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

Bovine adrenalzona fasciculata cells (AZF) express a noninactivatingK⁺ current(I_AC) whoseinhibition by adrenocorticotropic hormone and ANG II may be coupled tomembrane depolarization andCa²⁺-dependentcortisol secretion. We studiedI_ACinhibition byCa²⁺ and theCa²⁺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,I_AC reachedmaximum current density of 25.0 ± 5.1 pA/pF(n = 16); raisingCa²⁺]_ito 2.0 µM reduced it 76%. In inside-out patches, elevatedCa²⁺]_idramatically reducedI_AC channelactivity. Ionomycin inhibited I_AC by 88 ± 4% (n = 14) without altering rapidlyinactivating A-type K⁺ current.Inhibition of I_ACby ionomycin was unaltered by adding calmodulin inhibitory peptide tothe pipette or replacing ATP with its nonhydrolyzable analog5'-adenylylimidodiphosphate.I_AC inhibition byionomycin was associated with membrane depolarization. WhenCa²⁺]_iwas buffered to 0.02 µM with 2 and 11 mM1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), ionomycin inhibitedI_AC 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 inhibitedI_AC significantlymore effectively when pipette BAPTA was reduced from 11 to 2 mM. RaisingCa²⁺]_iinhibits I_ACthrough a mechanism not requiring calmodulin or protein kinases,suggesting direct interaction withI_AC channels. ANGII may inhibitI_AC anddepolarize AZF by activating parallel signaling pathways, one of whichuses Ca²⁺ asa mediator.