TRH signal transduction in melanotrope cells of Xenopus laevis.
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.
Lieste
J R
JR
Department of Cellular Animal Physiology, University of Nijmegen, Toernooiveld 1, The Netherlands.
Schoenmakers
T J M
TJ
Scheenen
W J J M
WJ
Willems
P H G M
PH
Roubos
E W
EW
Jenks
B G
BG
D000818:Animals; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D004533:Egtazic Acid / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D009074:Melanocyte-Stimulating Hormones / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D010880:Piperidines / Q000494:pharmacology; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism; D015398:Signal Transduction; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*; D014982:Xenopus laevis / Q000502:physiology*
D004791:Enzyme Inhibitors; D010880:Piperidines; C069701:NPC 15437; D004533:Egtazic Acid; D013973:Thyrotropin-Releasing Hormone; D019284:Thapsigargin; D009074:Melanocyte-Stimulating Hormones; D011493:Protein Kinase C; D000252:Calcium-Transporting ATPases; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium