The role of calcium influx pathways in phospholipase D activation in bovine adrenal glomerulosa cells

The steroid hormone aldosterone maintains sodium homeostasis and is therefore important in the control of blood volume and pressure. Angiotensin II (AngII) and elevated extracellular potassium concentrations ([K⁺]_e), the prime physiologic regulators of aldosterone secretion from adrenal glomerulosa cells, activate phospholipase D (PLD) in these cells. The role of Ca²⁺ in the activation by these agents is unknown, although nitrendipine, a voltage-dependent Ca^2C channel antagonist, does not inhibit AngII-elicited PLD activation, despite the fact that this compound blocked elevated [K⁺]_e-stimulated PLD activity. PLD activation triggered by AngII was also unaffected by the T-type calcium channel inhibitor nickel. Nevertheless, Ca²⁺ influx was required for AngII-induced PLD activation in both primary cultures of bovine adrenal glomerulosa cells and a glomerulosa cell model, the NCI H295R adrenocortical carcinoma cell line. The involvement of store-operated Ca²⁺ (SOC) influx and Ca²⁺ release-activated Ca²⁺ (CRAC) influx pathways in PLD activation was investigated using thapsigargin, an endoplasmic reticulum Ca²⁺ pump inhibitor that empties the store to induce SOC influx, and the SOC inhibitor YM-58483 (BTP2), as well as a CRAC inhibitor, tyrphostin A9. In bovine glomerulosa cells, tyrphostin A9 inhibited AngII-induced PLD activation without affecting elevated [K⁺]_e-stimulated enzyme activity. On the other hand, differences were observed between the bovine adrenal glomerulosa and H295R cells in the involvement of Ca²⁺ influx pathways in PLD activation, with the involvement of the SOC pathway suggested in the H295R cells. In summary, our results indicate that Ca²⁺ entry only through certain Ca²⁺ influx pathways is linked to PLD activation.