Characterization of muscarinic receptors on guinea pig gallbladder smooth muscle

Muscarinic receptor M3 mediates human gallbladder contraction through voltage-gated Ca2+ channels and Rho kinase

OBJECTIVE

Muscarinic receptors mediate contraction of the human gallbladder through unclear receptor subtypes. The aim of the present study was to characterize muscarinic acetylcholine receptors mediating contraction of the human gallbladder.

MATERIALS AND METHODS

Contraction of human gallbladder muscle strips caused by agonists carbachol and muscarine was measured and the inhibition of carbachol-induced contraction by muscarinic receptor antagonists was evaluated. Reverse transcription polymerase chain reaction was performed to determine the existence of muscarinic receptor subtypes.

RESULTS

Carbachol and muscarine caused concentration-dependent contraction of gallbladder strips. Four receptor antagonists, including atropine, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), methoctramine, and pirenzepine, inhibited the carbachol-induced contraction. The relative inhibitory potency of these receptor antagonists was atropine > 4-DAMP > methoctramine > pirenzepine. The antagonist affinity estimates (pA(2) values) correlated with the known affinities at M(3), M(4), and M(5) muscarinic receptors. In addition, the M(4)-selective antagonist muscarinic toxin 3 did not inhibit and the M(5)-selective positive allosteric modulator VU0238429 did not potentiate carbachol-induced gallbladder contraction. This suggests that M(3) muscarinic receptors mediate the muscarinic response predominantly. The contractile response of carbachol was attenuated by the voltage-gated Ca(2+) channel inhibitor nifedipine and Rho-kinase inhibitor H-1152, but not affected by protein kinase C inhibitor chelerythrine. This implies the involvement of voltage-gated Ca(2+) channel and Rho kinase but not protein kinase C.

CONCLUSIONS

These results suggest a major role of M(3) muscarinic receptors mediating the human gallbladder contraction through voltage-gated Ca(2+) channels and Rho kinase. M(3)-selective muscarinic receptor antagonists could be of therapeutic importance in the treatment of biliary motility disorders.

Effect of Fructus Psoraleae on motility of gallbladder isolated smooth muscle strips from guinea pigs

AIM: To observe the effect of Fructus Psoraleae on motility of isolated gallbladder muscle strips of guinea pigs and its mechanism.

METHODS: Guinea pigs were hit to lose consciousness and the whole gallbladder was removed quickly. Two or three smooth muscle strips (8 mm × 3 mm) were cut along a longitudinal direction. The mucosa was gently removed. Every longitudinal muscle strip was suspended in a tissue chamber which was continuously perfused with 5 mL Krebs solution (37°C), pH 7.4, and aerated with 950 mL/L O₂ and 50 mL /L CO₂. The isometric response was recorded with an ink-writing recorder. After 2 h equilibration under 1 g-load, 50 μL Fructus Psoraleae (10, 20, 70, 200, 700, 1000 g/L) was added cumulatively into the tissue chamber in turn every 2 min to observe their effects on gallbladder muscle strips (cumulating final concentration of Fructus Psoraleae was 0.1, 0.3, 1.0, 3.0, 10.0, 20.0 g/L). The antagonists, including 4-DAMP, benzhydramine, hexamethonium, phentolamine, verapamil and idomethine were given 2 min before Fructus Psoraleae respectively to investigate the mechanisms involved.

RESULTS: Fructus Psoraleae dose-dependently increased the resting tension (r = 0.992, P < 0.001), decreased the mean contractile amplitude (r = 0.970, P < 0.001) and meanwhile increased the contractile frequency of the gallbladder muscle strip in vitro (r = 0.965, P < 0.001). The exciting action of Fructus Psoraleae on the resting tension could be partially blocked by 4-DAMP (the resting tension decreased from 1.37 ± 0.41 to 0.70 ± 0.35, P < 0.001), benzhydramine (from 1.37 ± 0.41 to 0.45 ± 0.38, P < 0.001), hexamethonium (from 1.37 ± 0.41 to 0.94 ± 0.23, P < 0.05), phentolamine ( from 1.37 ± 0.41 to 0.89 ± 0.22, P < 0.01) and verapamil (from 1.37 ± 0.41 to 0.94 ± 0.26, P < 0.05). But the above antagonists had no significant effect on the action of Fructus Psoraleae–induced mean contractile amplitude (P > 0.05). Moreover, the increase of the contractile frequency due to Fructus Psoraleae was inhibited by 4-DAMP (decreased from 8.3 ± 1.2 to 6.8 ± 0.5, P < 0.01) and hexamethonium (from 8.3 ± 1.2 to 7.0 ± 0.9, P < 0.05). Idomethine had no significant effect on the Fructus Psoraleae-induced responses (P > 0.05).

