Contraction Of Gallbladder Smooth Muscle Research Articles

The role of TRPP2 in agonist-induced gallbladder smooth muscle contraction.

TRPP2 channel protein belongs to the superfamily of transient receptor potential (TRP) channels and is widely expressed in various tissues, including smooth muscle in digestive gut. Accumulating evidence has demonstrated that TRPP2 can mediate Ca(2+) release from Ca(2+) stores. However, the functional role of TRPP2 in gallbladder smooth muscle contraction still remains unclear. In this study, we used Ca(2+) imaging and tension measurements to test agonist-induced intracellular Ca(2+) concentration increase and smooth muscle contraction of guinea pig gallbladder, respectively. When TRPP2 protein was knocked down in gallbladder muscle strips from guinea pig, carbachol (CCh)-evoked Ca(2+) release and extracellular Ca(2+) influx were reduced significantly, and gallbladder contractions induced by endothelin 1 and cholecystokinin were suppressed markedly as well. CCh-induced gallbladder contraction was markedly suppressed by pretreatment with U73122, which inhibits phospholipase C to terminate inositol 1,4,5-trisphosphate receptor (IP3) production, and 2-aminoethoxydiphenyl borate (2APB), which inhibits IP3 recepor (IP3R) to abolish IP3R-mediated Ca(2+) release. To confirm the role of Ca(2+) release in CCh-induced gallbladder contraction, we used thapsigargin (TG)-to deplete Ca(2+) stores via inhibiting sarco/endoplasmic reticulum Ca(2+)-ATPase and eliminate the role of store-operated Ca(2+) entry on the CCh-induced gallbladder contraction. Preincubation with 2 μmol L(-1) TG significantly decreased the CCh-induced gallbladder contraction. In addition, pretreatments with U73122, 2APB or TG abolished the difference of the CCh-induced gallbladder contraction between TRPP2 knockdown and control groups. We conclude that TRPP2 mediates Ca(2+) release from intracellular Ca(2+) stores, and has an essential role in agonist-induced gallbladder muscle contraction.

Orai1 forms a signal complex with SK3 channel in gallbladder smooth muscle

Orai1 is one of the key components of store-operated Ca2+ entry (SOCE) involved in diverse physiological functions. Orai1 may associate with other proteins to form a signaling complex. In the present study, we investigated the interaction between Orai1 and small conductance Ca2+-activated potassium channel 3 (SK3). With the use of RNA interference technique, we found that the SOCE and its associated membrane hyperpolarization were reduced while Orai1 was knocked down by a specific Orai1 siRNA in guinea pig gallbladder smooth muscle. However, with the use of isometric tension measurements, our results revealed that agonist-induced muscle contractility was significantly enhanced after Orai1 protein was knocked down or the tissue was treated by SK3 inhibitor apamin, but not affected by larger conductance Ca2+-activated potassium channel inhibitor iberiotoxin or intermediate conductance Ca2+-activated potassium channel inhibitor TRAM-34. In addition, in the presence of apamin, Orai1 siRNA had no additional effect on agonist-induced contraction. In coimmunoprecipitation experiment, SK3 and Orai1 pulled down each other. These data suggest that, Orai1 physically associated with SK3 to form a signaling complex in gallbladder smooth muscle. Ca2+ entry via Orai1 activates SK3, resulting in membrane hyperpolarization in gallbladder smooth muscle. This hyperpolarizing effect of Orai1-SK3 coupling could serve to prevent excessive contraction of gallbladder smooth muscle in response to contractile agonists.

Role of ROCK expression in gallbladder smooth muscle contraction.

