Growth hormone-releasing peptide-2 reduces inward rectifying K+ currents via a PKA-cAMP-mediated signalling pathway in ovine somatotropes.
Inward-rectifying potassium (Kir) channels are essential for maintaining the resting membrane potential near the K(+) equilibrium and they are responsible for hyperpolarisation-induced K(+) influx. We characterised the Kir current in primary cultured ovine somatotropes and examined the effect of growth hormone-releasing peptide-2 (GHRP-2) on this current and its related intracellular signalling pathways. The Kir current was, in most cases, isolated using nystatin-perforated patch-clamp techniques. In bath solution containing 5 mM K(+), the Kir current was composed of both transient (fast activated) and delayed (slowly activated) components. An increase in the external K(+) concentration from 5 to 25 mM induced an augmentation of approximately 4-fold in the delayed part of the Kir current and both BaCl(2) and CsCl dose-dependently inhibited this current, confirming the presence of the Kir current in ovine somatotropes. Moreover, this specific effect of high K(+) on the Kir current was only observed in the cells that showed positive staining with anti-growth hormone (GH) antibodies, or in GC cells that belong to a rat somatotrope cell line. Application of GHRP-2 (100 nM) reversibly and significantly reduced the Kir current in bath solutions with 5 or 25 mM K(+) in ovine somatotropes. In addition, we found that the reduction in the Kir current mediated by GHRP-2 was totally abolished by the pretreatments with H89 (1 microM) or Rp-cAMP (100 microM) or by intracellular dialysis of a specific protein kinase A (PKA) inhibitory peptide PKI (10 microM). The specific PKC blocker chelerythrine (1 microM) or inhibitory peptide PKC(19-36) (10 microM) did not show any effects on the GHRP-2-induced decrease in the Kir current. These results suggest that the inhibition of Kir current through PKA-cAMP pathways may play an integral role in GHRP-2-induced depolarisation and GH release in ovine somatotropes.
Xu
Ruwei
R
Prince Henry's Institute of Medical Research, Clayton, Victoria 3168, Australia.
Zhao
Yufeng
Y
Chen
Chen
C
D000818:Animals; D017609:Barium Compounds / Q000494:pharmacology; D002469:Cell Separation; D002478:Cells, Cultured; D002586:Cesium / Q000494:pharmacology; D002712:Chlorides / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology*; D004594:Electrophysiology; D013006:Growth Hormone / Q000378:metabolism; D007150:Immunohistochemistry; D008564:Membrane Potentials / Q000502:physiology; D009842:Oligopeptides / Q000494:pharmacology*; D018408:Patch-Clamp Techniques; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D024661:Potassium Channels, Inwardly Rectifying / Q000037:antagonists & inhibitors*; D051381:Rats; D012756:Sheep; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*
D017609:Barium Compounds; D002712:Chlorides; D009842:Oligopeptides; D024661:Potassium Channels, Inwardly Rectifying; C024986:barium chloride; D002586:Cesium; D013006:Growth Hormone; D000242:Cyclic AMP; C091874:growth hormone-releasing peptide-2; D017868:Cyclic AMP-Dependent Protein Kinases; C028019:cesium chloride; D011188:Potassium