The effect of several muscarine receptor antagonists on responses to carbachol (CCh) and McN-A-343 (McN) were compared in the perfused rabbit ear artery preparation stimulated via noradrenergic nerves at 3 Hz in the presence of cocaine (10 microM) and yohimbine (1 microM). The slope of the dose-response curve to McN was significantly less (P less than 0.05) than that for CCh although both agonists produced up to 100% inhibition of responses to nervous stimulation. All the antagonists investigated produced parallel shifts of the dose-response curve to the agonists and atropine, fenipramide or stercuronium gave a similar pA2 value with either agonist. Pirenzepine was a competitive antagonist when CCh was used, as judged by a slope of 0.96 +/- 0.10 for the Arunlakshana-Schild (A-S) plot (pKB 6.2). Displacement of 3H-(-)QNB binding by pirenzepine gave a pKI value of 6.0 which was not significantly different to the pKB value. When McN was used as the agonist, the dose-ratios obtained with pirenzepine (0.5 microM) were significantly different (P less than 0.01) to those with CCh as agonist and the slope of the A-S plot over the concentration range of 0.1 to 3 microM was significantly less than 1.0 (P less than 0.01), indicating that the inhibition was not a simple competitive interaction. It is suggested that the interaction of McN and pirenzepine may involve an allosteric mechanism.

1 The inhibitory effect of several muscarinic agonists on responses to sympathetic nerve stimulation of the isolated perfused ear artery of the rabbit was compared to that of acetylcholine in preparations pretreated with dyflos, cocaine and yohimbine. 2 In general the potency of the agonists was similar to that observed at peripheral muscarinic sites except for arecaidine propargyl ester and 4-(m-chlorophenylcarbamoyloxy)-2-butynyl trimethylammonium chloride

1. The effect of temperature reduction on the interaction of carbachol (CCh) and McN-A-343 (McN) with muscarinic receptors in the guinea-pig taenia caeci was investigated. 2. McN, a partial agonist, acted on the smooth muscle to produce contraction. The response was unaffected by tetrodotoxin and the pKB for inhibition by pirenzepine was 6.8, indicating that ganglionic M1 receptors were not involved in the response. 3. Reduction in temperature from 37 degrees C to 18 degrees C for 3 h led to a marked reduction in the contractile response to McN (2-200 microM) but no reduction in the response to CCh (0.1-3 microM). 4. The reduction in temperature was not accompanied by any change in the affinity of McN or CCh for muscarine receptors in binding experiments with [3H]-QNB. 5. The KA value for CCh determined after irreversible receptor inactivation with propylbenzilylcholine mustard followed by ca 60-min wash-out was 7.6 microM, a value similar to that obtained in binding experiments. 6. The EC50 for McN in producing contraction at 37 degrees C (2.1 microM) was similar to the KA value for the partial agonist obtained in experiments with the irreversible antagonist phenoxybenzamine (2.5 microM). It was also similar to the KB value determined at 18 degrees C (3.4 microM) when McN could be used as an antagonist of contractions to CCh. 7. At 18 degrees C, phosphatidylinositol (PI) hydrolysis by CCh was reduced to 23% of that at 37 degrees C. 8. It is concluded that reduction of muscarinic receptor activation of the PI pathway by cholinomimetics with lowering of the temperature could account for the findings with McN on contractility.