Possible involvement of a small G-protein sensitive to exoenzyme C3 of Clostridium botulinum in the regulation of myofilament Ca2+ sensitivity in beta-escin skinned smooth muscle of guinea pig ileum

Functions of Muscarinic Receptor Subtypes in Gastrointestinal Smooth Muscle: A Review of Studies with Receptor-Knockout Mice

Parasympathetic signalling via muscarinic acetylcholine receptors (mAChRs) regulates gastrointestinal smooth muscle function. In most instances, the mAChR population in smooth muscle consists mainly of M2 and M3 subtypes in a roughly 80% to 20% mixture. Stimulation of these mAChRs triggers a complex array of biochemical and electrical events in the cell via associated G proteins, leading to smooth muscle contraction and facilitating gastrointestinal motility. Major signalling events induced by mAChRs include adenylyl cyclase inhibition, phosphoinositide hydrolysis, intracellular Ca2+ mobilisation, myofilament Ca2+ sensitisation, generation of non-selective cationic and chloride currents, K+ current modulation, inhibition or potentiation of voltage-dependent Ca2+ currents and membrane depolarisation. A lack of ligands with a high degree of receptor subtype selectivity and the frequent contribution of multiple receptor subtypes to responses in the same cell type have hampered studies on the signal transduction mechanisms and functions of individual mAChR subtypes. Therefore, novel strategies such as genetic manipulation are required to elucidate both the contributions of specific AChR subtypes to smooth muscle function and the underlying molecular mechanisms. In this article, we review recent studies on muscarinic function in gastrointestinal smooth muscle using mAChR subtype-knockout mice.

The effects of the small GTPase RhoA on the muscarinic contraction of airway smooth muscle result from its role in regulating actin polymerization

The small GTPase RhoA increases the Ca(2+) sensitivity of smooth muscle contraction and myosin light chain (MLC) phosphorylation by inhibiting the activity of MLC phosphatase. RhoA is also a known regulator of cytoskeletal dynamics and actin polymerization in many cell types. In airway smooth muscle (ASM), contractile stimulation induces MLC phosphorylation and actin polymerization, which are both required for active tension generation. The objective of this study was to evaluate the primary mechanism by which RhoA regulates active tension generation in intact ASM during stimulation with acetylcholine (ACh). RhoA activity was inhibited in canine tracheal smooth muscle tissues by expressing the inactive RhoA mutant, RhoA T19N, in the intact tissues or by treating them with the cell-permeant RhoA inhibitor, exoenzyme C3 transferase. RhoA inactivation reduced ACh-induced contractile force by approximately 60% and completely inhibited ACh-induced actin polymerization but inhibited ACh-induced MLC phosphorylation by only approximately 20%. Inactivation of MLC phosphatase with calyculin A reversed the reduction in MLC phosphorylation caused by RhoA inactivation, but calyculin A did not reverse the depression of active tension and actin polymerization caused by RhoA inactivation. The MLC kinase inhibitor, ML-7, inhibited ACh-induced MLC phosphorylation by approximately 80% and depressed active force by approximately 70% but did not affect ACh-induced actin polymerization, demonstrating that ACh-stimulated actin polymerization occurs independently of MLC phosphorylation. We conclude that the RhoA-mediated regulation of ACh-induced contractile tension in ASM results from its role in mediating actin polymerization rather than from effects on MLC phosphatase or MLC phosphorylation.

Regulation of smooth muscle contraction by small GTPases

Next to changes in cytosolic [Ca(2+)], members of the Rho subfamily of small GTPases, in particular Rho and its effector Rho kinase, also known as ROK or ROCK, emerged as key regulators of smooth muscle function in health and disease. In this review, we will focus on the regulation of the contractile machinery by Rho/ROK signaling and its interaction with PKC and cyclic nucleotide signaling. We will briefly discuss the emerging evidence that remodeling of cortical actin is necessary for contraction.

Muscarinic receptor subtypes mediating Ca2+ sensitization of intestinal smooth muscle contraction: studies with receptor knockout mice

In the present study, we have characterized muscarinic receptor subtypes that mediate carbachol-induced Ca2+ sensitization of contraction in intestinal smooth muscle, using mutant mice lacking M(2) or M(3) muscarinic receptors or both receptor subtypes. In alpha-toxin-permeabilized muscle strips from wild-type (WT) mice, isometric tension responses to Ca2+ applied cumulatively (pCa 7.0-5.0) were increased when the muscarinic agonist carbachol (100 microM) was added to the medium, as judged from shifts of pCa-tension curves in both 50% effective concentration (EC(50)) and maximum response (E(max)) of pCa-tension curve. In preparations from M(2)-knockout (KO) mice, pCa-tension curves were also shifted by carbachol (100 microM), and the extents of the EC(50) and E(max) changes resembled those observed in preparations from WT mice. In preparations from M(3)-KO or M(2)/M(3)-double KO mice, however, no significant changes in pCa-tension curves were obtained after carbachol application. The G(q/11)-type G-protein inhibitor YM-254890 (1 microM) completely blocked the Ca2+ sensitization of contraction induced by carbachol in M(2)-KO or WT preparations. The results strongly support the idea that the muscarinic activation of Ca2+ sensitization in intestinal smooth muscles is mediated by the M(3) muscarinic receptor coupled to G(q/11)-type G-proteins, without any significant involvement of the other muscarinic receptor subtypes including M(2).

