Separate upstream and convergent downstream pathways of G-protein- and phorbol ester-mediated Ca2+ sensitization of myosin light chain phosphorylation in smooth muscle

Senescent murine femoral arteries undergo vascular remodelling associated with accelerated stress-induced contractility and reactivity to nitric oxide

This work explored the mechanism of augmented stress-induced vascular reactivity of senescent murine femoral arteries (FAs). Mechanical and pharmacological reactivity of young (12-25 weeks, y-FA) and senescent (>104 weeks, s-FAs) femoral arteries was measured by wire myography. Expression and protein phosphorylation of selected regulatory proteins were studied by western blotting. Expression ratio of the Exon24 in/out splice isoforms of the regulatory subunit of myosin phosphatase, MYPT1 (MYPT1-Exon24 in/out), was determined by polymerase chain reaction (PCR). While the resting length-tension relationship showed no alteration, the stretch-induced-tone increased to 8.3 ± 0.9 mN in s-FA versus only 4.6 ± 0.3 mN in y-FAs. Under basal conditions, phosphorylation of the regulatory light chain of myosin at S19 was 19.2 ± 5.8% in y-FA versus 49.2 ± 12.6% in s-FA. Inhibition of endogenous NO release raised tone additionally to 10.4 ± 1.2 mN in s-FA, whereas this treatment had a negligible effect in y-FAs (4.8 ± 0.3 mN). In s-FAs, reactivity to NO donor was augmented (pD₂  = -4.5 ± 0.3 in y-FA vs. -5.2 ± 0.1 in senescent). Accordingly, in s-FAs, MYPT1-Exon24-out-mRNA, which is responsible for expression of the more sensitive to protein-kinase G, leucine-zipper-positive MYPT1 isoform, was increased. The present work provides evidence that senescent murine s-FA undergoes vascular remodelling associated with increases in stretch-activated contractility and sensitivity to NO/cGMP/PKG system.

Mirabegron induces relaxant effects via cAMP signaling-dependent and -independent pathways in detrusor smooth muscle

OBJECTIVE

We previously found that mirabegron exerts a relaxant effect in the presence of the β₃ -adrenoceptor antagonist SR58894A during carbachol-induced contraction in human and pig detrusor. The aim of this study was to explore the possible mechanism underlying the relaxant effects of mirabegron using detrusor smooth muscle.

METHODS

Human tissue was obtained from urinary bladders of patients undergoing radical cystectomy at Kyushu University and Harasanshin Hospital. Pig tissue was obtained from an abattoir. Tension force (organ bath experiments) was measured in intact or permeabilised (α-toxin or β-escin) detrusor smooth muscle strips. The contribution of cAMP-dependent signaling and the inhibition of Ca²⁺ sensitization to the relaxant effects of mirabegron were characterized using 1 μM SR58894A, 100 μM SQ22536 (an adenylyl cyclase inhibitor), 10 μM H-89 (a protein kinase [PK] A inhibitor), 10 μM Y-27632 (a selective Rho kinase inhibitor), and 10 μM GF-109203X (a selective PKC inhibitor).

RESULTS

30 μM Mirabegron impaired carbachol (0.03-1 μM)-induced contraction in human detrusor smooth muscle. SR58894A only partially attenuated the relaxant effects of mirabegron in human and pig detrusor strips precontracted with 1 μM carbachol. In α-toxin-permeabilized detrusor strips, tension force at 1 μM [Ca²⁺ ]_i was decreased by mirabegron in a concentration-dependent manner. The relaxant effect of mirabegron was only slightly attenuated by H-89 and not significantly affected by SQ22536. Y-27632 potentiated the relaxation response to mirabegron, but attenuated responses to cAMP; GF-109203X had little effect. Mirabegron but not cAMP had a notable relaxant effect in the pig detrusor smooth muscle permeabilized with β-escin.

CONCLUSIONS

Mirabegron-induced relaxation of pig and human detrusor smooth muscle occurs via both a β₃ -adrenoceptor/cAMP-dependent and -independent pathway.

The cells and conductance mediating cholinergic neurotransmission in the murine proximal stomach

KEY POINTS

Enteric neurotransmission is essential for gastrointestinal (GI) motility, although the cells and conductances responsible for post-junctional responses are controversial. The calcium-activated chloride conductance (CaCC), anoctamin-1 (Ano1), was expressed by intramuscular interstitial cells of Cajal (ICC-IM) in proximal stomach and not resolved in smooth muscle cells (SMCs). Cholinergic nerve fibres were closely apposed to ICC-IM. Conductances activated by cholinergic stimulation in isolated ICC-IM and SMCs were determined. A CaCC was activated by carbachol in ICC-IM and a non-selective cation conductance in SMCs. Responses to cholinergic nerve stimulation were studied. Excitatory junction potentials (EJPs) and mechanical responses were evoked in wild-type mice but absent or greatly reduced with knockout/down of Ano1. Drugs that block Ano1 inhibited the conductance activated by carbachol in ICC-IM and EJPs and mechanical responses in tissues. The data of the present study suggest that electrical and mechanical responses to cholinergic nerve stimulation are mediated by Ano1 expressed in ICC-IM and not SMCs.

ABSTRACT

Enteric motor neurotransmission is essential for normal gastrointestinal (GI) motility. Controversy exists regarding the cells and ionic conductance(s) that mediate post-junctional neuroeffector responses to motor neurotransmitters. Isolated intramuscular ICC (ICC-IM) and smooth muscle cells (SMCs) from murine fundus muscles were used to determine the conductances activated by carbachol (CCh) in each cell type. The calcium-activated chloride conductance (CaCC), anoctamin-1 (Ano1) is expressed by ICC-IM but not resolved in SMCs, and CCh activated a Cl- conductance in ICC-IM and a non-selective cation conductance in SMCs. We also studied responses to nerve stimulation using electrical-field stimulation (EFS) of intact fundus muscles from wild-type and Ano1 knockout mice. EFS activated excitatory junction potentials (EJPs) in wild-type mice, although EJPs were absent in mice with congenital deactivation of Ano1 and greatly reduced in animals in which the CaCC-Ano1 was knocked down using Cre/loxP technology. Contractions to cholinergic nerve stimulation were also greatly reduced in Ano1 knockouts. SMCs cells also have receptors and ion channels activated by muscarinic agonists. Blocking acetylcholine esterase with neostigmine revealed a slow depolarization that developed after EJPs in wild-type mice. This depolarization was still apparent in mice with genetic deactivation of Ano1. Pharmacological blockers of Ano1 also inhibited EJPs and contractile responses to muscarinic stimulation in fundus muscles. The data of the present study are consistent with the hypothesis that ACh released from motor nerves binds muscarinic receptors on ICC-IM with preference and activates Ano1. If metabolism of acetylcholine is inhibited, ACh overflows and binds to extrajunctional receptors on SMCs, eliciting a slower depolarization response.

Diversity and plasticity in signaling pathways that regulate smooth muscle responsiveness: Paradigms and paradoxes for the myosin phosphatase, the master regulator of smooth muscle contraction

A hallmark of smooth muscle cells is their ability to adapt their functions to meet temporal and chronic fluctuations in their demands. These functions include force development and growth. Understanding the mechanisms underlying the functional plasticity of smooth muscles, the major constituent of organ walls, is fundamental to elucidating pathophysiological rationales of failures of organ functions. Also, the knowledge is expected to facilitate devising innovative strategies that more precisely monitor and normalize organ functions by targeting individual smooth muscles. Evidence has established a current paradigm that the myosin light chain phosphatase (MLCP) is a master regulator of smooth muscle responsiveness to stimuli. Cellular MLCP activity is negatively and positively regulated in response to G-protein activation and cAMP/cGMP production, respectively, through the MYPT1 regulatory subunit and an endogenous inhibitor protein named CPI-17. In this article we review the outcomes from two decade of research on the CPI-17 signaling and discuss emerging paradoxes in the view of signaling pathways regulating smooth muscle functions through MLCP.

A novel trigger for cholesterol-dependent smooth muscle contraction mediated by the sphingosylphosphorylcholine-Rho-kinase pathway in the rat basilar artery: a mechanistic role for lipid rafts

Hyperlipidemia is a risk factor for abnormal cerebrovascular events. Rafts are cholesterol-enriched membrane microdomains that influence signal transduction. We previously showed that Rho-kinase-mediated Ca(2+) sensitization of vascular smooth muscle (VSM) induced by sphingosylphosphorylcholine (SPC) has a pivotal role in cerebral vasospasm. The goals of the study were to show SPC-Rho-kinase-mediated VSM contraction in vivo and to link this effect to cholesterol and rafts. The SPC-induced VSM contraction measured using a cranial window model was reversed by Y-27632, a Rho-kinase inhibitor, in rats fed a control diet. The extent of SPC-induced contraction correlated with serum total cholesterol. Total cholesterol levels in the internal carotid artery (ICA) were significantly higher in rats fed a cholesterol diet compared with a control diet or a β-cyclodextrin diet, which depletes VSM cholesterol. Western blotting and real-time PCR revealed increases in flotillin-1, a raft marker, and flotillin-1 mRNA in the ICA in rats fed a cholesterol diet, but not in rats fed the β-cyclodextrin diet. Depletion of cholesterol decreased rafts in VSM cells, and prevention of an increase in cholesterol by β-cyclodextrin inhibited SPC-induced contraction in a cranial window model. These results indicate that cholesterol potentiates SPC-Rho-kinase-mediated contractions of importance in cerebral vasospasm and are compatible with a role for rafts in this process.

