to actin.

Isometric tension of cultured endothelial cells: new technical aspects

In this paper new technical aspects are discussed in the measurement of the low amount of force typically expressed in cultured endothelial cells. We illustrate how potential background noises interfere with signal acquisition. We present a new generation prototype that measures isometric tension in vitro in multiple samples and in more than on isometric vector. We report that thrombin increases isometric tension in at least two separate vectors that are directed in opposite directions. We also report that phorbol ester dibutyrate can randomly mediate a false relaxation (anisotropic contraction) in cultured PPAEC, when the force vector is directed opposite to the referenced isometric vector of the transducer. In contrast, stimulation of cultured HUVEC with the cAMP agonists, theophylline and forskolin, decreased isometric force in both vectors. Thus direction of the force vector needs to be considered when interpreting isometric tension in cultured endothelial cells.

Purine and pyrimidine nucleotide metabolism of vascular smooth muscle cells in culture

1. Cultures of vascular smooth muscle cells accumulate extracellular breakdown products of purine and pyrimidine nucleotides that, over 9 hr, represent 60 +/- 7 and 78 +/- 17%, respectively, of the intracellular nucleotide content. 2. The accumulation is stimulated during contracture with 20 mM KCl or 70 microM carbachol, consistent with the notion that both pyrimidine and purine nucleotides are involved in the energetics of smooth muscle contracture. 3. Because the intracellular levels of pyrimidine and purine nucleotides remain constant, it appears likely that rates of synthesis match the rates of release. 4. Ectonucleotidases are present that can degrade ATP, UTP, and CTP. High-energy nucleotides may be the primary products released.

Inhibitory effect of noradrenaline uptake inhibitors on contractions of rat aortic smooth muscle

1 The effects of noradrenaline (NA) uptake inhibitors on contractions induced by NA, high K+, and 12-O-tetradecanoylphorbol-13-acetate (TPA) in rat isolated aorta were investigated. 2 Protriptyline (0.3 microM) and amitriptyline (0.3 microM) produced an approximately parallel shift to the right in the dose-response curve to NA. Protriptyline (> 0.3 microM), amitriptyline (> 0.3 microM) and xylamine (0.01-1 microM) significantly reduced the maximal contractile response to NA. The IC50 values for inhibition of the contractile response to 3 microM NA were 1.58 microM for xylamine, 1.70 microM for amitriptyline and 2.57 microM for protriptyline. 3 Protriptyline and amitriptyline dose-dependently inhibited the high K+ (60 mM)-induced contraction (IC50 = 0.69 microM for protriptyline and IC50 = 3.15 microM for amitriptyline). In contrast, xylamine did not affect the high K(+)-induced contraction. 4 Protriptyline and amitriptyline dose-dependently inhibited TPA (1 microM)-induced contraction in calcium-free solution; xylamine (up to 30 microM) was without effect. Staurosporine (10 nM) completely inhibited the TPA- and NA-induced contraction. 5. Protriptyline (3 microM) and amitriptyline (3 microM) caused about 54% and 60% inhibition, respectively, of aortic contractions caused by endothelin-1 (10 nM) in the absence of endothelium. Xylamine (10 microM) was without effect. 6 Inhibitory effects of NA uptake inhibitors on contractions were independent of the presence of endothelium and were unaffected by the K+ channel blockers, tetraethylammonium ions (up to 3 mM) and glibenclamide (up to 30 microM). 7 These results indicate that tricyclic antidepressant drugs such as protriptyline and amitriptyline could act as both postsynaptic adrenoceptor antagonists and direct inhibitors of muscle contraction; whereas, xylamine, a structurally distinct NA uptake blocker might principally exert its action only at alpha-adrenoceptors on rat aortic smooth muscle.

Calcium mobilization and isometric tension in bovine tracheal smooth muscle: effects of salbutamol and histamine

We determined if decreases in relative free intracellular calcium concentration ([Ca2+]i) caused by salbutamol, a selective beta2-adrenoreceptor agonist, were paralleled by calcium egression from the cytosol in bovine trachealis muscle strips. [Ca2+]i, or tissue-surface extracellular calcium changes (Ts[Ca2+]ext), were monitored using Fluo-3 acetoxymethylester or Fluo-3 pentaammonium salt simultaneously with isometric tension. Salbutamol (1 microM) decreased histamine-induced isometric tension from an average peak tension of 128.5 +/- 18.4 to -4.9 +/- 0.3 mN/mm2, and reduced the associated sustained increases in [Ca2+]i from 100% at peak to 20.4 +/- 7.6%. Both histamine-induced elevation in [Ca2+]i and isometric tension were reversed completely by forskolin (1 microM). In muscle strip at active resting tension, salbutamol caused a decrease (49.6 +/- 12.1%) in [Ca2+]i. Following precontraction with histamine, salbutamol caused an immediate and sustained increase in Ts[Ca2+]ext which was not seen in a Na(+)-free solution. Finally, propranolol (10 microM) blocked both increases in Ts[Ca2+]ext and muscle relaxation caused by salbutamol. These findings indicate that in bovine trachealis muscle, the effect of salbutamol to decrease [Ca2+]i and isometric tension is via a beta2-adrenoceptor, and the changes in [Ca2+]i are by an increase in calcium egression via the Na(+)/Ca2+ exchanger, and reuptake by myoplasmic stores.