Cholelithiasis is a common medical condition whose incidence rate is increasing yearly, while its pathogenesis has yet to be elucidated. The present study assessed the expression of Rho-kinase (ROCK) in gallbladder smooth muscles and its effect on the contractile function of gallbladder smooth muscles during gallstone formation. Thirty male guinea pigs were randomly divided into three groups: The control group, the gallstone model group and the fasudil interference group. The fasting volume (FV) and bile capacity of the gallbladder (FB) as well as the total cholesterol (TC) and triglyceride (TG) contents of the gallbladder bile were determined. In addition, the gallbladder was dissected to identify whether any gallstones had formed. Part of the gallbladder tissue specimens were used for immunohistochemical analysis of ROCK expression in gallbladder smooth muscles. The results showed that four guinea pigs in the model group and eight in the fasudil group displayed gallstone formation, while there was no gallstone formation in the control group. The FV and FB were significantly increased in the model and fasudil groups. Similarly, the TC and TG contents of gallbladder bile were increased in these groups. The positive expression rate of ROCK in gallbladder smooth muscles in the model and fasudil groups was significantly reduced compared with that in the control group (P<0.05). The results of the present study indicated that the reduction of ROCK expression in guinea pig gallbladder smooth muscles weakened gallbladder contraction and thereby promoted gallstone formation.

The flavonoid chrysin, an endocrine disrupter, relaxes cholecystokinin- and KCl-induced tension in male guinea pig gallbladder strips through multiple signaling pathways

The bioflavonoids have effects on vascular smooth muscle and gastrointestinal smooth muscle. The flavone and phytoestrogen, chrysin, has been shown to have a vasorelaxant effect on resistance blood vessels. This effect was mediated by nitric oxide (NO). Chrysin inhibited aromatase/estrogen biosynthesis in postmenopausal women. The purpose of this study was to determine if chrysin had an effect on cholecystokinin- or KCl-induced tension in male guinea pig gallbladder strips. In addition, the second messenger(s) system(s) that mediated the effect were to be determined. A pharmacologic approach was used. Male guinea pig gallbladder strips were placed in in vitro chambers filled with Krebs solution, maintained at 37°C, and gassed with 95% O2–5% CO2. Changes in tension were recorded using a polygraph.It was shown that the PKA/cAMP second messenger system mediated part of the observed chrysin-induced relaxation of cholecystokinin-induced tension, the PKC system also mediated part of the relaxation, and the inhibition of both extracellular Ca2+ entry and intracellular Ca2+ release also mediated the chrysin-induced relaxation. This is the first report of chrysin having an effect on gallbladder smooth muscle contraction.

Emodin Inhibits Voltage‐Dependent Potassium Current in Guinea Pig Gallbladder Smooth Muscle

Emodin is known to prompt bile secretion in gallbladder and to be used in the treatment of cholesterol stones. We studied the effects of emodin on the contraction of gallbladder smooth muscle and voltage-dependent K(+) current in gallbladder smooth muscle cells. Gallbladder muscle strips were obtained from adult guinea pigs and the resting tension was recorded. Gallbladder smooth muscle cells were isolated by enzymatic digestion, and K(+) current was recorded by the whole-cell patch clamp method. Emodin increased the resting tension of gallbladder smooth muscle strips and inhibited voltage-dependent K(+) current in a dose-dependent manner. When 10 microM emodin was applied to gallbladder smooth muscle cells for 3-6 min., the amplitude of voltage-dependent K(+) current was decreased by 31.5 +/- 0.5% at +40 mV, and this inhibitory effect mostly recovered after washout. The steady-state inactivation curves were shifted in a hyperpolarizing direction by emodin. In the presence of the protein kinase C inhibitors staurosporine and chelerythrine, the effect of emodin on voltage-dependent K(+ )current was significantly attenuated. In conclusion, emodin promotes gallbladder contraction, mainly by inhibiting voltage-dependent K(+) current via the protein kinase C pathway. These findings provide theoretical foundation for the application of emodin in gallbladder motility disorders.