Regulation of GPCR-mediated smooth muscle contraction: implications for asthma and pulmonary hypertension

Contractile G-protein-coupled receptors (GPCRs) have emerged as key regulators of smooth muscle contraction, both under healthy and diseased conditions. This brief review will discuss some key topics and novel insights regarding GPCR-mediated airway and vascular smooth muscle contraction as discussed at the 7th International Young Investigators' Symposium on Smooth Muscle (2011, Winnipeg, Manitoba, Canada) and will in particular focus on processes driving Ca(2+)-mobilization and -sensitization.

Augmented PDBu-mediated contraction of bronchial smooth muscle of mice with antigen-induced airway hyperresponsiveness

To explore the role of protein kinase C (PKC) in the augmented bronchial smooth muscle (BSM) contraction observed in the antigen-induced airway hyperresponsive (AHR) mice, the effects of a PKC activator, phorbol 12,13-dibutylate (PDBu), on BSM contraction were compared between the AHR and control mice. Actively sensitized mice were repeatedly challenged by antigen inhalation. Twenty-four hours after the final antigen challenge the isometrical contractions of the BSMs were measured. The BSM contraction induced by acetylcholine, but not high K(+) depolarization, was significantly augmented in the AHR mice. In BSMs of control mice, PDBu caused a significant increase in tension when the tissues were precontracted with high K(+), although PDBu itself had no effect on basal tone. The PDBu-mediated contraction was markedly augmented in BSMs of the AHR mice. These findings suggest that an increase in the PKC-mediated signaling is involved in the augmented contraction of BSMs in the antigen-induced AHR mice.

Signalling pathways responsible for the methylisogermabullone-induced contraction of ileal longitudinal muscles

Objectives

We have previously reported that methylisogermabullone (MIGB) stimulates small bowel motility through activation of acetylcholinergic receptors. This study investigated the cellular signalling pathways implicated in the regulation of ileal contractility by MIGB.

Methods

The ileal longitudinal muscles prepared from rats were treated with MIGB isolated from radish roots, and muscle contractility and protein expression were measured by force transducer and Western blot, respectively.

Key findings

MIGB at 30 µm induced a sustained phasic contraction of ileal longitudinal muscles. Acetylcholine (ACh, 0.5 µm) and MIGB stimulated translocation of protein kinase C (PKC) to cell membrane of ileal longitudinal muscles, and these stimulatory effects were remarkably attenuated by atropine (0.5 µm). ACh and MIGB induced phosphorylation of ERK 1/2 and p38 MAPKs in ileal longitudinal muscles, and they also phosphorylated the caldesmon and 20-kDa regulatory light chain of myosin (MLC20). Additionally, PD-98058 (10 µm), a selective ERK 1/2 MAPK inhibitor, and SB-203580 (10 µm), a selective p38 MAPK inhibitor, significantly reduced the MIGB-induced contraction of ileal longitudinal muscles.

Conclusions

The muscarinic receptor activated by MIGB translocates the PKC to cell membrane which phosphorylates the ERK 1/2 and p38 MAPKs, resulting in subsequent phosphorylation of caldesmon and MLC20. These cellular events likely converge on the contraction of ileal longitudinal muscles in rats.

Raf-1 kinase regulates smooth muscle contraction in the rat mesenteric arteries

We investigated the potential role of Raf-1 kinase in mesenteric arterial contraction. Inhibitors of Raf-1 kinase, GW5074, L779450 and ZM 336372 reversed phenylephrine (PE)-induced mesenteric vascular contraction. Studies in vivo in rats showed that GW5074 inhibited PE-induced increase in mean arterial pressure in adult female Sprague-Dawley rats. Isometric tension studies in mesenteric arteries of rats showed that GW5074 did not change the KCl-evoked contraction but significantly inhibited the contractions to PE, 5-HT, U46619, endothelin 1, angiotensin II and phorbol 12, 13-dibutyrate (PDBu). In mesenteric vascular smooth muscle cells (VSMCs), PE stimulated increase in Raf-1 phosphorylation which was inhibited by GW5074. Measurement of [Ca(2+)](i) with Fura-2 showed that GW5074-mediated inhibition of PE-induced contraction was not associated with decreases in [Ca(2+)](i). VSMCs treated with PE exhibited higher levels of the contractile proteins, p-MYPT1 and p-MLC(20), which was inhibited by GW5074. Similarly, PDBu induced increases in phosphorylation of Raf-1, MLC(20) and MYPT1 and this was inhibited by GW5074. However, GW5074 did not have any significant effect on PE/PDBu-induced MEK/ERK activation. The results indicate that Raf-1 kinase plays an important role in the regulation of vascular contractility through regulation of calcium sensitization.

A minor role for Ca2+ sensitization in sustained contraction through activation of muscarinic receptor in circular muscle of rat distal colon

We previously demonstrated that Ca(2+) sensitization has an essential role for carbachol-induced contraction in the longitudinal muscle of the rat distal colon. In the present study, we extended these studies to clarify the role of Ca(2+) sensitization in contraction induced by the activation of muscarinic receptors in the circular muscle of the rat distal colon. Carbachol induced a rapid phasic contraction followed by a sustained contraction that was significantly lower than the phasic and was superimposed with the rhythmic contractions. The extent of increase in intracellular Ca(2+) concentration that was measured simultaneously with tension recording was dissociated from the phasic contraction, whereas it exhibited to a similar extent as sustained contraction. In alpha-toxin-permeabilized preparations, Ca(2+) induced contraction comprising a rapid phasic and a subsequent low sustained component. After Ca(2+)-induced sustained contraction reached a constant level, guanosine triphosphate (GTP) addition resulted in the enhancement of contractile force in a concentration-dependent manner. Carbachol in the presence of GTP caused a further minimal increase in tension (Ca(2+) sensitization). Chelerythrine, a protein kinase C (PKC) inhibitor, inhibited carbachol-induced Ca(2+) sensitization but not GTP-induced Ca(2+) sensitization. In contrast, Y-27632, a Rho kinase inhibitor, inhibited GTP-induced Ca(2+) sensitization but not that induced by carbachol. Phorbol 12,13-dibutyrate, a PKC activator, increased the sustained contraction. These results suggest that the activation of muscarinic receptor with carbachol induces Ca(2+) sensitization via activation of PKC, but this action is minor in the circular muscle of the rat distal colon as a result of limited coupling between muscarinic receptors and Ca(2+) sensitization via the PKC pathway.

Augmentation of endothelin-1-induced phosphorylation of CPI-17 and myosin light chain in bronchial smooth muscle from airway hyperresponsive rats

Airway hyperresponsiveness (AHR) associated with heightened airway resistance and inflammation is a characteristic feature of bronchial asthma. It has been demonstrated that contraclile responsiveness to endothelin-1 (ET-1) in repeated antigen challenge-induced airway hyperresponsive bronchial preparation was significantly increased. ET-1 is a potent contracting substance for various smooth muscles including airways. In addition to the classical Ca(2+)-mediated contraction, ET-1 also induced Ca(2+) sensitization of contraction. However, it is not clear whether ET-1 stimulation also activates the CPI-17 (PKC-potentiated inhibitory protein for heterotrimeric myosin light chain phosphatase of 17 kDa) pathway in airway smooth muscles. Therefore, the changes in ET-1-induced activation/phosphorylation of CPI-17 and myosin light chain (MLC) in bronchial smooth muscle of repeatedly antigen-challenged rats were examined. The levels of ET-1-induced phosphorylation of CPI-17 and MLC were increased much more markedly in the AHR group than in the sensitized control animals. It might be suggested that the augmented activation of CPI-17 observed in the hyperresponsive bronchial smooth muscle is responsible for the enhanced agonists-induced contraction of bronchial smooth muscle in AHR rats.

Role of Rho kinase in endothelin-1-induced phosphorylation of CPI-17 in rat bronchial smooth muscle

It has been reported that CPI-17 (protein kinase C (PKC)-potentiated inhibitory protein for heterotrimeric myosin light chain phosphatase (MLCP) of 17 kDa) was phosphorylated by excitatory agonists in smooth muscle contraction. However, endothelin-1 (ET-1)-mediated regulation of CPI-17 in bronchial smooth muscle has not been documented. We therefore investigated whether phosphorylation of CPI-17 is induced by ET-1 in rat bronchial smooth muscle. Moreover, the role of Rho kinase (ROCK; Rho-associated coiled-coil forming protein kinase) is investigated in phosphorylation of CPI-17 induced by ET-1 in rat bronchial smooth muscle. The ET-1-induced contraction was attenuated by Y-27632 (10(-6) M), a ROCK inhibitor. ET-1 induced a phosphorylation of CPI-17 with a phosphorylation of myosin light chain (MLC); those phosphorylation responses were significantly inhibited by Y-27632 (10(-6) M). These findings suggest that the activation of ROCK is involved in force development and CPI-17 phosphorylation induced by ET-1 stimulation in rat bronchial smooth muscle. Thus, RhoA/ROCK/CPI-17 pathway is considered to play an important role in the ET-1-induced Ca(2+) sensitisation of bronchial smooth muscle contraction.

