Roles of the plasma membrane in Ca^<2+> oscillations in smooth muscle cells

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The present study was made to elucidate the role of the plasma membrane and intracelullar Ca^<2+> stores in oscillations in cytosolic Ca^<2+> concentration following stimulation of muscarinic receptors in single smooth muscle cells of guinea-pig ileum.1. The muscarinic receptor-mediated Ca^<2+> oscillations occur differentially because of Ca^<2+> influx across the plasma membrane associated with action potential discharge and of Ca^<2+> release from internal stores by a second messenger, inositol trisphosphate.2. Ca^<2+> store-derived Ca^<2+> oscillations require a sustained Ca^<2+> influx for their persistence and regulation of the oscillation frequency. The L-type of Ca^<2+> channel and an as yet unidentified Ca^<2+> channel may be involved in the sustained Ca^<2+> influx.3. The plasma membrane-derived type of Ca^<2+> oscillation may have no or little contribution from Ca^<2+>-induced Ca^<2+> release ; Ca^<2+> entering the cell during action potential discharge activtes ryanodine-sensitive, Ca^<2+>-gated Ca^<2+> channels in the internal store to release Ca^<2+> from there.4. During generation of Ca^<2+> store-derived Ca^<2+> oscillations, membrane potential oscillates coincidentally in time with them. The membrane potential oscillations result each from potentiation of the activity of muscarinic receptor-operated nonselective cation channels by a rise in cytosolic Ca^<2+> concentration.The results of no.1,2 and 3 have been published in international technical journals and those of no.4 are under submission.