The ionic mechanism of action of probucol, a known lipid-lowering agent, was examined in rat pituitary GH3 cells. Whole-cell voltage-clamp was used to measure hyperpolarization-activated K+ currents in GH3 cells bathed in a high-K+, Ca2+-free solution to determine the effect of probucol on the erg-like inwardly rectifying K+ current (IK(IR)). Probucol reversibly suppressed the amplitude of IK(IR) in a concentration-dependent manner. The IC50 value of probucol-induced inhibition of IK(IR) was 1 μM. Probucol shifted the steady-state inactivation curve of IK(IR) to less negative potentials and also prolonged the recovery of IK(IR) inactivation. The K+ inward current in response to hyperpolarizing voltage pulses was also inhibited by haloperidol (10 μM) and bepridil (10 μM) but not by reduced glutathione (10 mM) or superoxide dismutase (500 U/ml). Pretreatment with t-butyl hydroperoxide (1 mM) or thimerosal (1 mM) did not prevent the probucol-mediated inhibition of IK(IR). Probucol (10 μM) caused a slight reduction in the amplitude of voltage-dependent L-type Ca2+ current, but did not affect Ca2+-activated and voltage-dependent K+ currents. In the current-clamp configuration, probucol (10 μM) increased the firing frequency of action potentials. The present study provides substantial evidence that in addition to the presence of antioxidant activity, probucol is a selective blocker of IK(IR), and implies that probucol-mediated blockade of this current may affect membrane excitability and hormonal secretion in GH3 cells.
All Science Journal Classification (ASJC) codes
- Drug Discovery