Changes of activation and inactivation gating of the transient potassium current of rat pituitary melanotrophs caused by micromolar Cd2+and Zn2+

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Abstract
We studied the effects of Zn2+and Cd2+on the behaviour of IK(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 IK(f). The KDfor the half-maximal shift of the activation curve was 278 μM for Cd2+and 93 μM 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 μM Cd2+and 500 μM Zn2+. We suggest that Cd2+and Zn2+interact specifically with a binding site on or electrically close to the IK(f) channel and in so doing modify the electric field "seen" by the voltage sensors.Key words: fast transient K+current, melanotrophs, divalent cations, activation gating, inactivation gating.