CONCLUSION: Fructus Psoraleae enhances the motility of isolated gallbladder muscle strips from guinea pigs, in a dose-dependent manner. The effect of Fructus Psoraleae is partly related to M₃, N receptor, α receptor, H₁ receptor, Ca²⁺ channel, but not related to prostaglandin.

Rho kinase expression and its central role in ovine gallbladder contractions elicited by a variety of excitatory stimuli

Rho kinase has contractile activity, which induces Ca2+ sensitization in various cells. Several receptors are linked to the Rho/Rho-kinase pathway. Therefore, in this study we aimed to demonstrate the central importance of this novel pathway for diverse excitatory stimuli in the smooth muscle of the sheep gallbladder. Accordingly, the effects of a Rho kinase inhibitor, (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexanecarboxamide dihydrochloride monohydrate (Y-27632, 10(-8)-3 x 10(-5) M), were investigated on cholecystokinin-8 (CCK-8, 10(-8) M), endothelin-1 (10(-8) M), carbachol (10(-6)-10(-5) M), 5-hydroxytryptamine (5-HT, 10(-6)-10(-5) M), histamine (10(-6)-10(-5) M), phenylephrine (10(-5)-10(-4) M), neurokinin A (10(-7)-10(-6) M), electrical field stimulation (40 V, 0.5 ms, 2, 4, 8, 16, 32 Hz, 15 s, 3 min intervals) and potassium chloride (KCl, 25-50 mM)-induced contractions as well as spontaneous contractile activity. Electrical field stimulation evoked tetrodotoxin (3 x 10(-6) M)-sensitive reproducible contractions, which were inhibited by atropine (2 x 10(-6) M) and potentiated by eserine (5 x 10(-7) M). EFS-induced contraction was significantly inhibited by Y-27632 (10(-5) M). In addition, spontaneous contractile activity was suppressed in the presence of the compound (10(-6)-10(-5) M). This Rho kinase inhibitor also dramatically decreased the contractions elicited by 5-HT, neurokinin A and carbachol. KCl-induced contraction, which was not atropine-sensitive, was also conspicuously attenuated by Y-27632. Moreover, Y-27632 (10(-8)-3 x 10(-5) M) relaxed gallbladder strips that were contracted by histamine, endothelin-1, CCK-8 and phenylephrine in a concentration-dependent manner. pEC50 values for Y-27632 were 6.25+/-0.10, 5.79+/-0.12, 5.83+/-0.09 and 5.70+/-0.13 for the contraction elicited by histamine, CCK-8, endothelin-1 and phenylephrine, respectively. Furthermore, we also demonstrated Rho kinase protein expression (ROCK-1 and ROCK-2) by Western blot analysis. In conclusion, ROCK is expressed in the smooth muscle of the ovine gallbladder, and it has a central role in the contractile activity induced by diverse excitatory stimuli.

