The in vitro regulation of growth hormone secretion by orexins.
Orexins, orexigenic neuropeptides, have recently been discovered in lateral hypothalamus and play an important role in the regulation of pituitary hormone secretion. Two subtypes of orexin receptors (orexin-1 and orexin-2) have been demonstrated in pituitaries. In this experiment, the effects of orexins on voltage-gated Ca2+ currents and the GH release in primary cultured ovine somatotropes were examined. Voltage-gated Ca2+ currents were isolated in ovine somatotropes as L, T, and N currents using whole-cell patch-clamp techniques and specific Ca2+ channel blocker and toxin. Application of orexin-A or orexin-B (100 nM) significantly, dose-dependently, and reversibly increased only nifedipine-sensitive L-type Ca2+ current. Inhibitors of PKC (calphostin C, PKC inhibitory peptide) but not inhibitors of PKA (H89, PKA inhibitory peptide) cancelled the increase in the L current by orexins. Co-administration of orexin-A and GHRH (10 nM) showed an additive effect on the L current. Specific intracellular Ca2+-store-depleting reagent, thapsigargin (1 microM), did not affect the orexin-induced increase in the L current. Orexin-B alone slightly increased GH release and co-administration of orexin-A and GHRH synergistically stimulated GH secretion in vitro. It is therefore suggested that orexins may play an important role in regulating GHRH-stimulated GH secretion through an increase in the L-type Ca2+ current and the PKC-mediated signaling pathways in ovine somatotropes.
Chen
Chen
C
Prince Henry's Institute of Medical Research, and Department of Physiology, PO Box 5152, Monash University, Clayton, Victoria 3168, Australia. chen.chen@phimr.monash.edu.au
Xu
Ruwei
R
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects*; D002352:Carrier Proteins / Q000494:pharmacology*; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000378:metabolism; D019382:Human Growth Hormone / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins; D015640:Ion Channel Gating / Q000502:physiology; D009479:Neuropeptides / Q000494:pharmacology*; D009543:Nifedipine / Q000494:pharmacology; D000068797:Orexins; D018408:Patch-Clamp Techniques; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D011863:Radioimmunoassay; D015290:Second Messenger Systems / Q000502:physiology; D012756:Sheep; D020960:omega-Conotoxins / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D002352:Carrier Proteins; C000596229:HCRT protein, human; D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D000068797:Orexins; D020960:omega-Conotoxins; D019382:Human Growth Hormone; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D017868:Cyclic AMP-Dependent Protein Kinases; D009543:Nifedipine; D002118:Calcium