Voltage-gated currents of tilapia prolactin cells.

The first recordings of neuron-like electrical activity from endocrine cells were made from fish pituitary cells. However, patch-clamping studies have predominantly utilized mammalian preparations. This study used whole-cell patch-clamping to characterize voltage-gated ionic currents of anterior pituitary cells of Oreochromis mossambicus in primary culture. Due to their importance for control of hormone secretion we emphasize analysis of calcium currents (ICa), including using peptide toxins diagnostic for mammalian neuronal Ca2+ channel types. These appear not to have been previously tested on fish endocrine cells. In balanced salines, inward currents consisted of a rapid TTX-sensitive sodium current and a smaller, slower ICa; there followed outward potassium currents dominated by delayed, sustained TEA-sensitive K+ current. About half of cells tested from a holding potential (Vh) of −90mV showed early transient K+ current; most cells showed a small Ca2+-mediated outward current. I–V plots of isolated ICa with 15mM [Ca2+]o showed peak currents (up to 20pA/pF from Vh −90mV) at ∼+10mV, with ∼60% ICa for Vh −50mV and ∼30% remaining at Vh −30mV. Plots of normalized conductance vs. voltage at several Vhs were nearly superimposable. Well-sustained ICa with predominantly Ca2+-dependent inactivation and inhibition of ∼30% of total ICa by nifedipine or nimodipine suggests participation of L-type channels. Each of the peptide toxins (ω-conotoxin GVIA, ω-agatoxin IVA, SNX482) alone blocked 36–54% of ICa. Inhibition by any of these toxins was additive to inhibition by nifedipine. Combinations of the toxins failed to produce additive effects. ICa of up to 30% of total remained with any combination of inhibitors, but 0.1mM cadmium blocked all ICa rapidly and reversibly. We did not find differences among cells of differing size and hormone content. Thus, ICa is carried by high voltage-activated Ca2+ channels of at least three types, but the molecular types may differ from those characterized from mammalian neurons.