Methoctramine and gallamine inhibit PI hydrolysis in guinea-pig gallbladder

The present study aimed to determine the effect of two M2/M4-selective muscarinic receptor antagonists on blocking the hydrolysis of carbachol (CCh) stimulated phospho-inositide (PI) breakdown in order to address the possibility that a muscarinic receptor other than the M(3) receptor is involved in PI hydrolysis in this tissue. Gallbladder tissue slices labeled with myo-[2-3H] inositol were incubated with increasing concentrations of antagonists and agonist. After the reactions were terminated by the addition of chloroform/methanol, labeled inositol phosphates were separated using anion exchange chromatography. Muscarinic M2 antagonists methoctramine and gallamine both inhibited carbachol-induced PI breakdown at high concentrations, with log IC50 values of -5.145 and -6.049, respectively. Gallamine at 10(-5)M concentration failed to displace the dose-response curve for carbachol-induced accumulation of inositol triphosphate (IP3). Our data suggest that M(3) receptors play a major role in stimulation of PI hydrolysis in the guinea-pig gallbladder.

Identification of a spontaneously active, Na+-permeable channel in guinea pig gallbladder smooth muscle

The action potential in gallbladder smooth muscle (GBSM) is caused by Ca2+ entry through voltage-dependent Ca2+ channels (VDCC), which contributes to the GBSM contractions. Action potential generation in GBSM is critically dependent on the resting membrane potential (about -50 mV), which is approximately 35 mV more positive of the K+ equilibrium potential. We hypothesized that a tonic, depolarizing conductance is present in GBSM and contributes to the regulation of the resting membrane potential and action potential frequency. GBSM cells were isolated from guinea pig gallbladders, and the whole cell patch-camp technique was used to record membrane currents. After eliminating the contribution of VDCC and K+ channels, we identified a novel spontaneously active cation conductance (I(cat)) in GBSM. This I(cat) was mediated predominantly by influx of Na+. Na+ substitution with N-methyl-D-glucamine (NMDG), a large relatively impermeant cation, caused a negative shift in the reversal potential of the ramp current and reduced the amplitude of the inward current at -50 mV by 65%. Membrane potential recordings with intracellular microelectrodes or in current-clamp mode of the patch-clamp technique indicated that the inhibition of I(cat) conductance by NMDG is associated with membrane hyperpolarization and inhibition of action potentials. Extracellular Ca2+, Mg2+, and Gd3+ attenuated the I(cat) in GBSM. Muscarinic stimulation did not activate the I(cat). Our results indicate that, in GBSM, an Na+-permeable channel contributes to the maintenance of the resting membrane potential and action potential generation and therefore plays a critical role in the regulation of GBSM excitability and contractility.

Effect of rhubarb on contractile response of gallbladder smooth muscle strips isolated from guinea pigs

AIM: To investigate the effect of rhubarb on contractile response of isolated gallbladder muscle strips from guinea pigs and its mechanism.

METHODS: Guinea pigs were killed to remove the whole gallbladder. Two or three smooth muscle strips (8 mm×3 mm) were cut along the longitudinal direction. The mucosa on each strip was carefully removed. Each longitudinal muscle strip was suspended in a tissue chamber containing 5 mL Krebs solution (37 °C), bubbled continuously with 950 mL/L O₂ and 50 mL/L CO₂. The resting tension (g), mean contractile amplitude (mm), and contractile frequency (waves/min) were simultaneously recorded on recorders. After 2-h equilibration, rhubarb (10, 20, 70, 200, 700, 1000 g/L) was added cumulatively to the tissue chamber in turns every 2 min to observe their effects on gallbladder. Antagonists were given 3 min before administration of rhubarb to investigate the possible mechanism.

RESULTS: Rhubarb increased the resting tension (from 0 to 0.40±0.02, P<0.001), and decreased the mean contractile amplitude (from 5.22±0.71 to 2.73±0.41, P<0.001). It also increased the contractile frequency of the gallbladder muscle strips in guinea pigs (from 4.09±0.46 to 6.08±0.35, P<0.001). The stimulation of rhubarb on the resting tension decreased from 3.98±0.22 to 1.58±0.12 by atropine (P<0.001), from3.98±0.22 to 2.09±0.19 by verapamil (P<0.001) and from 3.98±0.22 to 2.67±0.43 by phentolamine (P<0.005). But the effect was not inhibited by hexamethonium (P>0.05). In addition, the action of mean amplitude and frequency was not inhibited by the above antagonists.

