Early and late effects of angiotensin-II on Ca2+ fluxes in bovine adrenal zona glomerulosa cells.

Previous studies have yielded conflicting results concerning the effects of angiotensin-II (Ang II) on Ca2+ fluxes in adrenal zona glomerulosa (ZG) cells. The present study was designed to investigate the kinetics and dose-dependency of Ang II-mediated changes in Ca2+ influx and efflux in cultured bovine ZG cells. At a high (10 nM) Ang II concentration, cytosolic Ca2+ (Cai) shows a peak-plateau response for the first 15 min, with small Cai transients commonly observed with longer stimulations. At 50 pM Ang II, more sustained Cai changes were elicited, typically consisting of Cai oscillations. The underlying changes in Ca2+ influx and efflux were studied. The early modifications of Ca2+ influx after 2 min of agonist stimulation were biphasic, with uptake increased by 90% between 1-100 pM Ang II and inhibited by 30% at 10 nM Ang II. Furthermore, high (10 nM) Ang II doses inhibited extracellular K(+)-stimulated Ca2+ influx. After 30 min of Ang II stimulation, the later dose response of Ca2+ influx was of similar magnitude but shifted to the left, showing a maximal influx at 10 pM Ang II and a modest enhancement at 10 nM. Basal Ca2+ efflux followed a two-compartment exponential decay, reflecting rapid Ca2+ displacement from extracellular sites (k1) and active Ca2+ transport (k2). A high (10 nM) Ang II concentration induced a transient large increase (130%) in k2 during the initial phase of Ang II stimulation, which returned to basal values within 10 min. A low (50 pM) Ang II concentration induced a small sustained increase (30%) in k2. A 10-nM Ang II concentration markedly reduced the exchangeable Ca2+ pool, as Ca2+ mobilized from intracellular stores into the cytosol was rapidly extruded, while Ca2+ influx was inhibited. A more physiological (50 pM) concentration of Ang II did not significantly alter the total exchangeable Ca2+ pool due to modest stimulation of both Ca2+ efflux and influx. In summary, the initial transient Cai response to high Ang II results from a large Ca2+ mobilization combined with inhibition of Ca2+ influx, which does not allow for the refilling of Ca2+ stores. At later times, small increases in Ca2+ influx allow for the eventual recovery of exchangeable cell Ca2+ and an enhanced elevation in Cai. At low Ang II concentrations, stimulation of both Ca2+ influx and efflux are concurrent and maintained, allowing for a sustained increase in Cai with little change in exchangeable cell Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)