Isoflurane attenuates cAMP-mediated vasodilation in rat microvessels

BACKGROUND

Endothelium-dependent vasodilation mediated by cGMP is known to be attenuated by the inhalational anesthetic isoflurane. The present study examines the effect of isoflurane on beta-adrenergic and cAMP-mediated vasodilation.

METHODS AND RESULTS

Fifty-three subepicardial coronary arteries (diameter, 103 +/- 13 microns) from Wistar rats were studied in vitro in a pressurized (40 mm Hg), no-flow state with use of optical density video detection system. After preconstriction of vessels with the thromboxane A2 analogue U46619 10(-6) mol/L, concentration response curves to the nonselective beta-adrenergic agonist isoproterenol, the Gs protein activator sodium fluoride, the adenylate cyclase activator forskolin, the cAMP analogue 8-Br-cAMP, or the phosphodiesterase inhibitor RO20-1724 were obtained either in the presence of absence (control) of 2% isoflurane. Relaxations to all the agents tested were significantly reduced in the presence of isoflurane compared with controls.

CONCLUSIONS

Isoflurane attenuates cAMP-mediated vasodilation. The impairment appears to be distal to adenylate cyclase and is not due to enhancement of cAMP phosphodiesterase.

Involvement of protein phosphatase-1 in cytoskeletal organization of cultured endothelial cells

The phosphorylation and dephosphorylation of cytoskeletal proteins regulate the shape of eukaryotic cells. To elucidate the role of serine/threonine protein phosphatases (PP) in this process, we studied the effects of calyculin A (CLA), a potent and specific inhibitor of protein phosphatases 1 (PP-1) and 2A (PP-2A) on the cytoskeletal structure of cultured human umbilical vein endothelial cells (HUVECs). The addition of CLA (5 min) caused marked alterations in cell morphology, such as cell constriction and bleb formation. Microtubules and F-actin were reorganized, becoming markedly condensed around the nucleus. Although the fluorescence intensity of phosphoamino acids was not significantly different according to immunocytochemistry between cells with and without CLA, polypeptides of 135, 140, 158, and 175 kDa were specifically phosphorylated on serine and/or threonine residues. There was no significant effect on tyrosine residues. The effects of CLA on cytoskeletal changes and protein phosphorylation were almost completely inhibited by the non-selective kinase inhibitor, K-252a. The effect of CLA on cell morphology was at least 100 times more potent than that of okadaic acid, consistent with the inhibitory potency against PP-1. The catalytic subunit of PP-1 was also identified in HUVECs by Western blotting with its monoclonal antibody antibody. These results suggest that PP-1 is closely involved in sustaining the normal structure of the cytoskeleton.

Cytoskeletal reorganization of human platelets induced by the protein phosphatase 1/2 A inhibitors okadaic acid and calyculin A

Okadaic acid (OA) and calyculin A (CLA), which are potent and specific inhibitors of serine/threonine protein phosphatases type 1 and 2A, have been shown to induce drastic changes in platelet morphology. The aim of this study was to analyse the molecular mechanisms of OA- or CLA-induced cytoskeletal reorganization, with a specific focus on microtubules and actin filaments. Confocal fluorescence microscopy revealed that OA or CLA altered the distribution of microtubules from marginal band arrangements to homogeneous patterns, consistent with the transmission-electron-microscopic finding that microtubules were fragmented and redistributed into pseudopod-like processes. In thrombin-activated platelets, OA or CLA induced extremely long pseudopods containing an array of microtubules and actin filaments, and a condensed mass of actin filaments in the centre of platelets. OA or CLA induced the constriction of actin filaments without an increase in filamentous (F)-actin, and also rather significantly inhibited actin polymerization in thrombin-activated platelets. Furthermore, neither OA or CLA enhanced phosphorylation of myosin light chain (MLC). By immunoprecipitation of platelet lysate with anti-alpha-tubulin antibody, a 90 kDa protein was co-precipitated with tubulin and was predominantly phosphorylated in the presence of OA. As the time-dependent phosphorylation of 90 kDa protein correlated well with the reorganization of microtubules, these data suggest that phosphorylation and dephosphorylation of this protein might play a role in the regulation of microtubule organization. These findings indicate that OA or CLA induces reorganization of microtubules and actin filaments via the phosphorylation of a microtubule-associated 90 kDa protein and an MLC-phosphorylation-independent mechanism. mechanism.