Post-statin approaches to hyperlipidaemia

The advent of statins has virtually resolved the treatment of a majority of essential hypercholesterolaemic patients. Nevertheless, other abnormalities in lipoprotein metabolism, including such lipoprotein disturbances as hypertriglyceridaemia, mixed hyperlipidaemia, accumulation of small dense low density lipoprotein (LDL), high levels of lipoprotein (a) (Lp[a]) and hypo-HDL-cholesterolaemia, although also highly atherogenic, are not as efficiently treated as essential hypercholesterolaemia. Pharmaceutical companies are improving new molecules directed against old targets (PPARalpha: fibrates) or creating original molecules directed against new targets (acyl CoA:cholesterol acyltransferase (ACAT), microsomal triglyceride transfer protein (MTP), retinoid X receptor (RXR)). Of the multitude of ACAT inhibitors, only a few have reached preliminary clinical studies: e.g., F-1394, Sch48461 and CI-1011. They reduce LDL-cholesterol and atherosclerosis development in animals, partly by directly inhibiting cholesteryl ester formation in the artery wall. BW-USC-148 is a fibric acid derivative with ACAT-inhibiting activity. The hypocholesterolaemic activity for this novel ureido fibrate analogue was found to be over 100-fold greater than that of any 'second generation' fibrate in cholesterol-fed rats, mainly through its fibrate activity (PPARalpha activation) but not its ACAT activity. Targretin (LGD1069), a member of the rexinoid family (RXR activator), was shown to decrease triglyceridaemia and to increase HDL levels in hypertriglyceridaemic rats. Microsomal triglyceride transfer protein inhibitors are potent inhibitors of the synthesis of all the atherogenic apolipoprotein B-containing particles and are under development, but in vivo data are not yet available in literature. Vitamin E, an old molecule, should be used in the near future as a potent anti-atherosclerotic treatment due to its anti-oxidant power. Results of preliminary gene therapy studies of homozygous familial hypercholesterolaemic patients and of hypo-HDL-cholesterolaemia in animals are promising but do not show hope for significant clinical use in the near future. The improvement in the understanding of the molecular mechanisms of dyslipoproteinaemia and atherosclerosis development, taken together with new strategies in drug design and drug synthesis, has led to the discovery of potent normolipidaemic drugs.

Mechanisms involved in carbachol-induced Ca(2+) sensitization of contractile elements in rat proximal and distal colon

1. Mechanisms involved in Ca(2+) sensitization of contractile elements induced by the activation of muscarinic receptors in membrane-permeabilized preparations of the rat proximal and distal colon were studied. 2. In alpha-toxin-permeabilized preparations from the rat proximal and distal colon, Ca(2+) induced a rapid phasic and subsequent tonic component. After Ca(2+)-induced contraction reached a plateau, guanosine 5'-triphosphate (GTP) and carbachol (CCh) in the presence of GTP further contracted preparations of both the proximal and distal colon (Ca(2+) sensitization). Y-27632, a rho-kinase inhibitor, inhibited GTP plus CCh-induced Ca(2+) sensitization more significantly in the proximal colon than in the distal colon. 3. Y-27632 at 10 microm had no effect on Ca(2+)-induced contraction or slightly inhibited phorbol-12,13-dibutyrate-induced Ca(2+) sensitization in either proximal or distal colon. Chelerythrine, a protein kinase C inhibitor, inhibited GTP plus CCh-induced Ca(2+) sensitization in the distal colon, but not in the proximal colon. The component of Ca(2+) sensitization that persisted after the chelerythrine treatment was completely inhibited by Y-27632. 4. In beta-escin-permeabilized preparations of the proximal colon, C3 exoenzyme completely inhibited GTP plus CCh-induced Ca(2+) sensitization, but PKC(19-31) did not. In the distal colon, C3 exoenzyme abolished GTP-induced Ca(2+) sensitization. It inhibited CCh-induced sensitization by 50 % and the remaining component was inhibited by PKC(19-31). 5. These results suggest that both protein kinase C and rho pathways in parallel mediate the Ca(2+) sensitization coupled to activation of muscarinic receptors in the rat distal colon, whereas the rho pathway alone mediates this action in the proximal colon.

Receptor signaling mechanisms underlying muscarinic agonist-evoked contraction in guinea-pig ileal longitudinal smooth muscle

1 In guinea-pig ileal longitudinal muscle, muscarinic partial agonists, 4-(N-[3-chlorophenyl]-carbomoyloxy)-2-butynyl-trimethylammonium (McN-A343) and pilocarpine, each produced parallel increases in tension and cytosolic Ca(2+) concentration ([Ca(2+)]c) with a higher EC(50) than that of the full agonist carbachol. The maximum response of [Ca(2+)]c or tension was not much different among the three agonists. The Ca(2+) channel blocker nicardipine markedly inhibited the effects of all three agonists 2 The contractile response to any agonist was antagonized in a competitive manner by M(2) receptor selective antagonists (N,N'-bis[6-[[(2-methoyphenyl)methyl]amino]hexyl]-1,8-octanediamine tetrahydrochloride and 11-[[2-[(diethlamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4] benzodiazepine-6-one), and the apparent order of M(2) antagonist sensitivity was McN-A343>pilocarpine>carbachol. M(3) receptor selective antagonists, 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide and darifenacin, both severely depressed the maximum response for McN-A343, while darifenacin had a similar action in the case of pilocarpine. Both M(3) antagonists behaved in a competitive manner in the case of the carbachol response. 3 McN-A343 failed to release Ca(2+) from the intracellular stores, and the Ca(2+)-releasing action of pilocarpine was very weak compared with that of carbachol. All three agonists were capable of increasing Ca(2+) sensitivity of the contractile proteins. 4 McN-A343 rarely produced membrane depolarization, but always accelerated electrical spike discharge. Pilocarpine effect was more often accompanied by membrane depolarization, as was usually seen using carbachol. 5 The results suggest that muscarinic agonist-evoked contractions result primarily from the integration of Ca(2+) entry associated with the increased spike discharge and myofilaments Ca(2+) sensitization, and that Ca(2+) store release may contribute to the contraction indirectly via potentiation of the electrical membrane responses. They may also support the idea that an interaction of M(2) and M(3) receptors plays a crucial role in mediating the contraction response.

