Determination of dissociation constants and relative efficacies of some potent muscarinic agonists at postjunctional muscarinic receptors

Subtype-selective blockade of cardiac muscarinic receptors inhibits vagal chronotropic responses in cats

This study was designed to determine if chronotropic responses induced by neurally released acetylcholine are modified by subtype-selective blockade of cardiac muscarinic cholinoreceptors. In anesthetized cats, a single burst of vagal stimulation was generated with an incremental time delay after the P wave of the atrial electrogram (P-Stimulus interval). The slope of the relationships between P-Stimulus and P-P intervals was used to assess changes in responsiveness of cardiac pacemaker to vagal effects throughout the cardiac cycle. An increase in P-Stimulus interval over the initial portion (approximately 120 ms) of the cardiac cycle produced a significant increment in lengthening of the P-P interval. Once the maximal negative chronotropic response was achieved, a further increase in P-Stimulus interval by only approximately 25 ms resulted in profound (by 80-90%) reductions in vagal effects, thus yielding a bimodal vagal phase response curve. Antagonists of M1 (pirenzepine), M2 (methoctramine and gallamine), and M3 (4-DAMP) muscarinic cholinoreceptors produced a reduction in the magnitude of maximal lengthening of cardiac cycle as well as an increase in latency of vagal effects. However, the increment in prolongation of P-P interval induced by a given change in timing of vagal stimulation during cardiac cycle was reduced by M1 and M2 muscarinic receptor blockers, but was unaffected by 4-DAMP. None of the antagonists modified the range of P-Stimulus intervals over which the maximum-to-minimum change of vagal responses occurred. Taken together, these data suggest different contribution of various subtypes of cardiac muscarinic receptors into the negative chronotropic responses induced by brief bursts of vagal stimulation.

The use of 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine (4-DAMP mustard) for estimating the apparent affinities of some agonists acting at muscarinic receptors in guinea-pig ileum

1. 4-Diphenylacetoxy-N-(2-chloroethyl)-piperidine (4-DAMP mustard), which is known to block muscarinic M3 receptors in preference to muscarinic M2 receptors, was used to estimate the apparent affinity constants of some agonists acting at muscarinic receptors in guinea-pig ileum. Estimates for carbachol and n-pentyl-trimethyl ammonium iodide were similar to published values obtained in similar conditions: those for n-hexyl-trimethyl ammonium iodide were slightly lower. 2. The results for the agonists, n-pentyl- and n-hexyl-trimethyl ammonium iodides and for the partial agonist, n-heptyl-trimethyl ammonium iodide were not as regular as was suggested by Stephenson, though there is an overall increase in apparent affinity with chain length. 3. Estimates of apparent affinity may be affected by hexamethonium, usually present in experiments on ileum. Its absence had little effect on the results with carbachol but reduced the estimates obtained with n-pentyl trimethyl ammonium, which has strong nicotinic effects compared with its muscarinic effects. On ileum treated with tetrodotoxin the values for n-pentyl trimethyl ammonium were similar to those obtained in the presence of hexamethonium (0.28 mM): slightly higher estimates of affinity were obtained in the presence of indomethacin (2.8 microM). The nicotinic effects of n-pentyl ammonium may involve the release of prostaglandins. 4. The estimates of apparent affinity did not depend on the method used to calculate them as the 'null' method and the 'operational' method give similar answers. Estimates of the transducer-ratio for the partial agonist, n-heptyl-trimethyl ammonium iodide, were numerically the same as those of its efficacy. 5. This work illustrates the use of 4-DAMP mustard as a tool for measuring the apparent affinity of agonists acting at muscarinic M3 receptors.

Different muscarinic receptor subtypes mediating the phasic activity and basal tone of pig isolated intravesical ureter

1. We have studied the effects of muscarinic cholinoceptor agonists and specific antagonists on both phasic activity and basal tone of the isolated intravesical ureter of the pig by means of isometric techniques in vitro. 2. Acetylcholine in the presence and absence of physostigmine increased both phasic activity and basal tone of ureteral strips in a concentration-dependent manner. Moreover carbachol, methacholine and oxotremorine-M increased both contractile parameters while bethanechol and McN-A-343 evoked only increases in tone without affecting the frequency of the phasic contractions. 3. The nicotinic receptor blocker, hexamethonium (10(-6)-10(-4) M), failed to modify the contractions evoked by a single dose of carbachol (10(-5) M), whilst the muscarinic antagonist, atropine inhibited both phasic and tonic responses. 4. The muscarinic M1 (pirenzepine), M2 (AF-DX 116 and methoctramine), M3 (4-DAMP, HHSiD and p-F-HHSiD), and putative M4 receptor (tropicamide) antagonists significantly reversed increases in both frequency of phasic activity and baseline tone induced by a submaximal dose of carbachol (10(-5) M). The pIC50 values for inhibition of the induced phasic activity were: atropine (10.16) > 4-DAMP (9.12) > HHSiD (8.22) = methoctramine (7.98) = p-F-HHSiD (7.88 > tropicamide (7.62) = pirenzepine (7.53) = AF-DX 116 (7.45) and for inhibition of basal tone were: atropine (10.73) > 4-DAMP (9.32) > HHSiD (8.65) = pirenzepine (8.43) = p-F-HHSiD (8.38) > methoctramine (7.79) > tropicamide (7.53) > AF-DX 116 (7.04). 5. The antagonist profile indicates that an M1 receptor mediates the tonic response while the phasic activity could involve either both M2 and M3 or an M4 muscarinic receptor. These results suggest that different muscarinic receptor subtypes mediate the phasic and tonic contractile activity induced by a submaximal concentration of carbachol in the porcine intravesical ureter.

