Okadaic acid uncouples myosin light chain phosphorylation and tension in smooth muscle

Angiotensin II, a unique vasoactive agent dissociates myosin light chain phosphorylation from contraction

Angiotensin II (100 nM) induced bi-phasic increases in cytosolic Ca²⁺ level ([Ca²⁺]_i) through the activation of angiotensin II type 1 receptor. Pharmacological examinations using 10 µM verapamil, 30 µM La³⁺, and 1 µM thapsigargin indicated that the first phase of the [Ca²⁺]_i-increase was mediated by Ca²⁺ release from sarcoplasmic reticulum (SR) and Ca²⁺ influx independently of voltage dependent Ca²⁺ channel (VDC). In contrast, the second phase of [Ca²⁺]_i-increase was mediated by Ca²⁺ influx through VDC. Although both [Ca²⁺]_i and myosin light chain (MLC)-phosphorylation at the first phase was apparently exceeded the threshold for contraction as estimated by high K⁺-induced responses, there was no appreciable contraction, indicating the dissociation between MLC phosphorylation and force during this phase. In contrast, the second phase of [Ca²⁺]_i was associated with the increases in both MLC phosphorylation and force. These results suggest that angiotensin II is a unique agonist which dissociates MLC-phosphorylation from muscle force during the Ca²⁺ releases from SR.

Protein phosphatases 1 and 2A and their naturally occurring inhibitors: current topics in smooth muscle physiology and chemical biology

Protein phosphatases 1 and 2A (PP1 and PP2A) are the most ubiquitous and abundant serine/threonine phosphatases in eukaryotic cells. They play fundamental roles in the regulation of various cellular functions. This review focuses on recent advances in the functional studies of these enzymes in the field of smooth muscle physiology. Many naturally occurring protein phosphatase inhibitors with different relative PP1/PP2A affinities have been discovered and are widely used as powerful research tools. Current topics in the chemical biology of PP1/PP2A inhibitors are introduced and discussed, highlighting the identification of the gene cluster responsible for the biosynthesis of calyculin A in a symbiont microorganism of a marine sponge.

Role of serine-threonine phosphoprotein phosphatases in smooth muscle contractility

The degree of phosphorylation of myosin light chain 20 (MLC20) is a major determinant of force generation in smooth muscle. Myosin phosphatases (MPs) contain protein phosphatase (PP) 1 as catalytic subunits and are the major enzymes that dephosphorylate MLC20. MP regulatory targeting subunit 1 (MYPT1), the main regulatory subunit of MP in all smooth muscles, is a key convergence point of contractile and relaxatory pathways. Combinations of regulatory mechanisms, including isoform splicing, multiple phosphorylation sites, and scaffolding proteins, modulate MYPT1 activity with tissue and agonist specificities to affect contraction and relaxation. Other members of the PP1 family that do not target myosin, as well as PP2A and PP2B, dephosphorylate a range of proteins that affect smooth muscle contraction. This review discusses the role of phosphatases in smooth muscle contractility with a focus on MYPT1 in uterine smooth muscle. Myometrium shares characteristics of vascular and other visceral smooth muscles yet, during healthy pregnancy, undergoes hypertrophy, hyperplasia, quiescence, and labor as physiological processes. Myometrium presents an accessible model for the study of normal and pathological smooth muscle function, and a better understanding of myometrial physiology may allow the development of novel therapeutics for the many disorders of myometrial physiology from preterm labor to dysmenorrhea.

Ablation of smooth muscle caldesmon affects the relaxation kinetics of arterial muscle

Smooth muscle caldesmon (h-CaD) is an actin- and myosin-binding protein that reversibly inhibits the actomyosin ATPase activity in vitro. To test the function of h-CaD in vivo, we eliminated its expression in mice. The h-CaD-null animals appeared normal and fertile, although the litter size was smaller. Tissues from the homozygotes lacked h-CaD and exhibited upregulation of the non-muscle isoform, l-CaD, in visceral, but not vascular tonic smooth muscles. While the Ca(2+) sensitivity of force generation of h-CaD-deficient smooth muscle remained largely unchanged, the kinetic behavior during relaxation in arteries was different. Both intact and permeabilized arterial smooth muscle tissues from the knockout animals relaxed more slowly than those of the wild type. Since this difference occurred after myosin dephosphorylation was complete, the kinetic effect most likely resulted from slower detachment of unphosphorylated crossbridges. Detailed analyses revealed that the apparently slower relaxation of h-CaD-null smooth muscle was due to an increase in the amplitude of a slower component of the biphasic tension decay. While the identity of this slower process has not been unequivocally determined, we propose it reflects a thin filament state that elicits fewer re-attached crossbridges. Our finding that h-CaD modulates the rate of smooth muscle relaxation clearly supports a role in the control of vascular tone.

New insights into myosin phosphorylation during cyclic nucleotide-mediated smooth muscle relaxation

Nitrovasodilators and agonists, via an increase in intracellular cyclic nucleotide levels, can induce smooth muscle relaxation without a concomitant decrease in phosphorylation of the regulatory light chains (RLC) of myosin. However, since cyclic nucleotide-induced relaxation is associated with a decrease in intracellular [Ca²⁺], and hence, a decreased activity of MLCK, we tested the hypothesis that the site responsible for the elevated RLC phosphorylation is not Ser19. Smooth muscle strips from gastric fundus were isometrically contracted with ET-1 which induced an increase in monophosphorylation from 9 ± 1 % under resting conditions (PSS) to 36 ± 1 % determined with 2D-PAGE. Electric field stimulation induced a rapid, largely NO-mediated relaxation with a half time of 8 s, which was associated with an initial decline in RLC phosphorylation to 18 % within 2 s and a rebound to 34 % after 30 s whereas relaxation was sustained. In contrast, phosphorylation of RLC at Ser19 probed with phosphospecific antibodies declined in parallel with force. LC/MS and western blot analysis with phosphospecific antibodies against monophosphorylated Thr18 indicate that Thr18 is significantly monophosphorylated during sustained relaxation. We therefore suggest that (i) monophosphorylation of Thr18 rather than Ser19 is responsible for the phosphorylation rebound during sustained EFS-induced relaxation of mouse gastric fundus, and (ii) that relaxation can be ascribed to dephosphorylation of Ser19, the site considered to be responsible for regulation of smooth muscle tone.