Agonist-induced changes in the phosphorylation of the myosin- binding subunit of myosin light chain phosphatase and CPI17, two regulatory factors of myosin light chain phosphatase, in smooth muscle

The inhibition of myosin light chain phosphatase (MLCP) enhances smooth muscle contraction at a constant [Ca2+]. There are two components, myosin-binding subunit of MLCP (MBS) and CPI17, thought to be responsible for the inhibition of MLCP by external stimuli. The phosphorylation of MBS at Thr-641 and of CPI17 at Thr-38 inhibits the MLCP activity in vitro. Here we determined the changes in the phosphorylation of MBS and CPI17 after agonist stimulation in intact as well as permeabilized smooth muscle strips using phosphorylation-site-specific antibodies as probes. The CPI17 phosphorylation transiently increased after agonist stimulation in both alpha-toxin skinned and intact fibres. The time course of the increase in CPI17 phosphorylation after stimulation correlated with the increase in myosin regulatory light chain (MLC) phosphorylation. The increase in CPI17 phosphorylation was significantly diminished by Y27632, a Rho kinase inhibitor, and GF109203x, a protein kinase C inhibitor, suggesting that both the protein kinase C and Rho kinase pathways influence the change in CPI17 phosphorylation. On the other hand, a significant level of MBS phosphorylation at Thr-641, an inhibitory site, was observed in the resting state for both skinned and intact fibres and the agonist stimulation did not significantly alter the MBS phosphorylation level at Thr-641. While the removal of the agonist markedly decreased MLC phosphorylation and induced relaxation, the phosphorylation of MBS was unchanged, while CPI17 phosphorylation markedly diminished. These results strongly suggest that the phosphorylation of CPI17 plays a more significant role in the agonist-induced increase in myosin phosphorylation and contraction of smooth muscle than MBS phosphorylation in the Ca2+-independent activation mechanism of smooth muscle contraction.

Origin of Ca2+ necessary for carbachol-induced contraction in longitudinal muscle of the proximal colon of rats

The origin of Ca2+ necessary for carbachol (CCh)-induced contraction of longitudinal muscle of the proximal colon of rats was studied. CCh induced contraction of the muscle consisting of two phases, phasic and tonic phases, with a concomitant biphasic increase in [Ca2+]i. After removal of Ca2+ from the bathing solution of the colonic segments, CCh-induced contraction was rapidly inhibited; there was almost complete inhibition 1 min after the removal. Nicardipine, a blocker of voltage-dependent calcium channel, also significantly inhibited CCh-induced contraction. On the other hand, treatment of the colonic segments with thapsigargin, an inhibitor of sarcoplasmic reticulum (SR) Ca2+-ATPase, did not significantly affect the contraction except causing a slight decrease in the rate of contraction. These results suggest that Ca> entering through voltage-dependent calcium channels, but not released from SR, is essential for CCh-induced contraction of longitudinal muscle of the proximal colon of rats. This strict dependency of the CCh-induced contraction on extracellular Ca2+ was discussed in relation to the results obtained in the fundus of rats.

Activation of Rho signaling contributes to lysophosphatidic acid-induced contraction of intact ileal smooth muscle of guinea-pig

To elucidate the possible role of Rho A/Rho-kinase on lysophosphatidic acid (LPA)-induced contraction in intact guinea-pig ileal smooth muscle, we examined effects of pretreatment with a specific inhibitor of Rho-kinase (Y-27632) on the LPA-induced contraction and MLC20 phosphorylation. In addition, we investigated whether LPA actually elicits an activation of Rho A by studying subcellular distribution of Rho A in unstimulated and stimulated smooth muscles by LPA. LPA induced a less intense, but sustained, contraction compared with ACh, and was accompanied by significant increases in MLC20 phosphorylation. The effects of LPA on tension and MLC20 phosphorylation were inhibited by Y-27632. The ACh-induced contraction, but not increases in MLC20 phosphorylation, was partially inhibited by Y-27632. High K+-induced contraction was unaffected by the inhibitor. LPA stimulated translocation of Rho A from the cytosol to the membrane fraction of the muscle. Translocation of Rho A was also induced by ACh and high K+. These results suggest that LPA-induced contraction of intact ileal smooth muscle is dominated through activation of Rho A and Rho-kinase and subsequent increases in MLC20 phosphorylation.Key words: lysophosphatidic acid, Rho, Rho-kinase, ileal smooth muscle.

