Growth hormone-releasing hormone decreases voltage-gated potassium currents in GH4C1 cells.
The electrophysiological properties of anterior pituitary somatotropes integrally involve the function of voltage-gated K+ currents. In this study, we have used GH4C1 cell lines to investigate the effect of human GHRH on voltage-gated K+ currents. Because of a clear 'rundown' of the K+ current with classic whole cell recording (WCR) without ATP in pipette solution, nystatin-perforated WCR was the major recording configuration used. Using a low Ca2+ (0.5 mM) bath solution containing Co2+ (1 mM) and TTX (1 microM), GH4 cells predominantly exhibited an outward delayed rectifier K+ current (IK). Local application of growth hormone releasing hormone (GHRH) (100 nM) reversibly reduced the amplitude of the K+ currents (to 83% of control). There was no effect of GHRH on the activation curve of the K+ current and no difference observed using 2.5 mM Ca2+ or low Ca2+ (0.5 mM Ca2++1 mM Co2+) bath solutions. Under the condition of low Ca2+ bath solution, the application of apamin (1 microM) or charybdotoxin (1 microM), two specific blockers of the Ca2+-activated K+ current, did not alter the K+ current or the response to GHRH. This reduction in the K+ current by GHRH was also observed with classic WCR with a pipette solution containing ATP (2 mM). The GHRH-induced reduction in the K+ current was completely abolished by the presence of GDP-beta-s (500 microM) in the pipette solution or by addition of PKC inhibitors, calphostin C (100 nM) and chelerythrine (1 microM), in bath solution. Inhibitor for cAMP-PKA system (Rp-cAMP and H89) did not affect the K+ current response to GHRH. These results suggest that GHRH reduces the voltage-gated K+ current in GH4C1 cells, a response that is mediated by G-proteins and PKC system but not by cAMP-PKA system. The reduction in the K+ current may partially contribute to the GHRH-stimulated growth hormone (GH) secretion.
Xu
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Prince Henry's Institute of Medical Research, Clayton, VIC, Australia.
Clarke
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IJ
Chen
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Chen
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D001030:Apamin / Q000494:pharmacology; D002118:Calcium / Q000493:pharmacokinetics; D002460:Cell Line; D018999:Charybdotoxin / Q000494:pharmacology; D004553:Electric Conductivity; D004558:Electric Stimulation; D019204:GTP-Binding Proteins / Q000378:metabolism; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D006153:Guanosine Diphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects*; D007476:Ionophores / Q000494:pharmacology; D017551:Microdialysis; D009761:Nystatin / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010902:Pituitary Gland / Q000737:chemistry / Q000166:cytology* / Q000201:enzymology; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000502:physiology*; D011493:Protein Kinase C / Q000378:metabolism; D013873:Thionucleotides / Q000494:pharmacology
D007476:Ionophores; D015221:Potassium Channels; D013873:Thionucleotides; D018999:Charybdotoxin; D009761:Nystatin; D006153:Guanosine Diphosphate; D001030:Apamin; C023427:guanosine 5'-O-(2-thiodiphosphate); D013007:Growth Hormone-Releasing Hormone; D011493:Protein Kinase C; D019204:GTP-Binding Proteins; D011188:Potassium; D002118:Calcium