Muscarinic receptors stimulate Ca2+ influx via phospholipase A2 pathway in ileal smooth muscles.

Receptor-mediated changes in intracellular Ca2+ in ileal longitudinal smooth muscles of guinea pig were studied. Stimulation of muscarinic receptors by carbachol induced both Ca(2+)-influx through plasma membranes and Ca(2+)-release from intracellular stores. Pretreatment of the muscle with an inhibitor of phospholipase A2 abolished the Ca2+ influx but not the Ca(2+)-release, whereas an inhibitor of phospholipase C showed opposite effects. The inhibitors of cyclooxygenase and lipoxygenase enhanced the muscarinic receptor-induced Ca(2+)-influx. Addition of arachidonic acid mimic the receptor-induced Ca(2+)-influx. Treatment of muscles with pertussis toxin abolished the receptor-induced release of arachidonic acid as well as Ca(2+)-influx, but was less effective on the IP3 formation and the Ca(2+)-release. Taken together, our results suggest that phospholipase A2 but not phospholipase C pathway is involved in the muscarinic receptor-induced Ca(2+)-influx in ileal smooth muscle.