The relaxant effect of okadaic acid on canine basilar artery involves activation of PKCalpha and phosphorylation of the myosin light chain at Thr-9

Vasodilator responses induced by okadaic acid were investigated in canine basilar artery precontracted with 80 mM KCl. Okadaic acid (1 microM) relaxed the artery and this relaxant effect was partially inhibited by Gö6976, a conventional protein kinase C inhibitor, and calphostin C, an inhibitor of conventional and novel PKCs. Rottlerin, a specific inhibitor of PKCdelta, did not influence okadaic acid's effect. KCl increased phosphorylation of 20,000-Dalton myosin light chain (MLC(20)) at Ser-19. Okadaic acid additionally increased MLC(20) phosphorylation at Thr-18 and Thr-9, resulting in triphosphorylation of MLC(20). This phosphorylation was inhibited by Gö6976. Okadaic acid stimulated phosphorylation of PKCalpha and 17,000-Dalton PKC-potentiated inhibitory phosphoprotein (CPI-17), and Gö6976 inhibited these phosphorylations. These results suggest that okadaic acid's relaxant effect involves MLC(20) triphosphorylation through a direct phosphorylation by PKCalpha and an indirect phosphorylation by inhibition of myosin light chain phosphatase through PKCalpha-mediated CPI-17 phosphorylation.

Augmentation of α1-Adrenoceptor-Mediated Contraction by Warming Without Increased Phosphorylation of Myosin in Rat Caudal Arterial Smooth Muscle

We previously reported the relationship between alpha1-adrenoceptor-mediated contraction and phosphorylation of 20-kDa myosin light chain (LC20) in de-endothelialized rat caudal arterial smooth muscle at room temperature (Mita M, Walsh MP. Biochem J. 1997;327:669-674). We now describe the effect of increasing the temperature to 37 degrees C on this relationship. The EC50 value (76.6 +/- 18.2 nM) for cirazoline (alpha1-adrenergic agonist)-induced contraction of the strips at room temperature (23 degrees C) was significantly greater than that (14.5 +/- 1.9 nM) at 37 degrees C. The initial rate of the contraction to a sub-maximal concentration of cirazoline (0.3 microM) was similar at the two temperatures. However, cirazoline-induced maximal force at 37 degrees C was approximately 1.8 times that at room temperature. LC20 phosphorylation in response to cirazoline at room temperature and 37 degrees C closely matched the time courses of contraction, but values were not significantly different at the two temperatures: resting phosphorylation levels were 0.09 +/- 0.04 mol P(i)/mol LC20 at 37 degrees C and 0.22 +/- 0.06 mol P(i)/mol LC20 at room temperature; maximal cirazoline-stimulated LC20 phosphorylation levels were 0.58 +/- 0.08 mol P(i)/mol LC20 at room temperature and 0.49 +/- 0.05 mol P(i)/mol LC20 at 37 degrees C. We conclude, therefore, that the enhanced cirazoline-induced contraction at 37 degrees C is not due to increased LC20 phosphorylation.

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.

Alpha1-adrenergic signaling mechanisms in contraction of resistance arteries

Our goal in this review is to provide a comprehensive, integrated view of the numerous signaling pathways that are activated by alpha(1)-adrenoceptors and control actin-myosin interactions (i.e., crossbridge cycling and force generation) in mammalian arterial smooth muscle. These signaling pathways may be categorized broadly as leading either to thick (myosin) filament regulation or to thin (actin) filament regulation. Thick filament regulation encompasses both "Ca(2+) activation" and "Ca(2+)-sensitization" as it involves both activation of myosin light chain kinase (MLCK) by Ca(2+)-calmodulin and regulation of myosin light chain phosphatase (MLCP) activity. With respect to Ca(2+) activation, adrenergically induced Ca(2+) transients in individual smooth muscle cells of intact arteries are now being shown by high resolution imaging to be sarcoplasmic reticulum-dependent asynchronous propagating Ca(2+) waves. These waves differ from the spatially uniform increases in [Ca(2+)] previously assumed. Similarly, imaging during adrenergic activation has revealed the dynamic translocation, to membranes and other subcellular sites, of protein kinases (e.g., Ca(2+)-activated protein kinases, PKCs) that are involved in regulation of MLCP and thus in "Ca(2+) sensitization" of contraction. Thin filament regulation includes the possible disinhibition of actin-myosin interactions by phosphorylation of CaD, possibly by mitogen-activated protein (MAP) kinases that are also translocated during adrenergic activation. An hypothesis for the mechanisms of adrenergic activation of small arteries is advanced. This involves asynchronous Ca(2+) waves in individual SMC, synchronous Ca(2+) oscillations (at high levels of adrenergic activation), Ca(2+) sparks, "Ca(2+)-sensitization" by PKC and Rho-associated kinase (ROK), and thin filament mechanisms.

Membrane depolarization-induced contraction of rat caudal arterial smooth muscle involves Rho-associated kinase

Depolarization of the sarcolemma of smooth muscle cells activates voltage-gated Ca2+ channels, influx of Ca2+ and activation of cross-bridge cycling by phosphorylation of myosin catalysed by Ca2+/calmodulin-dependent myosin light-chain kinase (MLCK). Agonist stimulation of smooth muscle contraction often involves other kinases in addition to MLCK. In the present study, we address the hypothesis that membrane depolarization-induced contraction of rat caudal arterial smooth muscle may involve activation of Rho-associated kinase (ROK). Addition of 60 mM K+ to de-endothelialized muscle strips in the presence of prazosin and propranolol induced a contraction that peaked rapidly and then declined to a steady level of force corresponding to approx. 30% of the peak contraction. This contractile response was abolished by the Ca2+-channel blocker nicardipine or the removal of extracellular Ca2+. An MLCK inhibitor (ML-9) inhibited both the phasic and tonic components of K+-induced contraction. On the other hand, the ROK inhibitors Y-27632 and HA-1077 abolished the tonic component of K+-induced contraction, and slightly reduced the phasic component. Phosphorylation levels of the 20-kDa light chain of myosin increased rapidly in response to 60 mM K+ and subsequently declined to a steady-state level significantly greater than the resting level. Y-27632 abolished the sustained and reduced the phasic elevation of the phosphorylation of the 20-kDa light chain of myosin, without affecting the K+-induced elevation of cytosolic free Ca2+ concentration. These results indicate that ROK activation plays an important role in the sustained phase of K+-induced contraction of rat caudal arterial smooth muscle, but has little involvement in the phasic component of K+-induced contraction. Furthermore, these results are consistent with inhibition of myosin light-chain phosphatase by ROK, which would account for the sustained elevation of myosin phosphorylation and tension in response to membrane depolarization.

Extensive skinning of cell membrane diminishes the force-inhibiting effect of okadaic acid on smooth muscles of Guinea pig hepatic portal vein

The influences of cell membrane permeabilization (skinning) on the okadaic acid-induced inhibition of vascular smooth muscle contraction were studied in guinea pig hepatic portal vein. Pretreatment by 1 microM okadaic acid in the absence of Ca(2+) suppressed subsequent submaximal Ca(2+)-induced contraction in preparations permeabilized with Staphylococcus aureus alpha-toxin or beta-escin, but not in those treated with saponin or Triton X-100. The SDS-PAGE of elutants from the preparation suggests that the loss of the inhibitory effect of okadaic acid in preparations skinned with saponin or Triton X-100 results from the leakage of some cellular components with a molecular mass of 67 to 200 kDa.

