CHANGES OF ACTIVATION AND INACTIVATION GATING OF THE TRANSIENT POTASSIUM CURRENT OF RAT PITUITARY MELANOTROPHS CAUSED BY MICROMOLAR CD2+ AND ZN2+

We studied the effects of Zn2+ and Cd2+ on the behaviour of I-K(f), a transient outward potassium current in acutely dissociated melanotrophs of the pars intermedia of the rat pituitary gland. Micromolar concentrations of external Cd2+ or Zn2+ caused parallel and nearly equal rightward shifts along the voltage axis of the activation and steady-state inactivation curves for I-K(f). The K-D for the half-maximal shift of the activation curve was 278 mu M for Cd2+ and 93 mu M for Zn2+; the maximal shifts of the activation curve were 32.5 and 34 mV, for Cd2+ and Zn2+, respectively. The times to half-activation and half-inactivation were shifted rightward by 30-60 mV in both 500 mu M Cd2+ and 500 mu M Zn2+. We suggest that Cd2+ and Zn2+ interact specifically with a binding site on or electrically close to the I-K(f) channel and in so doing modify the electric field ''seen'' by the voltage sensors.