Emodin increases Ca 2+ influx through L-type Ca 2+ channel in guinea pig gallbladder smooth muscle

Emodin is known to be used in the treatment of cholesterol stones and cholecystitis. This study sought to investigate the effects of emodin on the contraction of gallbladder smooth muscle (GBSM), intracellular Ca 2+ concentration and L-type calcium current in GBSM cells. Gallbladder muscle strips were obtained from adult guinea pigs and the resting tension was recorded. Gallbladder smooth muscle cells were isolated by enzymatic digestion. Cells were loaded with fluo-3/AM and [Ca 2+] i was determined by a laser confocal microscope. Calcium current was recorded by the whole-cell patch clamp method. Emodin increased the resting tension of GBSM strips in a dose-dependent manner. Emodin elevated [Ca 2+] i in GBSM cells, and this effect was attenuated by pretreatment with nifedipine. In addition, Emodin increased L-type calcium current at concentrations of 1 to 30 µM (at + 10 mV, 10 µM, 45.1 ± 5.2% compared to control, EC 50 = 3.11 µM). In the presence of protein kinase C (PKC) inhibitor, Staurosporine, emodin did not significantly affect the calcium current. However, phorbol 12, 13-dibutyrate mimicked emodin in enhancement of the calcium current. These results suggest that emodin promotes gallbladder contraction by increasing Ca 2+ influx through L-type calcium channel via PKC pathway.

Role of Rho-kinase in guinea-pig gallbladder smooth muscle contraction

Guinea-pig gallbladder smooth muscle contractions can be elicited pharmacologically by a range of mechanisms. The involvement of Rho-kinase in contractions mediated by receptor-dependent and receptor-independent mechanisms was investigated using the Rho-kinase inhibitor (+)-( R)- trans-4-(1-aminoethyl)- N-(4-pyridyl) cyclohexane carboxamide (Y-27632). In a separate series of experiments, the role of Rho-kinase in the contractile response to Ca 2+ entry through store-operated Ca 2+ channels and to electrical field stimulation was also examined. Y-27632 (10 μM), which caused a significant decrease ( P < 0.0005) in basal resting tone, significantly inhibited gallbladder contractions evoked by cumulative additions of the G-protein-coupled agonists, carbachol (1 nM–100 μM; P < 0.05) and cholecystokinin (10 nM–1 μM; P < 0.005). Y-27632 also inhibited the contractions evoked by a single addition of the sarcoplasmic reticulum ATPase inhibitor, thapsigargin (1 μM; P < 0.0005) and cumulative additions of KCl (10–85 mM; P < 0.0005). The contractile response to Ca 2+ entry through store-operated Ca 2+ channels was significantly inhibited by Y-27632 ( P < 0.05) as were the contractile responses evoked by electrical field stimulation (2–25 Hz; P < 0.0005). In contrast, Y-27632 had no significant effect on contractions evoked by phorbol 12,13-dibutyrate (0.1 nM–1 μM; a protein kinase C activator) or by the phosphatase inhibitor, cantharidin (100 μM). In conclusion, Rho-kinase contributes to the contractile response in guinea-pig gallbladder smooth muscle evoked by both G-protein-coupled and non-G-protein-coupled mechanisms in addition to contributing to the maintenance of basal tone. It also contributes to the contractile responses resulting from electrical field stimulation and store-operated Ca 2+ channel entry.