Effects of fasudil, a Rho-kinase inhibitor, on myocardial preconditioning in anesthetized rats

The aim of this study was to examine the effects of fasudil, a Rho-kinase inhibitor, on ischemic preconditioning and carbachol preconditioning in anesthetized rats. The total number of ventricular ectopic beats was markedly augmented with fasudil at 0.3 mg/kg and depressed with fasudil at 10 mg/kg. Fasudil at 10 mg/kg also markedly decreased the ventricular tachycardia incidence. Ischemic preconditioning, induced by 5 min coronary artery occlusion and 5 min reperfusion, decreased the incidence of ventricular tachycardia and abolished the occurrence of ventricular fibrillation. The incidences of ventricular tachycardia and ventricular fibrillation in the fasudil (10 mg/kg) + ischemic preconditioning group were found to be similar to the ischemic preconditioning group. However, low doses of fasudil (0.3 and 1 mg/kg) appeared to prevent the antiarrhythmic effects of ischemic preconditioning. Carbachol (4 microg/kg/min for 5 min) induced marked reductions in mean arterial blood pressure, heart rate and abolished ventricular tachycardia. Marked reductions in ventricular ectopic beats and ventricular tachycardia were noted in the fasudil (10 mg/kg) + carbachol preconditioning group. Lactate levels were markedly reduced in the ischemic preconditioning group and this reduction was prominently inhibited with fasudil at 1 mg/kg. Ischemic preconditioning caused a marked decrease in plasma malondialdehyde levels. Fasudil (10 mg/kg), ischemic preconditioning and carbachol preconditioning each generated marked reductions in ischemic myocardial malondialdehyde levels. Decreases in infarct size were observed with fasudil (10 mg/kg) treatment, ischemic preconditioning and carbachol preconditioning when compared to control. These results suggest that low doses of fasudil (0.3 and 1 mg/kg) appeared to prevents the effects of ischemic preconditioning and carbachol preconditioning, but a high dose of fasudil (10 mg/kg) was able to produce cardioprotective effects on myocardium against arrhythmias, infarct size or biochemical parameters and mimic the effects of ischemic preconditioning in anesthetized rats.

Divergent kinase signaling mediates agonist-induced phosphorylation of phosphatase inhibitory proteins PHI-1 and CPI-17 in vascular smooth muscle cells

Phosphatase holoenzyme inhibitor (PHI)-1 is one of the newest members of the family of protein phosphatase inhibitor proteins. In isolated enzyme systems, several kinases, including PKC and rho kinase (ROCK), have been shown to phosphorylate PHI-1. However, it is largely unknown whether PHI-1 is phosphorylated in response to agonist stimulation in intact cells. We investigated this question in primary cultured rat aortic vascular smooth muscle cells (VSMCs). Using two-dimensional polyacrylamide gel electrophoresis and immunoblot, we found that there are two major PHI-1 spots under resting conditions: a minor spot with an acidic isoelectric point (pI) and a major spot with a more alkaline pI. Interestingly, U-46619, a G protein-coupled receptor agonist, caused a significant increase in the acidic spot, suggesting that it may represent a phosphorylated form of PHI-1. This was confirmed by phosphatase treatment and by a specific phospho-PHI-1 antibody. Furthermore, we found that angiotensin II, thrombin, and U-46619 increased phosphorylated PHI-1 from 9% of total PHI-1 in resting cells to 18%, 18%, and 30%, respectively. We also found that inhibition of ROCK by Y-27632 or H-1152 selectively diminished U-46619-induced CPI-17 phosphorylation, whereas it did not affect PHI-1 phosphorylation. Activation of ROCK by expressing V14RhoA selectively induced CPI-17 phosphorylation without affecting PHI-1 phosphorylation. In contrast, inhibition of PKC by GF-109203X or by PKC downregulation selectively diminished U-46619-induced PHI-1 phosphorylation without significantly affecting U-46619-induced CPI-17 phosphorylation. Activating PKC by PMA induced PHI-1 phosphorylation. Together, our results show for the first time that agonist induces PHI-1 phosphorylation in VSMCs and divergent kinase signaling couples agonist stimulation to PHI-1 and CPI-17 phosphorylation.

Involvement of RhoA/Rho kinase pathway in myogenic tone in the rabbit facial vein

Myogenic tone (MT), a fundamental stretch-sensitive vasoconstrictor property of resistance arteries and veins, is a key determinant of local blood flow regulation. We evaluated the pathways involved in MT development. The role of the RhoA/Rho kinase, p38 MAP kinase, and HSP27 in MT was investigated in the rabbit facial vein (RFV), previously shown to possess MT at a pressure level equivalent to 20 mm Hg. Venous MT is poorly understood, although venous diseases affect a large proportion of the population. Stretched RFV are characterized by a temperature-sensitive MT, which is normal at 39 degrees C but fails to develop at 33 degrees C. This allows for the discrimination of the pathways involved in MT from the multiple pathways activated by stretch. Isolated RFV segments were mounted in organ baths and stretched. Temperature was then set at 33 degrees C or 39 degrees C. MT was associated to the translocation of RhoA to the plasma membrane and the Rho kinase inhibitor Y27632 decreased stretch-induced MT by 93.1+/-4.9%. MT was also associated to an increase in p38 (131.0+/-12.5% at 39 degrees C versus 100% at 33 degrees C) and HSP27 phosphorylation (196.1+/-13.3% versus 100%), and the p38 MAP kinase inhibitor SB203580 decreased MT by 36.5+/-8.1%. (39 degrees C, compared with RFV stretched at 33 degrees C). Finally, phosphorylation of p38 was blocked by Y27632 and HSP27 phosphorylation was inhibited by SB203580 and Y27632. Thus, MT and the associated p38 and HSP27 phosphorylation seem to depend on RhoA/Rho kinase activation in stretch RFV.

Possible involvement of CPI-17 in augmented bronchial smooth muscle contraction in antigen-induced airway hyper-responsive rats

Airway hyper-responsiveness (AHR) associated with heightened airway resistance and inflammation is a characteristic feature of asthma. It has been demonstrated that contractile responsiveness and Ca(2+) sensitization to acetylcholine (ACh) in repeated antigen challenge-induced airway hyper-responsive bronchial preparation were significantly increased. The CPI-17 (PKC-potentiated inhibitory protein for heterotrimeric myosin light chain phosphatase of 17 kDa) is activated by protein kinase C and acts on a myosin light-chain phosphatase-specific target. The aim of the present study was to explore the role of CPI-17 in hyper-responsiveness of bronchial smooth muscle in antigen-induced AHR rats. In immunoblotting, the levels of expression of CPI-17 mRNA and protein were significantly increased in bronchus from rats that were repeatedly challenged with antigen. ACh-induced CPI-17 phosphorylation and translocation to membrane fraction were also significantly increased in bronchus from antigen-challenged rats. In conclusion, we suggest that augmented expression and activation of CPI-17 observed in the hyper-responsive bronchial smooth muscle might be responsible for the enhanced ACh-induced Ca(2+) sensitization of bronchial smooth muscle contraction associated with AHR.

Acetylcholine-induced phosphorylation of CPI-17 in rat bronchial smooth muscle: the roles of Rho-kinase and protein kinase C

It has been demonstrated that CPI-17 provokes an inhibition of myosin light chain phosphatase to increase myosin light chain phosphorylaton and Ca2+sensitivity during contraction of vascular smooth muscle. However, expression and agonist-mediated regulation of CPI-17 in bronchial smooth muscle have not been documented. Thus, expression and phosphorylation of CPI-17 mediated by PKC and ROCK were investigated using rat bronchial preparations. Acetylcholine (ACh)-induced contraction and Ca2+sensitization were both attenuated by 10–6mol Y-27632 /L, a ROCK inhibitor, 10–6mol calphostin C/L, a PKC inhibitor, and their combination. A PKC activator, PDBu, induced a Ca2+sensitization in α-toxin-permeabilized bronchial smooth muscle. In this case, the Ca2+sensitizing effect was significantly inhibited by caphostin C but not by Y-27632. An immunoblot study demonstrated CPI-17 expression in the rat bronchial smooth muscle. Acetylcholine induced a phosphorylation of CPI-17 in a concentration-dependent manner, which was significantly inhibited by Y-27632 and calphostin C. In conclusion, these data suggest that both PKC and ROCK are involved in force development, Ca2+sensitization, and CPI-17 phosphorylation induced by ACh stimulation in rat bronchial smooth muscle. As such, RhoA/ROCK, PKC/CPI-17, and RhoA/ROCK/CPI pathways may play important roles in the ACh-induced Ca2+sensitization of bronchial smooth muscle contraction.Key words: CPI-17, bronchial smooth muscle, acetylcholine, ROCK, protein kinase C.

