Ca2+ depolarizes adrenal cortical cells through selective inhibition of an ATP-activated K+ current

Bovine adrenal zona fasciculata cells (AZF) express a noninactivating K+ current (IAC) whose inhibition by adrenocorticotropic hormone and ANG II may be coupled to membrane depolarization and Ca2+-dependent cortisol secretion. We studied IAC inhibition by Ca2+ and the Ca2+ ionophore ionomycin in whole cell and single-channel patch-clamp recordings of AZF. In whole cell recordings with intracellular (pipette) Ca2+ concentration ([Ca2+]i) buffered to 0.02 microM, IAC reached maximum current density of 25.0 +/- 5.1 pA/pF (n = 16); raising [Ca2+]i to 2.0 microM reduced it 76%. In inside-out patches, elevated [Ca2+]i dramatically reduced IAC channel activity. Ionomycin inhibited IAC by 88 +/- 4% (n = 14) without altering rapidly inactivating A-type K+ current. Inhibition of IAC by ionomycin was unaltered by adding calmodulin inhibitory peptide to the pipette or replacing ATP with its nonhydrolyzable analog 5'-adenylylimidodiphosphate. IAC inhibition by ionomycin was associated with membrane depolarization. When [Ca2+]i was buffered to 0.02 microM with 2 and 11 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid (BAPTA), ionomycin inhibited IAC by 89.6 +/- 3.5 and 25.6 +/- 14.6% and depolarized the same AZF by 47 +/- 8 and 8 +/- 3 mV, respectively (n = 4). ANG II inhibited IAC significantly more effectively when pipette BAPTA was reduced from 11 to 2 mM. Raising [Ca2+]i inhibits IAC through a mechanism not requiring calmodulin or protein kinases, suggesting direct interaction with IAC channels. ANG II may inhibit IAC and depolarize AZF by activating parallel signaling pathways, one of which uses Ca2+ as a mediator.