On the mechanism of selective action of probucol on the inwardly rectifying potassium current in GH(3) lactotrophs

The ionic mechanism of action of probucol, a known lipid-lowering agent, wa s examined in rat pituitary GH(3) cells. Whole-cell voltage-clamp was used to measure hyperpolarizatioin-activated K+ currents in GH(3) cells bathed i n a high-K+, Ca2+-free solution to determine the effect of probucol on the erg-like inwardly rectifying K+ current (I-K(IR)). Probucol reversibly supp ressed the amplitude of I-K(IR) in a concentration-dependent manner. The IC 50 value of probucol-induced inhibition of I-K(IR) was 1 muM. Probucol shif ted the steady-state inactivation curve of I-K(IR) to less negative potenti als and also prolonged the recovery of I-K(IR) inactivation. The K+ inward current in response to hyperpolarizing voltage pulses was also inhibited by haloperidol (10 muM) and bepridil (10 muM) but not by reduced glutathione (10 mM) or superoxide dismutase (500 U/ml). Pretreatment with t-butyl hydro peroxide (1 mM) or thimerosal (1 mM) did not prevent the probucol-mediated inhibition of I-K(IR). Probucol (10 muM) caused a slight reduction in the a mplitude of voltage-dependent L-type Ca2+ current, but did not affect Ca2+- activated and voltage-dependent K+ currents. In the current-clamp configura tion, probucol (10 muM) increased the firing frequency of action potentials . The present study provides substantial evidence that in addition to the p resence of antioxidant activity, probucol is a selective blocker of I-K(IR) , and implies that probucol-mediated blockade of this current may affect me mbrane excitability and hormonal secretion in GH(3) cells. (C) 2001 Wiley-L iss, Inc.