Influence of lithium pretreatment and of cooling on the responsiveness of the rat isolated jejunum and urinary bladder to muscarinic agonists

1. The aim of the present study was to see whether contractile responses induced by muscarinic agonists in the rat jejunum and urinary bladder were differently affected by procedures that mainly influence the steps following agonist-receptor interaction. Thus, the effects of ex vivo lithium pretreatment (6.8 mmol kg-1 i.p. for 3 days) and in vitro cooling from 37 degrees C to 20 degrees C) on the contractile responses to full and partial agonists, carbachol, oxotremorine, muscarine and pilocarpine were studied. 2. Lithium pretreatment did not affect muscarinic responses on the urinary bladder. It significantly reduced responses to carbachol and oxotremorine but not to muscarine and pilocarpine on the rat jejunum. 3. A decrease of the bath temperature from 37 degrees C to 20 degrees C potentiated responses to carbachol, muscarine and oxotremorine and abolished those to pilocarpine in the urinary bladder. The same lowering of the bath temperature potentiated responses to carbachol, did not affect those to muscarine and to oxotremorine and reduced those to pilocarpine in the jejunum. 4. Together the findings indicate that differences exist in the stimulus-response coupling induced by muscarinic agonists between the two tissues and that, in a given tissue, differences exist among agonists in their ability to activate the agonist-receptor-transducer complex.

Analysis of muscarinic cholinoceptors mediating phosphoinositide hydrolysis in guinea pig cardiac muscle

The muscarinic receptor mediating stimulation of PI hydrolysis in guinea pig atria and ventricles has been studied. The non-selective muscarinic agonist (+)-cis-dioxolane elicited this response, concentration-dependently, with a potency indicative of a low receptor reserve. The potency of a novel, M2-selective agonist, L-660,863 (-log EC50 = 6.3, atria; 6.0, ventricles) was observed to be lower than its apparent affinity (-log KA = 7.6) for M2 receptors, indicating an action probably mediated by a population distinct from that producing negative inotropy in the same tissue. The inhibition of the response to (+)-cis-dioxolane by several muscarinic antagonists (atropine, pirenzepine, AF-DX 116, methoctramine, HHSiD and pFHHSiD) generated an affinity profile for this receptor also dissimilar to that described for the receptor mediating the classical cardiac 'M2' response. Although no other muscarinic receptor mRNA has been detected in this tissue, these data suggest the presence of a second population of muscarinic sites, which may signify an M2 receptor diversity.

Comparison of the muscarinic receptors of the guinea-pig oesophageal muscularis mucosae and trachea in vitro

1. The muscarinic receptor profile of the guinea-pig oesophageal muscularis mucosae has been compared to that of the trachea in vitro. There was no significant difference in the potency of the following muscarinic agonists at muscarinic receptors in the two tissues: carbachol, RS-86, ethoxyethyltrimethyl-ammonium, bethanechol and pilocarpine. RS-86 was 6-fold more potent at receptors in the muscularis mucosae in comparison to the trachea. There was no difference in the affinity of either carbachol or RS 86 at receptors in the two tissues. 2. The affinities (pA2) of the majority of antagonists (atropine, 4 diphenylacetoxy-N-methylpiperidine methiodide, AF-DX 116, silabenzhexol, methoctramine and gallamine) were similar at receptors in the muscularis mucosae and trachea. In contrast, the affinity of pirenzepine was approximately 5-fold greater at receptors in the muscularis mucosae (pA2 = 7.4) in comparison to receptors in the trachea (pA2 = 6.8). 3. It is concluded that the muscarinic receptor profile of the oesophageal muscularis and trachea are similar and only differ slightly with respect to the affinity of pirenzepine. Consequently, it is difficult to justify receptor heterogeneity in these two tissues.

Muscarinic M1- and M2-receptors mediating opposite effects on neuromuscular transmission in rabbit vas deferens

Twitch contractions of the rabbit vas deferens elicited by electrical field stimulation were inhibited by tetrodotoxin, guanethidine, bretylium and alpha,beta-methylene-ATP but were unaffected by hexamethonium, physostigmine, 1,1-dimethyl-4-phenylpiperazinium and prazosin, suggesting that they resulted from ATP released following postganglionic sympathetic nerve stimulation. McN-A-343 inhibited but carbachol and several other muscarinic agonists potentiated the twitch contractions; these effects were not modified by hexamethonium or physostigmine. Muscarinic agonists had no effect on the tension in unstimulated organs whereas contractions elicited by ATP, noradrenaline and KCl were potentiated by carbachol but remained unaffected by McN-A-343. The responses of the twitch contractions to McN-A-343 and carbachol were inhibited to different degrees by antimuscarinic drugs: the affinity (pA2) of atropine, secoverine and himbacine against McN-A-343 and carbachol was similar. However, pirenzepine, telenzepine, trihexyphenidyl, dicyclomine and hexahydro-sila-difenidol displayed preferential antagonism of the responses to McN-A-343 whereas the converse was true for AF-DX 116 and gallamine. The highly significant correlation between the pA2 values obtained for 10 antagonists against carbachol responses in rabbit vas deferens and rat left atrium suggests that the receptors may be similar. The data support the presence of a presynaptic M1-receptor mediating inhibition and a postsynaptic, cardiac-like M2-receptor responsible for enhancing neurogenic contractions in rabbit vas deferens.