EARLY AND LATE EFFECTS OF ANGIOTENSIN-II ON CA2+ FLUXES IN BOVINE ADRENAL ZONA GLOMERULOSA CELLS

Previous studies have yielded conflicting results concerning the effec ts of angiotensin-II (Ang II) on Ca2+ fluxes in adrenal zona glomerulo sa (ZG) cells. The present study was designed to investigate the kinet ics 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 concentra tion, cytosolic Ca2+ (Ca(i)) shows a peak-plateau response for the fir st 15 min, with small Ca(i) transients commonly observed with longer s timulations. At 50 pm Ang II, more sustained Ca(i) changes were elicit ed, typically consisting of Ca(i) oscillations. The underlying changes in Ca2+ influx and efflux were studied. The early modifications of Ca 2+ influx after 2 min of agonist stimulation were biphasic, with uptak e increased by 90% between 1-100 pm Ang II and inhibited by 30% at 10 nm Ang II. Furthermore, high (10 nm) Ang 11 doses inhibited extracellu lar K+-stimulated Ca2+ influx. After 30 min of Ang II stimulation, the later dose response of Ca'' influx was of similar magnitude but shift ed 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 exp onential decay, reflecting rapid Ca2+ displacement from extracellular sites (k1) and active Ca2+ transport (k2). A high (10 nm) Ang II conce ntration induced a transient large increase (130%) in k2 during the in itial phase of Ang II stimulation, which returned to basal values with in 10 min. A low (50 pm) Ang II concentration induced a small sustaine d increase (30%) in k2. A 10-nm Ang II concentration markedly reduced the exchangeable Ca2+ pool, as Ca2+ mobilized from intracellular store s into the cytosol was rapidly extruded, while Ca2+ influx was inhibit ed. A more physiological (50 pm) concentration of Ang II did not signi ficantly alter the total exchangeable Ca2+ pool due to modest stimulat ion of both Ca2+ efflux and influx. In summary, the initial transient Ca(i) response to high Ang II results from a large Ca2+ mobilization c ombined with inhibition of Ca2+ influx, which does not allow for the r efilling of Ca2+ stores. At later times, small increases in Ca2+ influ x allow for the eventual recovery of exchangeable cell Ca2+ and an enh anced elevation in Ca(i). At low Ang II concentrations, stimulation of both Ca2+ influx and efflux are concurrent and maintained, allowing f or a sustained increase in Ca(i) with little change in exchangeable ce ll Ca2+. Thus, Ca2+ transport pathways are modulated differently depen ding on the Ang II concentration and the duration of Ang II stimulatio n. Contradictory reports from previous studies may result from differe nces in the sensitivity of ZG cell preparations and in the examination of Ang II effects at different doses and times.