DEVELOPMENT OF MUSCARINIC-CHOLINERGIC STIMULATION OF INOSITOL PHOSPHATE PRODUCTION IN CULTURED EMBRYONIC CHICK ATRIAL CELLS - EVIDENCE FOR A SWITCH IN GUANINE-NUCLEOTIDE-BINDING PROTEIN COUPLING

We have demonstrated that muscarinic stimulation of inositol phosphate production in cultured atrial cells from chicks at 14 days in ovo is partially sensitive to inhibition by pertussis toxin. In these cells, muscarinic agonist binding is coupled to phospholipase C activity via at least two guanine-nucleotide-binding proteins (G-proteins), one sensitive to pertussis toxin and the other (G(p)) insensitive to pertusssis toxin [Barnett, Shamah, Lassegue, Griendling and Galper (1990) Biochem. J. 271, 437-442]. In the current study we demonstrate that during embryonic development of the chick heart, muscarinic stimulation of inositol phosphate production decreases by 50% between days 5 and 14 in ovo in cells cultured from both atrium and ventricle. In atrial cells, however, pertussis toxin-sensitive muscarinic stimulation of inositol phosphate production increased from undetectable levels at day 5 in ovo to 40% of total stimulation at day 12 in ovo. Muscarinic stimulation of inositol phosphate production in the ventricle did not become sensitive to pertussis toxin at any age studied. In permeabilized atrial cells from embryonic chicks at 5 days in ovo, guanosine 5'-[γ-thio]triphosphate (GTP[S]) stimulated InsP1 levels by 40 ± 10% (mean ± S.E.M., n = 3), InsP2 levels by 117 ± 18% and InsP3 levels by 51 ± 8%, suggesting that at day 5 in ovo all of the muscarinic-stimulated inositol phosphate production was coupled to phospholipase C via G(p). H.p.l.c. analysis demonstrated that, in spite of these changes in coupling of phospholipase C to different G-proteins, no changes could be demonstrated in the isomers of InsP3 produced in response to carbamylcholine at both days 5 and 14 in ovo. These data demonstrate that embryonic development of the chick atrium is associated with a switch in coupling of muscarinic receptors to phospholipase C from G(p) to a pertussis toxin substrate. This developmental switch in coupling of G-proteins may be related to possible developmental switches in levels of muscarinic receptor isoforms or switches in the subtype of phospholipase C.