CONCLUSION: Rhubarb can stimulate the motility of isolated gallbladder muscle strips from guinea pigs. The stimulation of rhubarb might be relevant with M receptor, Ca²⁺ channel and α receptor partly.

Characteristics and cholinergic control of the 'minute rhythm' in ovine antrum, small bowel and gallbladder

Eight adult conscious rams were used to characterize further the minute rhythm and to determine the role of cholinergic receptors in nervous control of this event. In chronic experiments, the myoelectrical and motor activity of the gastrointestinal tract and gallbladder were recorded. Physiological experiments were performed in fasted or non-fasted rams before, during and after feeding, and the occurrence of minute rhythm during various phases of the migrating motor myoelectric complex was observed. The pattern occurred most frequently in the small intestine, where it exhibited mostly the propagating character. It was also detectable in the ileum. In the gallbladder, the minute rhythm arrived systematically and its character was irregular, propagating, retropropagating or stationary. In all episodes observed, it was well correlated with that in the small intestine. In the pyloric antrum, the minute rhythm was identified occasionally. During pharmacological experiments, 0.15 M NaCl or graded doses of hexamethonium, atropine and pirenzepine were administered intravenously during various phases of the migrating motor myoelectric complex, in fasted and non-fasted animals, before and during feeding. The drugs inhibited the minute rhythm in the small bowel for a longer period than in the gallbladder. However, the smallest dose of pirenzepine (0.02 mg/kg) exerted a non-significant effect both in the small intestine and in the gallbladder. It is concluded, that in normal conditions the minute rhythm occurs regularly in the entire small intestine and in the gallbladder. In the small intestine the pattern is organized more precisely. The minute rhythm is controlled by nicotinic receptors and by muscarinic receptor subtypes.

Muscarinic receptor knockout mice: role of muscarinic acetylcholine receptors M(2), M(3), and M(4) in carbamylcholine-induced gallbladder contractility

Muscarinic receptors play a major role in gallbladder function, although the muscarinic receptor(s) mediating smooth muscle contractility is unclear. This study compared smooth muscle contractile responses to carbamylcholine (10(-7)-10(-3) M) in isolated gallbladder from wild-type and M(2), M(3), and M(4) receptor knockout mice. Carbamylcholine-induced contraction in gallbladder was associated with tachyphylaxis and the release of a cyclooxygenase product because indomethacin (10(-6) M) inhibited carbamylcholine-induced contraction. The M(3) receptor was the major muscarinic receptor involved in contraction because carbamylcholine-induced contractility was inhibited in gallbladder from M(3) receptor knockout mice. Furthermore, the muscarinic receptor antagonists 11-[[[2-diethylamino-O-methyl]-1-piperidinyl]acetyl]-5,11-dihydrol-6H-pyridol[2,3-b][1,4]benzodiazepine-6-one (AF-DX 116) and pirenzepine dextrally shifted contraction to carbamylcholine in gallbladder from wild-type, M(2), and M(4) receptor knockout mice, with affinities consistent with M(3) receptor interaction. In addition, maximal contraction to carbamylcholine was reduced in gallbladder from M(2) receptor knockout mice and affinities for AF-DX 116 and pirenzepine in gallbladder from M(3) receptor knockout mice were consistent with their affinities at M(2) receptors. In M(4) receptor knockout mice, contraction to carbamylcholine was dextrally shifted, although the affinities for AF-DX 116 and pirenzepine in gallbladder from M(2) or M(3) knockout mice were not similar to their affinities at M(4) receptors. The M(4) receptor may serve as an accessory protein necessary for optimal potency of M(2) and M(3) receptor-mediated responses. Thus, muscarinic receptor knockout mice provided direct and unambiguous evidence that M(3), and to a lesser extent, M(2) receptors are the predominant muscarinic receptors mediating gallbladder contractility, and M(4) receptors appear necessary for optimal potency of carbamylcholine in gallbladder contraction.

