Discrete store‐operated calcium influx into an intracellular compartment in rabbit arteriolar smooth muscle

This study tested the hypothesis that store‐operated channels (SOCs) exist as a discrete population of Ca 2+ channels activated by depletion of intracellular Ca 2+ stores in cerebral arteriolar smooth muscle cells and explored their direct contractile function. Using the Ca 2+ indicator fura‐PE3 it was observed that depletion of sarcoplasmic reticulum (SR) Ca 2+ by inhibition of SR Ca 2+ ‐ATPase (SERCA) led to sustained elevation of [Ca 2+ ] i that depended on extracellular Ca 2+ and slightly enhanced Mn 2+ entry. Enhanced background Ca 2+ influx did not explain the raised [Ca 2+ ] i in response to SERCA inhibitors because it had marked gadolinium (Gd 3+ ) sensitivity, which background pathways did not. Effects were not secondary to changes in membrane potential. Thus SR Ca 2+ depletion activated SOCs. Strikingly, SOC‐mediated Ca 2+ influx did not evoke constriction of the arterioles, which were in a resting state. This was despite the fura‐PE3‐indicated [Ca 2+ ] i rise being greater than that evoked by 20 m m [K + ] o (which did cause constriction). Release of endothelial vasodilators did not explain the absence of SOC‐mediated constriction, nor did a change in Ca 2+ sensitivity of the contractile proteins. We suggest SOCs are a discrete subset of Ca 2+ channels allowing Ca 2+ influx into a ‘non‐contractile’ compartment in cerebral arteriolar smooth muscle cells.