Effect of ramosetron on short-circuit current response in rat colonic mucosa

We investigated the effects of ramosetron (YM060, (−)-(R)-5-[(1-methyl-1H-indol-3-yl)carbonyl]-4,5,6,7-tetrahydro-1H-benzimidazole monohydrochloride) on the short-circuit current (I_sc) responses to 5-HT receptor agonists in the rat distal colon, and compared its potency to that of other 5-HT₃ receptor antagonists. 5-Hydroxytryptamine (5-HT) concentration-dependently increased I_sc. The I_sc response to 5-HT was partially reduced by tetrodotoxin and ramosetron, and strongly inhibited by GR113808 ([[1-[(2-methylsulphonyl)amino]ethyl]-4-piperidin-yl]methyl 1-methyl-1H-indole-3-carboxylate). 2-Methyl-5-HT and 5-methoxytryptamine also increased I_sc. The former response was inhibited by ramosetron, and the latter was abolished by GR113808. Ramosetron, YM114 (KAE-393, (−)-(R)-5-[(1-indolinyl)carbonyl]-4,5,6,7-tetrahydro-1H-benzimidazole monohydrochloride) and granisetron concentration-dependently antagonized the I_sc responses to 2-methyl-5-HT with reduction in the maximal response at higher concentrations. Apparent pA₂ values for these antagonists were 10.40, 10.37 and 8.99, respectively. Ondansetron produced clear rightward shifts of the concentration-response curves to 2-methyl-5-HT, with a pA₂ value of 8.53. These results suggest that 5-HT increases I_sc through the 5-HT₃ and 5-HT₄ receptors, and that ramosetron is a potent and selective 5-HT₃ receptor antagonist in rat colonic mucosa.     

Compensation of muscarinic bronchial effects of talsaclidine by concomitant sympathetic activation in guinea pigs

The aim of the present investigation was to determine the reasons why the muscarinic receptor agonist talsaclidine (WAL 2014 FU, 1-azabicyclo[2.2.2] octane,3-(2-propynyloxy)-, (R)-,(E)-2-butenedioate) is devoid of bronchospastic effects in anaesthetized guinea pigs but causes contracture in isolated tracheal muscle from this species. Effects on airway resistance were assessed with a modified Konzett–Rössler method in guinea pigs anaesthetized with urethane. Intravenous injection of 1–64 mg/kg talsaclidine did not cause substantial bronchospasm in control animals. After blockade of β-adrenoceptors, the muscarinic receptor agonist induced dose-dependent bronchospasm which could be blocked by atropine. In despinalized animals and in animals with spinal transection, talsaclidine was bronchospastic but ED₅₀ values were higher and maximal effects were smaller than in intact animals after β-adrenoceptor blockade. In adrenalectomized guinea pigs, talsaclidine was nearly as bronchospastic as after blockade of β-adrenoceptors. In contrast, the muscarinic ganglion stimulant McN-A-343, 4-(m-chlorophenylcarbamoyloxy)-2-butyn-trimethyl-ammonium chloride, (2–32 mg/kg i.v.), which has a muscarinic receptor profile similar to that of talsaclidine, i.e., full muscarinic agonism and highest affinity at muscarinic M₁ receptors, partial agonism at muscarinic M₃ receptors, but in contrast to talsaclidine does not penetrate the blood–brain barrier, caused dose-dependent bronchospasm in control animals. These results indicate that talsaclidine has bronchospastic potential which, however, does not become evident in vivo because of functional antagonism via β-adrenoceptors resulting from concomitant activation of the sympathetic nervous system in general and the adrenals in particular. It can be concluded that the unique profile of action of talsaclidine is due to partial agonism at bronchial muscarinic M₃ receptors, a prerequisite for susceptibility to functional antagonism, and to its ability to penetrate the blood–brain barrier readily and to induce sympathetic activation as a result of full agonism at peripheral ganglionic and adrenal as well as central muscarinic M₁ receptors.     

