Reevaluation of the electrophysiological actions of thyrotropin-releasing hormone in a rat pituitary cell line (GH3)

The electrophysiological actions of TRH were examined in the clonal pituitary cell line GH3 with the use of the perforated patch variation of the standard whole cell patch-clamp technique. The action of TRH on spontaneously spiking cells was to cause a brief hyperpolarization (first phase action), followed by a period during which action potential behavior was significantly modified (second phase action). The modifications during second phase action included a reduction in the slope of the up-stroke, a reduced peak potential, an increase in duration, and a depolarizing shift of the after-hyperpolarization. The modification of voltage- and calcium-dependent conductances that underlie these changes were investigated in voltage clamp experiments. During first phase action TRH was found to increase calcium-dependent potassium current. During second phase action TRH was found to significantly reduce the L-type calcium current (35%), with no alteration in the T-type calcium current. The second phase action of TRH on calcium-dependent potassium conductance was complex. First, a decrease was observed. This was followed by an increase that did not become fully manifest until after TRH was washed from the cell. TRH caused no change in voltage-dependent potassium current. These results indicate that the second phase action of TRH on action potential behavior in GH3 cells is mediated by a reduction in L-type calcium current and alterations in the behavior of calcium-dependent potassium currents, but not through changes in voltage-dependent potassium currents.