The role of the inwardly rectifying K+ current in resting potential and thyrotropin-releasing-hormone-induced changes in cell excitability of GH3 rat anterior pituitary cells

Abstract

Exposure of GH3 rat anterior pituitary cells to cholera toxin for 2-4 h significantly increased the thyrotropin-releasing-hormone(TRH)-induced inhibition of the inwardly rectifying K+ current studied in patch-perforated voltage-clamped cells. On the other hand, the current reduction became almost totally irreversible after washout of the neuropeptide. Comparison of the effects elicited by the toxin with those of 8-(4-chlorophenylthio)-cAMP or forskolin plus isobutylmethylxanthine indicated that, although the irreversibility may be due, at least in part, to elevations of cAMP levels, the enhancement of the TRH-induced inhibition of the current is not mediated by the cyclic nucleotide. Only reductions on the inwardly rectifying K+ current, but not those elicited by TRH on voltage-dependent Ca2+ currents, were increased by the treatment with cholera toxin. In current-clamped cells showing similar rates of firing, the second phase of enhanced action-potential frequency induced by TRH was also significantly potentiated by cholera toxin. Measurements of [Ca2+]i oscillations associated with electrical activity, using video imaging with fura-2-loaded cells, demonstrated that cholera toxin treatment causes a clear reduction of spontaneous [Ca2+]i oscillations. However, this did not prevent the stimulatory effect of TRH on oscillations due to the action potentials. In cholera-toxin-treated cells, the steady-state, voltage dependence of inactivation of the inward rectifier was shifted by nearly 20 mV to more negative values. These data suggest that the inwardly rectifying K+ current plays an important role in maintenance of the resting K+ conductance in GH3 cells. Furthermore, the TRH-induced reductions on this current may be an important factor contributing to the increased cell excitability promoted by the neuropeptide.

A novel inward‐rectifying K+ current with a cell‐cycle dependence governs the resting potential of mammalian neuroblastoma cells.

“…In fact this parameter was previously reported to vary in GH3 cells only upon addition of thyrotrophin-releasing hormone (Bauer et al 1990) or cholera toxin (Barros, Villalobos, Garcia-Sancho, del Camino & de la Pena, 1994). To a substantial degree the h(V) variability was attributable to the fact that cell scattering along the cycle phases was markedly reduced in synchronized cultures (Figs 7 and 8).…”

Role of BK Potassium Channels Shaping Action Potentials and the Associated [Ca²⁺]_i Oscillations in GH₃ Rat Anterior Pituitary Cells

“…Maintenance of GH 3 cells (ATCC-CCL 82.1) and conditions for perforated-patch recordings are described in detail elsewhere [13, 14, 18, 19, 21]. All recordings were performed at room temperature with the perforated-patch variant of the patch-clamp technique using a recording chamber of 0.2–0.3 ml continuously perfused at 0.5– 1.0 ml/min.…”

“…It is also known that similar mechanisms (i.e. ion channels) exist that control basal and agonist-modulated electrical activity in normal pituitary cells and clonal cell lines derived from them [1, 13, 14, 15, 16]. Furthermore, in all cases, analogous signaling pathway(s) seem to link hormonal receptors to cellular responses.…”

Characterization of Inhibition by Risperidone of the Inwardly Rectifying K+ Current in Pituitary GH3 Cells

“…This current was sensitive to caffeine, thyrotropin-releasing hormone, and class III antiarrhythmic agents (e.g., E-4031), and proposed to be an important determinant of the resting membrane potential (Bauer et al 1990;Barros et al 1994Barros et al , 1996Barros et al , 1997Weinsberg et al 1997;Bauer 1998;Schwarz and Bauer 1999). More importantly, the blockade of I K(IR) was noted to produce an increase in the firing rate of action potentials (Barros et al 1996;Weinsberg et al 1997;Schwarz and Bauer 1999) and to stimulate prolactin secretion in rat lactotrophs .…”