Muscarinic cholinergic stimulation of inositol phosphate production in cultured embryonic chick atrial cells. Evidence for a role of two guanine-nucleotide-binding proteins

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Abstract
These studies demonstrate a novel mechanism for the coupling of the muscarinic receptor to phospholipase C activity in embryonic chick atrial cells. In monolayer cultures of atrial cells from hearts of embryonic chicks at 14 days in ovo, carbamylcholine stimulated the sequential appearance of InsP3, InsP2 and InsP1 with an EC50 (concn. causing 50% of maximal stimulation) of 30 .mu.M. In the presence of 15 mM-Li, a 5 min exposure to carbamylcholine (0.1 mM) increased InsP3 levels to a maximum of 47.+-. 12% over basal, InsP2 to 108 .+-. 13% over basal and InsP1 to 42 .+-. 5% over basal. This effect was blocked by 5 .mu.M-atropine. Incubation of these cells with pertussis toxin (15 h; 0.5 ng/ml) inhibited carbamylcholine-stimulated InsP3, InsP2 and InsP1 formation by 42 .+-. 7%, 30 .+-. 3% and 48 .+-. 7% respectively. The IC50 (con. causing 50% inhibition) for pertussis toxin inhibited of all three inositol phosphates was 0.01 ng/ml, with a half-time of 6 h at 0.05 ng/ml. This partial sensitivity to pertussis toxin was not due to incomplete ADP-ribosylation of the guanine-nucleotide-binding protein (G-protein), since autoradiography of polyacrylamide gels of cell homogenates incubated with [32P]NAD+ in the presence of pertussis toxin demonstrated that incubation of cells with 0.05 ng of pertussis toxin/ml for 15 h resulted in complete ADP-ribosylation of pertussis toxin substrates by endogenous NAD+. In cells permeabilized with saponin (10 .mu.g/ml), 0.1 mM-GTP[S] (guanosine 5''-[.gamma.-thio]triphosphate) stimulated InsP1 by 102 .+-. 15% (mean .+-. S.E.M., n = 4), InsP2 by 421 .+-. 67% and InsP3 by 124 .+-.33% above basal. Incubation of cells for 15 h with 0.5 ng of pertussis toxin/ml decreased GTP[S]-stimulated InsP1 production in saponin-treated cells by 30 .+-. 10% (n = 3), InsP2 production by 45 .+-. 7% (n = 4) and InsP3 production by 49 .+-. 6% (n = 4). These data demonstrate that in embryonic chick atrial cells at least two independent G-proteins, a pertussis toxin-sensitive G-protein and a pertussis toxin-insensitive G-protein, play a role in coupling muscarinic agonist binding to phospholipase C activation and to inositol phosphate production.