Phospholipase C involvement in activation of the muscarinic receptor-operated cationic current in guinea pig ileal smooth muscle cells

In guinea pig single ileal smooth muscle cells held under voltage-clamp, the role of phospholipase C (PLC) in activation of the muscarinic receptor-operated cationic current (I-cat) was studied. U73122, a PLC inhibitor, prevented the generation of I-cat by the muscarinic agonist carbachol. The effect did not involve muscarinic receptor block since it also blocked I-cat which was evoked by GTPgammaS applied intracellularly to activate G proteins bypassing muscarinic receptors. Also, neither cationic channel block nor other possible nonspecific actions seemed to be involved since its analogue (U73343), structurally close but deficient of the PLC-inhibiting activity, did not significantly affect carbachol- or GTPgammaS-evoked I-cat. Antibodies against the a subunits of G(q)/G(11) proteins (Galpha(q)/Galpha(11)-antibody) blocked only the small component of carbachol-evoked I-cat, which was associated with an increase in [Ca2+](i) linked to an increase in G(q/11) protein-regulated PLC activity. 1-Oleoyl-2-acetyl-sn-glycerol (OAG), an analogue of diacylglycerol (DAG) produced via PLC-catalyzed metabolism, produced no or only a small current by itself, with the carbachol-evoked I-cat remaining unchanged. These results provide evidence for the importance of PLC in I-cat generation, and they also strongly suggest that the activity of PLC involved in the primary activation of I-cat is neither under regulation by G(q/11) proteins nor dependent on the action of DAG.