Models of the mechanism for crossbridge attachment in smooth muscle

The mechanism responsible for formation of attached, dephosphorylated crossbridges (latchbridges) in smooth muscle is controversial. Myosin light chain phosphorylation may be obligatory for crossbridge attachment; if this were the case, latchbridges would arise solely by dephosphorylation of attached, phosphorylated crossbridges. Alternatively, the presence of attached crossbridges could induce cooperative activation by allowing dephosphorylated crossbridges to attach to the thin filament. We evaluated whether four-state models based on dephosphorylation and/or cooperativity-regulated attachment could quantitatively predict smooth muscle contractile behaviour. Five quantitative models for transitions between crossbridge states were developed. Mechanisms for latchbridge formation included: (1) dephosphorylation, (2) cooperativity-regulated attachment dependent only on attached, phosphorylated crossbridges, (3) cooperativity-regulated attachment dependent on all attached crossbridges, (4) dephosphorylation and cooperativity-regulated attachment dependent only on attached, phosphorylated crossbridges, and (5) dephosphorylation and cooperativity-regulated attachment dependent on all attached crossbridges. All five models approximated the time course of contraction and the dependence of steady-state stress on myosin phosphorylation in the swine carotid artery. In the two models that had cooperative attachment regulated by all attached crossbridges, small increases in the rate constant for cooperativity-regulated attachment resulted in positive feedback and irreversible contraction. We suggest that a number of four-state crossbridge models can predict contractile behaviour in arterial smooth muscle. Potentially, latchbridges could be formed by both dephosphorylation and cooperativity-regulated attachment. If cooperativity-regulated latchbridge attachment does exist in smooth muscle, we suggest that it should be dependent only on the number of phosphorylated crossbridges rather than all attached crossbridges.

Role of protein kinase C in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage

This study investigated the role of protein kinase C (PKC) in the pathogenesis of vasospasm after experimental subarachnoid hemorrhage (SAH). PKC activation by intracisternal injection of a phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TP)] induced dose-dependent, slowly developing, severe contraction of the basilar artery. A single intracisternal injection of TP (5 x 10(-9) M in the CSF) induced sustained contraction lasting over 3 days, which almost paralleled the changes of membrane-bound PKC activity in the basilar arterial wall. In a two-hemorrhage SAH model, membrane-bound PKC activity in the basilar artery increased up to day 4 and returned to the control level by day 14, whereas angiographic contraction reached a maximum on day 7 and still persisted at a moderate level on day 14. Thus, there was a discrepancy between arterial PKC activity and arterial contraction. Multiple intracisternal injections of TP produced 30-40% sustained contraction of the basilar artery lasting for more than 10 days along with sustained activation of PKC to levels compatible with that observed in the SAH model. However, TP injection caused considerably milder histological changes in the basilar artery than those noted in the SAH model. We concluded that cerebral vasospasm after SAH cannot be explained solely on the basis of activation of the PKC pathway.

Activation of protein kinase C isozymes by contractile stimuli in arterial smooth muscle

Protein kinase C (PKC) has been proposed to be involved in the regulation of vascular smooth muscle (VSM) contractile activity. However, little is known in detail about the activation of this kinase or specific isozymes of this kinase by contractile stimuli in VSM. As an index of PKC activation, Ca(2+)- and phospholipid-dependent histone IIIS kinase activity was measured in the particulate fraction from individual strips of isometrically contracting carotid arterial smooth muscle. Phorbol 12,13-dibutyrate (PDB) increased PKC activity in the particulate fraction (155% over resting value by 15 min) with a time course which paralleled or preceded force development. Stimulation with the agonist histamine (10(-5) M) resulted in rapid increases in both force and particulate fraction PKC activity which was maximal by 2 min (increase of 139%) and partially sustained over 45 min (increase of 41%). KCl (109 mM), which evokes a sustained contractile response, caused a slow increase (124% by 45 min) in particulate fraction PKC activity. No significant increases in activator-independent histone kinase activity were observed in response to any stimulus tested. PKC alpha and PKC beta were identified as the principal Ca2+/phospholipid-dependent PKC isozymes expressed in this tissue. In unstimulated arterial tissue, the ratio of immunodetectable isozyme content (alpha:beta) was estimated to be 1:1 in the particulate and 1.5:1 in the cytosolic fractions. Upon stimulation with each of the three contractile stimuli, particulate fraction PKC content assessed by immunoblotting increased with a time course and to an extent comparable to the observed changes in PKC activity. There was no evidence of differential regulation of the PKC alpha or -beta isozymes by PDB compared to the other contractile stimuli. These results indicate that diverse contractile stimuli are capable of tonically activating PKC in preparations of functional smooth muscle, and are consistent with a functional role for PKC alpha and/or -beta in the regulation of normal smooth muscle contractile activity.