Agonist-dependent difference in the mechanisms involved in Ca2+ sensitization of smooth muscle of porcine coronary artery

This study was undertaken to explore possible signal-transduction mechanisms involved in the Ca2+-sensitizing effects of carbachol and endothelin-1 (ET-1) by using beta-escin-skinned smooth muscle of porcine coronary artery. Pretreatment with C3 exoenzyme of Clostridium botulinum, which selectively inactivates rho p21 by adenosine diphosphate (ADP) ribosylation, resulted in a significant inhibition of ET-1-induced Ca2+ sensitization, but had no effect on carbachol-induced Ca2+ sensitization. Whereas the protein kinase C (PKC) inhibitors calphostin C and staurosporine did not affect the Ca2+-sensitizing effect of carbachol, the tyrosine kinase inhibitors genistein and tyrphostin 25 greatly but incompletely suppressed it. In contrast, the Ca2+-sensitizing effect of ET-1 was significantly inhibited by either calphostin C or genistein. Although the inhibitory effect of calphostin C on ET-1-induced Ca2+ sensitization was less than that of genistein, the effects of calphostin C and genistein were additive. The genistein-sensitive component of ET-1-induced Ca2+ sensitization appeared to include the C3-sensitive one. However, a substantial enhancement by ET-1 of the Ca2+-induced contraction was observed even in the presence of the two inhibitors. In beta-escin-skinned smooth muscle of rabbit mesenteric artery, ET-1-induced Ca2+ sensitization was marginally affected by C3 pretreatment, calphostin C, and genistein. We conclude that, although PKC activation and rho p21 protein-dependent and -independent tyrosine phosphorylation each plays an important role in an increase in myofilament Ca2+ sensitivity, the contributions of these signaling pathways to Ca2+ sensitization are different depending on receptor agonists and tissues used. Furthermore, these data suggest the existence of an as yet undefined signal-transduction mechanism involved in Ca2+ sensitization caused by receptor agonists.

Changes in the mechanisms involved in uterine contractions during pregnancy in guinea-pigs

1. The mechanisms involved in contraction in guinea-pig myometrium were compared at mid- and late pregnancy. Tension was recorded simultaneously with either membrane potential or cytoplasmic calcium ([Ca2+]i) in strips exposed briefly to prostaglandin F2alpha (PGF). 2. PGF-induced increases in tension were underpinned by action potentials followed by sustained depolarization and biphasic increases in [Ca2+]i at mid- (peak, 879 +/- 199 nM; sustained, 298 +/- 35 nM, n = 11) and late pregnancy (peak, 989 +/- 302 nM; sustained 178 +/- 33 nM, n = 8). 3. At mid- and late pregnancy, nifedipine (10-6 M) reduced (a) the PGF-induced increase in tension to 84 and 35 %, (b) the level attained during the depolarization by 2 and 12 mV and (c) the peak rise in [Ca2+]i to 42 and 17 %. The sustained rises in [Ca2+]i were resistant to nifedipine. 4. In Ca2+-free solution (containing 1 mM EGTA), PGF elicited an increase in tension that was 26 % of that in 2.5 mM Ca2+ and an increase in [Ca2+]i (24 % of the sustained level) at mid-pregnancy but no increase in tension or [Ca2+]i at term. 5. At both stages of pregnancy, PGF decreased the level of [Ca2+]i required to elicit increases in tension comparable to those evoked by high K+o. The slope of the tension-[Ca2+]i curves were steeper in mid- than in late pregnancy. 6. In conclusion, at mid-pregnancy, the contractile response of the guinea-pig myometrium to PGF involves Ca2+ influx through L-type voltage-operated Ca2+ channels (VOCCs) and by receptor-operated mechanisms, release of Ca2+ from intracellular stores, and an increase in the sensitivity of the contractile apparatus to Ca2+. At term the situation is different: a modest increase in the sensitivity of the contractile apparatus to Ca2+ persists and there is a major reliance on Ca2+ influx through VOCCs.

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.

The Rho-related protein Rnd1 inhibits Ca2+ sensitization of rat smooth muscle

1. The small GTP-binding Rho proteins are involved in the agonist-induced Ca2+ sensitization of smooth muscle. The action and the expression of Rnd1, a new member of the Rho protein family constitutively bound to GTP, has been studied in rat smooth muscle. 2. Recombinant prenylated Rnd1 (0.01-0.1 mg ml-1) dose dependently inhibited carbachol- and GTPgammaS-induced Ca2+ sensitization in beta-escin-permeabilized ileal smooth muscle strips but had no effect on the tension at submaximal [Ca2+] (pCa 6.3). Rnd1 inhibited GTPgammaS-induced tension without shifting the dose-response curves to GTPgammaS. 3. pCa-tension relationships were not modified by Rnd1 and the rise in tension induced through the inhibition of myosin light chain phosphatase by calyculin A was not affected by Rnd1. 4. The Ca2+ sensitization induced by recombinant RhoA was completely abolished when RhoA and Rnd1 were applied together. 5. Rnd1 was expressed at a low level in membrane fractions prepared from intestinal or arterial smooth muscles. The expression of Rnd1 was strongly increased in ileal and aortic smooth muscle from rats treated with progesterone or oestrogen. Progesterone-treated ileal muscle strips showed a decrease in agonist-induced Ca2+ sensitization. 6. The present study shows that (i) Rnd1 inhibits agonist- and GTPgammaS-induced Ca2+ sensitization of smooth muscle by specifically interfering with a RhoA-dependent mechanism and (ii) an increase in Rnd1 expression may account, at least in part, for the steroid-induced decrease in agonist-induced Ca2+ sensitization.