Acetylcholine receptor agonists stimulate [H]taurine release from rat sympathetic ganglia

The endogenous taurine content, and the uptake and release of [³H]taurine were examined using the rat superior cervical ganglion. Taurine was found to be one of the most abundant amino acids in the superior cervical ganglion, and the superior cervical ganglion took up [³H]taurine from the incubation medium. Carbachol stimulated the release of [³H]taurine in a concentration-dependent manner with an EC₅₀ of 26 μM and maximal stimulation at 100 μM. The nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium stimulated release with the same potency but with greater efficacy than carbachol. The nicotinic receptor antagonist hexamethonium (1 mM) inhibited carbachol-stimulated release by 74% (±)-Muscarine stimulated release with an EC₅₀ of 8 μM but with a maximal effect of only 32% of that produced by 100 μM carbachol. Oxotremorine another muscarinic receptor agonist, was ineffective, even at 1 mM. The muscarinic receptor antagonist atropine inhibited carbachol-stimulated release by 30% at 10μM. These results show that ³H taurine release from rat superior cervical ganglion can be stimulated by cholinergic receptor agonists. Release is mediated predominantly by a nicotinic receptor and partially by a muscarinic receptor.     

Castration decreases amylase release associated with muscarinic acetylcholine receptor downregulation in rat parotid gland

1 The mechanism and receptor subtypes involved in carbachol-stimulated amylase release and its changes after castration were studied in parotid slices from male rats. 2 Carbachol induced both amylase release and inositol phosphate (IP) accumulation in parotid slices from control and castrated rats, but castration induced a decrease of carbachol maximal effect. The effect of castration was reverted by testosterone replacement. 3 The selective M(1) and M(3) muscarinic receptor antagonists, pirenzepine and 4-diphenylacetoxy-N-methylpiperidine methiodide, respectively, inhibited carbachol-stimulated amylase release and IP accumulation in a dose-dependent manner in parotid slices from control and castrated rats. 4 A diminution of binding sites of muscarinic receptor in parotid membrane from castrated rats was observed. Competition binding assays showed that both, M(1) and M(3) muscarinic receptor subtypes are expressed in membranes of parotid glands from control and castrated rats, M(3) being the greater population. 5 These results suggest that amylase release induced by carbachol in parotid slices is mediated by phosphoinositide accumulation. This mechanism appears to be triggered by the activation of M(1) and M(3) muscarinic receptor subtypes. Castration induced a decrease of the maximal effect of carbachol evoked amylase release and IP accumulation followed by a diminution in the number of parotid gland muscarinic acetylcholine receptors.     

Muscarinic interactions of bisindolylmaleimide analogues

We have used radioligand binding studies to determine the affinities of seven bisindolylmaleimide analogues, six of which are selective inhibitors of protein kinase C, at human muscarinic M₁–M₄ receptors. The compounds were most potent at M₁ receptors, and Ro-31-8220 was the most potent analogue, with a K_d of 0.6 μM at M₁ receptors. The weakest compounds, bisindolylmaleimide IV and bisindolylmaleimide V, had K_d values of 100 μM. If it is necessary to use protein kinase C inhibitors at concentrations of 10 μM or more in studies involving muscarinic receptors then bisindolylmaleimide IV may be the most appropriate inhibitor to use.     

