[Subtypes of the functionally active muscarinic receptors in the heart of the frog Rana temporaria].

Radioligand analysis of ventricular membranes from the frog Rana temporaria with muscarinic antagonist 3H-QNB (qunuclidinul benzilate) revealed homogeneous population of highly specific muscarinic receptors with high affinity (Kd = 0.502 +/- 0.020 nM and Bmax = 789 +/- 18 fmol/mg protein). Competitive binding of M1-antagonist pirenzepine revealed two binding sites on the membrane with Kd values 5.6 +/- 1.7 and 237 +/- 45 nM, and total capacity of 20.6 +/- 10.2 and 742 +/- 58 fmol/mg protein respectively, which may be attributed to M1- and M2-subtypes in accordance with Receptor Nomenclature. The inhibitory effect of pirenzepine on negative inotropic action of Ach has been studied in the intact electrically stimulated ventricle. Analysis of the data by Schild procedure showed that KB for pirenzepine is equal to 13.9 +/- 2.4 nM. The results obtained suggest that the frog ventricle contains muscarinic cholinoreceptors of M2-subtype (possibly, M4-subtype) which account for negative inotropic effect of Ach. Small population of M1-like receptors was also revealed, their functional role still remaining obscure.