Activation of myosin light chain phosphatase in intact arterial smooth muscle during nitric oxide-induced relaxation

We investigated whether myosin light chain phosphatase activity changes during nitric oxide-induced relaxation of contracted intact carotid media and how changes in phosphatase activity mediate this relaxation. We also investigated one mechanism for regulating this phosphatase. Myosin phosphatase activity, myosin light chain phosphorylation, guanosine 3',5'-cyclic monophosphate (cGMP) concentration, and phosphorylation of the inhibitory protein CPI-17 were all assayed in homogenates of one carotid media ring at each time point during nitric oxide-induced relaxation. The application of sodium nitroprusside to histamine-contracted media caused rapid declines in light chain phosphorylation and force. These were temporally correlated with a rapid elevation of cGMP and a large transient increase in myosin phosphatase activity. During the early response to nitroprusside, when force declined, increases in myosin phosphatase activity, concurrent with cGMP-mediated decreases in calcium and myosin light chain kinase activity, could accelerate light chain dephosphorylation. CPI-17 was dephosphorylated upon application of nitroprusside at the same time that myosin phosphatase activity increased, suggesting that the removal of inhibition by phospho-CPI-17 contributed to the increase in myosin phosphatase activity. After 20 min of nitroprusside, myosin phosphatase activity had declined to basal levels, however low force was sustained. Additional light chain phosphorylation-independent mechanisms may be involved in sustaining the relaxation.

Selected contribution: HSP20 phosphorylation in nitroglycerin- and forskolin-induced sustained reductions in swine carotid media tone

Cyclic nucleotide-induced relaxation of maximally activated arterial smooth muscle has two phases. 1) The initial relaxation transient is typically characterized by a rapid reduction in force associated with brief reductions in myoplasmic Ca(2+) concentration ([Ca(2+)](i)) and myosin regulatory light chain (MRLC) phosphorylation on serine (Ser)-19 (Ser(19)). 2) The sustained inhibitory response is typically associated with Ser(16) phosphorylation of heat shock protein 20 (HSP20) without sustained reductions in [Ca(2+)](i) or MRLC phosphorylation. We investigated whether the extent of Ser(16)-HSP20 phosphorylation quantitatively correlated with the sustained inhibitory response. With addition of nitroglycerin to histamine-stimulated swine carotid media, the initial relaxation transient was associated with a decrease in MRLC phosphorylation without an increase in Ser(16)-HSP20 phosphorylation. During the sustained phase of nitroglycerin-induced relaxation and during force redevelopment induced by washout of nitroglycerin in the continued presence of histamine, the level of Ser(16)-HSP20 phosphorylation, but not MRLC phosphorylation, correlated with inhibition of force. Forskolin, which increases cAMP concentration, also induced a sustained inhibitory response that was associated with increases in Ser(16)-HSP20 phosphorylation without reductions in MRLC phosphorylation levels. Forskolin increased Ser(16)-HSP20 phosphorylation to a greater extent and inhibited force more completely than that observed with nitroglycerin. Increases in Ser(16)-HSP20 phosphorylation correlated with the degree of force inhibition regardless of whether the relaxation was induced by nitroglycerin or forskolin. These data are consistent with the hypothesis that Ser(16)-HSP20 phosphorylation may be a cyclic nucleotide-dependent, yet MRLC phosphorylation-independent, inhibitor of smooth muscle contractile force.

Myosin light chain kinase- and PKC-dependent contraction of LES and esophageal smooth muscle

In smooth muscle cells enzymatically isolated from circular muscle of the esophagus (ESO) and lower esophageal sphincter (LES), ACh-induced contraction and myosin light chain (MLC) phosphorylation were similar. Contraction and phosphorylation induced by purified MLC kinase (MLCK) were significantly greater in LES than ESO. ACh-induced contraction and MLC phosphorylation were inhibited by calmodulin and MLCK inhibitors in LES and by protein kinase C (PKC) inhibitors in ESO. Contraction of LES and ESO induced by the PKC agonist 1,2-dioctanoylglycerol (DG) was unaffected by MLCK inhibitors. Caldesmon and calponin concentration-dependently inhibited ACh-induced contraction of ESO and not LES. In ESO, caldesmon antagonist GS17C reversed caldesmon- but not calponin-induced ACh inhibition. GS17C caused contraction of permeabilized ESO but had much less effect on LES. GS17C-induced contraction was not affected by MLCK inhibitors, suggesting that MLCK may not regulate caldesmon-mediated contraction. DG-induced contraction of ESO and LES was inhibited by caldesmon and calponinin, suggesting that these proteins may regulate PKC-dependent contraction. We conclude that calmodulin and MLCK play a role in ACh-induced LES contraction, whereas the classical MLCK may not be the major kinase responsible for contraction and phosphorylation of MLC in ESO. ESO contraction is PKC dependent. Caldesmon and/or calponin may play a role in PKC-dependent contraction.

Invited review: regulation of myosin phosphorylation in smooth muscle

Phosphorylation of the regulatory light chains of myosin II (rMLC) by the Ca(2+)/calmodulin-dependent myosin light-chain kinase (MLCK) and dephosphorylation by a type 1 phosphatase (MLCP), which is targeted to myosin by a regulatory subunit (MYPT1), are the predominant mechanisms of regulation of smooth muscle tone. The activities of both enzymes are modulated by several protein kinases. MLCK is inhibited by the Ca(2+)/calmodulin-dependent protein kinase II, whereas the activity of MLCP is increased by cGMP and perhaps also cAMP-dependent protein kinases. In either case, this results in a decrease in the Ca(2+) sensitivity of rMLC phosphorylation and force production. The activity of MLCP is inhibited by Rho-associated kinase, one of the effectors of the monomeric GTPase Rho, and protein kinase C, leading to an increase in Ca(2+) sensitivity. Hence, smooth muscle tone appears to be regulated by a network of activating and inactivating intracellular signaling cascades.

The paradox of smooth muscle physiology

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (contraction) and the release of force (relaxation). The signaling events that activate contraction include Ca(2+)-dependent myosin light chain phosphorylation. The signaling events that mediate relaxation include the removal of a contractile agonist (passive relaxation) and activation of cyclic nucleotide-dependent signaling pathways in the continued presence of a contractile agonist (active relaxation). The major questions that remain in contractile physiology include (1) how is tonic force maintained when intracellular Ca(2+) levels and myosin light chain phosphorylation have returned to basal levels; and (2) what is the mechanism of cyclic nucleotide-dependent relaxation? This review focuses on these specific controversies surrounding the molecular mechanisms of contraction and relaxation of vascular smooth muscle.