Spinal muscarinic receptors are activated during low or high frequency TENS-induced antihyperalgesia in rats

Transcutaneous electrical nerve stimulation (TENS) is a non-pharmacological modality used clinically to relieve pain. Central involvement of serotonin and endogenous opioids are implicated in TENS-induced analgesia. Activation of spinal cholinergic receptors is antinociceptive and these receptors interact with opioid and serotonin receptors. In the current study, the possible involvement of spinal cholinergic receptors in TENS analgesia was investigated in rats. Hyperalgesia was induced by inflaming one knee joint with 3% kaolin–carrageenan and assessed by measuring paw withdrawal latency (PWL) to heat before and 4 h after injection. The non-selective nicotinic antagonist mecamylamine (50 μg), non-selective muscarinic antagonist atropine (30 μg) or one of the muscarinic subtype antagonists: pirenzepine (M₁, 10 μg), methoctramine (M₂, 10 μg), 4-DAMP (M₃, 10 μg), or saline was administered intrathecally just prior to TENS treatment. Low or high frequency TENS was then applied to the inflamed knee and PWL was determined again. Atropine, pirenzepine and 4-DAMP significantly attenuated the antihyperalgesic effects of low and high frequency TENS while mecamylamine and methoctramine had no effects, compared to saline control. The results show that TENS-induced antihyperalgesia is mediated partially by activation of spinal muscarinic receptors but not spinal nicotinic receptors. Further, the results also indicate that spinal M₁ and M₃ muscarinic receptor subtypes mediate the muscarinic component of TENS antihyperalgesia.     

Decreased binding affinity of olanzapine and clozapine for human muscarinic receptors in intact clonal cells in physiological medium

The binding affinity of olanzapine, clozapine and atropine for muscarinic receptor subtypes in clonal Chinese hamster ovary (CHO) cell lines was compared in intact cells in physiological media to disrupted cells in hypotonic buffer. The affinity of olanzapine and clozapine, but not atropine, for muscarinic receptor subtypes (M₁–M₅) was significantly reduced in intact cells in physiological medium compared to disrupted cells in hypotonic buffer. The affinity of olanzapine for muscarinic M₁ receptors was most affected with a reduction of K_i value from 2.5 to 73 nM in intact cells. These data suggest that the affinity of olanzapine and clozapine for muscarinic receptors have been significantly overestimated.     

Effects of phorbol ester on carbachol-induced contraction in bovine ciliary muscle: possible involvement of protein kinase C

The aim of the research was to characterize muscarinic receptors of bovine ciliary muscle and to investigate the desensitization process. The role of protein kinase C was analyzed. The results show that muscarinic receptors of bovine ciliary muscle have the pharmacological characteristics of the M₃ subtype. Acute exposure to phorbol esters (1 μM phorbol 12,13-dibutyrate, PDB, or 0.1 μM phorbol 12-myristate 13-acetate, PMA, for 15 and 5 min, respectively) resulted in antagonism of muscarinic receptor-mediated contraction. Long-term pretreatment (18 h) with PMA to down-regulate protein kinase C resulted in potentiation of carbachol-induced contraction, reduction of agonist-induced desensitization and loss of phorbol ester-induced desensitization. Staurosporine (3 μM) and H₇ [1-(5-isoquinolinesulfonyl)-2-methyl-piperazine] (1 μM), protein kinase C inhibitors, produced a significant potentiation of the contractile effect of carbachol, reduced the desensitization produced by repeated addition of carbachol and suppressed that induced by phorbol esters. In vitro incubation with carbachol, PDB or PMA did not cause any modification of the binding of labeled [³H]quinuclidinyl benzilate. In vitro incubation with PDB and PMA produced, as expected, a significant translocation of protein kinase C from the cytosol to the membrane. The incubation of the ciliary muscle with carbachol, using the protocol of exposure that induced maximal desensitization of contractile responses, produced a significant redistribution of the enzyme from the cytosol to the membrane. These findings suggest that agonist-induced modulation of functional cholinergic sensitivity in ciliary muscle is correlated, at least partially, to the translocation of protein kinase C from the cytosol to the membrane. The desensitization by phorbol esters is completely due to protein kinase C activation; during the desensitization process, direct modification of the density and affinity of muscarinic receptors is not involved.     

The role of muscarinic M1 and M2 receptors in airway constriction in the cat.

