Properties of ATP-dependent K⁺ channels in adrenocortical cells

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 Ca(2+) conductance. In symmetrical K(+) solutions with physiological concentrations of divalent cations (M(2+)), I(AC) channels were outwardly rectifying with outward and inward chord conductances of 94.5 and 27.0 pS, respectively. In the absence of M(2+), 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 (PPP(i)) dramatically enhanced I(AC) activity. In current-clamp recordings, nucleotides and PPP(i) 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 PPP(i). Furthermore, they suggest a model for I(AC) gating that is controlled through a cycle of ATP binding and hydrolysis.

ACTH-induced Cl−current in bovine adrenocortical cells: correlation with cortisol secretion

“…B: this graph drawn from the recording shown in A illustrates the change of both the membrane potential (left axis, F) and the membrane conductance (right axis, E) during the exposure of the Cs ϩ -loaded cell to ACTH. mechanisms involved in this process are likely similar to those extensively studied by Enyeart and co-workers (7,9,10,17,30), depolarization associated with a decrease in membrane conductance was not further characterized in the present work.…”

Caffeic Acid Esters Activate TREK-1 Potassium Channels and Inhibit Depolarization-Dependent Secretion

“…The activity of native bTREK-1 channels in AZF cells is enhanced by acidification of the pipette solution and by PPP i (Xu and Enyeart, 2001;Enyeart et al, 2002). The rectifying properties of bTREK-1, activated by acidifying the pH of the pipette solution to 6.4 and adding 5 mM PPP i , were compared with those of the CDC-activated current using ramp voltage protocols.…”

“…For small spherical cells such as AZF cells, these molecules reach a maximum intracellular concentration in the cytoplasm within seconds (Pusch and Neher, 1988). In previous studies, we have shown that bTREK-1 channels are effectively activated by nucleotides and inorganic polytriphosphates, applied through the patch pipette (Enyeart et al, 1997;Xu and Enyeart, 2001). …”

An ACTH- and ATP-regulated Background K+ Channel in Adrenocortical Cells Is TREK-1

“…In whole cell patch clamp recordings, I AC appears as a noninactivating, outwardly rectifying K ϩ current that displays weak voltage dependence and grows spontaneously over many minutes (1,3,4). At the single channel level, the conductance and rectifying properties of these channels depend strongly on the presence of divalent cations (5). In symmetrical K ϩ , with physiological concentrations of Ca 2ϩ and Mg 2ϩ , the channels are outwardly rectifying, whereas they become nearly ohmic in the absence of divalent cations (4,5).…”