Capacitative calcium entry in guinea pig gallbladder smooth muscle in vitro

This study investigates the involvement of capacitative Ca 2+ entry in excitation-contraction coupling in guinea pig gallbladder smooth muscle. Thapsigargin (0.1 nM–1 μM, a sarcoplasmic reticulum Ca 2+-ATPase inhibitor) produced slowly developing sustained tonic contractions in guinea pig isolated gallbladder strips. All contractions approached 50% of the response to carbachol (10 μM) after 55 min. Contractile responses to thapsigargin (1 μM) were abolished in a Ca 2+-free medium. Subsequent re-addition of Ca 2+ (2.5 mM) produced a sustained tonic contraction (99 ± 6% of the carbachol response). The contractile response to Ca 2+ re-addition following incubation of tissues in a Ca 2+-free bathing solution in the absence of thapsigargin was significantly less than in its presence (79 ± 4 % vs 100 ± 7 % of carbachol; p < 0.05). Contractile responses to Ca 2+ re-addition following treatment with thapsigargin were attenuated by (a) the L-type voltage-operated Ca 2+ channel antagonist, nifedipine (10 μM) and (b) the general inhibitor of Ca 2+ entry channels including store-operated channels, SK&F96365 (50 μM and 100 μM). In separate experiments, responses to Ca 2+ re-addition were essentially abolished by the tyrosine kinase inhibitor, genistein (100 μM). These results suggest that capacitative Ca 2+ entry provides a source of activator Ca 2+ for guinea pig gallbladder smooth muscle contraction. Contractile responses to Ca 2+ re-addition following depletion of sarcoplasmic reticulum Ca 2+ stores with thapsigargin, are mediated in part by Ca 2+ entry through voltage-operated Ca 2+ channels and by capacitative Ca 2+ entry through store-operated Ca 2+ channels which can be blocked by SK&F96365. Furthermore, capacitative Ca 2+ entry in this tissue may be modulated by tyrosine kinase.

Myosin light chain phosphorylation correlates with contractile force in guinea pig gallbladder muscle.

Acetylcholine (ACh)-induced gallbladder smooth muscle contraction involves myosin light chain phosphorylation (MLCP). ACh-induced contraction is dose dependent. Whether MLCP by ACh is also dose dependent and how it correlates with contractile force have not been carefully evaluated. This study investigated the correlation between gallbladder muscle contraction and MLCP. Guinea pig gallbladder muscle strips were studied isometrically and frozen after different doses of ACh (0, 0.1, 5, 100 microM) for different periods of incubation (0.5, 1, 2, 3, 4 min). MLCP was determined using gel electrophoresis. Both contraction and MLCP in response to ACh were concentration dependent. Peak MLCP to ACh 100 microM occurred at 30 sec. There was a high correlation between active force and MLCP (r = 0.991; P = 0.009 at 30 sec stimulation). Nifedipine 1 microM reduced ACh-induced contraction and MLCP by a similar degree (31% and 33%, respectively). In conclusion, gallbladder contractile force significantly correlates with MLCP. This is consistent with the hypothesis that initiation of gallbladder cholinergic contraction is dependent on phosphorylation of myosin light chains.

Actions of histamine on muscle and ganglia of the guinea pig gallbladder

Histamine is an inflammatory mediator present in mast cells, which are abundant in the wall of the gallbladder. We examined the electrical properties of gallbladder smooth muscle and nerve associated with histamine-induced changes in gallbladder tone. Recordings were made from gallbladder smooth muscle and neurons, and responses to histamine and receptor subtype-specific compounds were tested. Histamine application to intact smooth muscle produced a concentration-dependent membrane depolarization and increased excitability. In the presence of the H(2) antagonist ranitidine, the response to histamine was potentiated. Activation of H(2) receptors caused membrane hyperpolarization and elimination of spontaneous action potentials. The H(2) response was attenuated by the ATP-sensitive K(+) (K(ATP)) channel blocker glibenclamide in intact and isolated smooth muscle. Histamine had no effect on the resting membrane potential or excitability of gallbladder neurons. Furthermore, neither histamine nor the H(3) agonist R-alpha-methylhistamine altered the amplitude of the fast excitatory postsynaptic potential in gallbladder ganglia. The mast cell degranulator compound 48/80 caused a smooth muscle depolarization that was inhibited by the H(1) antagonist mepyramine, indicating that histamine released from mast cells can activate gallbladder smooth muscle. In conclusion, histamine released from mast cells can act on gallbladder smooth muscle, but not in ganglia. The depolarization and associated contraction of gallbladder smooth muscle represent the net effect of activation of both H(1) (excitatory) and H(2) (inhibitory) receptors, with the H(2) receptor-mediated response involving the activation of K(ATP) channels.

