ACTH inhibits the noninactivating K⁺ current in human AZF cells. (A) ACTH selectively inhibits noninactivating current. K⁺ currents were activated by voltage steps to 20 mV applied at 30-s intervals from a holding potential of −80 mV with (right traces) or without (left traces) 10-s prepulses to −20 mV. When the noninactivating current reached a stable maximum, the cell was superfused with 1 nM ACTH. Current amplitudes are plotted against time at right. (B) Voltage-independent inhibition by ACTH. After the noninactivating K⁺ current reach a maximum, K⁺ currents were activated by voltage steps applied at 30-s intervals from a holding potential of −80 mV to test potentials between −60 and 40 mV. I-V relationships were recorded before (left traces) and after (right traces) superfusing the cell with 1 nM ACTH. Noninactivating current amplitudes are plotted against voltage in the absence (closed circles) and presence (open circles) of ACTH in the graph at right.