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 alpha 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 [Ca(2+)](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.