Further evidence for the heterogeneity of functional muscarinic receptors in guinea pig gallbladder

Previous studies have suggested the presence of multiple muscarinic receptor subtypes in guinea pig gallbladder smooth muscle, although the relative abundance and functional role of these subtypes remains an area of significant research efforts. The present study utilized both radioligand kinetic and functional experiments to further probe the nature of the muscarinic receptors in gallbladder smooth muscle and their mode of coupling to intra- and extra-cellular Ca(2+) sources. Dissociation kinetic studies using [3H]N-methylscopolamine ([3H]NMS) indicated that the binding profile in guinea pig gallbladder smooth muscle could not be reconciled with that expected for a single muscarinic receptor subtype, the latter determined in parallel experiments conducted on the cloned muscarinic M(1)-M(5) subtypes in Chinese hamster ovary (CHO) cells. Furthermore, comparison of the gallbladder data with the dissociation characteristics of [3H]NMS in guinea pig urinary bladder revealed a significantly different kinetic profile, with the urinary bladder, but not the gallbladder, demonstrating biphasic radioligand dissociation kinetics. In functional experiments, carbachol caused a concentration-dependent contraction of guinea pig gallbladder smooth muscle strips in Ca(2+)-free or 5 mM Sr(2+)-substituted physiological salt solutions (PSS) with amplitudes of the maximal contractions corresponding to 45.8+/-8.0% and 33.2+/-6.6% of control responses in normal PSS, respectively. Furthermore, the stimulus-response characteristics of carbachol-mediated contraction appeared significantly altered in Ca(2+)-free PSS relative to normal or Sr(2+)-substituted PSS. The antagonist, methoctramine (1x10(-7)-3x10(-5) M), exerted only a slight inhibition of carbachol (10(-5) M)-induced contractions in 5 mM Sr(2+)-substituted medium, whereas it was significantly more potent in antagonizing gallbladder contractions in response to 10(-5) M carbachol in the absence of extracellular Ca(2+). Both atropine and tripitramine were equipotent in antagonizing carbachol-induced contractions in Ca(2+)-free (pIC(50): 6.85+/-0.11 for atropine and 5.75+/-0.32 for tripitramine) and Sr(2+)-substituted media (pIC(50): 6.88+/-0.25 for atropine and 5.70+/-0.16 for tripitramine), and pirenzepine was only slightly more potent in Ca(2+)-free PSS (pIC(50): 5.66+/-0.23) than in Sr(2+)-substituted PSS (pIC(50): 5.33+/-0.21). Taken together, our data indicate that carbachol contracts guinea pig gallbladder by stimulating two distinct muscarinic receptor subtypes linked to extracellular Ca(2+) influx and intracellular Ca(2+) release. These two subtypes may represent the muscarinic M(3) and M(4) receptors, although the presence of the muscarinic M(2) receptor subtype is also suggested from the binding data.

Muscarinic M(2) receptors are not primarily involved in the contraction of guinea-pig gallbladder smooth muscle

The presence of M(1)-M(4) receptors in guinea-pig gallbladder smooth muscle cells has been reported recently. The majority of these receptors are said to be of M(2) subtype. However, there are controversial reports about the functional muscarinic receptors that mediate contraction in this tissue. Similar to gallbladder, it was claimed that M(4) receptors mediate guinea-pig uterine contractions, but these receptors have appeared to be of M(2) subtypes later. Therefore, the antagonistic affinities of three M(2)-selective muscarinic antagonists were determined in contraction and radioligand binding experiments in guinea-pig gallbladder in the present study. The antagonistic affinity values (p K(i)) of gallamine, tripitramine and imperialine were as follows, respectively: 6.28+/-0.15, 8.65+/-0.10 and 6.55+/-0.07 against 0.250 n m [(3)H]QNB binding. All three antagonists displaced the concentration- response curves to carbachol to the right in parallel without affecting the maximum responses. The p A(2) values obtained from constrained Schild plots (-log K(B)) were 4.14+/-0.18 for gallamine, 6.79+/-0.09 for tripitramine, and 7.02+/-0.09 for imperialine. The antagonistic affinity values of gallamine, tripitramine and imperialine for M(2) receptors are reported to be 6. 3, 9.6, 7.7, respectively. The p A(2) values obtained in this study clearly indicate that the primary muscarinic receptors involved in carbachol-induced guinea-pig gallbladder contraction are not of M(2) subtype. The poor correlation between the antagonistic affinity values of these antagonists obtained at radioligand binding (p K(i)) and contraction (p A(2)) experiments also support the conclusion that the majority of muscarinic receptors which have been reported to be of M(2) do not mediate the contractile responses.