Endothelin increases myofilament Ca2+ sensitivity in alpha-toxin-permeabilized rabbit mesenteric artery

This study was designed to investigate the mechanism of endothelin-1 (ET-1) contractions in Staphylococcus alpha-toxin-permeabilized vascular smooth muscle. Rabbit small mesenteric arteries permeabilized with alpha-toxin were mounted for isometric or isotonic force recording or were processed for determination of myosin light chain (MLC) phosphorylation levels. Addition of 100 nM ET-1 plus 10 microM GTP significantly enhanced myofilament Ca2+ sensitivity as compared with the addition of Ca2+ alone (EC50, 0.47 microM Ca2+ for Ca2+ alone and 0.13 microM Ca2+ for ET-1 plus (GTP). This enhanced sensitivity was reversed by GDP beta S. ET-1-induced contractions were relaxed at a constant [Ca2+] by the addition of 30 microM cAMP or cGMP, demonstrating a direct effect of the cyclic nucleotides on contractile regulation. Inhibition of protein kinase C activity by 100 nM staurosporine relaxed ET-1 plus GTP-induced contractions, and pretreatment with 40 microM chelerythrine inhibited the ET-1 plus GTP increase in force. At 0.32 microM Ca2+, steady-state levels of shortening velocity were not increased by ET-1 plus GTP, although steady-state levels of MLC phosphorylation were significantly enhanced. The ET-1-induced increase in MLC phosphorylation was not altered by changes in [Ca2+], whereas the shortening velocity was Ca2+ dependent, suggesting that the increase MLC phosphorylation level may be the result of protein kinase C, rather than MLC kinase, activation. These results are consistent with the hypothesis that ET-1 increases myofilament Ca2+ sensitivity by a G protein-dependent pathway and subsequent activation of protein kinase C.(ABSTRACT TRUNCATED AT 250 WORDS)

Magnesium relaxes arterial smooth muscle by decreasing intracellular Ca2+ without changing intracellular Mg2+

Elevations in extracellular [Mg2+] ([Mg2+]o) relax vascular smooth muscle. We tested the hypothesis that elevated [Mg2+]o induces relaxation through reductions in myoplasmic [Ca2+] and myosin light chain phosphorylation without changing intracellular [Mg2+] ([Mg2+]i). Histamine stimulation of endothelium-free swine carotid medial tissues was associated with increases in both Fura 2- and aequorin-estimated myoplasmic [Ca2+], myosin phosphorylation, and force. Elevated [Mg2+]o decreased myoplasmic [Ca2+] and force to near resting values. However, elevated [Mg2+]o only transiently decreased myosin phosphorylation values: sustained [Mg2+]o-induced decreases in myoplasmic [Ca2+] and force were associated with inappropriately high myosin phosphorylation values. The elevated myosin phosphorylation during [Mg2+]o-induced relaxation was entirely on serine 19, the Ca2+/calmodulin-dependent myosin light chain kinase substrate. Myoplasmic [Mg2+] (estimated with Mag-Fura 2) did not significantly increase with elevated [Mg2+]o. These results are consistent with the hypothesis that increased [Mg2+]o induces relaxation by decreasing myoplasmic [Ca2+] without changing [Mg2+]i. These data also demonstrate dissociation of myosin phosphorylation from myoplasmic [Ca2+] and force during Mg(2+)-induced relaxation. This finding suggests the presence of a phosphorylation-independent (yet potentially Ca(2+)-dependent) mechanism for regulation of force in vascular smooth muscle.

Substrate based inhibitors of smooth muscle myosin light chain kinase

Activation of myosin light chain kinase is a prerequisite for smooth muscle activation. In this study, short peptide analogs of the phosphorylation site of the myosin light chain were studied for their effects on several contractile protein systems. The peptides inhibited phosphorylation of isolated ventricular and smooth muscle myosin light chains by smooth muscle myosin light chain kinase, but they were only weak inhibitors of phosphorylation of intact myosin and actomyosin. The peptides were also unable to block force development or myosin light chain phosphorylation in glycerol permeabilized fibers of swine carotid media. Apparently, the association of the myosin light chain with myosin changes its conformation such that substrate analogs which are potent inhibitors of the phosphorylation of isolated myosin light chains by myosin light chain kinase are ineffective at blocking phosphorylation of the intact molecule.

