Three phases of TRH-induced facilitation of exocytosis by single lactotrophs.
Membrane capacitance measurements were used to study neuropeptide modulation of exocytosis by perforated patch clamped rat lactotrophs. We report that depolarizing voltage-clamp pulses evoke exocytosis that is steeply dependent on Ca2+ influx through voltage-gated Ca2+ channels. Furthermore, we find that the neuropeptide TRH (thyrotropin-releasing hormone) acts in three phases to promote exocytosis. First, TRH transiently (within approximately 0.5 min) triggers depolarization- and extracellular Ca(2+)-independent exocytosis. Second, within 3 min of application, TRH facilitates depolarization-evoked exocytosis while inhibiting voltage-gated Ca2+ current. Finally, after 8 min, TRH further enhances depolarization-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 increasing Ca2+ current. Therefore, PKC can only account for the last effect of TRH. Thus, a single neuromodulator may employ several temporally distinct mechanisms to stimulate peptide secretion.
Fomina
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AF
Department of Pharmacology, University of Pittsburgh, Pennsylvania 15261, USA.
Levitan
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D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000502:physiology; D004594:Electrophysiology; D005089:Exocytosis / Q000187:drug effects*; D005260:Female; D015640:Ion Channel Gating; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D011388:Prolactin / Q000378:metabolism*; D011493:Protein Kinase C / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
D015220:Calcium Channels; D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D011493:Protein Kinase C; D002118:Calcium