Tyrosine phosphorylation as a convergent pathway of heterotrimeric G protein- and rho protein-mediated Ca2+ sensitization of smooth muscle of rabbit mesenteric artery

1. The aim of this study was to determine whether different signal transduction mechanisms underlie the Ca2+ sensitizing effects of guanosine 5'-O-(3-thiotriphosphate) (GTP(gamma)S) and receptor agonists on beta-escin-skinned smooth muscle of rabbit mesenteric artery. 2. In the homogenate of the beta-escin-skinned arterial strip, C3 exoenzyme of Clostridium botulinum catalyzed the [32P]-ADP-ribosylation of only one protein that had the same molecular mass as the protein detected in Western blots with anti-rho p21 antibody. Pretreatment of preparations with C3 resulted in great inhibition of GTP(gamma)S-induced Ca2+ sensitization, although the effect of GTP(gamma)S at higher concentrations (> or = 30 microM) was not completely blocked by this treatment. In contrast, the enhancement by phenylephrine and histamine, in the presence of guanosine 5'-triphosphate, of the Ca2+-induced contraction was not affected by C3 pretreatment. 3. The protein kinase C (PKC) inhibitors calphostin C and staurosporine completely eliminated the enhancement by phorbol ester 12,13-dibutyrate of the Ca2+-induced contraction. However, these PKC inhibitors had no effect on GTP(gamma)S- and receptor agonist-induced Ca2+ sensitization. 4. The tyrosine kinase inhibitors genistein and tyrphostin 25 caused an irreversible and complete block of the enhancement by GTP(gamma)S of the Ca2+-induced contraction without affecting this Ca2+ contraction. The inactive genistein analogue daidzein did not modify the effect of GTP(gamma)S. The Ca2+ sensitizing effects of phenylephrine and histamine were also blocked by these tyrosine kinase inhibitors. 5. These results suggest that rho p21 predominantly mediates GTP(gamma)S-induced Ca2+ sensitization of beta-escin-skinned smooth muscle of rabbit mesenteric artery, while the Ca2+ sensitizing actions of heterotrimeric G protein-coupled receptor agonists do not involve this small G protein. However, it seems that tyrosine phosphorylation, but not PKC activation, plays an important role in both of the rho p21 protein- and heterotrimeric G protein-mediated Ca2+ sensitization mechanisms.

Involvement of small GTPases in the regulation of smooth muscle contraction

Neurohumoral stimulation of smooth muscle leads to an increased responsiveness of the myofilaments to Ca2+. This review provides a summary of the data that suggest that the signalling from the membrane-bound serpentine receptors to the contractile apparatus leading to the increase in Ca(2+)-sensitivity requires the activation of the Ras-related low molecular mass GTPase Rho. In smooth muscle permeabilized with alpha-toxin or beta-escin, the increase in force elicited by different agonists at fixed [Ca2+] (Ca(2+)-sensitization) can be inhibited by bacterial toxins (EDIN, and exoenzyme C3) which ADP-ribosylate and inactivate Rho proteins. Moreover, the agonist-induced increase in Ca(2+)-sensitivity can be mimicked by constitutively active recombinant Rho proteins. The physiological relevance of this mechanism is suggested by the fact that toxins that are internalized into intact cells (toxin B from C. difficile and a chimeric toxin (DC3B) consisting of C3 and the (non-catalytic) B fragment of diphteria toxin (inhibit the tonic phase of an agonist-induced contraction. Toxin B inhibits contraction without affecting the intracellular Ca(2+)-transient determined with fura-2. However, it inhibits phosphorylation of the regulatory light chains of myosin (MLC). Rho has been suggested to activate a Rho-associated kinase which in turn phosphorylates the myosin binding subunit of the myosin light chain phosphatase. This would lead to an increase in phosphorylation of MLC and hence of force at constant Ca2+. The Ca(2+)-sensitizing effect of agonists is also inhibited by tyrosine kinase inhibitors. This suggests the possibility that in smooth muscle, like in non-muscle cells, there is a cross-talk between Rho and tyrosine kinases.

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.

Potentiation by neurotensin of carbachol-induced tension development in beta-escin-skinned smooth muscle of guinea-pig ileum

Effect of neurotensin (NT) on carbachol(CCh)-induced tension development due to Ca2+ release from intracellular stores was investigated in beta-escin-skinned smooth muscle of guinea-pig ileum. NT (10 nM) increased the tension development in response to CCh. NT also increased the tension response to caffeine, another store-Ca2+ releaser. NT did not exert such an effect in pertusis toxin (PTX)-treated preparations. Treatment with isoprenaline to elevate endogenous cyclic AMP levels or with dibutyryl cyclic AMP did not affect the effect of NT. A nonpeptide NT antagonist, SR 48692, failed to block the effect of NT. NT shifted the pCa-tension relationship in the lower direction of Ca2+ concentration. NT was incapable of releasing Ca2+ from intracellular stores. The results suggest that NT may cause an increase in Ca2+ sensitivity of contractile elements to potentiate the CCh-induced tension development due to release of stored Ca2+ and that the effect is mediated by SR 48692-insensitive NT receptors linked to a PTX-sensitive G protein which works with no relation to a change in cytosolic cyclic AMP levels.