The role of prejunctional inhibitory and facilitatory muscarinic receptors was investigated in cats with tracheal hyperresponsiveness to vagal stimulation. Intrathoracic airway caliber (total lung resistance (R_L) and dynamic compliance (C_dyn)) and the diameter of tracheal ring 4 were measured during vagal stimulation and local acetylcholine (ACh) injection before and after administration of the M₁ receptor antagonist pirenzepine or the M₂ receptor antagonist gallamine. The responses of tracheal ring 4, R_L, and C_dyn to ACh were unaltered by gallamine or pirenzepine. Changes in R_L and C_dyn during vagal stimulation were enhanced by gallamine, but the magnitude of tracheal constriction was unchanged. Vagally induced tracheal constriction was decreased by pirenzepine in hyperresponsive but not in control cats. The M₂ receptors limit intrathoracic airway constriction, but a functional role for M₂ receptors in the cervical trachea could not be demonstrated. However, these data suggest that M₁ excitatory receptors may play a role in vagally mediated tracheal hyperreactivity.     

Central muscarinic M2 cholinoceptors involved in cholinergic hypertension

Cholinomimetic agents increase blood pressure and heart rate via central muscarinic cholinoceptors in various aspects. It was reported that i.c.v. injection of the muscarinic M₁ and M₃ cholinoceptor selective antagonist, 4-DAMP (4-diphenylacetoxy-N-methyl-piperidine methiodide, inhibited the pressor response to physostigmine, while the M₁ selective antagonist, pirenzepine, was ineffective. In the present study, the involvement of muscarinic M₂ cholinoceptors in central cholinergic hypertension and tachycardia was investigated. Physostigmine (10–80 μg/kg i.v.), a cholinesterase inhibitor, and oxotremorine (20–40 μg/kg i.v.), a direct muscarinic cholinoceptor agonist, caused a dose-dependent increase in blood pressure. Additionally, physostigmine induced dose-dependent tachycardiac responses. I.c.v. administration of the muscarinic M₂ cholinoceptor antagonists, AF-DX 116 and methoctramine, inhibited both physostigmine (60 μg/kg) and oxotremorine (20 μg/kg)-induced pressor responses at their lower doses used in this study (100 nmol/rat and 10 nmol/rat, respectively). These findings indicate the partial involvement of postsynaptic muscarinic M₂ cholinoceptors. The higher doses of the antagonists (AF-DX 116 300 nmol/rat and methoctramine 30 nmol/rat) potentiated the blood pressure increase due to physostigmine but did not affect that due to oxotremorine. The physostigmine-induced tachycardiac responses were influenced similarly by these antagonists. These results suggest the presence and tonic influence of presynaptic inhibitory muscarinic M₂ cholinoceptors.     

Activation of nitric oxide synthase through muscarinic receptors in rat parotid gland

Muscarinic receptors play an important role in secretory and vasodilator responses in rat salivary glands. Nitric oxide synthase (NOS) activity was found coupled to muscarinic receptor activation as well as to nitric oxide-mediated amylase secretion elicited by carbachol. Parotid glands presented a predominant M(3) and a minor muscarinic M(1) acetylcholine receptor population, though carbachol stimulated NOS activity only through muscarinic M(3) receptors as revealed in the presence of 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and pirenzepine. Amylase secretion induced by carbachol appeared to be partly mediated by nitric oxide and nitric oxide-induced signaling since N-nitro-L-arginine methyl ester (L-NAME) inhibited the effect as well as did methylene blue. A negative regulation of NOS by protein kinase C activation in the presence of a high concentration of carbachol was seen in parotid glands and this inhibition was paralleled by amylase secretion.     

