Properties of ATP-dependent K+ channels in adrenocortical cells

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Abstract
Bovine adrenocortical zona fasciculata (AZF) cells express a novel ATP-dependent K+-permeable channel ( I AC). Whole cell and single-channel recordings were used to characterize I AC channels with respect to ionic selectivity, conductance, and modulation by nucleotides, inorganic phosphates, and angiotensin II (ANG II). In outside-out patch recordings, the activity of unitary I AC channels is enhanced by ATP in the patch pipette. These channels were K+ selective with no measurable Na+ or Ca2+ conductance. In symmetrical K+ solutions with physiological concentrations of divalent cations (M2+), I ACchannels were outwardly rectifying with outward and inward chord conductances of 94.5 and 27.0 pS, respectively. In the absence of M2+, conductance was nearly ohmic. Hydrolysis-resistant nucleotides including AMP-PNP and NaUTP were more potent than MgATP as activators of whole cell I AC currents. Inorganic polytriphosphate (PPPi) dramatically enhanced I AC activity. In current-clamp recordings, nucleotides and PPPi produced resting potentials in AZF cells that correlated with their effectiveness in activating I AC. ANG II (10 nM) inhibited whole cell I AC currents when patch pipettes contained 5 mM MgATP but was ineffective in the presence of 5 mM NaUTP and 1 mM MgATP. Inhibition by ANG II was not reduced by selective kinase antagonists. These results demonstrate that I AC is a distinctive K+-selective channel whose activity is increased by nucleotide triphosphates and PPPi. Furthermore, they suggest a model for I AC gating that is controlled through a cycle of ATP binding and hydrolysis.