Review article: control of gall-bladder motor function.

Muscular contraction of the gall-bladder is the primary determinant of bile delivery into the duodenum. Where bile goes following its secretion by the liver depends upon a co-ordinated series of pressure interrelations between the hepatic secretory pressure at the entrance to the biliary system, a low pressure conduit, and the pressure differences between the gall-bladder, cystic duct and sphincter of Oddi. During fasting, the relatively higher tone in the sphincter of Oddi fosters the entry of bile into the gall-bladder. The gall-bladder accommodates this influx without an increase in intravesicular pressure through its compliance or distensibility, which consists of active muscle relaxation and passive fibroelastic components. The concentrating function of the gall-bladder keeps the volume small. Once about every 120 min during the interdigestive period, gall-bladder emptying occurs coincident with intense duodenal contractions; all part of the migratory myoelectric complex. This helps maintain the enterohepatic circulation of bile salts. Motilin, which mediates these events during fasting, acts by stimulating intrinsic cholinergic nerves. Cholecystokinin is the major determinant of gallbladder emptying with eating. Cholecystokinin acts through pre-ganglionic cholinergic nerves, to initiate gall-bladder contraction. Agonists like cholecystokinin and acetylcholine cause contraction of gall-bladder smooth muscle through signal transduction, which increases intracellular calcium levels and so initiates the contractile machinery. Cholecystokinin also acts on the sphincter of Oddi via pre-ganglionic cholinergic nerves to release vasoactive intestinal polypeptide and nitric oxide, and so lower tone. These events are co-ordinated with motility and secretory events in the upper gastrointestinal tract, delivering bile at appropriate times into the duodenum.

Activation of tyrosine kinase pathway by vanadate in gallbladder smooth muscle∗

Vanadate, an inhibitor of tyrosine phosphatase activity, might induce gallbladder contraction through the stimulation of the tyrosine kinase pathway. The aim of this study was to characterize the effects of vanadate in the guinea pig gallbladder smooth muscle. Vanadate exerts contractile effects which are not mediated by neurotransmitter release. The tyrosine kinase inhibitor genistein nearly abolished vanadate contraction, suggesting that an increase in protein tyrosine phosphorylation mediates the actions of vanadate. This suggestion was confirmed by Western blot analysis. Vanadate contractions were reduced in the presence of methoxyverapamil or in Ca 2+-free medium, suggesting that vanadate may induce Ca 2+ influx. Neither inactivation of the Na +/K + pump nor reversal of the Na +/Ca 2+ exchanger can account for vanadate’s actions. Vanadate contractile effects were reduced by indomethacin, as well as mepacrine, the inhibitor of phospholipase A 2, but were not affected by phospholipase C inhibitors. Neither inhibitors of diacylglycerol lipase nor protein kinase C reduced the response induced by vanadate. These data indicate that the effects of vanadate on smooth muscle are mainly mediated by protein tyrosine phosphorylation and reveal a new link between tyrosine phosphorylation and arachidonic acid metabolism in the control of gallbladder smooth muscle contraction.

Calcium sources mediating human gallbladder smooth muscle contraction by acetylcholine and cholecystokinin

Calcium sources mediating human gallbladder smooth muscle contraction by acetylcholine and cholecystokinin

1,4,5-inositol trisphosphate mediates carbachol-induced contraction of gallbladder smooth muscle: T. von Schrenck, U. Matsui, A. de Weerth, P. Lossin, S. Schmidt, B. Mackensen∗, W. Schmitz∗∗, H. Greten. Dep. of Medicine, University Hospital Eppendorf, Hamburg, Dep. Medicine∗ and Dep. of Pharmacology and Toxicology∗∗, Münters, Germany

1,4,5-inositol trisphosphate mediates carbachol-induced contraction of gallbladder smooth muscle: T. von Schrenck, U. Matsui, A. de Weerth, P. Lossin, S. Schmidt, B. Mackensen∗, W. Schmitz∗∗, H. Greten. Dep. of Medicine, University Hospital Eppendorf, Hamburg, Dep. Medicine∗ and Dep. of Pharmacology and Toxicology∗∗, Münters, Germany

Role of myosin light-chain phosphorylation in guinea pig gallbladder smooth muscle contraction.

