Properties of a native cation channel activated by Ca2+ store depletion invascular smooth muscle cells

Depletion of intracellular Ca2+ stores activates capacitative Ca2+ influx i n smooth muscle cells, but the native store-operated channels that mediate such influx remain unidentified. Recently we demonstrated that calcium infl ux factor produced by yeast and human platelets with depleted Ca2+ stores a ctivates small conductance cation channels in excised membrane patches from vascular smooth muscle cells (SMC). Here we characterize these channels in intact cells and present evidence that they belong to the class of store-o perated channels, which are activated upon passive depletion of Ca2+ stores . Application of thapsigargin (TG), an inhibitor of sarco-endoplasmic retic ulum Ca2+ ATPase, to individual SMC activated single 3-pS cation channels i n cell-attached membrane patches. Channels remained active when inside-out membrane patches were excised from the cells. Excision of membrane patches from resting SMC did not by itself activate the channels. Load-ing SMC with BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid), which sl owly depletes Ca2+ stores without a rise in intracellular Ca2+, activated t he same 3-pS channels in cell-attached membrane patches as well as whole ce ll nonselective cation currents in SMC, TG- and BAPTA-activated 3-pS channe ls were cation-selective but poorly discriminated among Ca2+, Sr2+, Ba2+, N a+, K+, and Cs+. Open channel probability did not change at negative membra ne potentials but increased significantly at high positive potentials. Acti vation of 3-pS channels did not depend on intracellular Ca2+ concentration. Neither TG nor a variety of second messengers (including Ca2+, InsP3, InsP 4, GTP gammaS, cyclic AMP, cyclic GIMP, ATP, and ADP) activated 3-pS channe ls in inside-out membrane patches. Thus, 3-pS nonselective cation channels are present and activated by TG or BAPTA-induced depletion of intracellular Ca2+ stores in intact SMC. These native store-operated cation channels can account for capacitative Ca2+ influx in SMC and can play an important role in regulation of vascular tone.