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

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Abstract
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 studiedIACinhibition 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 μM,IAC reached maximum current density of 25.0 ± 5.1 pA/pF (n = 16); raising [Ca2+]ito 2.0 μM reduced it 76%. In inside-out patches, elevated [Ca2+]idramatically reducedIAC channel activity. Ionomycin inhibitedIAC by 88 ± 4% (n = 14) without altering rapidly inactivating A-type K+ current. Inhibition of IACby 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+]iwas buffered to 0.02 μM with 2 and 11 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA), ionomycin inhibitedIAC 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 inhibitedIAC significantly more effectively when pipette BAPTA was reduced from 11 to 2 mM. Raising [Ca2+]iinhibits IACthrough a mechanism not requiring calmodulin or protein kinases, suggesting direct interaction withIAC channels. ANG II may inhibitIAC and depolarize AZF by activating parallel signaling pathways, one of which uses Ca2+ as a mediator.