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.

Caldesmon and heat shock protein 20 phosphorylation in nitroglycerin- and magnesium-induced relaxation of swine carotid artery

Nitrovasodilators, high extracellular Mg(2+), and some other relaxing agents can cause smooth muscle relaxation without reductions in myosin regulatory light chain (MRLC) phosphorylation. Relaxations without MRLC dephosphorylation suggest that other regulatory systems, beyond MRLC phosphorylation, are present in smooth muscle. We tested whether changes in caldesmon phosphorylation, heat shock protein 20 (HSP20) phosphorylation, or intracellular pH (pH(i)) could be responsible for relaxation without MRLC dephosphorylation. In unstimulated tissues, caldesmon was phosphorylated 1.02+/-0.10 mol P(i)/mol caldesmon (mean+/-1 S.E.M.), HSP20 was phosphorylated 0.005+/-0.003 mol P(i)/mol HSP20, and estimated pH(i) was 7.21+/-0.07. Histamine stimulation induced a contraction, an intracellular acidosis, but did not significantly change caldesmon or HSP20 phosphorylation. Addition of nitroglycerin induced a relaxation, significantly increased HSP20 phosphorylation to 0.18+/-0.02 mol P(i)/mol HSP20, did not significantly change caldesmon phosphorylation, and pH(i) returned to near unstimulated values. Increase in extracellular Mg(2+) to 10 mM induced a relaxation, but did not significantly change HSP20 or caldesmon phosphorylation. These data suggest that changes in caldesmon phosphorylation, HSP20 phosphorylation, or pH(i) cannot be the sole explanation for relaxation without MRLC dephosphorylation. However, it is possible that HSP20 phosphorylation may be involved in nitroglycerin-induced relaxation without MRLC dephosphorylation.

Inhibition of CaM kinase II activation and force maintenance by KN-93 in arterial smooth muscle

Ca(+)/calmodulin-dependent protein kinase II (CaM kinase II) has been implicated in the regulation of smooth muscle contractility. The goals of this study were to determine: 1) to what extent CaM kinase II is activated by contractile stimuli in intact arterial smooth muscle, and 2) the effect of a CaM kinase II inhibitor (KN-93) on CaM kinase II activation, phosphorylation of myosin regulatory light chains (MLC(20)), and force. Both histamine (1 microM) and KCl depolarization activated CaM kinase II with a time course preceding maximal force development, and suprabasal CaM kinase II activation was sustained during tonic contractions. CaM kinase II activation was inhibited by KN-93 pretreatment (IC(50) approximately 1 microM). KN-93 inhibited histamine-induced tonic force maintenance, whereas early force development and MLC(20) phosphorylation responses during the entire time course were unaffected. Both force development and maintenance in response to KCl were inhibited by KN-93. Rapid increases in KCl-induced MLC(20) phosphorylation were also inhibited by KN-93, whereas steady-state MLC(20) phosphorylation responses were unaffected. In contrast, phorbol 12,13-dibutyrate (PDBu) did not activate CaM kinase II and PDBu-stimulated force development was unaffected by KN-93. Thus KN-93 appears to target a step(s) essential for force maintenance in response to physiological stimuli, suggesting a role for CaM kinase II in regulating tonic contractile responses in arterial smooth muscle. Pharmacological activation of protein kinase C bypasses the KN-93 sensitive step.

Identification of a novel actin binding motif in smooth muscle myosin light chain kinase

Phosphorylation of the 20-kDa regulatory light chain of myosin catalyzed by a Ca(2+)/calmodulin-dependent myosin light chain kinase is important in the initiation of smooth muscle contraction and other contractile processes in non-muscle cells. It has been previously shown that residues 1-142 of smooth muscle myosin light chain kinase are necessary for high-affinity binding to actin-containing filaments in cells (1). To further localize the region of the kinase required for binding, a series of N-terminal deletion mutants as well as several N-terminal glutathione S-transferase fusion proteins were constructed. Cosedimentation assays showed that a peptide containing residues 1-75 binds to purified smooth muscle myofilaments. Furthermore, the N-terminal peptide was sufficient for high-affinity binding to actin stress fibers in smooth muscle cells in vivo. Alanine scanning mutagenesis in the fusion protein identified residues Asp-30, Phe-31, Arg-32, and Leu-35 as important for binding in vitro. There are two additional DFRXXL motifs located at residues 2-7 and 58-63. The DFR residues in these three motifs were individually replaced by alanine residues in the full-length kinase. Each of these mutations significantly decreased myosin light chain kinase binding to myofilaments in vitro, and each abolished high-affinity binding to actin-containing filaments in smooth muscle cells in vivo. These results identify a unique structural motif comprised of three repeat consensus sequences in the N terminus of myosin light chain kinase necessary for high-affinity binding to actin-containing filaments.

Properties of filament-bound myosin light chain kinase

Myosin light chain kinase binds to actin-containing filaments from cells with a greater affinity than to F-actin. However, it is not known if this binding in cells is regulated by Ca2+/calmodulin as it is with F-actin. Therefore, the binding properties of the kinase to stress fibers were examined in smooth muscle-derived A7r5 cells. Full-length myosin light chain kinase or a truncation mutant lacking residues 2-142 was expressed as chimeras containing green fluorescent protein at the C terminus. In intact cells, the full-length kinase bound to stress fibers, whereas the truncated kinase showed diffuse fluorescence in the cytoplasm. After permeabilization with saponin, the fluorescence from the truncated kinase disappeared, whereas the fluorescence of the full-length kinase was retained on stress fibers. Measurements of fluorescence intensities and fluorescence recovery after photobleaching of the full-length myosin light chain kinase in saponin-permeable cells showed that Ca2+/calmodulin did not dissociate the kinase from these filaments. However, the filament-bound kinase was sufficient for Ca2+-dependent phosphorylation of myosin regulatory light chain and contraction of stress fibers. Thus, dissociation of myosin light chain kinase from actin-containing thin filaments is not necessary for phosphorylation of myosin light chain in thick filaments. We note that the distance between the N terminus and the catalytic core of the kinase is sufficient to span the distance between thin and thick filaments.

