Effects of diltiazem on calcium metabolism in cultured bovine glomerulosa cells: relationships to the actions of angiotensin II and potassium

Abstract

The effects of diltiazem on Ca++ metabolism in bovine glomerulosa cells maintained in primary monolayer culture were examined to characterize further the pathways by which angiotensin II (AII) and K+ promote Ca++ influx. Diltiazem (0.05-0.5 mM) inhibited 45Ca++ uptake by control cells in a concentration-dependent manner. At concentrations of 0.1 mM or more, diltiazem also mobilized an intracellular Ca++ store and transiently increased the cytosolic [Ca++]. In cells pretreated with drug for 15 min, half-maximal inhibition of K+ (15 mM)-stimulated increases in both Ca++ influx and cytosolic [Ca++] was achieved at approximately 0.02 mM diltiazem. Increases in Ca++ influx and cytosolic [Ca++] caused by AII (1 nM) were less sensitive to inhibition by diltiazem than were those caused by K+. The AII-stimulated Ca++ signal was affected by diltiazem at concentrations of 0.1 mM or more, whereas the AII-stimulated Ca++ influx was inhibited only at concentrations greater than 0.5 mM. In general, the AII-stimulated peak in cytosolic [Ca++] was more sensitive to inhibition by diltiazem than was the plateau and the extent of inhibition increased with the concentration and duration of treatment. The effect of diltiazem on aldosterone secretion in control cells correlated closely with its ability to inhibit Ca++ influx. In K(+)-stimulated cells, in contrast, the IC50 for diltiazem inhibition of aldosterone secretion was 10 to 20-fold less than the IC50 for inhibition of Ca++ influx or cytosolic [Ca++]. In AII-stimulated cells, the relationships between Ca++ uptake, cytosolic [Ca++] and aldosterone secretion varied with the duration of diltiazem treatment. The effect of diltiazem on the initial secretory response to AII correlated most closely with its effect on Ca++ uptake; the effect of diltiazem on the sustained secretory response correlated with its effect on the initial, AII-stimulated calcium signal. The present results indicate that diltiazem has multiple, concentration-dependent actions in glomerulosa cells and that the mechanism(s) by which the drug inhibits aldosterone secretion differ(s) under basal, K(+)-stimulated and AII-stimulated conditions. Furthermore, they support the premise that K+ and AII promote Ca++ influx by way of different mechanisms.