Receptor-coupled shortening of alpha-toxin-permeabilized single smooth muscle cells from the guinea-pig stomach

Role of Ca2+ stores in acetylcholine-induced all-or-none shortening of smooth muscle cells from guinea-pig taenia caecum

1. We have reported previously that isolated single smooth muscle cells from guinea-pig taenia caecum respond to acetylcholine (ACh) in an all-or-none manner. 2. To clarify the roles of intracellular Ca(2+) stores in the all-or-none response of isolated smooth muscle cells from guinea-pig taenia caecum to ACh, we examined the inositol 1,4,5-trisphosphate (IP(3))-induced contractile response in Staphylococcus aureus alpha-toxin-permeabilized smooth muscle cells and the effect of depletion of intracellular Ca(2+) stores on the all-or-none response to ACh in intact smooth muscle cells. 3. alpha-Toxin-permeabilized smooth muscle cells responded to 3-30 nmol/L or 0.3-3 nmol/L IP(3) in the presence of 0.2 micromol/L Ca(2+) with 1 mmol/L EGTA or 0.1 mmol/L EGTA, respectively, in an all-or-none manner. These results suggest that Ca(2+) release induced by IP(3) is Ca(2+) dependent and is evoked in an all-or-none manner. 4. In the presence of the Ca(2+) ionophore A23187 (0.1 micromol/L) or the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor cyclopiazonic acid (1 micromol/L), the shortening of intact smooth muscle cells induced by increasing concentrations of ACh showed a graded response, but not an all-or-none response. 5. In conclusion, the results suggest that Ca(2+) release from Ca(2+) stores induced by IP(3) plays an important role in the all-or-none response of intact smooth muscle cells to ACh.

Contractile responses and myosin phosphorylation in reconstituted fibers of smooth muscle cells from the rat cerebral artery

String-shaped reconstituted smooth muscle fibers were prepared in rectangular wells by thermal gelation of a mixed solution of collagen and cultured smooth muscle cells derived from the rat cerebral artery. The fibers contracted in response to KCl, 5-hydroxytryptamine (5-HT), noradrenaline, endothelin-1, endothelin-2, angiotensin II, prostaglandin F2alpha and prostaglandin E2. 5-HT-induced contraction was partially inhibited by the L-type voltage-dependent Ca2+ channel inhibitor nifedipine, putative non-selective cationic channel inhibitor SKF96365 and intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM), and completely abolished by the myosin light chain kinase inhibitor ML-9. The fibers pre-contracted by 5-HT were completely relaxed by the Rho kinase inhibitor Y-27632, serine/threonine kinase inhibitor staurosporine, 8-bromo cyclic GMP and papaverine, and partially relaxed by dibutyryl cyclic AMP. Moreover, 5-HT as well as endothelin-1 and KCl enhanced 20-kDa myosin light chain phosphorylation in the fibers. These results suggested that the characteristics of contraction of the fibers reflect typical contractilities of vascular smooth muscle tissues. This technique will allow us to directly address questions relating to heterogeneity of receptor mechanisms and intracellular pathways of vascular smooth muscle contraction as a function of vessel type.

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

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

Agonist-induced isometric contraction of smooth muscle cell-populated collagen gel fiber

String-shaped reconstituted smooth muscle (SM) fibers were prepared in rectangular wells by thermal gelation of a mixed solution of collagen and cultured SM cells derived from guinea pig stomach. The cells in the fiber exhibited an elongated spindle shape and were aligned along the long axis. The fiber contracted in response to KCl (140 mM), norepinephrine (NE; 10(-7) M), epinephrine (10(-7) M), phenylephrine (10(-6) M), serotonin (10(-6) M), and histamine (10(-5) M), but not acetylcholine (10(-5) M). Phentolamine (10(-7) M) produced a parallel rightward shift of the NE dose-response curve. Moreover, NE-induced contraction was partially inhibited by nifedipine and completely abolished by the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, the myosin light chain kinase inhibitor ML-9, the Rho kinase inhibitor Y-27632, and papaverine. A [(3)H]quinuclidinyl benzilate binding study revealed that the loss of response to acetylcholine was due to the loss of muscarinic receptor expression during culture. The expression of contractile proteins in the fibers was similar to that in cultured SM cells. These results suggest that, although the fiber is not a model for fully differentiated SM, contractile mechanisms are maintained.

