Characterization of ionic currents and electrophysiological properties of goldfish somatotropes in primary culture

Growth hormone release in goldfish is partly dependent on voltage-sensitive Ca2+ channels but somatotrope electrophysiological events affecting such channel activities have not been elucidated in this system. The electrophysiological properties of goldfish somatotropes in primary culture were studied using the whole-cell and amphotericin B-perforated patch-clamp techniques. Intracellular Ca2+ concentration ([Ca2+]i) of identified somatotropes was measured using Fura-2/AM dye. Goldfish somatotropes had an average resting membrane potential of −78.4 ± 4.6 mV and membrane input resistance of 6.2 ± 0.2 GΩ. Voltage steps from a holding potential of −90 mV elicited a non-inactivating outward current and transient inward currents at potentials more positive than 0 and −30 mV, respectively. Isolated current recordings indicate the presence of 4-aminopyridine- and tetraethylammonium (TEA)-sensitive K+, tetrodotoxin (TTX)-sensitive Na+, and nifedipine (L-type)- and ω-conotoxin GVIA (N-type)-sensitive Ca2+ channels. Goldfish somatotropes rarely fire action potentials (APs) spontaneously, but single APs can be induced at the start of a depolarizing current step; this single AP was abolished by TTX and significantly reduced by nifedipine and ω-conotoxin GVIA. TEA increased AP duration and triggered repetitive AP firing resulting in an increase in [Ca2+]i, whereas TTX, nifedipine and ω-conotoxin GVIA inhibited TEA-induced [Ca2+]i pulses. These results indicate that in goldfish somatotropes, TEA-sensitive K+ channels regulate excitability while TTX-sensitive Na+ channels together with N- and L-type Ca channels mediates the depolarization phase of APs. Opening of voltage-sensitive Ca2+ channels during AP firing leads to increases in [Ca2+]i.

Research highlights
► Electrophysiological properties of goldfish somatotropes.
► Presence of TEA- and 4AP-sensitive outward K+ and TTX-sensitive inward Na+ currents, and high-voltage activated L- and/or N-type Ca2+ current, but not transient T currents and TTX-insensitive Na+ currents.
► Spontaneous action potential firing lacking but TEA-sensitive K+ channels modulates sensitivity to evoked action potential firing.
► L- and N-type Ca2+ currents, as well as TTX-sensitive Na+ currents, are components of the depolarizing phase of evoked action potentials.
► Opening of L- and N-type channels leads to increase in [Ca2+]i.