Muscarinic receptor subtype in porcine coronary artery

The specific binding of (-)-[³H]QNB (quinuclidinyl benzilate) in membrane fractions of porcine coronary artery was saturable, of high affinity and stereoselective. It has been shown that there exist (-)-[³H]QNB binding sites with high (K_i = 12 nM)- and low(K_i = 1010 nM)-affinity for pirenzepine in the coronary artery but predominantly low-affinity sites in cardiac muscle. AF-DX 116 and gallamine showed a lower affinity to ( - )-[³H]QNB binding sites in the coronary artery compared to cardiac muscle. Thus, the present study suggests that porcine coronary artery contains a significant number of muscarinic receptors, probably both M₁ and M₂ subtypes.     

Pharmacological characterization and distribution of muscarinic receptors in human placental syncytiotrophoblast brush-border and basal plasma membranes

Based on the existence of choline acetyltransferase and acetylcholine in human placenta, we have investigated the presence of muscarinic acetylcholine receptors in brush-border and basal plasma membranes from human term placenta. Radioligand binding assay, using [³H]N-methyl-scopolamine as tracer, showed the existence of acetylcholine muscarinic receptors in brush-border (K_d 0.28±0.04 nM; B_max 9.4±1.6 fmol/mg protein) and basal plasma membranes (K_d 0.24±0.05 nM; B_max 34.3±6.3 fmol/mg protein). In order to perform a pharmacological characterization of these receptors, competition binding experiments were carried out using the muscarinic receptor antagonists pirenzepine, (11(2-diethyl-amino)methyl)-1-piperidinylacetyl-5-11-dihydro-6H-pyrido(14)benzodiazepine (AF-DX 116), himbacine, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), dicyclomine and hexahydro-sila-difenidol (HHSD). The results obtained showed that the muscarinic receptors in brush-border and basal plasma membranes belong to different subtypes. In brush-border membranes, the receptor found match in terms of affinity for the antagonists with the muscarinic M₁ receptor subtype (K_i pirenzepine, 13.6±8.2 nM; K_i AF-DX 116, 1680±271 nM; K_i himbacine, 212±6.5 nM; K_i 4-DAMP, 1.5±0.4 nM; K_i dicyclomine, 5.1±0.8 nM; K_i HHSD, 34.3±7.3 nM), whereas the receptor in basal plasma membrane seems to be of the muscarinic M₂ receptor subtype (K_i pirenzepine, 202±48 nM; K_i AF-DX 116, 124±60 nM; K_i himbacine, 20.6±4.8 nM; K_i 4-DAMP, 4.5±1.2 nM; K_i dicyclomine, 54.6±22 nM; K_i HHSD, 89.2±15.8 nM). The results obtained show the existence of muscarinic acetylcholine receptors in brush-border and basal plasma membranes from human term placenta with a different distribution pattern in terms of number of receptors and distribution of different subtypes. The functional significance of these findings is as yet unknown, but these receptors probably mediate different functions as they belong to different subtypes and are coupled to different second messengers.     

Deferoxamine blocks interactions of fluoride and carbachol in isolated mammalian cardiac preparations

In papillary muscles, carbachol reduced the positive inotropic effects of isoprenaline (10 nmol/l). The negative inotropic effects of carbachol in isoprenaline-stimulated guinea pig papillary muscles were attenuated by additionally applied sodium fluoride (3 mmol/l). These effects of sodium fluoride were blocked by deferoxamine (200 μmol/l). In guinea pig left atria, sodium fluoride alone greatly reduced force of contraction. These effects in atria were blocked by 200 μmol/l deferoxamine, and positive inotropic effects of sodium fluoride were observed. It is suggested that the cardiac effects of muscarinic M₂ receptor agonists in the ventricle involve, at least in part, the activation of phosphatases which are blocked by fluoride and reactivated by deferoxamine.     

Site-directed mutagenesis of the putative human muscarinic M2 receptor binding site.

