Muscarinic receptors mediating the positive inotropic effect of carbachol in embryonic chick ventricle have a low affinity for pirenzepine.

Electrically driven ventricular strips from 3 day old chick embryos were used to investigate the ability of pirenzepine, a muscarinic antagonist that distinguishes between subtypes of muscarinic receptors, to antagonize the positive inotropic effect of carbachol. A low pA2 value was found for pirenzepine (6.40) whereas the pA2 value for atropine was 1000-fold higher (9.36). It can be concluded that the positive inotropic effect of carbachol on this tissue is mediated by muscarinic receptors others than M1-receptors.