Control of cross-bridge cycling by myosin light chain phosphorylation in mammalian smooth muscle

This review focuses on experiments in which the single turnover of myosin-bound ADP is used to characterize the regulation of the cross-bridge cycle by myosin light chain phosphorylation in mammalian smooth muscle. Under isometric conditions, at rest, when the myosin light chain is not phosphorylated, myosin cycles very slowly (about 0.004 s-1), while phosphorylation of the light chain results in a 50-fold increase in cycling rate of 0.2 s-1. Experiments consistently show that some myosin does not increase its cycling rate although its light chain is phosphorylated. Studies at low levels of myosin light chain phosphorylation show that phosphorylation also induces an increase in the cycling rate of unphosphorylated myosin. The fast cycling phosphorylated myosin is the main determinant of suprabasal myosin ATPase activity, while the cycling rate of cooperatively activated unphosphorylated myosin is slow and appears to depend on the extent of phosphorylation of the entire thick filament. Single turnover experiments measuring the rate of phosphorylation and dephosphorylation of myosin light chain show that the turnover of light chain phosphate can be very rapid (0.3-0.4 s-1) at suprabasal calcium concentrations. The expected effect of such a rapid turnover of light chain phosphorylation on the turnover of myosin-bound ADP is not observed. The effects of low levels of myosin light chain phosphorylation on the single turnover of myosin suggest that the same small pool of myosin remains phosphorylated for relatively long periods of time rather than the entire pool of myosin spending a small fraction of its cycle time in the phosphorylated state.

Inhibitory mechanisms for cross-bridge cycling: the nitric oxide-cGMP signal transduction pathway in smooth muscle relaxation

Relaxation follows sequestration of Ca2+ mobilized by an excitatory stimulus in striated muscle. Removal of excitatory stimuli also relaxes smooth muscle in vitro after reductions in the myoplasmic [Ca2+] and dephosphorylation of the myosin regulatory light chains. However, there are several experimental procedures that produce relaxation in the presence of excitatory stimuli and elevated Ca(2+)-dependent cross-bridge phosphorylation. Of potential widespread physiological importance are treatments that increase myoplasmic [cGMP] owing to the ubiquity of nitric oxide (NO) as a signalling molecule for endothelial-mediated vasodilation and inhibitory nerves in most types of smooth muscle. Several mechanisms are implicated in the NO-cGMP mediated relaxation. Most studies support reductions in myoplasmic Ca2+. However, there is evidence that increases in cGMP also lower the Ca(2+)-sensitivity of cross-bridge phosphorylation. This would contribute to a decline in force through actions on the myosin light chain kinase/phosphatase system. In addition, changes in the dependence of force on phosphorylation are observed in tissues partially relaxed by treatments that elevate cGMP. This demonstrates that either the attachment and cycling of phosphorylated cross-bridges is impaired or blocked, or that the formation of dephosphorylated, force-generating cross-bridges ('latch-bridges') is reduced. Protein kinase G-catalysed phosphorylation of either a thin filament protein that blocks attachment of cross-bridges or a protein that inhibits myosin light chain phosphatase may explain the NO-induced relaxation with elevated cross-bridge phosphorylation.

cGMP mediates corpus cavernosum smooth muscle relaxation with altered cross-bridge function

We tested the prevailing paradigm that relaxation of corpus cavernosum smooth muscle (CCSM) and penile erection depends upon nitric oxide-induced elevation of myoplasmic cGMP and reduced Ca2+-dependent myosin regulatory light chain phosphorylation levels. This hypothesis invokes a reversal of normal activation pathways. Upon stimulation with 250 microM phenylephrine, phosphorylation of the 20 kD myosin regulatory light chains of rabbit or human CCSM increased approximately 4-fold coincident with contraction. Removal of the agonist was followed by a slow reduction in cross-bridge phosphorylation and force to basal levels. The NO donor, sodium nitroprusside elicited a dose-dependent increase in tissue [cGMP] associated with a rapid relaxation in the continued presence of phenylephrine, although cross-bridge phosphorylation remained significantly elevated. Thus the NO-cGMP inhibitory pathway in CCSM is not simply a reversal of excitatory signal transduction mechanisms. An unidentified mechanism contributes to relaxation by decreasing the rate of cross-bridge recruitment through phosphorylation.

alpha1-Adrenoceptor-mediated phosphorylation of myosin in rat-tail arterial smooth muscle

The mechanism of alpha1-adrenoceptor-mediated contraction was investigated in helical strips of the rat-tail artery. Muscle strips with the endothelium removed contracted in response to the alpha1-adrenoceptor agonist cirazoline, with half-maximal contraction at 0.23 microM. The contractile response to a submaximal concentration of cirazoline (0.3 microM) was biphasic, with a rapid phasic component peaking at approx. 30 s, followed by sustained tonic contraction. Phosphorylation of the 20 kDa light chain of myosin (LC20) in response to 0.3 microM cirazoline was also biphasic and closely matched the time-course of contraction. Resting LC20 phosphorylation levels were 0.22+/-0.06 mol of Pi/mol of LC20 (n=3) and reached a maximum of 0.58+/-0.08 mol of Pi/mol of LC20 (n=3). Phosphopeptide mapping and phosphoamino acid analysis revealed that LC20 phosphorylation occurred exclusively at serine-19. The sustained phase of contraction was eliminated by removal of extracellular Ca2+ and the phasic response was eliminated by depletion of endogenous Ca2+ stores. Both phases of the contractile response were restored by re-addition of Ca2+ to the bathing medium. LC20 phosphorylation and both phases of the contractile response to 0.3 microM cirazoline were inhibited by the myosin light-chain kinase inhibitor ML-9 (30 microM). Resting LC20 phosphorylation, however, was unaffected by ML-9. Finally, both phasic and tonic responses to 0.3 microM cirazoline were partially inhibited by chloroethylclonidine (50 microM), suggesting the involvement of both alpha1A and alpha1B adrenoceptors in these contractile responses.

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.

The effects of caldesmon extraction on mechanical properties of skinned smooth muscle fibre preparations

The role of caldesmon in the regulation of smooth muscle contraction was investigated in chemically skinned smooth muscle fibres from the guinea-pig taenia coli. A 19-kDa C-terminal fragment of caldesmon gave a minor (<5%) reduction of force in fully thiophosphorylated fibres, but reduced force by about 50% at intermediate activation levels without affecting the level of light chain phosphorylation. An extraction procedure was developed using incubation in solutions containing high Mg2+ concentrations. Protein analysis revealed a selective decrease in the amount of caldesmon in the fibres. Maximal active force per cross-sectional area was unaffected. The Ca2+ dependence of active force was shifted towards lower Ca2+ concentrations and became less steep. The effects of extraction of caldesmon could in part be reversed by incubation in a solution containing purified caldesmon. The results are consistent with the hypothesis that caldesmon in smooth muscle thin filaments inhibits force generation and plays a role in regulating cooperative attachment of cross-bridges at sub-maximal levels of activation in smooth muscle.

