3 PHASES OF TRH-INDUCED FACILITATION OF EXOCYTOSIS BY SINGLE LACTOTROPHS

Membrane capacitance measurements were used to study neuropeptide modu lation of exocytosis by perforated patch damped rat lactotrophs. We re port that depolarizing voltage-clamp pulses evoke exocytosis that is s teeply dependent on Ca2+ influx through voltage-gated Ca2+ channels. F urthermore, we find that the neuropeptide TRH (thyrotropin-releasing h ormone) acts in three phases to promote exocytosis. First, TRH transie ntly (within similar to 0.5 min) triggers depolarization- and extracel lular Ca2+-independent exocytosis. Second, within 3 min of application , TRH facilitates depolarization-evoked exocytosis while inhibiting vo ltage-gated Ca2+ current. Finally, after 8 min, TRH further enhances d epolarization-evoked exocytosis by increasing high-voltage-activated ( HVA) Ca2+ channel current. Activation of protein kinase C (PKC) with a phorbol ester also stimulates depolarization-evoked exocytosis by inc reasing Ca2+ current. Therefore, PKC can only account for the last eff ect of TRH. Thus, a single neuromodulator may employ several temporall y distinct mechanisms to stimulate peptide secretion.