Independent pathways regulate the cytosolic [Ca2+] initial transient and subsequent oscillations in individual cultured arterial smooth muscle cells responding to extracellular ATP

Stimulation with extracellular ATP causes a rapid initial transient rise followed by asynchronous periodic oscillations in cytosolic calcium ion activity ([Ca2+]i) in individual aortic smooth muscle cells in either HEPES-buffered or HCO3(-)-buffered saline. The dose at which one-half of the cells display an initial rise in cytosolic calcium is 0.11 microM ATP in the presence of external Ca2+ and 0.88 microM ATP in the absence of external Ca2+; the corresponding value for oscillations in the presence of external Ca2+ is 2.6 microM ATP. While the initial transient displays rapid desensitization, the oscillations persist for greater than 30 min in the continuous presence of ATP. The presence of the agonist ATP is also absolutely required for the maintenance of the oscillations, presumably to provide continuous activation of P2 purinoceptors. The average frequency of oscillation is approximately 0.9 min-1. The frequency depends only slightly on the concentration of ATP, and oscillations do not collapse into a prolonged elevated [Ca2+]i at high concentrations of ATP. Both Ca2+ influx and release from internal stores participate in the initial transient. Oscillations are not produced in the absence of external Ca2+ but are initiated upon the addition of external Ca2+ in the continued presence of ATP. Oscillations in progress are abolished by the removal of extracellular Ca2+ with one additional peak occurring after the Ca2+ removal. These data suggest that extracellular Ca2+ influx is required for the maintenance of the posttransient oscillations, presumably to provide the Ca2+ necessary for refilling intracellular Ca2+ pools that are the source of the oscillating [Ca2+]i. The Ca2+ influx is not regulated by voltage-gated Ca2+ channels. The data in this report are consistent with the view that the initial transient has contributions from two receptor-mediated pathways, and the oscillations are controlled either by a mechanism separate from the ones that control the initial transient or by steps whose control diverges before the point of desensitization.

Na(+)-Ca2+ exchange, myoplasmic Ca2+ concentration, and contraction of arterial smooth muscle

Na(+)-Ca2+ exchange is proposed to be an important regulator of myoplasmic intracellular Ca2+ concentration ([Ca2+]i) and contraction in vascular smooth muscle. We investigated the role of Na(+)-Ca2+ exchange in regulating [Ca2+]i in swine carotid arterial tissues that were loaded with aequorin to allow simultaneous measurement of [Ca2+]i and force. Reversal of Na(+)-Ca2+ exchange, by reduction of extracellular Na+ concentration ([Na+]o) to 1.2 mM, induced a large increase in aequorin-estimated [Ca2+]i and a low [Ca2+]i sensitivity. The contraction induced by 1.2 mM [Na+]o was partially caused by depolarization and opening of L-type Ca2+ channels because 10 microM diltiazem partially attenuated the 1.2 mM [Na+]o-induced increases in [Ca2+]i. High dose ouabain (10 microM), a putative endogenous Na+,K(+)-ATPase inhibitor, increased both [Ca2+]i and force. However, the increases in [Ca2+]i and force were mostly blocked by 10 microM phentolamine, suggesting the predominant effect of ouabain was to increase norepinephrine release from nerve terminals. In the presence of 10 microM phentolamine, 10 microM ouabain slightly accentuated 1 microM histamine-induced increases in [Ca2+]i and force. The ouabain dose necessary to induce contraction in the absence of phentolamine was significantly less than the ouabain dose necessary to accentuate histamine-induced contractions in the presence of phentolamine. These results suggest that Na(+)-Ca2+ exchange exists in swine arterial smooth muscle. These data also suggest that ouabain (which should increase [Na+]i and inhibit Na(+)-Ca2+ exchange) primarily enhances contractile function in the swine carotid artery by releasing catecholamines from nerve terminals; direct action of Na+,K(+)-ATPase inhibitors on smooth muscle appears to occur only with very high doses.

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.

Cyclic AMP relaxes swine arterial smooth muscle predominantly by decreasing cell Ca2+ concentration

1. Our objective was to evaluate the mechanism of cyclic AMP-dependent arterial smooth muscle relaxation. Cyclic AMP-dependent relaxation has been proposed to result from either (a) a decrease in intracellular [Ca2+] or (b) a decrease in [Ca2+] sensitivity of myosin light chain kinase by protein kinase A-dependent phosphorylation of myosin kinase. 2. We evaluated these proposed mechanisms by examining forskolin-induced changes in aequorin-estimated myoplasmic [Ca2+], [cyclic AMP], myosin phosphorylation and stress generation in agonist-stimulated or KCl-depolarized swine common carotid media tissues. 3. Forskolin, an activator of adenylyl cyclase, increased [cyclic AMP] and reduced [Ca2+], myosin phosphorylation and stress in tissues pre-contracted with phenylephrine or histamine. This relaxation was not associated with an alteration of the [Ca2+] sensitivity of phosphorylation, nor the dependence of stress on phosphorylation. 4. Forskolin pre-treatment attenuated, but did not abolish, agonist-induced increases in [Ca2+] and stress. 5. These results suggest that cyclic AMP-induced relaxation of the agonist-stimulated swine carotid media is primarily caused by cyclic AMP-mediated decreases in myoplasmic [Ca2+].