Role of G proteins in agonist-induced Ca2+ sensitization of tracheal smooth muscle

Increased sensitivity to intracellular Ca2+ concentration ([Ca2+]) is an important mechanism for agonist-induced contraction of airway smooth muscle, but the signal transduction pathways involved are uncertain. We studied Ca2+ sensitization with acetylcholine (ACh) and endothelin (ET)-1 in porcine tracheal smooth muscle by measuring contractions at a constant [Ca2+] in strips permeabilized with alpha-toxin or beta-escin. The peptide inhibitor G protein antagonist 2A (GP Ant-2A), which has selectivity for Gq over Gi, inhibited contractile responses to ET-1, ACh, and guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS), but the proportional inhibition of ACh responses was less than that of ET-1. Pretreatment with pertussis toxin reduced ACh contractions but had no effect on those of ET-1 or GTPgammaS. Clostridium botulinum C3 exoenzyme, which inactivates Rho family monomeric G proteins, caused similar reductions in contractile responses to ACh, ET-1, and GTPgammaS. Farnesyltransferase inhibition, which inhibits Ras G proteins, reduced responses to ET-1. We conclude that the heterotrimeric G proteins Gq and Gi both contribute to Ca2+ sensitization by ACh, whereas ET-1 responses involve Gq but not Gi. Both Gq and Gi pathways likely involve Rho family small G proteins. A Ras-mediated pathway also contributes to Ca2+ sensitization by ET-1 in airway smooth muscle.

M2 and M3 muscarinic receptors couple, respectively, with activation of nonselective cationic channels and potassium channels in intestinal smooth muscle cells

Smooth muscle cells of guinea pig ileum express both M2 and M3 subtypes of muscarinic receptors. Under voltage clamp, activation of the muscarinic receptors with carbachol (CCh) induces Ca2+-activated K+ current (I[K-Ca]) and nonselective cationic current (Icat). Receptor subtypes mediating the current responses were characterized by using pirenzepine, AF-DX116, 4-DAMP and atropine, which have different profiles of the affinity constants for muscarinic receptor subtypes. The muscarinic antagonists inhibited either CCh-evoked I(K-Ca) or Icat with different potencies. Their relative potencies for I(K-Ca) and Icat inhibition resembled the relative affinity constants for M3 and M2 subtypes, respectively. Thus, the I(K-Ca) is mediated via the M3 subtype and the Icat via the M2 subtype.

Clostridium difficile toxin B inhibits carbachol-induced force and myosin light chain phosphorylation in guinea-pig smooth muscle: role of Rho proteins

1. Clostridium difficile toxin B glucosylates the Ras-related low molecular mass GTPases of the Rho subfamily thereby inactivating them. In the present report, toxin B was applied as a tool to test whether Rho proteins participate in the carbachol-induced increase in the Ca2+ sensitivity of force and myosin light chain (MLC) phosphorylation in intact intestinal smooth muscle. 2. Small strips of the longitudinal muscle of guinea-pig small intestine were incubated in toxin B (40 ng ml-1) overnight. Carbachol-induced force and intracellular [Ca2+], and, in a separate series, force and MLC phosphorylation, were determined. 3. Carbachol induced a biphasic contraction: an initial rapid increase in force (peak 1) followed by a partial relaxation and a second delayed increase in force (peak 2). The peak of the Ca2+ signal measured with fura-2 preceded peak 1 of force and then declined to a lower suprabasal steady-state level. Peak 2 was not associated with a significant increase in [Ca2+]. Toxin B nearly completely inhibited peak 2 while peak 1 was not significantly inhibited. Toxin B had no effect on the Ca2+ transient. 4. In control strips, MLC phosphorylation at peak 2 was 27.7% which was significantly higher than the resting value (18.6%). The inhibition of the second, delayed, rise in force induced by toxin B was associated with complete inhibition of the increase in MLC phosphorylation. The resting MLC phosphorylation was not significantly different from that of the control strips. 5. The initial increase in MLC phosphorylation determined 3 s after exposure to carbachol was 54% in the control strips. Toxin B also inhibited this initial phosphorylation peak despite the fact that the Ca2+ transient and the initial increase in force were not inhibited by toxin B. This suggests that Rho proteins play an important role in setting the balance between MLC phosphorylation and dephosphorylation reactions even at high levels of intracellular Ca2+. 6. These findings are consistent with the hypothesis that the delayed rise in force elicited by carbachol is due to an increase in the Ca2+ sensitivity of MLC phosphorylation mediated by Rho proteins.

Inhibition of carnitine synthesis protects against left ventricular dysfunction in rats with myocardial ischemia

During myocardial ischemia, inhibition of the carnitine-mediated transportation of fatty acid may be beneficial because it facilitates glucose utilization and prevents an accumulation of fatty acid metabolites. We orally administered 3-(2,2,2-trimethyl hydrazinium) propionate (MET), an inhibitor of carnitine synthesis, for 20 days to rats. Then we evaluated left ventricular (LV) function during brief ischemia by using a buffer-perfused isovolumic heart model. After 15 min of reoxygenation after the transient ischemia, LV peak systolic pressure (PSP) almost completely returned to the baseline level in rats given MET (96 +/- 4%), whereas it was only partially (77 +/- 16%) recovered in the placebo-treated rats. We induced myocardial infarction in other rats by ligating the left anterior descending coronary artery. Then the animals were given MET for 20 days, and LV function was compared. In the placebo-treated rats (with myocardial infarction, but without drug treatment), LVPSP was lower than that in the sham group [108 +/- 19 (n = 10) vs. 136 +/- 15 mm Hg (n = 13); p < 0.05], and the time constant (T) of LV pressure decay was elongated (36 +/- 4 vs. 30 +/- 7 ms; p < 0.05). In MET-treated groups, however, neither PSP nor T differed from those in the sham group. In conclusion, inhibition of the carnitine-mediated transportation of fatty acid by MET protected against left ventricular dysfunction in acute and chronic myocardial ischemia.

