Expression of the Ca2+ mobilizing muscarinic system in the chick embryo correlates with morphogenesis

We describe muscarinic receptors and intracellular Ca2+ mobilization after cholinergic stimulation in cell suspensions prepared from chick embryos between day 2 (stage 12/13) and day 13 (stage 40) of development. Cell suspensions are prepared from whole chick embryos and from embryonic hearts, heads or brains, limb buds, and trunks. Muscarinic receptors are measured using [3H]quinuclidinylbenzilate as specific ligand. Intracellular Ca2+ mobilization is determined by changes of chlorotetracycline fluorescence. (1) Considerable amounts of muscarinic receptors are found in all parts of the embryo and at all stages tested. (2) The intracellular Ca2+ response after stimulation by muscarinic agonist shows a peak at day 3-4 (stage 23). (3) The pharmacological profile of the Ca2+ response remains constant during embryonic development and differs from the profiles of most adult systems. (4) The 'embryonic muscarinic system' is uniformly expressed in cells from neural and non-neural tissues. It appears and disappears independently of innervation.