Kinetics of Cytosolic Calcium and Aldosterone Responses in Rat Adrenal Glomerulosa Cells*

Abstract

To evaluate the relationship between cytosolic calcium (Cai) and aldosterone production, rat adrenal zona glomerulosa (ZG) cells were studied during long-term stimulation by different secretagogues. Cai was measured in single ZG cells using microspectrofluorimetry, and aldosterone was determined in cell populations using a superfusion system. For external potassium (K+), Cai increases are sustained, with only a slight decrement over time, a feature shared by aldosterone production. The relationship between aldosterone output and Cai is nonlinear, with a Cai value for half-maximal stimulation of approximately 500 nM. Furthermore, the sustained changes in Cai with external K+ indicate that ZG cells can use an amplitude-based Cai signal to stimulate aldosterone production. Cai changes stimulated by angiotensin-II (Ang-II) show a complex dose-response pattern, with high concentrations (greater than or equal to 1 nM) of Ang-II eliciting a peak-plateau signal and lower doses (0.1 nM to 10 pM) producing repeated Cai oscillations. The peak amplitude of the Cai response in individual cells is not dose dependent, with the ZG cell experiencing peak levels repeatedly at the lowest Ang-II concentrations. However, the Cai transients are more frequent with increasing Ang-II concentrations between 0.1 nM and 10 pM. When integrated over time, the mean Cai signal also shows only modest dose-dependency during the sustained phase of Ang-II stimulation. Unlike the integrated Cai signal, aldosterone production increases steeply between 10 pM and 0.1 nM Ang-II, indicating that the Cai signal is likely to be frequency-based. Conversely, the steroid response to high Ang-II closely mirrors the kinetics of the more sustained Cai signals, including the diminished Cai and aldosterone levels during sustained stimulation with the highest Ang-II doses. Arginine vasopressin stimulated Cai and aldosterone responses, which closely resemble those elicited by 0.1 nM Ang-II, except that both Cai and aldosterone return to basal values within 20 min of continuous presentation of arginine vasopressin. Each ZG secretagogue produces a distinct pattern of Cai and aldosterone response. In addition, Cai response patterns can be divided into two general classes: a sustained Cai response, which appears to modulate cell activation by the amplitude of the Cai signal, and an oscillating Cai response, which uses the frequency of the Cai transients to control the magnitude of stimulation.