Effects of Y-27632, a selective Rho-kinase inhibitor, on myocardial preconditioning in anesthetized rats

The objective of this study was to examine the effects of Y-27632, a selective Rho-kinase inhibitor, on ischemic preconditioning (IP) and carbachol preconditioning (CP) in anesthetized rats. Administration of Y-27632 (0.1 mg/kg) produced slight, but not significant, reduction in mean arterial blood pressure and suppressed the total number of ventricular ectopic beats (VEBs). IP, induced by 5 min coronary artery occlusion and 5 min reperfusion, decreased the incidence of ventricular tachycardia (VT) from 100 (n=30) to 25% (n=24) and abolished the occurrence of ventricular fibrillation (VF) (40% in control group) during 30 min of ischemia. The incidences of VT and VF in Y-27632+IP group were found to be similar to IP group. Carbachol (4 microg/kg/min for 5 min) induced marked depressions in mean arterial blood pressure, heart rate and attenuated the total number of VEBs, but significant reductions in VT and VF incidences were noted in Y-27632+CP group. Y-27632 infusion for 5 min abolished VF occurrence. Marked reductions in plasma lactate levels were observed in all treatment and preconditioning groups. IP led to marked decrease in malondialdehyde levels. Decreases in infarct size were also observed with all groups when compared to control. These results suggest that infusion of Y-27632 was able to produce cardioprotective effects on myocardium against arrhythmias, infarct size or biochemical parameters and mimic the effects of ischemic preconditioning in anesthetized rats. Therefore, it is likely that inhibition of Rho-kinase is involved in the signaling cascade of myocardial preconditioning.

Uncoupling of GPCR and RhoA-induced Ca2+-sensitization of chicken amnion smooth muscle lacking CPI-17

Ca2+-sensitization of smooth muscle occurs through inhibition of myosin light chain phosphatase (MLCP) leading to an increase in the MLCK:MLCP activity ratio. MLCP is inhibited through phosphorylation of its regulatory subunit (MYPT-1) following activation of the RhoA/Rho kinase (ROK) pathway or through phosphorylation of the PP1c inhibitory protein, CPI-17, by PKC delta or ROK. Here, we explore the crosstalk between these two modes of MLCP inhibition in a smooth muscle of a natural CPI-17 knockout, chicken amnion. GTPgammaS elicited Ca2+-sensitized force which was relaxed by GDI or Y-27632, however, U46619, carbachol and phorbol ester failed to induce Ca2+-sensitized force, but were rescued by recombinant CPI-17, and were sensitive to Y-27632 inhibition. In the presence, but not absence, of CPI-17, U46619 also significantly increased GTP.RhoA. There was no affect on MYPT-1 phosphorylation at T695, however, T850 phosphorylation increased in response to GTPgammaS stimulation. Together, these data suggest a role for CPI-17 upstream of RhoA activation possibly through activation of another PP1 family member targeted by CPI-17.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II: modulated by G proteins, kinases, and myosin phosphatase

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II reflects the ratio of activities of myosin light-chain kinase (MLCK) to myosin light-chain phosphatase (MLCP) and is a major, regulated determinant of numerous cellular processes. We conclude that the majority of phenotypes attributed to the monomeric G protein RhoA and mediated by its effector, Rho-kinase (ROK), reflect Ca2+ sensitization: inhibition of myosin II dephosphorylation in the presence of basal (Ca2+ dependent or independent) or increased MLCK activity. We outline the pathway from receptors through trimeric G proteins (Galphaq, Galpha12, Galpha13) to activation, by guanine nucleotide exchange factors (GEFs), from GDP. RhoA. GDI to GTP. RhoA and hence to ROK through a mechanism involving association of GEF, RhoA, and ROK in multimolecular complexes at the lipid cell membrane. Specific domains of GEFs interact with trimeric G proteins, and some GEFs are activated by Tyr kinases whose inhibition can inhibit Rho signaling. Inhibition of MLCP, directly by ROK or by phosphorylation of the phosphatase inhibitor CPI-17, increases phosphorylation of the myosin II regulatory light chain and thus the activity of smooth muscle and nonmuscle actomyosin ATPase and motility. We summarize relevant effects of p21-activated kinase, LIM-kinase, and focal adhesion kinase. Mechanisms of Ca2+ desensitization are outlined with emphasis on the antagonism between cGMP-activated kinase and the RhoA/ROK pathway. We suggest that the RhoA/ROK pathway is constitutively active in a number of organs under physiological conditions; its aberrations play major roles in several disease states, particularly impacting on Ca2+ sensitization of smooth muscle in hypertension and possibly asthma and on cancer neoangiogenesis and cancer progression. It is a potentially important therapeutic target and a subject for translational research.

Chronic treatment with interleukin-1beta attenuates contractions by decreasing the activities of CPI-17 and MYPT-1 in intestinal smooth muscle

Interleukin-1beta (IL-1beta) is a proinflammatory cytokine that plays a central role in inflammatory bowel disease (IBD). In order to elucidate the mechanism of motility disorders frequently observed in IBD, we investigated the long term effects of IL-1beta on rat ileal smooth muscle contractility by using an organ culture system. When ileal smooth muscle strips were cultured with IL-1beta (10 ng/ml), contractions elicited by high K+ and carbachol were inhibited in a time-dependent manner. IL-1beta more strongly inhibited the carbachol-induced contractions than high K+ with decreasing myosin light chain phosphorylation. In the alpha-toxin-permeabilized ileal muscle, carbachol with GTP or guanosine 5'-3-O-(thio)triphosphate increased the Ca2+ sensitivity of contractile elements, and this G protein-coupled Ca2+ sensitization was significantly reduced in the IL-1beta-treated ileum. Among the functional proteins involved in the smooth muscle Ca2+ sensitization, CPI-17 expression was significantly reduced after the culture with IL-1beta, whereas the expressions of RhoA, ROCK-I, ROCK-II, MYPT-1, myosin light chain kinase, and myosin phosphatase (PP1) were unchanged. The phosphorylation level of CPI-17 by carbachol was low in accordance with the decrease in CPI-17 expression due to IL-1beta treatment. In contrast, constitutively phosphorylated MYPT-1 was also decreased in the IL-1beta-treated muscles. These results suggest that long term treatment with IL-1beta decreases either CPI-17 expression or MYPT-1 phosphorylation, which may result in an increase in myosin phosphatase activity to reduce force generation. Based on these findings, we consider IL-1beta to be an important mediator of gastrointestinal motility disorders in IBD, and CPI-17 and MYPT-1 are key molecules in the decreased smooth muscle contractility due to IL-1beta.

Involvement of Rho-kinase in contraction of guinea-pig aorta induced by prostanoid EP3 receptor agonists

1. The mechanism of contraction of guinea-pig isolated aorta induced by the prostanoid EP(3) receptor agonist sulprostone (0.1-300 nM) has been investigated. In 60% of the experiments, the sulprostone log concentration-response curve (maximum=15-40% of 100 nM U-46619 response; low-responders) was unaffected by the removal of extracellular Ca(2+), blockade of L-type Ca(2+) channels with nifedipine and depletion of internal Ca(2+) stores. In the remaining preparations (35-65% of 100 nM U-46619 response; high-responders), contractions to higher sulprostone concentrations showed a nifedipine-sensitive component, which was enhanced by charybdotoxin. 2. In Ca(2+)-free Krebs solution, established contractions to 300 nM sulprostone were abolished by the Rho-kinase inhibitors H-1152, Y-27632 and HA-1077 (IC(50) values=190, 770 and 2030 nM). The PKA/Rho-kinase inhibitor H-89 (10 nM-10 micro M) caused enhancement progressing to inhibition. The selective PKC inhibitor Ro 32-0432 (3 micro M) had no effect, while staurosporine, recently shown to be a potent Rho-kinase inhibitor, abolished sulprostone responses (IC(50) approximately 47 nM), but its action was slow. The MAP kinase inhibitors SB 202190, SB 203580 and PD 80958 produced little inhibition. 3. In normal Krebs solution, H-1152 and Y-27632 abolished established contractions to 300 nM sulprostone and 1 micro M phenylephrine, and partially inhibited 10 micro M phenylephrine and 50 mM K(+) responses. 4. The results are discussed in relation to the reported potencies of the protein kinase inhibitors in enzyme assays. Activation of the Rho-kinase pathway appears to be a primary mechanism of contraction induced by EP(3) receptor agonists in guinea-pig aorta.

Microtubule-dependent PKC-alpha localization in A7r5 smooth muscle cells

Using laser scanning confocal, fluorescence resonance energy transfer (FRET), and atomic force (AFM) microscopy, we investigated association of protein kinase C (PKC)-alpha with microtubules during stimulus-induced relocalization in A7r5 smooth muscle cells. Confocal microscopy with standard immunostaining techniques confirmed earlier observations that colchicine disruption of microtubules blocked PKC-alpha localization in the perinuclear region of the cell caused by phorbol 12,13-dibutyrate (PDBu; 10-6M). Dual immunostaining suggested colocalization of PKC-alpha and beta-tubulin in both unstimulated and PDBu-treated cells. This finding was verified by FRET microscopy, which indicated that association of PKC-alpha was heterogeneous in distribution and confined primarily to microtubules in the perinuclear region. FRET analysis further showed that association between the molecules was not lost during colchicine-induced dissolution of microtubules, suggesting formation of tubulin-PKC-alpha complexes in the cytosol. Confocal imaging indicated that perinuclear microtubular structure was more highly sensitive to colchicine dissolution than other regions of the cell. Topographic imaging of fixed cells by AFM indicated a well-defined elevated structure surrounding the nucleus that was absent in colchicine-treated cells. It was calculated that the volume of the nuclear sleevelike structure of microtubules increased approximately fivefold in PDBu-treated cells, suggesting a probable increase in microtubular mass. In light of PKC-alpha localization, increased colchicine sensitivity, and their volume change in stimulated cells, the results suggest that perinuclear microtubules form a specialized structure that may be more dynamically robust than in other regions of the cell. PKC-alpha could contribute to this dynamic activity. Alternatively, perinuclear microtubules could act as a scaffold for regulatory molecule interaction at the cell center.