Immunocytochemical localization of caldesmon and calponin in chicken gizzard smooth muscle

The distribution of caldesmon and calponin in chicken gizzard smooth muscle was investigated with immunofluorescence and immunogold electron microscopy. Immunofluorescence microscopy showed that in verapamil treated (relaxed) muscles the distributions of caldesmon and myosin appeared to be uniform throughout the cytoplasm, but clearly more textured than that of actin filaments as revealed by the distribution of tropomyosin. In shortened muscles both caldesmon and myosin became segregated, in contrast to the distribution of actin, which remained uniform. The distribution of calponin was even more textured, with no similarity to those of caldesmon or myosin. Instead, considerable overlap was observed between calponin and the cytoskeletal protein desmin and, to a lesser extent, beta-actin. By immunogold electron microscopy caldesmon appeared mostly near and around myosin filaments in both relaxed and shortened muscle. Calponin, on the other hand, was found primarily at the periphery of cytoskeletal structures in the same general region as desmin, and very often adjacent to beta-actin, which is mainly in the core. These observations indicated that caldesmon and calponin are associated with different subsets of actin filaments, caldesmon with contractile actin, while calponin with cytoskeletal actin. Thus the in situ localization of caldesmon is consistent with its proposed regulatory function. Calponin, on the other hand, is unlikely to directly regulate actomyosin interactions in these cells; instead, it may function as a bridging protein between the actin and the intermediate filament networks.

Force-inhibiting effect of okadaic acid on skinned rat uterus permeabilized with alpha-toxin

Effects of okadai acid (OA) on contractile force in rat uterine uterine muscles permeabilized with alpha-toxin were examined. (1) Contractile force activated by Ca2+(10(-6.5) M to 10(-4.4) M) was suppressed by relatively low concentrations of OA (30 to 300 nM). The suppressed force was further decreased after washed out of OA. (2)Addition of 10 microM OA enhanced force. Whereas, the increased tension level fell to less than the control level after washed out of OA. (3)Okadaic acid methyl ester (methyl okadaate), an OA derivative without protein phosphatase inhibition, did not affect contraction. These results suggest that the force-inhibiting effect of OA is a result of interference with contractile elements through inhibition of protein phosphatases (PPs) activity.

Modulation of Ca2+ sensitivity in smooth muscle by genistein and protein tyrosine phosphorylation

Genistein, a potent tyrosine kinase inhibitor, inhibits contraction of several types of smooth muscle, suggesting that protein tyrosine phosphorylation may be an important regulatory mechanism for smooth muscle contraction. We suspected that one site between activation of smooth muscle and contraction which might be modulated by protein tyrosine phosphorylation involved mechanisms for control of Ca2+ sensitivity. Since smooth muscle permeabilized with staphylococcal alpha-toxin permits direct assessment of agonist-induced Ca2+ sensitivity, we studied the effects of genistein on potential coupling between tyrosine phosphorylation and Ca2+ sensitivity in permeabilized ileal smooth muscle. Results show that contraction of intact preparations with carbachol is markedly and reversibly inhibited by 40% at 4 micrograms genistein/ml and by 60% at 20 micrograms genistein/ml. Permeabilized preparations that are contracted with a submaximal [Ca2+] in the presence of GTP relax when genistein is added to the medium. Genistein also reversibly inhibits contractions induced in permeabilized muscle with either a submaximal or maximal [Ca2+] in the presence of GTP, as well as receptor-coupled activation of Ca2+ sensitization with 10 microM carbachol/10 microM GTP. Activation of permeabilized preparations at pCa 4.6 in the presence of 100 microM GTP promotes time-dependent tyrosine phosphorylation of several substrates. Both phosphorylation and force are inhibited by genistein. However, relatively high levels of myosin light chain phosphorylation persist during genistein-induced inhibition of Ca2+ sensitivity. In contrast, genistein has no effect on Ca(2+)-activated contraction in Triton-skinned preparations in either the presence or the absence of GTP. This shows that it does not directly inhibit actin-myosin interaction and suggests that its target(s) may be a cytosolic or membrane-bound regulatory protein(s) that is leached from the preparations during Triton-skinning. Taken together, these new data suggest that (a) tyrosine phosphorylation of one or more substrates may be coupled to mechanisms which regulate Ca2+ sensitivity and (b) the inhibitory effects of genistein are probably due to inhibition of agonist-induced Ca2+ sensitivity.

pHi, [Ca2+]i, and myosin phosphorylation in histamine- and NH4(+)-induced swine carotid artery contraction

We examined the interaction among changes in pHi, [Ca2+]i, myosin light-chain phosphorylation, and contraction in arterial smooth muscle stimulated by histamine, NH4+, Tris+, and/or changes in extracellular pH (pHo). We loaded swine carotid medial tissues with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein to measure pHi or aequorin to measure [Ca2+]i. Incubation of tissues in NH4+ increased pHi, [Ca2+]i, myosin phosphorylation, and force. Washout of NH4+ decreased pHi and transiently further increased in [Ca2+]i and force. Incubation of tissues in a similar concentration of Tris+ or increasing pHo also increased pHi; however, there were only modest changes in [Ca2+]i and force. Increasing extracellular pH coincidentally with washout of NH4+ prevented the decrease in pHi but did not affect the NH4+ washout-induced contraction. These data suggest that NH4+ altered [Ca2+]i and contraction by mechanisms other than its effects on pHi. The type of pH buffer did not affect the [Ca2+]i, myosin phosphorylation, or stress response to histamine stimulation. The time course of changes in pHi was much slower than the time course of histamine-induced changes in [Ca2+]i, myosin phosphorylation, and stress. Addition of 10 mmol/L NH4+ concurrently with histamine aborted the histamine-induced decrease in pHi and significantly slowed the histamine-induced increase in [Ca2+]i, myosin phosphorylation, and stress. There was little effect on histamine-induced increases in [Ca2+]i, myosin phosphorylation, or contraction when three other protocols aborted the histamine-induced decrease in pHi. These data show that incubation in NH4+ can alter [Ca2+]i and contraction in both unstimulated and histamine-stimulated smooth muscle. However, these effects were not caused by NH4(+)-dependent changes in pHi.(ABSTRACT TRUNCATED AT 250 WORDS)

Myosin phosphorylation and the control of myometrial contraction/relaxation

In summary, phosphorylation of the regulatory light chain of myosin by Ca2+/CaM-dependent MLCK plays an important role in smooth muscle contraction. Although there have been major advances in our understanding of the regulation and physiological functions of contractile proteins in smooth muscle in recent years, very little information exists on the functional status of these proteins in human myometrium during pregnancy. The simple view that contractile force in smooth muscle is proportionate to cytoplasmic Ca2+ concentrations (Ca2+i) and myosin light chain phosphorylation is now more complex as more experiments provide insights into mechanisms of regulation of the contractile elements. MLCK can be phosphorylated, which desensitizes its activation by Ca2+/CaM, and protein phosphatase activity toward myosin may also be regulated. Examples in smooth muscle tissue are sparse, and the different mechanisms by which these processes may be adapted in uterine smooth muscle during pregnancy are not well-defined. Much research is needed to define further the cellular, biochemical, and molecular basis for these physiological processes involved in the regulation of uterine smooth muscle contraction and relaxation.