Protein kinase C promotes spontaneous relaxation of streptolysin-O-permeabilized smooth muscle cells from the guinea-pig stomach

Isolated single smooth muscle cells from the fundus of a guinea-pig stomach were permeabilized by use of streptolysin-O (0.5 U/ml). Most of the permeabilized cells responded to 0.6 microM Ca2+, but not to 0.2 microM Ca2+, with a resulting maximal cell shortening to approximately 71% of the resting cell length. These cells were relaxed again by washing with the Ca2+-free solution (2.5 nM free Ca2+) for 3-5 min. Addition of 10 microM acetylcholine (ACh) resulted in both a marked decrease in the concentration of Ca2+ required to trigger a threshold response and an increase in the maximal cell shortening, indicating that the cells retained the muscarinic receptor function. When the cell treated with a protein kinase C (PKC) inhibitor, K-252b (1 microM), for 3 min was exposed to 10 microM ACh in the presence of K-252b, the cell shortened within 2 min with a maximal cell shortening. When the cell shortening was induced by 10 microM ACh plus 1 microM Ca2+ in the presence of K-252b (1 microM) or more selective PKC inhibitors, such as calphostin C (1 microM) or PKC pseudosubstrate peptide (100 microM), the extension of the shortened cells, by washing with the Ca2+-free solution, was significantly inhibited. In contrast, K-252b (1 microM) did not inhibit the relaxation of Ca2+-induced shortened cells. These results suggest that the receptor-mediated activation of PKC in the process of ACh-induced cell shortening plays a role in the subsequent relaxation of the shortened cells.

Inhibition of muscarinic receptor-operated Ca2+ sensitization by short-term desensitization of alpha-toxin-permeabilized smooth muscle cells from guinea pig stomach

1. Isolated single smooth muscle cells from the guinea pig stomach were permeabilized with Staphylococcus aureus alpha-toxin. 2. The permeabilized single cells showed a shortening in response to Ca2+ in an all-or-none manner. Moreover, the addition of acetylcholine (ACh) or guanosine 5'-triphosphate (GTP) resulted in a decrease in concentration of Ca2+ required to trigger a threshold response, suggesting that Ca2+ sensitization is induced by the stimulation of muscarinic acetylcholine receptors (mAChRs) or GTP-binding protein(s). 3. Short-term desensitization was induced by incubating the permeabilized cells with 100 microM ACh for 10 min. 4. In desensitized cells, the concentration of Ca2+ required to trigger a threshold response in the presence of ACh was increased, however, the cell shortening in response to Ca2+ in the absence of ACh and GTP-induced Ca2+ sensitization was not affected by short-term desensitization. 5. These results suggest that the receptor-operated augmentation of Ca2+ sensitivity is inhibited by short-term desensitization and that the development of short-term desensitization is due to an uncoupling of mAChR/GTP-binding protein(s).

Effects of phorbol-12,13-dibutyrate and protein kinase C inhibitors on Mn(2+)-dependent norepinephrine-induced contractions involving increase in Mn2+ sensitivity in Ca(2+)-depleted vas deferens of the guinea pig