Essential role of rho kinase in the Ca2+ sensitization of prostaglandin F(2alpha)-induced contraction of rabbit aortae

Inhibition of dephosphorylation of the 20 kDa myosin light chain (MLC(20)) is an important mechanism for the Ca(2+)-induced sensitization of vascular smooth muscle contraction. We investigated whether this mechanism operates in prostaglandin F(2alpha) (PGF(2alpha))-induced contraction of rabbit aortic smooth muscle and, if so, whether protein kinase C (PKC) or rho-associated kinase (rho kinase) contribute to the inhibition of dephosphorylation. In normal medium, PGF(2alpha) (10 microM) increased the phosphorylation of MLC(20) and developed tension. The rho-kinase inhibitors fasudil and hydroxyfasudil inhibited these changes, despite having no effect on a phorbol-ester-induced MLC(20) phosphorylation. After treatment with verapamil or chelation of external Ca(2+) with EGTA, PGF(2alpha) increased the MLC(20) phosphorylation and tension without an increase in [Ca(2+)](i), all of which were sensitive to fasudil and hydroxyfasudil. ML-9, a MLC kinase inhibitor, quickly reversed the KCl-induced MLC(20) phosphorylation and contraction to the resting level. However, fractions of PGF(2alpha)-induced contraction and MLC(20) phosphorylation were resistant to ML-9 but were sensitive to fasudil. Ro31-8220 (10 microM), a PKC inhibitor, did not affect the phosphorylation of MLC(20) and the tension caused by PGF(2alpha), thus excluding the possibility of the involvement of PKC in the PGF(2alpha)-induced MLC(20) phosphorylation. PGF(2alpha) increased phosphorylation at Thr654 of the myosin binding subunit (MBS) of myosin phosphatase, which is a target of rho kinase, and fasudil decreased the phosphorylation. These data suggest that the PGF(2alpha)-induced contraction is accompanied by the inhibition of MLC(20) dephosphorylation through rho kinase-induced MBS phosphorylation, leading to Ca(2+) sensitization of contraction. An actin-associated mechanism may also be involved in the PGF(2alpha)-induced sensitization.

Ca2+-independent phospholipase A2 is required for agonist-induced Ca2+ sensitization of contraction in vascular smooth muscle

Excitatory agonists can induce significant smooth muscle contraction under constant free Ca(2+) through a mechanism called Ca(2+) sensitization. Considerable evidence suggests that free arachidonic acid plays an important role in mediating agonist-induced Ca(2+)-sensitization; however, the molecular mechanisms responsible for maintaining and regulating free arachidonic acid level are not completely understood. In the current study, we demonstrated that Ca(2+)-independent phospholipase A(2) (iPLA(2)) is expressed in vascular smooth muscle tissues. Inhibition of the endogenous iPLA(2) activity by bromoenol lactone (BEL) decreases basal free arachidonic acid levels and reduces the final free arachidonic acid level after phenylephrine stimulation, without significant effect on the net increase in free arachidonic acid stimulated by phenylephrine. Importantly, BEL treatment diminishes agonist-induced Ca(2+) sensitization of contraction from 49 +/- 3.6 to 12 +/- 1.0% (p < 0.01). In contrast, BEL does not affect agonist-induced diacylglycerol production or contraction induced by Ca(2+), phorbol 12,13-dibutyrate (a protein kinase C activator), or exogenous arachidonic acid. Further, we demonstrate that adenovirus-mediated overexpression of exogenous iPLA(2) in mouse portal vein tissue significantly potentiates serotonin-induced contraction. Our data provide the first evidence that iPLA(2) is required for maintaining basal free arachidonic acid levels and thus is essential for agonist-induced Ca(2+)-sensitization of contraction in vascular smooth muscle.

Regulation of arterial tone by smooth muscle myosin type II

The initiation of contractile force in arterial smooth muscle (SM) is believed to be regulated by the intracellular Ca2+ concentration and SM myosin type II phosphorylation. We tested the hypothesis that SM myosin type II operates as a molecular motor protein in electromechanical, but not in protein kinase C (PKC)-induced, contraction of small resistance-sized cerebral arteries. We utilized a SM type II myosin heavy chain (MHC) knockout mouse model and measured arterial wall Ca2+ concentration ([Ca2+](i)) and the diameter of pressurized cerebral arteries (30-100 microm) by means of digital fluorescence video imaging. Intravasal pressure elevation caused a graded [Ca2+](i) increase and constricted cerebral arteries of neonatal wild-type mice by 20-30%. In contrast, intravasal pressure elevation caused a graded increase of [Ca2+](i) without constriction in (-/-) MHC-deficient arteries. KCl (60 mM) induced a further [Ca2+](i) increase but failed to induce vasoconstriction of (-/-) MHC-deficient cerebral arteries. Activation of PKC by phorbol ester (phorbol 12-myristate 13-acetate, 100 nM) induced a strong, sustained constriction of (-/-) MHC-deficient cerebral arteries without changing [Ca2+](i). These results demonstrate a major role for SM type II myosin in the development of myogenic tone and Ca2+ -dependent constriction of resistance-sized cerebral arteries. In contrast, the sustained contractile response did not depend on myosin and intracellular Ca2+ but instead depended on PKC. We suggest that SM myosin type II operates as a molecular motor protein in the development of myogenic tone but not in pharmacomechanical coupling by PKC in cerebral arteries. Thus PKC-dependent phosphorylation of cytoskeletal proteins may be responsible for sustained contraction in vascular SM.

Sphingosylphosphorylcholine is a novel messenger for Rho-kinase-mediated Ca2+ sensitization in the bovine cerebral artery: unimportant role for protein kinase C

Although recent investigations have suggested that a Rho-kinase-mediated Ca2+ sensitization of vascular smooth muscle contraction plays a critical role in the pathogenesis of cerebral and coronary vasospasm, the upstream of this signal transduction has not been elucidated. In addition, the involvement of protein kinase C (PKC) may also be related to cerebral vasospasm. We recently reported that sphingosylphosphorylcholine (SPC), a sphingolipid, induces Rho-kinase-mediated Ca2+ sensitization in pig coronary arteries. The purpose of this present study was to examine the possible mediation of SPC in Ca2+ sensitization of the bovine middle cerebral artery (MCA) and the relation to signal transduction pathways mediated by Rho-kinase and PKC. In intact MCA, SPC induced a concentration-dependent (EC50=3.0 micromol/L) contraction, without [Ca2+]i elevation. In membrane-permeabilized MCA, SPC induced Ca2+ sensitization even in the absence of added GTP, which is required for activation of G-proteins coupled to membrane receptors. The SPC-induced Ca2+ sensitization was blocked by a Rho-kinase inhibitor (Y-27632) and a dominant-negative Rho-kinase, but not by a pseudosubstrate peptide for conventional PKC, which abolished the Ca2+-independent contraction induced by phorbol ester. In contrast, phorbol ester-induced Ca2+ sensitization was resistant to a Rho-kinase inhibitor and a dominant-negative Rho-kinase. In primary cultured vascular smooth muscle cells, SPC induced the translocation of cytosolic Rho-kinase to the cell membrane. We propose that SPC is a novel messenger for Rho-kinase-mediated Ca2+ sensitization of cerebral arterial smooth muscle and, therefore, may play a pivotal role in the pathogenesis of abnormal contraction of the cerebral artery such as vasospasm. The SPC/Rho-kinase pathway functions independently of the PKC pathway.

PKC-alpha shows variable patterns of translocation in response to different stimulatory agents

Reports from numerous laboratories suggest that protein kinase C (PKC) translocation to substrate target sites may vary depending on cell type and experimental conditions. We have proposed that acutely variable targeting of PKC to different substrate sites could greatly expand the functional properties of individual isoforms in individual cell types (Li et al., 2001). Confocal microscopy and PKC alpha-enhanced green fluorescent protein (PKC alpha-EGFP) fusion protein expression were utilized to investigate the spatial and temporal pattern of PKC alpha translocation to different stimulating agents in A7r5 smooth muscle cells. Phorbol 12, 13 dibutyrate (PDBu 10(-8) M) caused a slow but irreversible relocation of the fusion protein from the cytosol to the plasmalemma. By comparison, thapsigargin (10(-5) M) and A23 187 (2 x 10(-5) M) induced a rapidly transient translocation to the cell membrane which was completed within 4 min. In contrast to these agents, angiotensin II (Ang II, 10(-6) M) caused only partial relocalization of cytosolic PKC alpha-EGFP to brightly fluorescing patches at the cell periphery. Localization at peripheral patches was completed within seconds and the fusion protein returned to the cytosol within 2 min. The PKC inhibitor staurosporine blocked cellular contraction to PDBu but not A(23 187) and had no effect on PKC alpha-EGFP translocation. By comparison, the calcium chelators EDTA and BAPTA-AM blocked the contraction to A(23 187), attenuated the contraction to PDBu, and abolished the translocation of PKC alpha-EGFP by both agents. The results show that in a single cell type the spatial and temporal characteristics of individual PKC isoform translocation may differ markedly. This further suggests the existence of potentially complex mechanisms which regulate the rate and location of target site availability.

