THE ROLE OF THE INWARDLY RECTIFYING K-RELEASING-HORMONE-INDUCED CHANGES IN CELL EXCITABILITY OF GH(3) RAT ANTERIOR-PITUITARY-CELLS( CURRENTIN RESTING POTENTIAL AND THYROTROPIN)

Exposure of GH(3) rat anterior pituitary cells to cholera toxin for 2- 4h significantly increased the thyrotropin-releasing-hormone(TRH)-indu ced inhibition of the inwardly rectifying K+ current studied in patch- perforated voltage-clamped cells. On the other hand, the current reduc tion became almost totally irreversible after washout of the neuropept ide. Comparison of the effects elicited by the toxin with those of 8-( 4-chlorophenyl-thio)-cAMP of forskolin plus isobutylmethylxanthine ind icated that, although the irreversibility may be due, at least in part , to elevations of cAMP levels, the enhancement of the TRH-induced inh ibition of the current is not mediated by the cyclic nucleotide. Only reductions on the inwardly rectifying K+ current, but not those elicit ed by TRH on voltage-dependent Ca2+ currents, were increased by the tr eatment with cholera toxin. In current-clamped cells showing similar r ates of firing, the second phase of enhanced action-potential frequenc y induced by TRH was also significantly potentiated by cholera toxin. Measurements of [Ca2+](i) oscillations associated with electrical acti vity, 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 o f the inward rectifier was shifted by nearly 20 mV to more negative va lues. These data suggest that the inwardly rectifying K+ current plays an important role in maintenance of the resting K+ conductance in GH( 3) cells. Furthermore, the TRH-induced reductions on this current may be an important factor contributing to the increased cell excitability promoted by the neuropeptide.