Subtypes of muscarinic receptors regulating gallbladder cholinergic contractions

The aim of this study was to determine the functional role of muscarinic receptor subtypes regulating gallbladder cholinergic contractions. Electrical field stimulation (EFS; 16 Hz) produced contractile responses of guinea pig gallbladder muscle strips in vitro that were inhibited by 1 microM tetrodotoxin (2 +/- 2% of control) and 1 microM atropine (1 +/- 1% of control), indicating activation of intrinsic cholinergic nerves. Exogenous ACh (5 microM)-induced contractions were inhibited by atropine (1 +/- 1% of control) but not tetrodotoxin (102 +/- 1% of control), indicating a direct effect on smooth muscle. The M1 receptor antagonist pirenzepine (10 nM) had no effect on ACh-induced contractions but inhibited EFS-induced contractions by 11 +/- 3%. The M2 antagonist methoctramine (10 nM) had no effect on ACh-induced contractions but augmented EFS-induced contractions by 5 +/- 2%. The M3 antagonist 4-DAMP (10 nM) inhibited ACh-induced contractions by 14 +/- 4% and EFS-induced contractions by 22 +/- 5%. In conclusion, specific M1, M2, and M3 receptors modulate gallbladder muscle contractions by regulating ACh release from cholinergic nerves and mediating the contraction. Cholinergic contractions are mediated by M3 receptors directly on the smooth muscle. M2 receptors are on cholinergic nerves and function as prejunctional inhibitory autoreceptors. M1 receptors are on cholinergic nerves and function as prejunctional facilitatory autoreceptors.

Contraction of guinea-pig gallbladder: muscarinic M3 or M4 receptors?

The muscarinic receptor mediating contraction of the guinea-pig isolated gallbladder, currently being disputed to belong either to the M3 or M4 subtype, was characterized by subtype-preferring agonists and discriminating antagonists. Highly significant correlations of agonist potencies to contract the gallbladder, e.g., arecaidine propargyl ester, oxotremorine, 5-methylfurtrethonium > arecoline, arecaidine 2-butyne-1,4-diyl bisester > (R)-nipecotic acid ethyl ester > 4-[[N-(4-chlorophenyl)carbamyl]oxy]-2-butynyltrimethylammonium iodide (4-Cl-McN-A-343), (S)-nipecotic acid ethyl ester > 4-[[N-(3-chlorophenyl)carbamoyl]oxy]-2-butynyltrimethylammonium chloride (McN-A-343) were found with muscarinic M3 receptors mediating contraction of the guinea-pig ileum and vasodilation in rat perfused kidney. Functional affinities at guinea-pig gallbladder muscarinic receptors of antagonists known to distinguish between native or cloned muscarinic M3/m3 and M4/m4 receptors, e.g., himbacine, methoctramine, mefurtramine, tripitramine, idaverine, zamifenacin and 11-[[4-[4-(diethylamino)butyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyr ido(2,3-b)(1,4)benzodiazepin-6-one (AQ-RA 741), were consistent with those at guinea-pig ileal muscarinic M3 receptors but not with published data at recently defined muscarinic M4 receptors in rabbit anococcygeus muscle or at muscarinic M1 and M2 receptors in rabbit vas deferens. Antagonist affinities at guinea-pig gallbladder correlated also best with published binding data on native or cloned muscarinic M3/m3 receptors but not with those for muscarinic M4/m4 receptors. The agonist potencies and antagonist affinities suggest that smooth muscle contraction elicited by muscarinic stimuli in guinea-pig gallbladder is mediated by functional muscarinic M3 receptors.

