Adenosine triphosphate activates a noninactivating K+ current in adrenal cortical cells through nonhydrolytic binding

Bovine adrenal zona fasciculata (AZF) cells express a noninactivating K+ current (I-AC) that is inhibited by adrenocorticotropic hormone and angiotensin II at subnanomolar concentrations. Since I-AC appears to set the membrane potential of AZF cells, these channels may function critically in coupling peptide receptors to membrane depolarization, Ca2+ entry, and cortisol secretion. I-AC channel activity may be tightly linked to the metabolic state of the cell. In whole cell patch clamp recordings, MgATP applied intracellularly through the patch electrode at concentrations above 1 mM dramatically enhanced the expression of I-AC K+ current. The maximum I-AC current density varied from a low of 8.45 +/- 2.74 pA/pF (n = 17) to a high of 109.2 +/- 26.3 pA/pF (n = 6) at pipette MgATP concentrations of 0.1 and 10 mM, respectively. In the presence of 5 mM MgATP, I-AC K+ channels were tonically active over a wide range of membrane potentials, and voltage-dependent open probability increased by only similar to 30% between -40 and +40 mV. ATP (5 mM) in the absence of Mg2+ and the nonhydrolyzable ATP analog AMP-PNP (5 mM) were also effective at enhancing the expression of I-AC, from a control value of 3.7 +/- 0.1 pA/pF (n = 3) to maximum values of 48.5 +/- 9.8 pA/pF (n = 11) and 67.3 +/- 23.2 pA/pF (n = 6), respectively. At the single channel level, the unitary I-AC current amplitude did not vary with the ATP concentration or substitution with AMP-PNP. In addition to ATP and AMP-PNP, a number of other nucleotides including CTP, UTP, GDP, and UDP all increased the outwardly rectifying I-AC current with an apparent order of effectiveness: MgATP > ATP = AMP-PNP > GTP = UTP > ADP >> GDP > AMP and ATP-gamma-S. Although ATP, GTP, and UTP all enhanced I-AC amplitude with similar effectiveness, inhibition of I-AC by ACTH (200 pM) occurred only in the presence of ATP. As little as 50 mu M MgATP restored complete inhibition of I-AC, which had been activated by 5 mM UTP. Although the opening of I-AC channels may require only ATP binding, its inhibition by ACTH appears to involve a mechanism other than hydrolysis of this nucleotide. These findings describe a novel form of K+ channel modulation by which I-AC channels are activated through the nonhydrolytic binding of ATP. Because they are activated rather than inhibited by ATP binding, I-AC K+ channels may represent a distinctive new variety of K+ channel. The combined features of I-AC channels that allow it to sense and respond to changing ATP levels and to set the resting potential of AZF cells, suggest a mechanism where membrane potential, Ca2+ entry, and cortisol secretion could be tightly coupled to the metabolic state of the cell through the activity of I-AC K+ channels.