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

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 patchperforated 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.