Mibefradil Potently Blocks ATP-Activated K⁺ Channels in Adrenal Cells

Abstract

Mibefradil is a novel Ca²⁺ channel antagonist that preferentially blocks T-type Ca²⁺ channels in many cells. Using whole-cell and single-channel patch-clamp recording, we found that mibefradil also potently blocked an ATP-activated K⁺channel (I_AC) expressed by adrenal zona fasciculata cells. I_AC channels were inhibited by mibefradil with an IC₅₀ value of 0.50 μM, a concentration 2-fold lower than that required to inhibit T-type Ca²⁺ channels under similar conditions in the same cells. The inhibition of I_AC by mibefradil was independent of the membrane potential. Mibefradil also reversibly blocked, with similar potency, unitary I_ACcurrents recorded in outside-out membrane patches. An analysis of dwell time histograms indicated the presence of two closed and one open state. Mibefradil (1 μM) increased the duration of the two closed time constants (τ_c1 and τ_c2) from 2.30 ± 0.18 and 27.9 ± 4.7 ms to 4.32 ± 0.61 and 62.5 ± 13.8 ms, respectively, but did not alter the open time constant (τ_o). Mibefradil also failed to reduce the size of the unitary I_AC current. A voltage-gated A-type K⁺current was also inhibited by mibefradil at concentrations approximately 10-fold higher than those required to block I_AC (IC₅₀ = 4.65 μM). These results identify mibefradil as a potent inhibitor of ATP-activated K⁺ channels in adrenal zona fasciculata cells. It appears to function by stabilizing closed states of these channels. In contrast to its selective block of T-type Ca²⁺ channels, mibefradil may be a potent but less-selective K⁺ channel blocker. In this regard, the block of K⁺ channels may produce some of the toxicity associated with mibefradil in cardiovascular pharmacology.