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.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

Effects of okadaic acid on the human isolated bronchus

The effects of okadaic acid, a polyether derivative of a 38-carbon monocarboxylic fatty acid obtained from a culture of the marine dinoflagellate, Prorocentrum lima, were studied on the human isolated bronchus. In low concentrations (0.01 and 0.03 microM), okadaic acid had no significant effect of its own on the human isolated bronchus, but in higher concentrations (0.1-10 microM) it induced a series of contractions and relaxations. The first contraction was of low intensity (5% of maximum response to acetylcholine 3 mM) and occurred early. The second contraction had a higher amplitude (30% of maximum response to acetylcholine 3 mM) and reached its peak with okadaic acid 0.3 microM. At higher concentrations (1-10 microM), following a relaxation phase, a later rebound contraction occurred between 70 and 120 min and corresponded to 40% of the maximum response to acetylcholine 3 mM. In addition, okadaic acid inhibited or abolished the contractile response evoked by either KCl 60 mM or acetylcholine 3 mM with IC50 of 0.04 and 0.12 microM, respectively. The second contraction evoked by 0.3 microM okadaic acid was partially inhibited in the presence of the Ca2+ channel blocker, nicardipine 1 microM, or after incubation of the human bronchus in a Ca(2+)-free solution and it was completely abolished in the presence of CdSO4 0.1 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of calyculin A on tension and myosin phosphorylation in skinned smooth muscle of the rabbit mesenteric artery

1. Using beta-escin and ionomycin-treated skinned smooth muscle strips of the rabbit mesenteric artery, the effects of calyculin A (CL-A, an inhibitor of type 1 and 2A phosphatases) on mechanical activities, phosphorylation of myosin light chain (MLC) and the relationship between the two were studied in Ca(2+)-free solution containing 4 mM EGTA and these effects were compared with those evoked by Ca2+. 2. The threshold concentration of Ca2+ required to increase either tension or MLC-phosphorylation was 0.1 microM and maximum effects were obtained at 10 microM. MLC was mainly monophosphorylated, rather than diphosphorylated, in the presence of Ca2+. ED50 value for Ca2+ was 0.54 microM for either tension or MLC-phosphorylation. The relationship between tension and MLC-phosphorylation is linear in the pCa range 7-5.5. 3. In Ca(2+)-free solution (containing either 20 mM EGTA or 4 mM EGTA with or without 4 mM BAPTA), 3 microM CL-A produced a contraction, the maximum amplitude of which was similar to that evoked by 10 microM Ca2+. CL-A (0.03-3 microM) concentration-dependently increased both tension and MLC-phosphorylation in Ca(2+)-free solution containing 4 mM EGTA. The threshold concentration of CL-A required for the increase in either tension or MLC-phosphorylation was 0.03 microM and maximum effects were obtained at 3 microM. In the presence of CL-A, MLC was not only monophosphorylated but also diphosphorylated. ED50 values for CL-A were 0.39 microM for tension, 0.44 microM for the monophosphorylated form of MLC and 0.54 microM for all phosphorylated (mono + di) forms. The relationship between tension and the monophosphorylated form of MLC was linear over the concentration range studied and was similar to that for Ca2+. 4. H-7 (3 microM, an inhibitor of protein kinase C) inhibited neither the tension nor phosphorylation of MLC induced by 10 microM Ca2+ or 3 microM CL-A. At a high concentration (30 microM), H-7 slightly inhibited both the tension and phosphorylation of MLC induced by either stimulant without a change in the tension-MLC-phosphorylation relationship. KN-62, an inhibitor of Ca(2+)-calmodulin-dependent protein kinase II, did not modify either the tension or the phosphorylation of MLC induced by 10 microM Ca2+ or 3 microM CL-A. CK-II, another inhibitor of Ca(2+)-calmodulin-dependent protein kinase II, did not inhibit the contraction induced by 3 microM CL-A. 5. SM-1 (0.03-0.3 mM) and ML-9 (0.1 and 0.3 mM), inhibitors of MLC-kinase, each lowered the resting level of MLC-phosphorylation in Ca2+-free solution and also inhibited both the tension and MLC-phosphorylation induced by 10 microM Ca2+ or 3 microM CL-A, in a concentration-dependent manner.Neither SM-1 nor ML-9 modified the relationship between tension and either monophosphorylated or all phosphorylated (mono + di) forms of MLC in the presence of Ca2+ or CL-A.6. In a solution containing MgITP (the substrate for myosin ATPase but not for MLC-kinase) with no MgATP, 10 microM Ca2+ failed to produce contraction. Under these conditions, the amplitude of the contraction induced by 3 microM CL-A was greatly diminished in comparison with that induced in the presence of MgATP.7. The present results suggest that in smooth muscle cells of the rabbit mesenteric artery, CL-A in Ca2+-free solution, produces a maximum contraction through an indirect activation of Ca2+-calmodulin independent(constitutively active) MLC-kinase via its inhibitory action on MLC-phosphatases. Based on this evidence, it is hypothesized that, in these cells, a constitutively active MLC-kinase may be present, though its action may be concealed by that of endogenous MLC-phosphatase.

Caldesmon and a 20-kDa actin-binding fragment of caldesmon inhibit tension development in skinned gizzard muscle fiber bundles

Caldesmon is known to inhibit actin-activated myosin ATPase activity in solution, to inhibit force production when added to skeletal muscle fibers, and to alter actin movement in the in vitro cell motility assay. It is less clear that caldesmon can inhibit contraction in smooth muscle cells in which caldesmon is abundant. We now show that caldesmon and its 20-kDa actin-binding fragment are able to inhibit force in chemically skinned gizzard fiber bundles, which are activated by a constitutively active myosin light-chain kinase in the presence and absence of okadaic acid. This inhibitory effect is reversed by high concentrations of Ca2+ and calmodulin. Therefore, caldesmon may act by increasing the level of myosin phosphorylation required to obtain full activation. Our results also suggest that caldesmon does not act to maintain force in smooth muscle by cross-linking myosin with actin since competition of binding of caldesmon with myosin does not cause a reduction in tension.

