ELECTRICAL MEMBRANE-PROPERTIES AND IONIC CURRENTS IN CULTURED GOLDFISH GONADOTROPHS

The electrical membrane properties and ionic currents of cultured iden tified goldfish gonadotrophs were characterized using whole-cell patch -clamp recordings. Under current-clamp recording conditions, goldfish gonadotrophs had a resting membrane potential of approximate to-60 mV and an input resistance of 8.2 +/- 1.6 G Ohm. In response to depolariz ing current injections, one or more action potentials could be induced from resting membrane potential or more hyperpolarized holding potent ials. In addition, some cells exhibited spontaneous action potential a ctivity. Under voltage-clamp recording conditions, several outward and inward currents were isolated and characterized using pharmacological and ionic substitution studies. The outward currents included a fast- activating, transient current and an inactivation-resistant current si milar to I-A-type and delayed rectifier currents, respectively. Unlike mammalian gonadotrophs, apamin-sensitive K+ currents were not detecte d. Inward currents included a fast-activating and inactivating Na+ cur rent and a high-voltage activated, dihydropyridine-sensitive Ca2+ curr ent. The value for half-maximal steady-state inactivation of the Na+ a nd Ca2+ currents was -50.8 and -16.7 mV, respectively, indicating that a significant proportion of both Na+ and Ca2+ channels are available for activation at the resting membrane potential of these cells.