Translocation of rhoA associated with Ca2+ sensitization of smooth muscle

We determined the relationship between the localization of rhoA and Ca2+ sensitization of force in smooth muscle. In alpha-toxin-permeabilized rabbit portal vein at pCa 6.5, the particulate hydrophobic fraction of rhoA (10 +/- 1.6% of the total) was significantly increased by phenylephrine to 18 +/- 5.5% at 5 min, by AlF4- to 26 +/- 8.4% at 20 min, and dose-dependently up to 62 +/- 9.5% by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS; 0.3-50 microM). Translocation of rhoA was selective (Rac1 and Cdc42 were not translocated) and was quantitatively correlated (up to approximately 50%; r = 0.91, p < 0.05) with Ca2+ sensitization; high GTPgammaS concentrations (>/=10 microM) further increased translocation without increasing force. The initial recruitment of rhoA to the membrane paralleled the time course of contraction, but sensitization could be reversed without a decrease in particulate rhoA. High [Ca2+] (pCa 4.5) also increased particulate rhoA to 31 +/- 5.8%. Membrane-associated rhoA in unstimulated portal vein was a good substrate for in vitro ADP-ribosylation, whereas the large amount translocated by GTPgammaS was not. We conclude that 1) translocation of rhoA plays a causal role in Ca2+ sensitization, and 2) membrane-bound rhoA can exist in two or more states.

Down-regulation of G-protein-mediated Ca2+ sensitization in smooth muscle

Prolonged treatment with guanosine 5'-[gamma-thio]triphosphate (GTP gamma S; 5-16 h, 50 microM) of smooth muscle permeabilized with Staphylococcus aureus alpha-toxin down-regulated (abolished) the acute Ca2+ sensitization of force by GTP gamma S, AIF-4, phenylephrine, and endothelin, but not the response to phorbol dibutyrate or a phosphatase inhibitor, tautomycin. Down-regulation also abolished the GTP gamma S-induced increase in myosin light chain phosphorylation at constant [Ca2+] and was associated with extensive translocation of p21rhoA to the particulate fraction, prevented its immunoprecipitation, and inhibited its ADP ribosylation without affecting the immunodetectable content of G-proteins (p21rhoA, p21ras, G alpha q/11, G alpha i3, and G beta) or protein kinase C (types alpha, beta 1, beta 2, delta, epsilon, eta, theta, and zeta). We conclude that the loss of GTP gamma S- and agonist-induced Ca2+ sensitization through prolonged treatment with GTP gamma S is not due to a decrease in the total content of either trimeric (G alpha q/11, G alpha i3, and G beta) or monomeric (p21rhoA and p21ras) G-protein or protein kinase C but may be related to a structural change of p21rhoA and/or to down-regulation of its (yet to be identified) effector.

Role of Rho proteins in carbachol-induced contractions in intact and permeabilized guinea-pig intestinal smooth muscle

1. The aim of this study was to determine whether the low molecular mass GTPase RhoA or related proteins are involved in carbachol- and high-K(+)-induced contractions in intact intestinal smooth muscle as well as the carbachol-induced increase in Ca2+ sensitivity of the myofilaments in permeabilized preparations. 2. The carbachol-induced increase in the Ca2+ sensitivity of force production in beta-escin-permeabilized intestinal smooth muscle was enhanced in preparations that were loaded with the constitutively active mutant of RhoA, Val14RhoA, and was inhibited by exoenzyme C3 from Clostridium botulinum, which ADP-ribosylates and inactivates small GTPases of the Rho family. The effect of C3 on Ca2+ sensitivity in the absence of the agonist was negligible, while the maximal Ca(2+)-activated force was inhibited by about 20%. 3. Inhibition of carbachol-induced force was associated with an increase in ADP-ribosylation of a protein band with a molecular mass of approximately 22 kDa, corresponding to Rho, and was partially reversed in the presence of Ile41RhoA, which is not a substrate for C3. Val14RhoA did not restore carbachol-induced Ca2+ sensitization in C3-treated smooth muscle. 4. In intact intestinal smooth muscle, toxin B from Clostridium difficile, which monoglucosylates members of the Rho family, inhibited high-K(+)-induced contractions and the initial phasic response to carbachol by about 30%. The delayed contractile response to carbachol was completely inhibited. 5. In smooth muscle preparations that were permeabilized with beta-escin after treatment with toxin B, carbachol-and GTP gamma S-induced Ca2+ sensitization was significantly inhibited. 6. These findings are consistent with a role for Rho or Rho-like proteins in agonist-induced increase in Ca2+ sensitivity of force production in intact and permeabilized intestinal smooth muscle.