Vasodilatory effects of okadaic acid on the canine cerebral artery

We investigated the in vivo and in vitro vasodilatory effects of okadaic acid, an inhibitor of protein phosphatases, in canine basilar arteries. Angiography revealed that the intracisternal injection of okadaic acid produced a long-lasting increase in the internal diameter of the canine basilar artery. The maximal increases in diameter induced by 1 and 10 nmol of okadaic acid were 23.3 +/- 13.5 and 33.8 +/- 11.9%, respectively. Okadaic acid in the concentrations of 10(-7) and 10(-6) M also exerted a dose-dependent, long-lasting relaxation without any contraction in isolated basilar arteries, even in the resting condition. Similar effects (ED50 values and maximal relaxation) were observed in arterial strips precontracted with K+, prostaglandin F2 alpha, and phorbol 12-myristate 13-acetate. These in vito and in vivo results suggest that inhibition of protein phosphatases by okadaic acid produces a vasodilation in the cerebral artery.

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.

Mechanism of relaxing action of the antiasthmatic drug, azelastine, in isolated porcine tracheal smooth muscle

Azelastine (1-300 microM) inhibited contractions of isolated porcine trachea induced by high K+, carbachol and endothelin-1 (ET-1) with a decrease in [Ca2+]cyt (as measured by fura-2-fluorescence). Verapamil (0.1-10 microM) also inhibited the high K(+)-induced increases in [Ca2+]cyt and contraction, although it only partially inhibited the responses evoked by carbachol or ET-1. In the absence of extracellular Ca2+ (with 0.5 mM EGTA), carbachol induced a transient increase in [Ca2+]cyt and force by releasing Ca2+ from cellular stores. Azelastine (100 microns) completely inhibited these contransient changes. In the absence of extracellular Ca2+, carbachol and 12-deoxyphorbol 13-isobutyrate (DPB) induced small sustained contractions without increasing [Ca2+]cyt. Azelastine inhibited these contractions. In muscle permeabilized with alpha-toxin, Ca2+ (0.3-3 microM) induced contraction in a concentration-dependent manner. DPB (without GTP) and carbachol or ET-1 (with GTP) enhanced the Ca(2+)-induced contraction. Azelastine partially inhibited the contraction induced by 0.3 microM Ca2+ but not the contraction induced by 3 microM Ca2+, and strongly inhibited the potentiating effects of DPB, carbachol and ET-1. Azelastine had no effect on the content of cyclic AMP or cyclic GMP. These results suggest that azelastine inhibits smooth muscle contraction by (i) decreasing [Ca2+]cyt, by inhibition of Ca2+ channels, (ii) decreasing agonist-induced Ca2+ release, and (iii) direct inhibition of contractile elements.

Myosin II phosphorylation and the dynamics of stress fibers in serum-deprived and stimulated fibroblasts

The actin-based cytomatrix generates stress fibers containing a host of proteins including actin and myosin II and whose dynamics are easily observable in living cells. We developed a dual-radioisotope-based assay of myosin II phosphorylation and applied it to serum-deprived fibroblasts treated with agents that modified the dynamic distribution of stress fibers and/or altered the phosphorylation state of myosin II. Serum-stimulation induced an immediate and sustained increase in the level of myosin II heavy chain (MHC) and 20-kDa light chain (LC20) phosphorylation over the same time course that it caused stress fiber contraction. Cytochalasin D, shown to cause stress fiber fragmentation and contraction, had little effect on myosin II phosphorylation. Okadaic acid, a protein phosphatase inhibitor, induced a delayed but massive cell shortening preceded by a large increase in MHC and LC20 phosphorylation. Staurosporine, a kinase inhibitor known to effect dissolution but not contraction of stress fibers, immediately caused an increase in MHC and LC20 phosphorylation followed within minutes by the dephosphorylation of LC20 to a level below that of untreated cells. We therefore propose that the contractility of the actin-based cytomatrix is regulated by both modulating the activity of molecular motors such as myosin II and by altering the gel structure in such a manner as to either resist or yield to the tension applied by the motors.

Nitrovasodilators relax arterial smooth muscle by decreasing [Ca2+]i and uncoupling stress from myosin phosphorylation

Elevations in guanosine 3',5'-cyclic monophosphate concentration ([cGMP]) are proposed to induce arterial smooth muscle relaxation by either 1) decreasing myoplasmic [Ca2+] ([Ca2+]i), 2) decreasing the [Ca2+]i sensitivity of phosphorylation, or 3) uncoupling force from myosin phosphorylation. We evaluated the importance of each of these mechanisms by measuring changes in [cGMP], aequorin- and fura-2-estimated [Ca2+]i, myosin light chain phosphorylation, and stress in histamine-stimulated swine carotid arteries. In tissues submaximally stimulated with 3 microM histamine, nitroprusside (NP) induced a proportional decrease in myoplasmic [Ca2+] and myosin phosphorylation, suggesting that the relaxation was at least partially induced by decreases in [Ca2+]i without a change in the [Ca2+]i sensitivity of phosphorylation. In tissues maximally stimulated with 10 microM histamine, NP and nitroglycerin produced significant relaxations that were not associated with significant sustained reductions in [Ca2+]i or myosin phosphorylation. With both submaximal and maximal histamine stimulation, nitrovasodilators produced more substantial relaxation than that expected from the nitrovasodilator-induced reduction in myosin phosphorylation. These results suggest that nitrovasodilators relax histamine-stimulated swine arterial smooth muscle by at least two mechanisms: 1) reducing [Ca2+]i, an effect observed in submaximally stimulated tissues, and 2) uncoupling of stress from myosin phosphorylation.

Steady-state dependence of stress on cross-bridge phosphorylation in the swine carotid media

Tonic contractions of the swine carotid media are typically characterized by initial transients in myoplasmic [Ca2+] and cross-bridge phosphorylation followed by force maintenance with reduced intracellular [Ca2+] and cross-bridge phosphorylation ("latch"). The presence of effective mechanisms in the carotid media to limit steady-state myoplasmic [Ca2+] and cross-bridge phosphorylation to modest increases over resting values has limited experimental attempts to determine the dependence of active stress (force/tissue cross-sectional area) on cross-bridge phosphorylation. In this study, we employed stimulation protocols that combined effective contractile agonists with inhibitors of Ca2+ extrusion or sequestration to achieve high steady-state levels of cross-bridge phosphorylation (up to 60%). Increases in cross-bridge phosphorylation from 30 to 60% were not associated with significant increases in stress in agreement with the predictions of Hai and Murphy [Am. J. Physiol. 254 (Cell Physiol. 23): C99-C106, 1988] four-state cross-bridge model for the carotid media. Thus cross-bridge phosphorylation may suffice to determine force generation in vascular smooth muscle if both phosphorylated and dephosphorylated attached cross bridges (or latch bridges) contribute to active stress.