In acetylcholine (ACh)-stimulated gallbladder smooth muscle, we have previously shown that phosphorylation of the 20,000-Da myosin light chains is necessary for the initiation of contraction, that myosin is stably phosphorylated at steady state, and that dephosphorylation of cross bridges is not necessary for the slowing of cross-bridge cycling rates during the period of steady-state isometric stress. The present studies were undertaken to determine whether 1) K+ (60 or 80 mM) or cholecystokinin (CCK, 10(-8) M) stimulation is accompanied by changes in myosin light-chain phosphorylation in gallbladder smooth muscle and 2) dephosphorylated noncycling cross bridges exist in K(+)- or CCK-stimulated gallbladder smooth muscle. Isometric stress, isotonic shortening velocity, and myosin light-chain phosphorylation were determined during contraction with K+ or CCK. Steady-state isometric stress was reached within 2.5 min of stimulation with K+ or CCK and was maintained for the duration of the stimulation. Stimulation with K+ or CCK was associated with rapid increases in myosin light-chain phosphorylation and maintenance of myosin light-chain phosphorylation during the stimulation. In contrast, isotonic shortening velocity was maximal at 1 min of stimulation with either K+ or CCK and then declined significantly to values that were only 26-32% of the peak velocity. These data, along with data from previous experiments with ACh, suggest that myosin light-chain phosphorylation is essential in the initiation of contraction in gallbladder smooth muscle, regardless of the source of Ca2+ or of the contractile agonist.(ABSTRACT TRUNCATED AT 250 WORDS)

The source of calcium for CCK-induced contraction of the guinea-pig gall bladder

The sources of calcium for cholecystokinin octapeptide (CCK-OP)-induced gallbladder smooth muscle contraction are considered both extracellular and intracellular, but the relative need for intracellular calcium especially at low, physiological concentrations is not clear. To better define the calcium sources responsible for guinea-pig gallbladder contractions in vitro, we inhibited calcium influx using the calcium channel blocker, methoxyverapamil, and a calcium-free Krebs' solution. Availability and release of intracellular calcium stores were depleted by strontium substitution and ryanodine. CCK-OP was compared to bethanechol and potassium chloride (KCl). Preventing calcium influx with 10 −5 M methoxyverapamil depressed the responses to CCK-OP, bethanechol and KCl. Methoxyverapamil, however, had little effect on the time-dependent generation of tension to CCK-OP, but significantly reduced the response to bethanechol and KCl, each at ED 50. The duration of the contractile response in the calcium-free Krebs' solution to CCK-OP was longer than that for bethanechol. Strontium (2.5 mM) significantly attenuated the response to CCK-OP and bethanechol, but not to KCl. Ryanodine significantly reduced contractions induced by CCK-OP but not for bethanechol, both at low dose ED 25. These results indicate that contraction of the guinea-pig gallbladder induced by CCK-OP, bethanechol and KCl requires extracellular calcium influx. Further, the initiation and maintenance of contraction by CCK-OP and bethanechol necessitates calcium mobilisation from intracellular stores. CCK-OP may have a greater penchant for these calcium stores, particularly at physiological doses.

Monochloramine induces contraction of guinea pig gallbladder via two different pathways.