Functional comparison between nuvenzepine and pirenzepine on different guinea pig isolated smooth muscle preparations

The antimuscarinic agents nuvenzepine and pirenzepine were tested on four guinea pig isolated smooth muscle preparations in order to better investigate the existence of differences in the functional activities of such antagonists, as suggested by previous reports. The effects of both compounds were compared to those of atropine. Nuvenzepine showed a four-fold higher affinity than pirenzepine in competitively antagonizing acetylcholine-induced contractions on isolated ileal musculature (pA2 = 7.08 +/- 0.15) and on longitudinal ileum dispersed cells (pA2 = 7.11 +/- 0.19). By contrast, unlike pirenzepine which was ineffective, nuvenzepine inhibited histamine-induced ileal motor activity in a dualistic manner, behaving as an irreversible competitive H1 antagonist (pA2 = 5.02 +/- 0.11). Nuvenzepine was almost equipotent to pirenzepine in competitively preventing bethanechol-induced gall-bladder contractions (pA2 = 7.23 +/- 0.16) and it displayed a four-fold higher potency than pirenzepine in blocking vagal-stimulated tracheal constrictions (pIC50 = 6.77 +/- 0.06). Both compounds were definitely less potent than atropine. On the whole, these findings indicate that, on the selected preparations, nuvenzepine substantially shares the antimuscarinic properties of pirenzepine but it is also endowed with a (weak) H1-blocking action. Furthermore, based on some observations, the presence in gallbladder smooth muscle of muscarinic receptors distinguishable from those of ileum could be speculated.

Signal transduction pathway of the muscarinic receptors mediating gallbladder contraction

In gallbladder smooth muscle, carbachol interacts with M3 receptors to mediate contraction. To examine components of the intracellular second messenger system that is coupled to these receptors we have tested whether carbachol stimulates the formation of inositol phosphates (IP) to cause contraction. Guinea pig gallbladder muscle strips were prelabeled with [3H]inositol and were incubated with 0.1 mmol/l carbachol, a concentration causing maximal contraction. [3H]inositol monophosphates, [3H]inositol bisphosphates and [3H]inositol trisphosphates and contraction were measured at various times (0-90 s). To examine whether a pertussis toxin-sensitive guanine nucleotide binding protein is coupled to the muscarinic receptors, guinea pigs were pretreated with pertussis toxin (180 micrograms/kg i.v./24 h). The effectiveness of pertussis toxin treatment was determined by measuring [32P]ADP-ribosylation of a approximately 40/41 kDa protein from gallbladder homogenates. Carbachol caused a significant time-dependent increase in the formation of [3H]inositol monophosphates, [3H]inositol bisphosphates and [3H]inositol trisphosphates. The time course of [3H]inositol trisphosphate turnover caused by carbachol was biphasic, and was detectable at 15 s and maximal at 60 s; at 75 s and 90 s formation of [3H]inositol trisphosphates decreased, whereas the time course of carbachol-induced contraction of the gallbladder smooth muscle strips reached a plateau after 90 s. The effects of carbachol on [3H]inositol trisphosphates and on contraction were abolished by atropine.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic acetylcholine receptor subtypes in smooth muscle

Muscarinic acetylcholine M2 and M3 receptor subtypes are coexpressed in many types of smooth muscle including gastrointestinal smooth muscle, urinary bladder and vascular and airway tissue. Activation of M3 receptors, via the G protein Gq, results in increased polyphosphoinositide hydrolysis, release of Ca2+ ions from the sarcoplasmic reticulum and consequently causes contraction. Quantitation of the relative expression of M2 and M3 receptors has shown that the proportion of M2 receptors often predominates over the M3 receptor population by 4:1 or more. Although it is established that M2 receptors preferentially link, via a pertussis-toxin-sensitive G protein Gi, to inhibition of adenylate cyclase activity, relatively little is known concerning the physiological role of the M2 receptor population. In this review, Richard Eglen and colleagues discuss recent data concerning the possible role(s) of muscarinic receptor subtypes in smooth muscle and appraise the pharmacological methods for dissecting the function of muscarinic receptor subtypes in tissues co-expressing multiple receptors.