Cultured melanotrophs of the adult rat pituitary possess a voltage-activated fast transient outward current.

1. Whole-cell voltage-clamp recordings were made from cultured melanotrophs obtained from adult rats and maintained in vitro using conventional cell culture procedures. 2. The outward current recorded in the presence of Na+ and Ca2+ channel blockers was normally comprised of two components: a slowly activating, slowly inactivating current (IK(s] and a fast transient current (IK(f]. The selective blockade of IK(s) by 20 mM-tetraethylammonium (TEA+) allowed the properties of IK(f) to be analysed in isolation. 3. The activation threshold for IK(f) was normally between -20 and -10 mV and the current-voltage relationship was linear. At positive potentials the decay of IK(f) was well fitted by a single exponential having a time constant of 20-35 ms. At -70 mV recovery from inactivation was best described by a single-exponential function with a time constant of 20-40 ms. IK(f) was fully activatable at -60 mV and was fully inactivated at -10 mV; the half-inactivation potential was approximately -25 mV. 4. Since IK(f) was reduced by raising the external concentration of K+, was blocked by Ba2+ and Cs+, and persisted in Ca2+-free medium it is attributed to a voltage-activated K+ conductance. The amplitude of IK(f) was unaffected either by 5 mM-4-aminopyridine (4-AP) or 50 microM-quinidine. 5. The electrical properties of IK(f) suggest that by affecting the amplitude and/or duration of the action potential IK(f) may modulate Ca2+ influx and consequently hormone release.