Regulation by GDI of RhoA/Rho-kinase-induced Ca2+ sensitization of smooth muscle myosin II

We characterized the role of guanine nucleotide dissociation inhibitor (GDI) in RhoA/Rho-kinase-mediated Ca2+ sensitization of smooth muscle. Endogenous contents (approximately 2-4 microM) of RhoA and RhoGDI were near stoichiometric, whereas a supraphysiological GDI concentration was required to relax Ca2+ sensitization of force by GTP and guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). GDI also inhibited Ca2+ sensitization by GTP. G14V RhoA, by alpha-adrenergic and muscarinic agonists, and extracted RhoA from membranes. GTPgammaS translocated Rho-kinase to a Triton X-114-extractable membrane fraction. GTP. G14V RhoA complexed with GDI also induced Ca2+ sensitization, probably through in vivo dissociation of GTP. RhoA from the complex, because it was reversed by addition of excess GDI. GDI did not inhibit Ca2+ sensitization by phorbol ester. Constitutively active Cdc42 and Rac1 inhibited Ca2+ sensitization by GTP. G14V RhoA. We conclude that 1) the most likely in vivo function of GDI is to prevent perpetual "recycling" of GDP. RhoA to GTP. RhoA; 2) nucleotide exchange (GTP for GDP) on complexed GDP. RhoA/GDI can precede translocation of RhoA to the membrane; 3) activation of Rho-kinase exposes a hydrophobic domain; and 4) Cdc42 and Rac1 can inhibit Ca2+ sensitization by activated GTP. RhoA.

Potentiation by aminoethylisothiourea of the extra-cellular Ca(2+) component of norepinephrine-induced contraction in rat femoral arteries

Aminoethylisothiourea (AET) is a potent inhibitor of inducible nitric oxide synthase (NOS). The present study was performed to investigate whether AET and its rearrangement products might modulate vascular contraction independently of its effects as a NOS inhibitor in rat small femoral arteries. AET caused an endothelium-independent increase in contraction induced by norepinephrine (NE). This effect was not affected by either N(omega)-nitro-L-arginine methyl ester, nitro-L-arginine, indomethacin or propanolol, but it was suppressed in Ca(2+)-free medium. AET enhanced extracellular Ca(2+) component of NE-induced contraction, and this effect was prevented by the receptor-mediated Ca(2+) entry blocker, 1-{beta-[3-(p-methoxyphenyl)-propyloxyl]-p-methoxyphenetyl}- 1H-imidaz ole hydrochloride (SK&F 96365), but not by the voltage-dependent Ca(2+) channel blocker, nitrendipine. AET did not alter the response to CaCl(2) in vessels exposed to KCl depolarization. The protein kinase C (PKC) inhibitor, 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl) (GF 109203X), prevented the potentiating effect of AET on the NE response. AET failed to produce an increase in tone in the presence of NE and GTP in permeabilized arteries. Among the AET rearrangement products, mercaptoethylguanidine produced an endothelium-independent increase in the NE response. 2-aminothiazoline had no effect, and guanidinoethyldisulphide produced relaxation. The effect of mercaptoethylguanidine was dependent on extracellular Ca(+) and was prevented by GF 109203X. These results indicate that AET is able to potentiate the contraction to NE in rat femoral resistance arteries independently of its inhibitory effect on either NOS or cyclo-oxygenase. Its effect occurs via an enhancement of SK&F 96365-sensitive Ca(2+) entry. A PKC inhibitor-sensitive mechanism also appears to be involved in the AET effect. Mercaptoethylguanidine potentiates NE response through a mechanism similar to AET.

Contractile properties and proteins of smooth muscles of a calponin knockout mouse

The role of h1-calponin in regulating the contractile properties of smooth muscle was investigated in bladder and vas deferens of mice carrying a targeted mutation in both alleles designed to inactivate the basic calponin gene. These calponin knockout (KO) mice displayed no detectable h1-calponin in their smooth muscles. The amplitudes of Ca2+ sensitization, force and Ca2+ sensitivity were not significantly different in permeabilized smooth muscle of KO compared with wild-type (WT) mice, nor were the delays in onset and half-times of Ca2+ sensitization, initiated by flash photolysis of caged GTPgammaS, different. The unloaded shortening velocity (Vus) of thiophosphorylated fibres was significantly (P<0.05) faster in the smooth muscle of KO than WT animals, but could be slowed by exogenous calponin to approximate WT levels; the concentration dependence of exogenous calponin slowing of Vus was proportional to its actomyosin binding in situ. Actin expression was reduced by 25-50%, relative to that of myosin heavy chain, in smooth muscle of KO mice, without any change in the relative distribution of the actin isoforms. We conclude that the faster Vus of smooth muscle of the KO mouse is consistent with, but does not prove without further study, physiological regulation of the crossbridge cycle by calponin. Our results show no detectable role of calponin in the signal transduction of the Ca2+-sensitization pathways in smooth muscle.

PKCalpha translocation is microtubule-dependent in passaged smooth muscle cells

The translocation of protein kinase C (PKC) isozymes from their inactive cell locus to a variety of cytoskeletal, organelle, and plasmalemmal sites is thought to play an important role in their activation and substrate specificity. We have utilized confocal microscopy to compare phorbol 12, 13 dibutyrate (PDB) - stimulated translocation of PKCalpha in cultured cells derived from rat vascular smooth muscle. In enzymatically dispersed, passaged smooth muscle cells, PKCalpha was uniformly distributed throughout the unstimulated cell. PDB stimulation resulted in extensive association of the PKCalpha into filamentous strands with subsequent accumulation of the isoform in the peri-nuclear region of the cell. Dual immunostaining indicated that PKCalpha was extensively colocalized with microtubules in the interval immediately following PDB stimulation but was largely disassociated from microtubules at 10 min, at which time the translocation of PKCalpha to the peri-nucleus/nucleus was nearly complete. It was further found that the use of colchicine to disrupt the microtubules caused the loss of PKCalpha translocation to the peri-nuclear region. By comparison, cytochalasin B disruption of actin microfilaments had no significant effect on this parameter. The data suggest that PDB stimulation results in a transient association of PKCalpha with cell microtubules and that the microtubules play an important role in the translocation of PKCalpha from the cytosol in passaged cells derived from rat aortic smooth muscle.

Inhibition of PKCalpha and rhoA translocation in differentiated smooth muscle by a caveolin scaffolding domain peptide

Receptor-coupled contraction of smooth muscle involves recruitment to the plasma membrane of downstream effector molecules PKCalpha and rhoA but the mechanism of this signal integration is unclear. Caveolins, the principal structural proteins of caveolar plasma membrane invaginations, have been implicated in the organization and regulation of many signal transducing molecules. Thus, using laser scanning confocal immunofluorescent microscopy, we tested the hypothesis that caveolin is involved in smooth muscle signaling by investigating caveolin isoform expression and localization, together with the effect of a peptide inhibitor of caveolin function, in intact differentiated smooth muscle cells. All three main caveolin isoforms were identified in uterine, stomach, and ileal smooth muscles and assumed a predominantly plasma membranous localization in myometrial cells. Cytoplasmic introduction of a peptide corresponding to the caveolin-1 scaffolding domain-an essential region for caveolin interaction with signaling molecules--significantly inhibited agonist-induced translocation of both PKCalpha and rhoA. Translocation was unimpaired by a scrambled peptide and was unaltered in sham-treated cells. The membranous localization of caveolins, and direct inhibition of receptor-coupled PKCalpha and rhoA translocation by the caveolin-1 scaffolding domain, supports the concept that caveolins can regulate the integration of extracellular contractile stimuli and downstream intracellular effectors in smooth muscle.

Sustained muscle contraction induced by agonists, growth factors, and Ca(2+) mediated by distinct PKC isozymes

The role of protein kinase C (PKC) in sustained contraction was examined in intestinal circular and longitudinal muscle cells. Initial contraction induced by agonists (CCK-8 and neuromedin C) was abolished by 1) inhibitors of Ca(2+) mobilization (neomycin and dimethyleicosadienoic acid), 2) calmidazolium, and 3) myosin light chain (MLC) kinase (MLCK) inhibitor KT-5926. In contrast, sustained contraction was not affected by these inhibitors but was abolished by 1) the PKC inhibitors chelerythrine and calphostin C, 2) PKC-epsilon antibody, and 3) a pseudosubstrate PKC-epsilon inhibitor. GDPbetaS abolished both initial and sustained contraction, whereas a Galpha(q/11) antibody inhibited only initial contraction, implying that sustained contraction was dependent on activation of a distinct G protein. Sustained contraction induced by epidermal growth factor was inhibited by calphostin C, PKC-alpha,beta,gamma antibody, and a pseudosubstrate PKC-alpha inhibitor. Ca(2+) (0.4 microM) induced an initial contraction in permeabilized muscle cells that was blocked by calmodulin and MLCK inhibitors and a sustained contraction that was blocked by calphostin C and a PKC-alpha,beta,gamma antibody. Thus initial contraction induced by Ca(2+), agonists, and growth factors is mediated by MLCK, whereas sustained contraction is mediated by specific Ca(2+)-dependent and -independent PKC isozymes. G protein-coupled receptors are linked to PKC activation via distinct G proteins.

