Calcium influx factor directly activates store-operated cation channels in vascular smooth muscle cells

Recently, we described a novel 3-pS Ca2+-conducting channel that is activated by BAPTA and thapsigargin-induced passive depletion of intracellular Ca2+ stores and likely to be a native store-operated channel in vascular smooth muscle cells (SMC). Neither Ca2+ nor inositol 1,4,5-trisphosphate or other second messengers tested activated this channel in membrane patches excised from resting SMC. Here we report that these 3-pS channels are activated in inside-out membrane patches from SMC immediately upon application of Ca2+ influx factor (CIF) extracted from mutant yeast, which has been previously shown to activate Ca2+ influx in Xenopus oocytes and Ca2+ release-activated Ca2+ current in Jurkat cells. In bioassay experiments depletion of Ca2+ stores in permeabilized human platelets resulted in the release of endogenous factor, which activated 3-pS channels in isolated inside-out membrane patches excised from SMC and exposed to permeabilized platelets. The same 3-pS channels in excised membrane patches were also activated by acid extracts of CIF derived from human platelets with depleted Ca2+ stores, which also stimulated Ca2+ influx upon injection into Xenopus oocytes, Specific high pressure liquid chromatography fractions of platelet extracts were found to have CIF activity when injected into oocytes and activate 3-pS channels in excised membrane patches. These data show for the first time that CIF produced by mammalian cells and yeast with depleted Ca2+ stores directly activates native 3-pS cation channels, which in intact SMC are activated by Ca2+ store depletion.