Length-dependent myosin phosphorylation and contraction of arterial smooth muscle

Ca2+, calmodulin-dependent myosin light chain phosphorylation is generally considered to be an important regulatory mechanism of smooth muscle contraction. We investigated the length dependence of myosin phosphorylation and active stress induced by K+ depolarization in arterial smooth muscle by measuring the two variables in the swine carotid media held at three steady-state tissue lengths-optimal length for contraction (Lo), 1.5 Lo, and slack length. We found that the length dependence of peak and steady-state myosin phosphorylation with respect to tissue length was different. Peak myosin phosphorylation was highest at Lo but lower at both slack length and 1.5 Lo, whereas steady-state myosin phosphorylation was similar at both Lo and 1.5 Lo, but lower at slack length. Stretching tissues to 1.5 Lo did not significantly change the steady-state myosin phosphorylation induced by K+ depolarization, but releasing tissues to slack length was associated with a 42% decrease in the steady-state myosin phosphorylation induced by K+ depolarization. These data indicated that one or more steps coupling membrane depolarization and Ca(2+)-dependent myosin phosphorylation were length sensitive. Additional data from skinned tissue experiments indicated that the length-sensitive step was not the coupling between Ca2+ and myosin phosphorylation. Therefore, these data together suggest that one or more steps coupling membrane depolarization and the increase in cytosolic Ca2+ concentration are length sensitive.

Possible role of protein kinase C-dependent smooth muscle contraction in the pathogenesis of chronic cerebral vasospasm

In the present study, we investigate the possible role of protein kinase C (PKC)-dependent smooth muscle contraction in cerebral vasospasm following subarachnoid hemorrhage (SAH), employing the beagle "two-hemorrhage" model. The occurrence of chronic vasospasm was angiographically confirmed on day 7 in the basilar artery, which was exposed via the transclival approach. The artery was superfused with aerated Krebs-Henseleit solution containing various agents, and the subsequent changes in the basilar artery diameter were recorded by successive angiography. The preexisting spasm was not ameliorated by local application of neurotransmitter antagonists (atropine, methysergide, phentolamine, and diphenhydramine), calmodulin inhibitors (R24571 and W-7), or a calcium antagonist, nicardipine. However, the application of PKC inhibitors such as H-7 and staurosporine induced significant dilation of the artery. In another experiment, an intrinsic PKC activator, 1,2-diacylglycerol (DAG), in the basilar artery, the CSF, and the cisternal clot of beagles exposed to two hemorrhages was measured on days 1, 2, 4, 7, and 14 using the DAG kinase method. On days 2, 4, and 7, the DAG content of the basilar artery showed a significant and prolonged increase (150-190% of control), whereas it was unchanged on days 1 and 14. Throughout the experimental period, there was a significant linear correlation between the DAG content and the angiographical diameter of the basilar artery. The above results indicate that SAH leads to an increase in the DAG level within the cerebral artery through an as yet unknown mechanism and that subsequent activation of the PKC-dependent contractile system participates in the occurrence of chronic vasospasm.

Regulation of a smooth muscle contraction: a hypothesis based on skinned fiber studies

It seems clear that a simple Ca2+ dependent switch (MLC phosphorylation) cannot completely explain all of the disparate mechanical and energetic results obtained under numerous experimental conditions in numerous laboratories. Some of the problems of the simple switch model are that: 1. Force can be developed in the complete absence of increases in MLC phosphorylation; 2. Crossbridge cycling rate, as measured by either shortening velocity or directly by ATPase activity, can be regulated independent of changes in MLC phosphorylation; and 3. Ca2+ can directly influence both force and crossbridge cycling rate. Thus, we believe that there are two distinct Ca2+ dependent regulatory systems which normally act in parallel to contract smooth muscle. One of these is the Ca2+ dependent MLC phosphorylation-dephosphorylation. system which is likely to be responsible for the rapid development of force. The other is the hypothesized Ca2+ dependent system which is probably responsible for the slow development of force as well as the maintenance of previously developed force, represented in Figure 5 as K8. This second system involves a calmodulin-like protein with a higher Ca2+ sensitivity than that for the Ca(2+)-calmodulin-MLC kinase system. Under most conditions, the total force attained by smooth muscle in response to stimulation is the result of the concerted activation of both of these regulatory systems. The available information is consistent with this hypothesis of two regulatory systems functioning in parallel. In addition to the information presented in this chapter, work from a number of laboratories (Moreland and Ford, 1982; Fujiwara et al., 1989; Kitazawa et al., 1989; Somlyo et al., 1989; Kubota et al., 1990; Kitazawa and Somlyo, this volume) have suggested the possibility that a regulated MLC phosphatase may functionally alter the Ca2+ sensitivity of the contractile filaments. There is evidence suggesting that the sensitivity of MLC kinase to activation by Ca2+ and calmodulin may be regulated (Stull et al., this volume). Protein kinase C has been postulated to play an important role in the regulation of myofilament Ca2+ sensitivity (Nishimura et al., this volume). MgADP has been suggested to affect the kinetics of latchbridge attachment and detachment (Kerrick and Hoar, 1987; Nishimura and van Breemen, 1989). Cooperativity between crossbridges as described by Somlyo et al. (1988) and Siegman et al. (this volume) might also be an important component in the regulation of smooth muscle contraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium dependent regulation of vascular smooth muscle contraction