Calphostin C-sensitive enhancements of force by lysophosphatidylinositol and diacylglycerols in mesenteric arteries from the rat

1. A pharmacological characterization was made of the effects of lysophosphatidyl-inositol (lysoPI) and -ethanolamine (lysoPE) on the Ca(2+)-sensitivity of contraction in alpha-toxin permeabilized rat mesenteric arteries. The effect of GTP gamma S (G-protein activator), diacylglycerols (DAGs, dioctanoyl glycerol (diC8) and 1-stearoyl-2-arachidonoyl-sn-glycerol) and phorbol myristate acetate (PMA, protein kinase C (PKC) activator) on Ca(2+)-sensitivity was also assessed. 2. LysoPI increased the Ca(2+)-sensitivity, demonstrated by both an increase in tension induced by 1 microM [Ca2+]free and an increase in the Ca(2+)-sensitivity of Ca2+ concentration-tension curves. LysoPE did not enhance force or Ca(2+)-sensitivity. 3. GTP gamma S enhanced force at constant Ca2+, increased the Ca(2+)-sensitivity, and increased force under Ca(2+)-free conditions. PMA also increased force at constant Ca2+ and increased Ca(2+)-sensitivity, but caused no force development under Ca(2+)-free conditions. 4. DAGs, both diC8 and the more physiological relevant DAG, 1-stearoyl-2-arachidonoyl-sn-glycerol, enhanced force at constant Ca2+ and increased the Ca(2+)-sensitivity. DiC8, in contrast to 1-stearoyl-2-arachidonoyl-sn-glycerol, caused force development under Ca(2+)-free conditions and substantially enhanced force at maximal Ca(2+)-induced contraction. GDP-beta-S abolished the increased Ca(2+)-sensitization induced by noradrenaline, but not that by DAGs. 5. The PKC inhibitor calphostin C completely abolished Ca(2+)-sensitization induced by all of the Ca(2+)-sensitizing agents. 6. These results show that lysoPI can increase the Ca(2+)-sensitivity of smooth muscle contraction, and the Ca(2+)-sensitization induced by DAGs was not completely G-protein mediated, because it was not inhibited by GDP-beta-S. A central role for PKC in regulation of Ca(2+)-sensitization in rat mesenteric small arteries was indicated by the abolishment of Ca(2+)-sensitization by calphostin C.

Role of guanine nucleotide-binding proteins--ras-family or trimeric proteins or both--in Ca2+ sensitization of smooth muscle

The purpose of this study was to identify guanine nucleotide-binding proteins (G proteins) involved in the agonist- and guanosine 5'-[gamma-thio]triphosphate (GTP[gamma-S])-induced increase in the Ca2+ sensitivity of 20-kDa myosin light chain (MLC20) phosphorylation and contraction in smooth muscle. A constitutively active, recombinant val14p21rhoA.GTP expressed in the baculovirus/Sf9 system, but not the protein expressed without posttranslational modification in Escherichia coli, induced at constant Ca2+ (pCa 6.4) a slow contraction associated with increased MLC20 phosphorylation from 19.8% to 29.5% (P < 0.05) in smooth muscle permeabilized with beta-esein. The effect of val14p21rhoA.GTP was inhibited by ADP-ribosylation of the protein and was absent in smooth muscle extensively permeabilized with Triton X-100. ADP-ribosylation of endogenous p21rho with epidermal cell differentiation inhibitor (EDIN) inhibited Ca2+ sensitization induced by GTP [in rabbit mesenteric artery (RMA) and rabbit ileum smooth muscles], by carbachol (in rabbit ileum), and by endothelin (in RMA), but not by phenylephrine (in RMA), and only slowed the rate without reducing the amplitude of contractions induced in RMA by 1 microM GTP[gamma-S] at constant Ca2+ concentrations. AlF(4-)-induced Ca2+ sensitization was inhibited by both guanosine 5'-[beta-thio]diphosphate (GDP[beta-S]) and by EDIN. EDIN also inhibited, to a lesser extent, contractions induced by Ca2+ alone (pCa 6.4) in both RMA and rabbit ileum. ADP-ribosylation of trimeric G proteins with pertussis toxin did not inhibit Ca2+ sensitization. We conclude that p21rho may play a role in physiological Ca2+ sensitization as a cofactor with other messengers, rather than as a sole direct inhibitor of smooth muscle MLC20 phosphatase.

MCI-826 is a potent and selective antagonist of peptide leukotrienes (p-LTs) and has characteristics distinctive from those of FPL 55712

Antagonistic effects of a newly synthesized compound, (E)-2,2-diethyl-3'-[2-[2-(4-isopropyl)thiazolyl]ethenyl]succinanilic+ ++ acid sodium salt (MCI-826) on the contraction of the isolated guinea pig trachea and human bronchus induced by various agonists including peptide leukotrienes (p-LTs), histamine, acetylcholine (ACh), prostaglandin (PG) D2 and others were investigated and compared with the effects of a p-LT antagonist, FPL 55712, in some experiments. MCI-826 potently antagonized LTD4- and LTE4-induced contractions at extremely low concentrations in the isolated guinea pig trachea with pA2 values of 8.3 and 8.9, respectively, on a molar basis. These values indicated that MCI-826 is over 100 times stronger than FPL 55712. Similarly, MCI-826 at 10(-8) g/ml (2.4 x 10(-8) M) markedly antagonized LTD4-induced contractions of the isolated human bronchus. Although FPL 55712 fairly inhibited the 10(-9) g/ml LTC4-induced contraction of the isolated guinea pig trachea, MCI-826 had little effect on the contraction at high concentrations like 3 x 10(-6) g/ml (7.1 x 10(-6) M). MCI-826 modestly affected the other agonist-induced contractions and the resting tonus of the isolated guinea pig trachea at 10(-6) g/ml (2.4 x 10(-6) M) or higher concentrations, but FPL 55712 caused fair inhibition of some of those contractions and gradually lowered the resting tonus with time. These results indicate that MCI-826 is a highly potent and selective antagonist of LTD4 and LTE4 and can be a useful tool for biological and pharmacological experiments on p-LTs.