1. In Ca(2+)-depleted Mn(2+)-loaded vas deferens from the guinea pig (Mn-loaded preparations), norepinephrine (NE) induced a tonic contraction dose dependently without extracellular Ca2+ and Mn2+. 2. In the beta-escin skinned vas deferens, Mn2+ as well as Ca2+ induced contractions. Guanosine triphosphate and NE increased the sensitivity of contractile mechanisms to these divalent cations. 3. Phorbol-12,13-dibutyrate (PDBu) did not induce contractions in normal (Mn(2+)-unloaded Ca(2+)-contained) preparations, whereas it induced slow sustained contractions dose dependently in Mn-loaded preparations. Although cumulative applications of PDBu desensitized the preparations to this phorbol ester, the desensitization did not affect Mn(2+)-dependent NE-induced contractions. PDBu did not affect the dose-response relation of NE in Mn-loaded preparations. 4. Staurosporine (10-100 nM) preferentially inhibited NE-induced contractions in normal and in Mn-loaded preparations to that induced by K+ in normal preparations. However, bisindolylmaleimide I (1 microM) did not inhibit NE-induced contractions in normal and Mn-loaded preparations but abolished PDBu-induced contractions. 5. These results suggest that the NE-induced increase in the Mn2+ sensitivity of contractile mechanisms contributes to Mn(2+)-dependent NE-induced contractions, which may not involve the activation of protein kinase C.

Modulation of the calcium sensitivity of the smooth muscle contractile apparatus: molecular mechanisms, pharmacological and pathophysiological implications

Smooth muscle contraction is the basis of the physiological reactivity of several systems (vascular, respiratory, gastrointestinal, urogenital ...). Hyperresponsiveness of smooth muscle may also contribute to a variety of problems such as arterial hypertension, asthma and spontaneous abortion. An increase in cytoplasmic calcium concentration ([Ca2+]i) is the key event in excitation-contraction coupling in smooth muscle and the relationship linking the [Ca2+]i value to the force of contraction represents the calcium sensitivity of the contractile apparatus (CaSCA). Recently, it has become evident that CaSCA can be modified upon the action of agonists or drugs as well as in some pathophysiological situations. Such modifications induce, at a fixed [Ca2+]i value, either an increase (referred to as sensitization) or a decrease (desensitization) of the contraction force. The molecular mechanisms underlying this modulation are not yet fully elucidated. Nevertheless, recent studies have identified sites of regulation of the actomyosin interaction in smooth muscle. Sensitization primarily results from the inhibition of myosin light chain phosphatase (MLCP) by intracellular messengers such as arachidonic acid or protein kinase C. In addition, phosphorylation of thin filament-associated proteins, caldesmon and calponin, increases CaSCA. Activation of small (monomeric) G-proteins such as rho or ras is also involved. Desensitization occurs as a consequence of phosphorylation of myosin light chain kinase (MLCK) by the calcium-calmodulin activated protein kinase II, or stimulation of MLCP by cyclic GMP-activated protein kinase. In the present review, examples of physiological modulation of CaCSA as well as pharmacological and pathophysiological implications are illustrated for some smooth muscles.

Ca(2+)-regulated dynamic compartmentalization of calmodulin in living smooth muscle cells

A key assumption of most models for calmodulin regulation of smooth and non-muscle contractility is that calmodulin is freely diffusible at resting intracellular concentrations of free Ca2+. However, fluorescence recovery after photobleaching (FRAP) measurements of three different fluorescent analogs of calmodulin in cultured bovine tracheal smooth muscle cells suggest that free calmodulin may be limiting in unstimulated cells. Thirty-seven % of microinjected calmodulin is immobile by FRAP and the fastest recovering component has an effective diffusion coefficient 7-fold slower than a dextran of equivalent size. Combining the FRAP data with extraction data reported in a previous paper (Tansey, M., Luby-Phelps, K., Kamm, K.E., and Stull, J.T. (1994) J. Biol. Chem. 269, 9912-9920), we estimate that at most 5% of total endogenous calmodulin in resting smooth muscle cells is unbound (freely diffusible). Examination of the Ca2+ dependence of calmodulin mobility in permeabilized cells reveals that binding persists even at intracellular Ca2+ concentrations as low as 17 nM. When Ca2+ is elevated to between 450 nM and 3 microM, some of the bound calmodulin is released, as indicated by an increase in the effective diffusion coefficient and the percent mobile fraction. At higher Ca2+, calmodulin becomes increasingly immobilized. In about 50% of the cell population, clamping Ca2+ at micromolar levels results in translocation of cytoplasmic calmodulin to the nucleus. The compartmentalization and complex dynamics of calmodulin in living smooth muscle cells have profound implications for understanding how calmodulin regulates contractility in response to extracellular signals.

