Diverse intracellular signalling systems used by growth hormone-releasing hormone in regulating voltage-gated Ca2+ or K channels in pituitary somatotropes.
Influx of Ca2+ via Ca2+ channels is the major step triggering exocytosis of pituitary somatotropes to release growth hormone (GH). Voltage-gated Ca2+ and K+ channels, the primary determinants of the influx of Ca2+, are regulated by GH-releasing hormone (GHRH) through G-protein-coupled intracellular signalling systems. Using whole-cell patch-clamp techniques, the changes of the Ca2+ and K+ currents in primary cultured ovine and human somatotropes were recorded. Growth hormone-releasing hormone (10 nmol/L) increased both L- and T-type voltage-gated Ca2+ currents. Inhibition of the cAMP/protein kinase A (PKA) pathway by either Rp-cAMP or H89 blocked this increase in both L- and T-type Ca2+ currents. Growth hormone-releasing hormone also decreased voltage-gated transient (IA) and delayed rectified (IK) K+ currents. Protein kinase C (PKC) inhibitors, such as calphostin C, chelerythrine or downregulation of PKC, blocked the effect of GHRH on K+ currents, whereas an acute activation of PKC by phorbol 12, 13-dibutyrate (1 micromol/L) mimicked the effect of GHRH. Intracellular dialysis of a specific PKC inhibitor (PKC19-36) also prevented the reduction in K+ currents by GHRH. It is therefore concluded that GHRH increases voltage-gated Ca2+ currents via cAMP/PKA, but decreases voltage-gated K+ currents via the PKC signalling system. The GHRH-induced alteration of Ca2+ and K+ currents augments the influx of Ca2+, leading to an increase in [Ca2+]i and the GH secretion.
Chen
C
C
Prince Henry's Institute of Medical Research, Clayton, Victoria, Australia. chen.chen@med.monash.edu.au
Xu
R
R
Clarke
I J
IJ
Ruan
M
M
Loneragan
K
K
Roh
S G
SG
D000470:Alkaloids; D000818:Animals; D053119:Benzophenanthridines; D015220:Calcium Channels / Q000378:metabolism*; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D006801:Humans; D007546:Isoquinolines / Q000494:pharmacology; D009281:Naphthalenes / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010617:Phenanthridines / Q000494:pharmacology; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism*; D015221:Potassium Channels / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism; D012756:Sheep; D015398:Signal Transduction / Q000187:drug effects; D013449:Sulfonamides
D000470:Alkaloids; D053119:Benzophenanthridines; D015220:Calcium Channels; D007546:Isoquinolines; D009281:Naphthalenes; D010617:Phenanthridines; D015221:Potassium Channels; D013449:Sulfonamides; D015240:Phorbol 12,13-Dibutyrate; D013007:Growth Hormone-Releasing Hormone; C016299:chelerythrine; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; C058819:calphostin C; C063509:N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide