Intracellular Ca2+-dependent protein kinase C activation mimics delayed effects of thyrotropin-releasing hormone on clonal pituitary cell excitability.

Biochemical and spectrophotometric studies of second messenger pathways transducing TRH signals in clonal pituitary (GH) cells have shown that TRH induces rapid turnover of phosphoinositides and changes in cytoplasmic Ca2+ as well as activation of protein kinase C (PKC) and secretion of PRL. Here we have used classical microelectrode and contemporary patch pipette recording techniques under current-clamp conditions to compare the effects of TRH receptor-coupled stimulation with direct activation of PKC on the excitability of GH3/B6 cells. With high resistance microelectrodes TRH induced a complex sequence of changes in membrane properties consisting of an initial 20- to 30-mV hyperpolarization associated with an increase in membrane conductance lasting less than a minute, followed by several minutes of low amplitude fluctuations and action potential activity superimposed on a modest increase in input resistance. Active phorbol ester induced a slowly developing hyperpolarization of about 5 mV and a modest increase in input resistance, followed by several minutes of low amplitude fluctuations and spontaneous action potential activity. Both the peptide- and phorbol ester-evoked changes in excitability were attenuated or completely lost during patch recordings in the whole cell mode. Dilute aqueous lysates of the clone restored various phases of the electrical response. The low amplitude fluctuations and action potential activity phase could be induced by either TRH or phorbol ester if the cells were dialyzed with intracellular electrolyte containing PKC and at least 50 nM Ca2+. These results demonstrate that the phosphoinositide/PKC circuit activated by TRH in clonal pituitary cells has electrically detectable effects on cell excitability, and these help to explain TRH's actions on electrical activity.