Voltage-activated ionic currents in goldfish pituitary cells.

The release of gonadotropin and growth hormone from goldfish pituitary cells has been shown to be dependent on the entry of extracellular Ca2+ through voltage-sensitive Ca2+ channels by pharmacological studies. As a first step to further investigate the involvement of voltage-dependent ion channels in the regulation of anterior pituitary hormone release in the goldfish, cell excitability and voltage-dependent ion currents were characterized using tight-seal whole-cell recordings in dispersed goldfish pituitary cells. Cultured goldfish pituitary cells had an average membrane potential of -36 +/- 3 mV. When held at membrane potentials more negative than -60 mV, these cells were excitable, responding to depolarizing current pulses or anode break with the firing of single action potentials. Results from total current voltage-clamp recordings suggested that all goldfish pituitary cells possess voltage-dependent Na+, Ca2+, and K+ currents. These currents were further characterized independently under isolated current recording conditions. The rapid, transient Na+ current activated at voltages more positive than -40 mV and was sensitive to tetrodotoxin. The steady state inactivation of this Na+ current was also voltage-dependent; at the measured resting potential, > or = 50% of the Na+ current was not available for activation. The voltage-dependence and activation kinetics of the tetraethylammonium-sensitive K+ current resembled those of the delayed rectifier K+ current. The K+ current activated slowly at potentials more positive than -40 mV, and showed little inactivation over the duration of a 1-sec depolarizing pulse. Steady-state inactivation characteristics indicated that < 50% of the K+ current was inactivated at resting potentials. Experiments with 4-aminopyridine indicated the presence of an early transient K+ current that activated in a similar voltage range as the delayed rectifier current. Using barium as the charge carrier to measure Ca2+ currents, a high-voltage activated, long-lasting Ca2+ current was revealed. This "L-type" Ca2+ current activated at potentials more positive than -30 mV and was inhibited by verapamil and nifedipine. This study indicates that goldfish pituitary cells possess the electrophysiological properties required for the participation of voltage-sensitive ion channels in the regulation of hormone release.