Subtypes of muscarinic receptor on cholinergic nerves and atrial cells of chicken and guinea‐pig hearts

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Abstract
Electrically driven chicken and guinea‐pig atria were used to investigate the negative inotropic effects of the muscarinic agonists methacholine and acetylcholine (ACh). The release of ACh from isolated hearts into the perfusate in response to (preganglionic) vagal or (pre‐and postganglionic) field stimulation was bioassayed on the guinea‐pig ileum or determined by labelling with [3H]‐choline. Concentration‐response curves for the negative inotropic effect of methacholine were shifted to the right by pirenzepine in various concentrations (0.03 to 10 μmol l−1). The pA2 values were 7.76 in chicken atria and 6.53 in guinea‐pig atria. Pirenzepine and atropine antagonized the negative inotropic response to 0.3 μmol l−1 ACh. The half‐maximally effective concentrations (IC50) of pirenzepine (Pz) and atropine were 40 and 5.4 nmol l−1 in chicken atria and 330 and 3.5 nmol l−1, respectively, in guinea‐pig atria. Thus, the respective potency ratios (IC50Pz/IC50atropine) were 7.4 and 94.3 in the two species. Pirenzepine in low concentrations increased the release of unlabelled and 3H‐labelled ACh from isolated hearts evoked by vagal and field stimulation only in chicken, but not in guinea‐pigs. The half‐maximally‐effective concentration of pirenzepine was about 30 nmol l−1 in the chicken heart, whereas, in the guinea‐pig heart, an increased release was observed at 300 nmol l−1. (+)‐Tubocurarine [(+)‐Tc; 100 μmol l−1] reduced the release of ACh evoked by (preganglionic) vagal stimulation to a (+)‐Tc‐resistant release of about 30%. The time‐course of the neuronal release of [3H]‐ACh was markedly altered: the onset was delayed and the termination was extended beyond the period of stimulation (1 min or 5 s) by several seconds. The (+)‐Tc‐resistant release was nearly abolished by 30 nmol l−1 pirenzepine. In conclusion, the pre‐and post‐synaptic muscarinic receptors of the parasympathetic neuroeffector junction of the heart both belong to the M1‐subtype in the chicken and to an M2‐subtype in the guinea‐pig. Block of the nicotinic ganglionic transmission in the chicken heart by (+)‐Tc unmasked a muscarinic transmission, which presumably was mediated through M1‐receptors stimulating a low and prolonged postganglionic release of ACh.