Positive and negative inotropic effects of carbachol on the embryonic chick atrium

The effect of carbachol on twitch tension of atrial preparations from chick embryos of different incubation ages (3-14 days) was studied. At every age carbachol evoked negative (at low concentrations) and positive (at higher concentrations) inotropic responses. Maximal response values for both effects increased with age; in 3- and 5-day atria the positive inotropic response prevailed. The muscarinic antagonist pirenzepine inhibited the positive (on 5-day atria) and negative (on strips of 14-day atria) inotropic effects of carbachol with pA2 values of 6.8 and 8.0, respectively, suggesting that muscarinic receptors mediating these effects belong to different receptor subtypes.