The experimental results discussed from our laboratory as well as from numerous other laboratories investigating the regulation of smooth muscle contraction have, in our opinion, clearly demonstrated that a simple Ca2+ dependent switch (MLC phosphorylation) cannot completely explain all of the mechanical and energetic findings. We and others have demonstrated that stress can be developed in the complete absence of increases in MLC phosphorylation, that crossbridge cycling rate can be regulated independent of changes in MLC phosphorylation, that Ca2+ can directly influence both stress and crossbridge cycling rate, and that protein kinase C can, apparently, directly initiate the development of stress supported by a specific population of crossbridges characterized by unphosphorylated MLC, low cycling rates, and weak binding characteristics. This information combined with the wealth of material demonstrating the important function played by the Ca2+ and calmodulin dependent MLC kinase is consistent with the hypothesis that there are two Ca2+ dependent regulatory systems acting in parallel in smooth muscle. One of these is the Ca2+ dependent MLC phosphorylation-dephosphorylation system responsible for the rapid development of stress and the second is a hypothesized Ca2+ dependent system responsible for the slow development of stress as well as the maintenance of previously developed stress. This second system has a higher Ca2+ sensitivity than that for MLC phosphorylation and may be activated by protein kinase C. The total stress attained by smooth muscle is activated by protein kinase C. The total stress attained by smooth muscle is the result of these two regulatory systems acting in concert. Although we believe the available information is consistent with this hypothesis of two regulatory systems functioning in parallel, it is by no means the only possibility. Early work from our laboratory and the recent work by the Somlyos and their colleagues and Kubota et al. suggest the possibility of a regulated MLC phosphatase which might functionally alter the Ca2+ sensitivity of the contractile filaments. Kerrick and Hoar and Nishimura and van Breemen have published data which imply a role for MgADP in latchbridge kinetics. These findings, as well as the discovery of several thin filament protein components which have been proposed as regulatory units, must all be taken into account in the final answer to the question: How does Ca2+ contract smooth muscle?

Phorbol 12,13-diacetate-induced contraction of the canine basilar artery: role of protein kinase C

The pharmacological and biochemical mechanisms of contractile responses to the protein kinase C (PKC) activator phorbol-12,13-diacetate (PDA) were investigated in canine basilar arteries. In the normal medium, PDA elicited a strong, dose-related, and slow-developing sustained contraction. Among the constrictors examined, including serotonin, prostaglandin F2 alpha, and endothelin, only PDA yielded contractions in a Ca2(+)-free medium. In both media, the PDA-induced contractions were virtually inhibited by either staurosporine, H-7, or quinacrine, while neither neurotransmitter blockades nor R24571 (calmidazolium) exerted significant effects. In addition, it was shown that 8-bromocyclic GMP, but not 8-bromocyclic AMP, markedly curtailed the PDA-induced contractions. Biochemical analysis, furthermore, showed that PDA induced increased phosphorylations of 27- and 96-kDa and proteins other than the myosin light chain (MLC) 20-kDa protein. Thus, the present results open up a novel mechanism of sustained cerebral artery contractions, where PKC activation rather than Ca2+/calmodulin/MLC system plays a key role that is regulated both by phospholipase A2 and by cyclic GMP.

Mechanical and biochemical events during hypoxia-induced relaxations of rabbit aorta

Hypoxic relaxation of norepinephrine contractions of isolated rabbit aorta is rapid, whereas relaxation of KCl contractions is slower and blunted. The data given here suggest that with receptor-evoked contractions of rabbit aorta, the energy-limitation of ATP-dependent K+ channels and other sarcolemmal channels, myosin light chain kinase, and actin-activated myosin ATPase are probably not involved in oxidative energy-contraction coupling. The data strongly support the hypothesis that the rate limiting, energy-dependent step is upstream to myosin light chain kinase, which is 50% inhibited at an ATP concentration of about 0.5 mM. This energy-dependent step may be in the inositol phospholipid transduction system, as we have previously postulated (Coburn et al., 1988). In contrast the energy-limited reaction during KCl contractions appears to be the actin-activated myosin ATPase which is 50% inhibited at a mean ATP concentration of about 0.1 mM.

