The T-type Ca²⁺ Channel Inhibitor Mibefradil Inhibits Voltage-Dependent K⁺ Channels in Rabbit Coronary Arterial Smooth Muscle Cells

Abstract

We examined the effects of mibefradil, a T-type Ca²⁺ channel inhibitor, on voltage-dependent K⁺ (Kv) channels in rabbit coronary arterial smooth muscle cells using the whole-cell patch clamp technique. Mibefradil reduced the Kv current amplitude in a dose-dependent manner, with an apparent K_d value of 1.08 μM. Kv current inhibition by mibefradil was highly voltage-dependent over the full activation voltage range (−30 to +10 mV). The decay rate of Kv channel inactivation was accelerated by mibefradil without altering the kinetics of current activation. The rate constants of association and dissociation were 2.23 ± 0.07 μM⁻¹·s⁻¹ and 2.40 ± 0.42 s⁻¹, respectively. Mibefradil had no significant effect on the steady-state activation or inactivation curves. In the presence of mibefradil, the recovery time constant from inactivation was decreased, and the application of train pulses (1 or 2 Hz) increased mibefradil-induced Kv channel inhibition, suggesting that the inhibitory effects of mibefradil were use-dependent. The inhibitory effect of mibefradil on Kv channels was unaffected by extracellular Ca²⁺-free conditions. Moreover, the absence of ATP inside the pipette did not alter the blocking effect of mibefradil. Therefore, we suggest that mibefradil directly inhibited the Kv current, independently of Ca²⁺ channel inhibition.