Angiotensin II priming of aldosterone secretion with agents that enhance Ca²⁺ influx

We have previously shown that angiotensin II (AngII) is able to prime, or sensitize, the secretory response of cultured bovine adrenal glomerulosa cells to the Ca²⁺ channel agonist, BAY K8644. We examined the ability of AngII to prime glomerulosa cells to an elevated extracellular K⁺ level, a physiological agonist that also triggers Ca²⁺ influx. K⁺ (9 mM) elicited enhanced secretion in AngII-primed cells compared to those with no prior exposure to the hormone, suggesting that AngII can sensitize glomerulosa cells to respond to increases in extracellular K⁺. The potential involvement of protein kinase C (PKC) in priming was investigated by determining whether enhanced Ca²⁺ influx could maintain the AngII-induced phosphorylation of the endogenous PKC substrate, myristoylated, alanine-rich C kinase substrate (MARCKS). Incubation with the AngII antagonist, saralasin, for 30 min following an AngII exposure reduced the AngII-induced increase in MARCKS phosphorylation. 100 nM BAY K8644 together with saralasin was unable to maintain AngII-stimulated MARCKS phosphorylation. On the other hand, phosphorylation of the steroidogenic acute regulatory (StAR) protein was sustained with saralasin exposure, both in the presence and absence of BAY K8644. This observation suggests that persistent StAR phosphorylation/activation may play a role in priming.