TRH signal transduction in melanotrope cells of Xenopus laevis

Abstract

TRH is a neuropeptide that activates phospholipase C and, when acting on secretory cells, usually induces a biphasic response consisting of a transitory increase in secretion (due to IP 3 mobilization of Ca 2+ from intracellular stores), followed by a sustained plateau phase of stimulated secretion (by protein kinase C-dependent influx of extracellular Ca 2+ through voltage-operated Ca 2+ channels). The melanotrope cell of the amphibian Xenopus laevis displays a unique secretory response to TRH, namely a broad transient but no sustained second phase, consistent with the observation that TRH induces a single Ca 2+ transient rather than the classic biphasic increase in [Ca 2+] i . The purpose of the present study was to determine the signal transduction mechanism utilized by TRH in generating this Ca 2+ signaling response. Our hypothesis was that the transient reflects the operation of only one of the two signaling arms of the lipase (i.e., either IP 3-induced mobilization of internal Ca 2+ or PKC-dependent influx of external Ca 2+). Using video-imaging microscopy it is shown that the TRH-induced Ca 2+ transient is dramatically attenuated under Ca 2+-free conditions and that thapsigargin has no noticeable effect on the TRH-induced transient. These observations indicate that an IP 3-dependent mechanism plays no important role in the action of TRH. PKC also does not seem to be involved because an activator of PKC did not induce a Ca 2+ transient and an inhibitor of PKC did not affect the TRH response. Experiments with a bis-oxonol membrane potential probe showed that the TRH response also does not underlie a PKC-independent mechanism that would induce membrane depolarization. We conclude that the action of TRH on the Xenopus melanotrope does not rely on the classical phospholipase C-dependent mechanism.