Store-operated Ca2+-permeable non-selective cation channels in smooth muscle cells

Over twenty years ago it was shown that depletion of the intracellular Ca2+ store in smooth muscle triggered a Ca2+ influx mechanism. The purpose of this review it to describe recent electrophysiological data which indicate that Ca2+ influx occurs through discrete ion channels in the plasmalemma of smooth muscle cells. The effect of external Ca2+ on the amplitude and reversal potential of whole-cell and single channel currents suggests that there are at least two, and probably more, distinct store-operated channels (SOCs) which have markedly different permeabilities to Ca2+ ions. Two activation mechanisms have been identified which involve Ca2+ influx factor and protein kinase C (PKC) activation via diacylglycerol. In addition, in rabbit portal vein cells there is evidence that stimulation of alpha-adrenoceptors can stimulate SOC opening via PKC in a store-independent manner. There is at present little knowledge on the molecular identity of SOCs but it has been proposed that TRPC1 may be a component of the functional channel. We also summarise the data showing that SOCs may be involved in contraction and cell proliferation of smooth muscle. Finally, we highlight the similarities and differences of SOCs and receptor-operated cation channels that are present in native rabbit portal vein myocytes.