CALCIUM CHANNELS AND CONTROL OF CYTOSOLIC CALCIUM IN RAT AND BOVINE ZONA GLOMERULOSA CELLS

Rat and bovine adrenal zona glomerulosa (ZG) cells possess a low-threshold, voltage-dependent Ca2+ current that was characterized using whole cell voltage clamp techniques. Activation of this current is observed at membrane potentials above -80 mV with maximal peak Ca2+ current elicited near -30 mV. Inactivation of the Ca2+ current was half-maximal between -74 and -58 mV, depending on the external Ca2+ concentration and was nearly complete at -40 mV. The voltage dependency of the current indicates that a calcium current could be sustained at membrane potentials between -80 and -40 mV and thereby elevates cytosolic calcium (Ca(i)) levels. Under basal conditions, Ca(i) is stable in single rat ZG cells, whereas more than half of the bovine ZG cells produce repeated Ca(i) transients. These Ca(i) transients, which are blocked by removal of external Ca2+ or addition of Ni2+, are likely due to repetitive electrical activity in bovine ZG cells. Ca(i) responses can be elicited by small increases in external K+ concentration (5-10 mM) in both rat and bovine ZG cells, indicating the opening of low-threshold Ca2+ channels. However, these Ca(i) changes remain robust at high external K+ concentrations (20-40 mM). In experiments combining Ca(i) measurements and whole cell voltage clamp, a steep dependence of Ca(i) on membrane potential was revealed beginning at depolarizing voltages near a holding membrane potential of -80 mV. A maximal increase in Ca(i) occurred near -30 mV (equivalent to an external K+ concentration of 40 mM), a membrane voltage at which sustained current through low-threshold Ca2+ channels should be negligible. These data raise the possibility of additional voltage-dependent pathways for Ca2+ influx.