Voltage-gated currents of tilapia prolactin cells

Abstract

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 ( I Ca), including using peptide toxins diagnostic for mammalian neuronal Ca 2+ 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 I Ca; there followed outward potassium currents dominated by delayed, sustained TEA-sensitive K + current. About half of cells tested from a holding potential ( V h) of −90 mV showed early transient K + current; most cells showed a small Ca 2+-mediated outward current. I– V plots of isolated I Ca with 15 mM [Ca 2+] o showed peak currents (up to 20 pA/pF from V h −90 mV) at ∼+10 mV, with ∼60% I Ca for V h −50 mV and ∼30% remaining at V h −30 mV. Plots of normalized conductance vs. voltage at several V hs were nearly superimposable. Well-sustained I Ca with predominantly Ca 2+-dependent inactivation and inhibition of ∼30% of total I Ca 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 I Ca. Inhibition by any of these toxins was additive to inhibition by nifedipine. Combinations of the toxins failed to produce additive effects. I Ca of up to 30% of total remained with any combination of inhibitors, but 0.1 mM cadmium blocked all I Ca rapidly and reversibly. We did not find differences among cells of differing size and hormone content. Thus, I Ca is carried by high voltage-activated Ca 2+ channels of at least three types, but the molecular types may differ from those characterized from mammalian neurons.