Experimental probing of the model of the muscarinic M₂ receptor binding site proposed by Hibert et al. [Hibert, M.F., Trumpp-Kallmeyer, S., Bruinsvels, A., Hoflak, K., 1991. Three-dimensional models of neurotransmitter G-binding protein-coupled receptors. Mol. Pharmacol. 40, 8–15.] was achieved by mutating each amino-acid proposed to interact with muscarinic ligands. Pharmacological analysis of the different mutant receptors transiently expressed in human embryonic kidney (HEK/293) cells was performed with a variety of agonists and antagonists. D103A, Y403A and N404A mutations prevented binding of [³H] N-methylscopolamine and [³H] quinuclidinyl benzilate with a reduction in affinity greater than 100-fold, indicating essential contributions of these residues to the binding site for the radioligands. W400A and W155A mutations had very large effects on the binding of [³H] N-methylscopolamine (150-fold, 960-fold) but modest effects on the binding of [³H] quinuclidinyl benzilate (4-fold, 17-fold). In addition, binding of oxotremorine-M, oxotremorine, arecoline and pilocarpine to W155A resulted in a greater than 100-fold decrease in affinity. Threonine mutations (T187A and T190A) alter binding of most agonists but not of antagonists. W99 makes little contribution (<10-fold) to the binding site of the M₂ receptor. D103, W155, W400, Y403 and N404 are likely to be part of the binding site for N-methylscopolamine and also to contribute to the binding site for quinuclidinyl benzilate. Some of the predicted residues do not seem to be part of the M₂ receptor binding site but W155 is important for proper ligand binding on the muscarinic M₂ receptor, as predicted by the proposed model.     

The actions of phenylglycine derived metabotropic glutamate receptor antagonists on multiple (1S,3R)-ACPD responses in the rat nucleus of the tractus solitarius

The effects of the metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocy-clopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] and a series of phenylglycine-derived putative mGluR antagonists were examined on electrophysiological responses mediated by glutamate and GABA receptors in the nucleus of the tractus solitarius (NTS) in transverse brainsten slices of the rat. Monosynaptic excitatory currents (EPSC's) evoked by electrical stimulation in the region of the tractus solitarius (TS) were reduced in the presence of (1S,3R)-ACPD in > 90% of neurons recorded in the dorsomedial subdivision of the NTS adjacent to the area postrema (AP). Monosynaptic evoked inhibitory currents (IPSC's) were similarly inhibited by (1S,3R)-ACPD. The inward current evoked by pressure application of -amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (I_AMPS) was potentiated in the presence of (1S,3R)-ACPD, whereas the outward current evoked by the γ-amino-butyric acid-A (GABA-A) receptor agonist muscimol (I_MUSC) was inhibited. (1S,3R)-APCD also produced a postsynaptic inward current (I_K(ACPD)) associated with a decrease in membrane conductance in approximately 50% of cells. The novel mGluR antagonists (S)-4-carboxy-3-hydroxy-phenylglycine (4C3H-PG), (R,S)-4-carboxy-phenylglycine (4C-PG) and (R,S)--methyl-4-carboxy-phenylglycine (M4C-PG) reversibly antagonized the effects of (1S,3R)-ACPD on EPSC's IPSC's, I_AMPA and I_MUSC. The first two compounds also displayed weak agonist activity. However, none of the antagonists significantly inhibited I_K(ACPD) concentrations which blocked (1S,3R)-ACPD effects on synaptic transmission. These results suggest that pharmacologically distinct mGluR's may be present in the NTS.     

Inhibitory effect of diazepam on muscarinic receptor-stimulated inositol 1,4,5-trisphosphate production in rat parotid acinar cells

