An embryonic Ca++ mobilizing muscarinic system in the chick embryo heart

In the embryonic and in the adult heart muscarinic stimulation reduces the heart rate. Here we demonstrate that in the embryonic heart an additional muscarinic system is present, which is characterized by Ca++ mobilization and corresponds to the embryonic muscarinic system in other organ anlagen. In suspensions of embryonic chick heart cells we measured release of Ca++ from intracellular stores and influx of extracellular Ca++ with fura-2 and chlorotetracycline after muscarinic stimulation and determined the [3H]quinuclidinylbenzilate binding sites. We observed intense Ca++ mobilization at day 4, weaker reactions between day 4.5 and 11, and no Ca++ response at day 13. The pharmacological profile was identical to the profile of Ca++ mobilization in cells from other embryonic tissues in which the general embryonic muscarinic system is expressed. In parallel, we studied the effect of muscarinic agonists and antagonists on the heart rate of isolated embryonic hearts at day 4 and 5 in a perfusion chamber. Oxotremorine and bethanechol being antagonists or weak partial agonists in Ca++ mobilization, behaved as full agonists in frequency regulation. Thus, the pharmacological profile of the transient embryonic muscarinic system was different from that of the definitive adult form.