IONOMYCIN ENHANCES CA2+ INFLUX BY STIMULATING STORE-REGULATED CATION ENTRY AND NOT BY A DIRECT ACTION AT THE PLASMA-MEMBRANE

In fura-2-loaded ECV304 cells ionomycin elicited a saturable biphasic change in intracellular Ca2+ concentration ([Ca2+](i)), where the initial phase represented mobilization of intracellular stores and the sustained component represented Ca2+ influx. To examine whether ionomycin could stimulate influx via a store-dependent mechanism, Mn2+ entry was monitored by the quenching of fura-2 fluorescence: influx was enhanced even after ionomycin wash-out, provided that internal stores were not refilled with Ca2+. Moreover, the maximal rate of histamine-stimulated Mn2+ entry was unaffected by ionomycin, suggesting a common route of entry. The Ca2+-entry blocker SK&F 96365 inhibited both the ionomycin-induced Mn2+ entry and the sustained [Ca2+](i) response to the ionophore (leaving the initial peak [Ca2+](i) response unaffected). In other experiments, although addition of ionomycin further increased the plateau phase induced by 100 mu M histamine, the increase was completely abolished by pretreatment with the store Ca2+-ATPase inhibitor cyclopiazonic acid (CPA). Furthermore, in store-depleted cells, re-addition of 1 mM extracellular Ca2+ (in the presence of CPA plus histamine) led to a rapid rise in [Ca2+](i), dependent on Ca2+ influx, with kinetics that were not enhanced by ionomycin. These data suggest that ionomycin acts primarily at the level of the internal Ca2+ stores, so that, at the concentrations used here (less than or equal to mu M), it increases Ca2+ (and Mn2+) influx via activation of endogenous entry pathways and not by plasmalemmal translocation.