1. This study examined the effect of diazepam (DZP) on phosphoinositide turnover, which plays an important role in the regulation of salivary secretion, in rat parotid acinar cells. 2. DZP (10(-9) M to 10(-5) M), a potent agonist of both central- and peripheral-type benzodiazepine receptors, dose-dependently decreased inositol 1,4,5-trisphosphate IP3 production stimulated by carbachol, a muscarinic receptor agonist, in the cells. 3. DZP produced a maximum inhibitory response at a concentration of 10(-5) M, with IP3 production decreased to 63% of maximal levels. The concentration inducing half maximal inhibition of IP3 production was approximately 3.5 x 10 (-8) M. 4. An inhibitory response to DZP was produced by a short-term pretreatment (<3 min) of the cells and prevented by antagonist and competing ligand for the central- and peripheral-type benzodiazepine receptors, flumazenil and PK 11195, respectively. 5. DZP showed a non-competitive inhibition of carbachol-stimulated IP3 production. It did not directly inhibit the activities of GTP-binding regulatory proteins and phosphatidylinositol 4,5-bisphosphate-specific phospholipase C (PLC) in the parotid gland membranes, though choline chloride inhibited PLC activity. 6. DZP (10(-5) M) attenuated the increase in the intracellular Ca2+ concentration ([Ca(2+)](i)) in the cells following stimulation of the muscarinic and alpha(1)-adrenoceptors. 7. These results suggest that in the parotid acinar cells, DZP inhibits muscarinic receptor-stimulated IP3 production through benzodiazepine receptors and that PLC activity which produces IP3 is inhibited by chloride. The decreases in IP3 and [Ca(2+)](i) in the cells may be connected with the suppression of salivary secretion induced by DZP.     

Differential coupling between muscarinic receptors and G-proteins in regions of the rat brain

The coupling of muscarinic receptors to G-proteins in various regions of the rat brain was assessed by measuring carbachol-stimulated, low-Km GTPase. The inhibition of carbachol-stimulated GTPase by the M1-selective antagonist pirenzepine was compared to the affinity of pirenzepine for various nuclei within the regions as measured autoradiographically. The rank order of potency of carbachol for stimulating GTPase in various brain regions was similar to that for binding to receptors in those areas. The maximal specific activity (efficacy) of carbachol-stimulated GTPase varied independently of the distribution of total receptors or receptor subtypes. The overall potency of pirenzepine for inhibiting carbachol-stimulated GTPase was not correlated with the overall affinity of pirenzepine for muscarinic receptors in the regions. Comparing results in various brain regions, the data suggest that there are differences in the efficiency of coupling between muscarinic receptors and G-proteins. For example, the pons-medulla appeared to have a small population of pirenzepine-sensitive (M1 or M4) receptors that were coupled very efficiently to G-proteins, whereas in the hippocampus all muscarinic receptors, most of which are pirenzepine-sensitive, appeared to be weakly coupled to G-proteins. It is suggested that variable interactions between receptors and G-proteins may be an important factor in the overall coupling between receptor occupancy and cellular responses to acetylcholine as well as other hormones and transmitters.     

Inhibitory effects of 2-guanidinebenzimidazole and 1-phenylbiguanide on gastric acid secretion in rats

In this study 2-guanidinebenzimidazole (GBI) and 1-phenylbiguanide (PBG) appear to be capable of decreasing gastric acid secretion, while the compounds dimethylbiguanide and cyanoguanidine do not. Thus, the antisecretory effect is present when the biguanide groups are associated with lipophilic molecules. GBI and PBG depress gastric acid secretion, even when it has been stimulated by carbamoylcholine (carbachol) or betazole. The antihistamine effects of GBI and PBG on betazole-stimulated gastric acid secretion were confirmed by the inhibitory activity of these compounds on the isolated guinea pig auricle stimulated by histamine. The antimuscarine activity of GBI and PBG on carbachol-stimulated gastric acid secretion in rats is also supported by the way in which these same drugs depress the motility of the duodenum and colon of the anaesthetized cat stimulated by prostigmine. The above mentioned effects of these compounds are also associated with myolytic activity, since they decrease the spontaneous and histamine-stimulated motility of the duodenum and colon. GBI and PBG probably depress gastric acid secretion by interfering with both histamine and acetylcholine receptors and with other sites involved in the secretory process.