Dual actions of lanthanides on ACTH-inhibited leak K(+) channels

Bovine adrenal zona fasciculata cells express background K(+) channels (I(AC) channels) whose activity is potently inhibited by ACTH. In whole cell patch clamp recordings, it was discovered that the trivalent lanthanides (Ln(3+)s) lanthanum and ytterbium interact with two binding sites to modulate K(+) flow through these channels. Despite large differences in ionic radii, these Ln(3+)s inhibited I(AC) channels half-maximally with IC(50) values near 50 microM. In addition, these Ln(3+)s blocked and reversed ACTH-mediated inhibition of I(AC) K(+) channels at similar concentrations. The Ln(3+)s did not alter inhibition of I(AC) by angiotensin II or cAMP. Ln(3+)-induced uncoupling of ACTH receptor activation from I(AC) inhibition was prevented by raising the external Ca(2+) concentration from 2 to 10 mM. The divalent cation Ni(2+) (500 microM) also blocked ACTH-dependent inhibition of I(AC) through a Ca(2+)-sensitive mechanism. The results are consistent with a model in which Ln(3+)s produce opposing actions on I(AC) K(+) currents through two separate binding sites. In addition to directly inhibiting I(AC), Ln(3+)s (and Ni(2+)) bind with high affinity to a Ca(2+)-selective site associated with the ACTH receptor. By displacing Ca(2+) from this site, Ln(3+)s prevent ACTH from binding and accelerate its dissociation. These results identify Ln(3+)s as a relatively potent group of noncompetitive ACTH receptor antagonists. Allosteric actions of trivalent and divalent metal cations on hormone binding, mediated through Ca(2+)-specific sites, may be common to a variety of peptide hormone receptors.