All-or-none augmentation of Ca2+ sensitivity in alpha-toxin-permeabilized single smooth muscle cells from guinea-pig taenia caecum

1. Isolated smooth muscle cells from guinea-pig taenia caecum were permeabilized by use of Staphylococcus aureus alpha-toxin, and the sarcoplasmic reticulum Ca2+ store was depleted by exposure to 0.1 microM A23187. 2. Shortening of alpha-toxin-permeabilized single smooth muscle cells was induced by increasing free Ca2+ but was not induced by 0.2 microM free Ca2+. 3. Shortening of the permeabilized cells was caused by application of acetylcholine (ACh) with free Ca2+ concentration held at 0.2 microM. Permeabilized smooth muscle cells responded to 0.3 microM or 1 microM ACh with 0.2 microM Ca2+ with maximal shortening. The concentration-response relationship to ACh had a very steep slope and the cell shortening appeared to be an all-or-none response rather than a graded response, as was the shortening of intact cells to ACh. 4. The shortening of permeabilized cells was also induced by application of guanosine 5'-triphosphate (GTP) with 0.2 microM free Ca2+, showing an all-or-none response. The threshold concentration of GTP that induced an all-or-none response was between 10 microM and 30 microM. 5. These results suggest that Ca2+ sensitivity is augmented by stimulation of the muscarinic receptor or GTP-binding protein(s) in an all-or-none manner. It seems probable that this contributes to the all-or-none response to ACh in intact smooth muscle cells.

Effects of GTP gamma S on muscarinic receptor-stimulated inositol phospholipid hydrolysis in permeabilized smooth muscle from the small intestine

1. Smooth muscle fragments from the longitudinal layer of the small intestine of the guinea-pig were permeabilized with Staphylococcus aureus alpha toxin (alpha-toxin) and used to investigate the role of G-protein activation in the regulation of muscarinic acetylcholine receptor (AChR)-stimulated inositol phospholipid hydrolysis. 2. The efficiency of alpha-toxin permeabilization was estimated by the release of [3H]-2-deoxyglucose ([3H]-2DG) after prior loading or lactate dehydrogenase (LDH) enzyme release from the smooth muscle fragments. 3. In alpha-toxin-permeabilized smooth muscle, but not in non-permeabilized muscle, GTP gamma S induced time- and concentration-dependent increases in labelled inositol phosphates. Carbachol (CCh) increased labelled inositol phosphates in both permeabilized and non-permeabilized muscle, although the increases were greater in non-permeabilized smooth muscle. The response to 100 microM CCh was severely reduced by 0.5 microM atropine. 4. In permeabilized muscle the effects of GTP gamma S or CCh on inositol phosphate levels were reduced by treatment with pertussis toxin (PTX) and completely inhibited by GDP beta S. 5. GTP gamma S caused a concentration-dependent inhibition of the CCh-induced increases in the levels of labelled inositol phosphates. Dibutyryl cyclic AMP or Sp-cAMPs (adenosine-3',5'-cyclic phosphorothiolate-Sp) reduced the effects of CCh on inositol phosphate levels. 6. The results suggest that muscarinic AChR activation induces inositol phospholipid hydrolysis via more than one G-protein in this smooth muscle and that several mechanisms may contribute to the modulation of both stimulatory and inhibitory responses observed.