The purpose of the present investigation was to examine the effect and mechanism of action of the reactive oxygen metabolites monochloramine (NH2Cl), hypochlorous acid (HOCl), and hydrogen peroxide (H2O2) on gallbladder smooth muscle contractility. All oxidants caused concentration-dependent increases in resting tension of gallbladder muscle; the rank order of potencies (half-maximal concentration) was NH2Cl (30 microM) greater than HOCl (70 microM) greater than H2O2 (100 microM). The oxidant concentrations employed are those found to exist in inflamed tissue. Tetrodotoxin (0.5 microM) had no effect on gallbladder muscle contraction caused by the oxidants, suggesting a direct, nonneural action. The maximal response induced by NH2Cl, HOCl, or H2O2 was significantly (P less than 0.05) inhibited by 5 microM indomethacin and 5 microM piroxicam. The calcium channel blocker verapamil partially inhibited the contractile effect of NH2Cl but had no effect on the contraction induced by exogenous cyclooxygenase products. Monochloramine induced significant prostaglandin E2 release from the gallbladder, which was blocked by indomethacin. Furthermore, the effect of NH2Cl on the smooth muscle was blocked by 5-aminosalicylic acid (1 mM). We conclude that reactive oxygen metabolites induce contraction of gallbladder smooth muscle by a direct action. The effect is mediated via cyclooxygenase metabolites and activation of calcium influx.

Contribution of intracellular calcium to gallbladder smooth muscle contraction

Studies were performed to evaluate the contribution of intracellular Ca2+ to gallbladder smooth muscle contraction under acetylcholine (ACh) or potassium stimulation. Gallbladder smooth muscle strips from adult guinea pigs were placed in tissue baths containing N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)-buffered physiological salt solution (PSS) and set to optimal length for contraction (Lo). The results were as follows, 1) A 20-min equilibration in zero Ca2(+)-0.1 mM ethylene glycol-bis( beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) PSS virtually abolished the response to potassium but not to ACh. 2) Substitution of strontium, an inhibitor of intracellular Ca2+ release, for Ca2+ significantly decreased the contractile response to ACh (3 X 10(-5), 10(-4), and 3 X 10(-4) M). Strontium had no effect on the response to 40 and 80 mM potassium. 3) Intracellular Ca2+ depletion significantly decreased gallbladder smooth muscle contraction to ACh (10(-4) M) but had no effect on the response to potassium (80 mM). 4) Ryanodine, a compound that inhibits Ca2+ storage by the sarcoplasmic reticulum, significantly decreased the contractile response to ACh (10(-4) M) but not to potassium (80 mM). These data support the observation that the use of intracellular Ca2+ by gallbladder smooth muscle for contraction is agonist dependent.

Calcium and gallbladder smooth muscle contraction in the guinea pig: Effect of pregnancy

Experiments were designed to evaluate the relative contributions of extracellular and intracellular calcium to the contractile process in gallbladder smooth muscle. Tissue strips from nonpregnant and pregnant guinea pigs were stimulated in vitro with maximally effective concentrations of potassium (80 mM), acetylcholine (10−4 M), and cholecystokinin-octapeptide (80 nM), both in the presence and absence of extracellular calcium. The results were as follows, (a) Potassium-induced contractions were completely abolished in the absence of extracellular calcium and in the presence of verapamil (10 μM). Tissues from nonpregnant and pregnant animals were affected equally, (b) Removal of extracellular calcium or the addition of verapamil reduced but did not abolish the acetylcholine and cholecystokinin-octapeptide responses. Acetylcholine-induced contractions were more sensitive to the elimination of extracellular calcium, (c) For both acetylcholine and cholecystokinin-octapeptide, the contractile responses of tissues from pregnant animals were reduced when compared with control responses. These data suggest that contractions induced by acetylcholine and cholecystokinin-octapeptide utilize calcium from both extracellular and intracellular stores, and that pregnancy affects the response to these agonists by reducing the contribution of the intracellular calcium pool.