Adenosine Triphosphate Activates a Noninactivating K+ Current in Adrenal Cortical Cells through Nonhydrolytic Binding

Abstract

Bovine adrenal zona fasciculata (AZF) cells express a noninactivating K+ current (IAC) that is inhibited by adrenocorticotropic hormone and angiotensin II at subnanomolar concentrations. Since IAC 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. IAC 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 IAC K+ current. The maximum IAC 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, IAC K+ channels were tonically active over a wide range of membrane potentials, and voltage-dependent open probability increased by only ∼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 IAC, 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 IAC 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 GTP, UTP, GDP, and UDP all increased the outwardly rectifying IAC current with an apparent order of effectiveness: MgATP > ATP = AMP-PNP > GTP = UTP > ADP >> GDP > AMP and ATP-γ-S. Although ATP, GTP, and UTP all enhanced IAC amplitude with similar effectiveness, inhibition of IAC by ACTH (200 pM) occurred only in the presence of ATP. As little as 50 μM MgATP restored complete inhibition of IAC, which had been activated by 5 mM UTP. Although the opening of IAC 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 IAC channels are activated through the nonhydrolytic binding of ATP. Because they are activated rather than inhibited by ATP binding, IAC K+ channels may represent a distinctive new variety of K+ channel. The combined features of IAC 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 IAC K+ channels.