Actin-severing and Ca(2+)-induced reversal of smooth muscle contraction that is independent of Ca2+

1. Intracellular actin filament organization of gastric smooth muscle cells of the guinea pig in primary culture was examined with rhodamine-labelled phalloidin using a confocal laser fluorescence microscope. 2. The resting cells, both in the presence and absence of Ca2+, showed an even distribution of microfilamentous actin fibers. 3. The characteristic image of the stimulated cells with 10 microM acetylcholine in the presence of 1.8 mM Ca2+ was that the actin filaments were located only on the periphery of the cell. 4. The characteristic image of the cells stimulated as above, but in the absence of Ca2+, was that the actin filaments were unevenly distributed in the cell. 5. The characteristic image of the cells stimulated in the presence of 1 microM Ca2+, which inhibits the above contraction, was pultaceous with the actin filaments absent, indicating severing of actin filaments by a Ca(2+)-activated system, such as gelsolin.

Fc epsilon RI-stimulated Ca(2+)-dependent secretion from rat basophilic leukemia (RBL-2H3) cells permeabilized with Staphylococcal alpha-toxin: Fc epsilon RI-operated signals are not mimicked by the actions of GTP gamma S

1. RBL-2H3 cells permeabilized with alpha-toxin responded to dinitrophenol (30-40 mol/mol)-conjugated human serum albumin, as antigen, to secrete [14C]serotonin in the micromolar range of free Ca2+. 2. Calcium ion alone did not cause substantial secretion. 3. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) (100 microM) in combination with Ca2+ produced only negligible [14C]serotonin secretion. 4. GTP gamma S, in the presence of cytochalasin D, caused optimal secretion of [14C]serotonin in a Ca(2+)-dependent manner.

All-or-none shortening of isolated single smooth muscle cells from different organs to acetylcholine

1. Single smooth muscle cells from the guinea-pig taenia caecum and the fundus of guinea-pig stomach were prepared by collagenase digestion under different, mild conditions. 2. Most of the cells either from taenia caecum or from the fundus of stomach responded repeatedly, showing an all-or-none response to acetylcholine (ACh). 3. The threshold concentrations of ACh were different for the cells of the two tissues. Although individual cells showed an all-or-none response to ACh, the average responses of all the cells were graded, like that of whole tissues. 4. Thus, isolated single smooth muscle cells from different tissues and under different conditions responded to ACh in an all-or-none manner such as the twitch observed in skeletal muscle. 5. These results suggest that the isolation of cells reveals the fundamental characteristics of smooth muscle cells as excitable.

GTP-binding protein amplifies contractile responses of alpha-toxin-permeabilized uterine smooth muscle to Ca2+

1. alpha-toxin of Staphylococcus aureus readily permeabilized rat uterine smooth muscle after incubation for a short time. 2. The permeabilized muscle responded to Ca2+ dose-dependently and repeatedly in the same manner. 3. The threshold concentration of Ca2+ for contraction was 0.1-0.3 microM and the maximal contraction was achieved with 1 or 3 microM Ca2+. 4. GTP gamma S or GTP augmented the contractile response to Ca2+. 5. GDP beta S or GDP suppressed the contraction. 6. The role of GTP-binding protein in sensitization of Ca(2+)-induced contractile response of smooth muscle is discussed.

Protein kinase C-independent sensitization of contractile proteins to Ca2+ in alpha-toxin-permeabilized smooth muscle cells from the guinea-pig stomach

Involvement of protein kinase C in receptor-operated Ca2+ sensitization of cell shortening was investigated by use of alpha-toxin-permeabilized smooth muscle cells from the fundus of the guinea-pig. Most of the isolated cells responded to 0.6 microM Ca2+ with a maximal shortening to approximately 65% of the resting cell length. Addition of acetylcholine (ACh) at a maximal concentration (10 microM) resulted in a marked decrease in the concentration of Ca2+ required to trigger a threshold response from 0.6 microM to 0.2 microM. The augmentation of Ca2+ sensitivity by ACh was not inhibited by specific protein kinase C inhibitors, calphostin C and K-252b at a concentration of 1 microM. These findings suggest that protein kinase C is not involved in the muscarinic receptor-operated augmentation of Ca2+ sensitivity.