Effect of a calcium sensitization modulator, Y-27632, on isolated human bronchus and pulmonary artery

A recently developed pyridine derivative, Y-27632, has been reported to inhibit smooth muscle contraction by inhibiting Ca(2+)sensitization in animal experiments. However, the effect of this compound in human tissues has not yet been elucidated. The aim of the present study was to evaluate the effect of Y-27632 on human bronchi and pulmonary arteries. The tissues were obtained from lung cancer patients undergoing lung resection. Tissue responses were assessed by isometric tension measurement. Y-27632 relaxed the bronchi at basal tone with an IC(50)(concentration causing 50% relaxation of the maximal response) of 2.0+/-0.3x10(-6)M. Y-27632 also dose-dependently relaxed the bronchi precontracted by acetylcholine (ACh), histamine and neurokinin A, and the IC(50)was 3.0+/-0.4x10(-6), 2.5+/-0.5x10(-6)and 1.8 +/-0.3x10(-6)M, respectively. The dilatory effect of Y-27632 was significantly smaller in ACh-precontracted tissues compared with those of basal or histamine- and neurokinin A-induced precontracted bronchi (P<0.05). Further, Y-27632 showed an inhibitory effect on cholinergic nerve stimulation- and ACh-induced bronchial contraction to the same degree, suggesting that a modulatory effect of this compound on ACh release from nerve terminals was unlikely. Y-27632 also dilated the pulmonary arteries precontracted by phenylephrine (IC(50)= 1.6+/-0.1x10(-6)M). These data suggest that Y-27632 has a dilatory capacity on human bronchi as well as on pulmonary arteries.

Chlorpromazine at comparatively low concentrations potentiates carbachol-induced tonic contraction of ileal longitudinal smooth muscle of the guinea-pig

The neuroleptic phenothiazine derivative chlorpromazine (CPZ) at high concentration (1 x 10(-5) M) decreased either the phasic or tonic contraction in response to carbachol and the carbachol-induced increase in [Ca2+]i in both phases in ileal muscle. In contrast, CPZ at low concentrations (8 x 10(-7) - 5 x 10(-6) M) decreased only the phasic contraction and potentiated the tonic contraction induced by carbachol. However, CPZ at these concentrations dose-dependently decreased the carbachol-induced increase in [Ca2+]i in both phases. These results suggested that CPZ dose-dependently decreased the initial phasic contraction in response to carbachol by inhibition of Ca2+ release from the intracellular storage sites. CPZ at low concentrations appears to increase Ca2+ sensitivity to contractile proteins in the carbachol-induced tonic phase. CPZ dose-dependently reduced the 60 mM K(+)-induced phasic and tonic responses and a concomitant decrease in [Ca2+]i in ileal muscle.

Signal transduction by G-proteins, rho-kinase and protein phosphatase to smooth muscle and non-muscle myosin II

We here review mechanisms that can regulate the activity of myosin II, in smooth muscle and non-muscle cells, by modulating the Ca2+ sensitivity of myosin regulatory light chain (RLC) phosphorylation. The major mechanism of Ca2+ sensitization of smooth muscle contraction and non-muscle cell motility is through inhibition of the smooth muscle myosin phosphatase (MLCP) that dephosphorylates the RLC in smooth muscle and non-muscle. The active, GTP-bound form of the small GTPase RhoA activates a serine/threonine kinase, Rho-kinase, that phosphorylates the regulatory subunit of MLCP and inhibits phosphatase activity. G-protein-coupled release of arachidonic acid may also contribute to inhibition of MLCP acting, at least in part, through the Rho/Rho-kinase pathway. Protein kinase C(s) activated by phorbol esters and diacylglycerol can also inhibit MLCP by phosphorylating and thereby activating CPI-17, an inhibitor of its catalytic subunit; this mechanism is independent of the Rho/Rho-kinase pathway and plays only a minor, transient role in the G-protein-coupled mechanism of Ca2+ sensitization. Ca2+ sensitization by the Rho/Rho-kinase pathway contributes to the tonic phase of agonist-induced contraction in smooth muscle, and abnormally increased activation of myosin II by this mechanism is thought to play a role in diseases such as high blood pressure and cancer cell metastasis.

A major role for the rho-associated coiled coil forming protein kinase in G-protein-mediated Ca2+ sensitization through inhibition of myosin phosphatase in rabbit trachea

1 G protein-mediated Ca2+ sensitization of airway smooth muscle contraction was investigated with respect to the relative importance of Rho-associated coiled coil forming protein kinase (ROCK) and protein kinase C (PKC). We examined the effects of Y-27632, a ROCK inhibitor, and GF 109203X, a PKC inhibitor, on guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS)-induced contraction in alpha-toxin- or beta-escin-permeabilized rabbit trachea. 2 Although pre-treatment with Y-27632 dose-dependently inhibited GTPgammaS (10 microM)-induced Ca2+ sensitization of alpha-toxin-permeabilized trachea, a Y-27632-insensitive component (approximately 16% of the maximum contraction) was retained during the early phase of the GTPgammaS response in the presence of Y-27632 (100 microM). 3 GF 109203X (5 microM) abolished 1 microM 4beta-phorbol 12, 13-dibutyrate (PDBu)-induced, but only partially inhibited the GTPgammaS-induced Ca2+ sensitization. A combination of Y-27632 (100 microM) and GF 109203X (5 microM) totally abolished the GTPgammaS response. 4 GTPgammaS caused only a small contraction in the absence of Ca2+. Wortmannin (30 microM), a myosin light chain kinase (MLCK) inhibitor, completely inhibited Ca2+-induced contraction. ATP-triggered contraction of the strip which had been treated with calyculin A (1 microM), a phosphatase inhibitor, in rigor solutions was markedly slowed by worthmannin (30 microM), but not by Y-27632 (100 microM), in the presence of GTPgammaS and Ca2+. 5 GTPgammaS, but not PDBu, contracted the beta-escin-permeabilized trachea in the absence of Ca2+, but the presence of Ca2+-independent MLCK. 6 We conclude that ROCK plays a primary role in G-protein-mediated Ca2+ sensitization, which requires MLCK activity, with minor contribution of PKC to the early phase of contraction, and PDBu utilizes conventional PKC(s) in airway smooth muscle.

Cellular redistribution of PKCalpha, rhoA, and ROKalpha following smooth muscle agonist stimulation

Efficient receptor-coupled activation of smooth muscle requires discrete coordination of many signal transducing events from the plasma membrane to the myofilaments. Recruitment of key factors to the plasma membrane is thought to be crucial for transduction of extracellular signals leading to contractility. We investigated, therefore, for the first time in intact differentiated smooth muscle cells, the distributions of three molecules important for receptor-coupled excitation: protein kinase Calpha (PKCalpha), rhoA, and rho kinase (ROK). We also directly confirmed, by single cell force measurements, carbachol-induced [Ca(2+)](i) sensitization of contractility. Laser scanning confocal immunofluorescent microscopy of central smooth muscle cell sections determined that, at rest, PKCalpha, rhoA, and ROKalpha were distributed predominantly throughout the cytosol. Muscarinic stimulation resulted in significant redistribution of each protein to the cell membrane. By digital image analysis, peripheral:cytosolic distributions of PKCalpha, rhoA, and ROKalpha were calculated as, respectively, 1.05 +/- 0.03 (8), 1.09 +/- 0.03 (5), and 1.26 +/- 0.04 (12) at rest, increasing significantly following stimulation to 2.09 +/- 0.22 (6), 2.02 +/- 0.12 (8), and 1.93 +/- 0.05 (10). It is proposed that this receptor-coupled recruitment to the cell periphery of the downstream signaling molecules PKCalpha, rhoA, and ROKalpha contributes to the efficacy of agonist-induced contractile activation of smooth muscle.

Enhanced Ca(2+) mobilization in airway smooth muscle contributes to airway hyperresponsiveness in an inbred strain of rat

The mechanisms underlying airway hyperresponsiveness are still unknown but increased contractility of airway smooth muscle may play a role. This study sought to demonstrate a relationship between in vivo airway responsiveness and a number of measures of airway smooth muscle responsiveness ex vivo, including intracellular Ca(2+) signaling, by comparing three inbred strains of rat with different degrees of airways responsiveness to methacholine. Lewis, ACI, and Fisher strains of rat were characterized for their pulmonary responses to 5-hydroxytryptamine (5HT) in vivo and Fisher rats were found to be hyperresponsive to 5HT compared with ACI and Lewis rats. The responsiveness of the airways from these strains of rat ex vivo revealed that intraparenchymal airways from Fisher rats significantly narrowed to a greater degree and at a faster rate to 5HT than Lewis rat airways, consistent with their differences in vivo. Intraparenchymal ACI airways, however, narrowed to the same degree as Fisher airways but took longer to do so at a high concentration of 5HT. 5HT caused concentration-dependent increases in intracellular Ca(2+) in airway smooth muscle cells from all three strains of rat, but Fisher and ACI displayed higher responses than Lewis airway smooth muscle. Our results demonstrate that the degree of intracellular Ca(2+) mobilization by 5HT in airway smooth muscle parallels the rate and degree of intraparenchymal airway narrowing and suggest that the degree of intracellular Ca(2+) mobilization plays a role in determining airway smooth muscle contractility.

Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The concept of pharmacomechanical coupling, introduced 30 years ago to account for physiological mechanisms that can regulate contraction of smooth muscle independently of the membrane potential, has since been transformed from a definition into what we now recognize as a complex of well-defined, molecular mechanisms. The release of Ca2+ from the SR by a chemical messenger, InsP3, is well known to be initiated not by depolarization, but by agonist-receptor interaction. Furthermore, this G-protein-coupled phosphatidylinositol cascade, one of many processes covered by the umbrella of pharmacomechanical coupling, is part of complex and general signal transduction mechanisms also operating in many non-muscle cells of diverse organisms. It is also clear that, although the major contractile regulatory mechanism of smooth muscle, phosphorylation/dephosphorylation of MLC20, is [Ca2+]-dependent, the activity of both the kinase and the phosphatase can also be modulated independently of [Ca2+]i. Sensitization to Ca2+ is attributed to inhibition of SMPP-1M, a process most likely dominated by activation of the monomeric GTP-binding protein RhoA that, in turn, activates Rho-kinase that phosphorylates the regulatory subunit of SMPP-1M and inhibits its myosin phosphatase activity. It is likely that the tonic phase of contraction activated by a variety of excitatory agonists is, at least in part, mediated by this Ca(2+)-sensitizing mechanism. Desensitization to Ca2+ can occur either through inhibitory phosphorylation of MLCK by other kinases or autophosphorylation and by activation of SMPP-1M by cyclic nucleotide-activated kinases, probably involving phosphorylation of a phosphatase activator. Based on our current understanding of the complexity of the many cross-talking signal transduction mechanisms that operate in cells, it is likely that, in the future, our current concepts will be refined, additional mechanisms of pharmacomechanical coupling will be recognized, and those contributing to the pathologenesis diseases, such as hypertension and asthma, will be identified.

From pharmacomechanical coupling to G-proteins and myosin phosphatase

A brief summary of recent studies of pharmacomechanical coupling is presented, with emphasis on the role of GTP-binding proteins and Ca(2+)-independent regulation of contraction (Ca(2+)-sensitization/desensitization) through regulatory myosin light chain (MLC20) phosphorylation and dephosphorylation. Pharmacomechanical regulation of cytosolic [Ca2+] is largely, though not solely, controlled by the phosphatidylinositol cascade and Ca(2+)-pumps of the plasma membrane and the sarcoplasmic reticulum. The monomeric GTPase, RhoA, is a major upstream component of Ca(2+)-sensitization. Its crystal structure and apparently obligatory translocation to the plasma membrane for activation of its downstream effectors are described. Inhibition of RhoA activity by a membrane-permeant ADP-ribosylating bacterial exoenzyme, DC3B, causes severe depression of the tonic component of agonist-induced contraction, suggesting that this component is largely due to Ca(2+)-sensitization. A relatively specific inhibitor (Y27632) of Rho-kinase, a downstream effector of Ca(2+)-sensitization (Uehata et al 1997), also inhibits oxytoxin-induced Ca(2+)-sensitization of myometrium. The major mechanism of physiological, G-protein-coupled Ca(2+)-sensitization is through inhibition of smooth muscle myosin phosphatase (SMPP-1M), whereas conventional or novel protein kinase Cs play very little or no role in this process. Mechanisms of Ca(2+)-desensitization include inhibition of myosin light chain kinase and activation of SMPP-1M. Activation of SMPP-1M in phasic smooth muscle can be attributed, at least in part, to the synergistic phosphatase activating activities of a cyclic nucleotide-dependent kinase and its major substrate, telokin.

Lipid second messengers derived from glycerolipids and sphingolipids, and their role in smooth muscle function

The processes that link activation of an external receptor to the internal mechanisms that elicit a physiological response have been the subject of extensive investigation. It has been established that rather than just being an inert barrier to protect the cell from environmental damage, there are populations of phospholipids located within the plasma membrane that act as a reservoir for signalling molecules and when a receptor binds its appropriate activating ligand a chain of events is initiated which leads to the breakdown of these lipids and the release of second messengers. Such processes are rapid enough for physiological responses to be effected. The purpose of this review is to examine the profile of lipid second messengers derived from glycerophospholipids and sphingolipids. In the former class are included phosphoinositide and phosphatidylcholine and the latter includes sphingomyelin. Hydrolysis of such parent compounds is mediated by phospholipases and the profile of metabolites appears to be agonist specific and modulated by a number of mechanisms including heterotrimeric G-protein subunits, small G-proteins, alterations in intracellular calcium concentration, protein kinase C and tyrosine kinases. The recent interest in sphingolipids, particularly in vascular smooth muscle cells, has been provoked by the observation that ceramide and sphingoid base formation is observed in response to vasoconstrictor hormones.

PKC-dependent signalling mechanisms in differentiated smooth muscle

Protein kinase C (PKC) is now known to play an important physiological role in essentially all cell types. This review will focus on what is known about the kinase in contractile differentiated smooth muscle. Current knowledge on the molecular structure of PKC isoforms will be discussed as they relate to mechanisms of translocation and targeting of the kinase within smooth muscle cells. Studies performed on PKC-dependent signalling pathways in differentiated smooth muscle cells will be discussed with emphasis on studies form our laboratory, especially discussing thin filament linked pathways. Thick filament linked PKC-dependent pathways will be described in more detail elsewhere in this monograph.

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.

The effects of the Rho-kinase inhibitor Y-27632 on arachidonic acid-, GTPgammaS-, and phorbol ester-induced Ca2+-sensitization of smooth muscle

The effects of the Rho-kinase inhibitor, Y-27632 [1] on Ca2+-sensitization of force induced by arachidonic acid (AA), phorbol 12,13-dibutyrate (PDBu), GTPgammaS, and by the stable thromboxane analog, 9,11-dideoxy-9alpha,11alpha-methanoepoxy-PGF2alpha (U-46619), were determined in alpha-toxin-permeabilized smooth muscles. Y-27632 relaxed (up to 99%) Ca2+-sensitization by GTPgammaS (10 microM) and U46619 (1 microM), but not by PDBu (20 microM), and reduced GTPgammaS-induced myosin light chain (MLC20) phosphorylation from 28% to 17% (P=0.002). GTPgammaS-induced force sensitization was inhibited by Y-27632 more potently when the inhibitor was added during the plateau of force than prior to stimulation. In alpha-toxin-permeabilized smooth muscle, Y-27632 inhibited AA (50 microM)-induced Ca2+-sensitization of force (by 66 +/- 1.3%) and reduced MLC20 phosphorylation. In contrast, Y-27632 did not relax force Ca2+-sensitized by AA in smooth muscle permeabilized with Triton X-100. We conclude that (i) AA induces Ca2+-sensitization through dual mechanisms, one mediated by Rho-kinase (or a related kinase), and (ii) Rho-kinase is not required for phorbol ester-induced Ca2+-sensitization.

Protein kinase C: modulation of vasopressin-induced calcium influx and release in A7r5 vascular smooth muscle cells

This study was guided by the hypothesis that specific isoforms of protein kinase C may participate in modulating increases in intracellular Ca2+ that are induced by stimulation of vascular smooth muscle cells with vasopressin. Immunoblot analysis revealed that A7r5 vascular smooth muscle cells expressed conventional (alpha), novel (delta and epsilon), and atypical (iota/lambda and mu) isoforms of protein kinase C. Stimulation of fura-2-loaded cells with 20 nM vasopressin induced a rapid transient increase in the intracellular concentration of calcium that was followed by a slowly declining component which was above baseline throughout the period of observation. Cell fractionation studies showed that the calcium response was associated with (a) transient translocation of the alpha and delta isoforms of protein kinase C from the cytosolic fraction to the particulate-membrane fraction, (b) sustained translocation of the epsilon isoform, and (c) no translocation of iota/lambda or mu isoforms. Ratiometric and isobestic fluorescence analysis showed that vasopressin-induced Ca2+ influx and release were markedly inhibited in cells that were preincubated with either 1 microM phorbol 12-myristate 13-acetate, or 10 microM 1, 2 dioctanoyl-sn-glycerol, two structurally different activators of protein kinase C. In contrast, vasopressin-induced increases in intracellular Ca2+ were not significantly altered following preincubation with either 1 microM 4alpha-phorbol or 4alpha-phorbol 12,13-didecanoate, analogs of phorbol 12-myristate 13-acetate that do not activate protein kinase C. Moreover, the inhibitory effects of phorbol 12-myristate 13-acetate were prevented by treatment with 1 microM GF109203X, a potent inhibitor of protein kinase C. Taken together, these results show that direct activation of protein kinase C can negatively modulate vasopressin-induced Ca2+ influx and release in cultured vascular smooth muscle cells. They also show that stimulation with vasopressin induces translocation of specific isoforms of protein kinase C, an observation suggesting that one or more of these isoforms may participate in modulation of vasopressin-induced increases in intracellular Ca2+.