Modulation of the [Ca2+] sensitivity of myosin phosphorylation in intact swine arterial smooth muscle

1. The [Ca2+] sensitivity of myosin light chain phosphorylation in vascular smooth muscle is dependent on the form of stimulation. Contractile agonist stimulation, when compared to high-KCl depolarization, is associated with an increase in [Ca2+] sensitivity of phosphorylation. I evaluated potential mechanisms for this stimulus-specific response by measuring aequorin-estimated myoplasmic [Ca2+], myosin phosphorylation, and isometric stress in swine carotid media. 2. The relative [Ca2+] sensitivity of phosphorylation depended on the type of stimulus (ranked high to low sensitivity): contractile agonists (histamine, phenylephrine) = endothelin (sustained contraction) = combination of histamine and NaF greater than NaF alone = endothelin (initial contraction) = combination of histamine and depolarization = combination of NaF and depolarization greater than depolarization = Bay K 8644 = combination of depolarization and low-dose phorbol diester. 3. Activation of L-type Ca2+ channels with Bay K 8644 induced a [Ca2+] sensitivity of phosphorylation similar to depolarization, suggesting that any other effects of high KCl (such as cellular swelling) were not responsible for the low [Ca2+] sensitivity of phosphorylation. 4. The addition of either histamine or NaF (an activator of G proteins) to depolarized tissues produced similar increases in the [Ca2+] sensitivity of phosphorylation, suggesting that NaF (possibly by activation of a G protein) can mimic contractile agonist-induced increases in the [Ca2+] sensitivity of phosphorylation. 5. Phorbol dibutyrate enhanced the contractile effect of depolarization, and this enhancement was primarily caused by increases in [Ca2+] rather than an alteration in the [Ca2+] sensitivity of phosphorylation. 6. These data suggest that the [Ca2+] sensitivity of phosphorylation in smooth muscle may be regulated by agonists (possible by G protein activation); however, the role of protein kinase C activation or depolarization induced Ca2+ compartmentalization requires further study.

Myosin light-chain phosphorylation and vascular resistance in canine anterior tibial arteries in situ

Two regulatory systems are believed to control the contractile state of vascular smooth muscle in vitro. In one, crossbridges are phosphorylated by myosin light-chain (MLC) kinase; these phosphorylated crossbridges are believed to predominate during force development. In the other system, the crossbridges are unphosphorylated (latchbridges); the latchbridges are thought to prevail during force maintenance. The role of these systems in the control of vascular resistance in vivo is unknown. This study compared MLC phosphorylation with vascular tone in canine anterior tibial arteries in vitro and in situ. The arteries were frozen at various times before and during stimulation with norepinephrine. Under both conditions, the levels of MLC phosphorylation were low at rest and increased transiently during norepinephrine stimulation. This increase was associated with stress development in vitro and with an increase in vascular resistance in situ. Thus, the biochemical changes measured in arteries in situ parallel those measured in vitro suggesting that the in vitro models are appropriate and useful for studying the physiological function of blood vessels. To our knowledge, this is the first demonstration of changing levels of MLC phosphorylation in an artery in situ.

Impairment of relaxations to acetylcholine and nitric oxide by a phorbol ester in rat isolated aorta

1. This study compared the abilities of acetylcholine (ACh) (endothelium-dependent) and nitric oxide (NO) (endothelium-independent and which may be the active component of the endothelium-derived relaxing factor) to relax rat isolated aortic rings contracted with equi-effective concentrations of noradrenaline (NA) or phorbol 12-myristate 13-acetate (PMA). 2. ACh and NO induced concentration-dependent relaxations of aortic rings contracted with NA (EC70 value: 0.2 microM). However, relaxations to both ACh and NO were markedly reduced in rings contracted with PMA (EC80 value: 0.5 microM). NO-induced relaxations of tissues were not affected by removal of the endothelium, but ACh-induced relaxations were confirmed to be endothelium-dependent. 3. ACh (10 microM) induced a 10 fold increase in guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels above control values in aortic rings contracted with NA (0.2 microM), but did not affect cyclic GMP levels in rings contracted with PMA (0.5 microM). 4. NO (3 microM) induced a 100 fold increase in cyclic GMP levels above control values in aortic rings contracted with NA (0.2 microM), but only an 11 fold increase in tissues contracted with PMA (0.5 microM). 5. It is concluded that the action (s) of EDRF (NO) are impaired in the presence of PMA by a mechanism that may involve the stimulation of protein kinase C in vascular smooth muscle cells.