The effect of two-day treatment of primary cultured ovine somatotropes with GHRP-2 on membrane voltage-gated K+ currents.
Long-term in vivo treatment with synthetic GH-releasing peptides (GHRPs) enhances the release of GH induced by endogenous GHRH. The mechanism for such an enhancement on GH release is unknown. In this experiment, somatotropes were obtained from ovine pituitaries by enzyme dissociation and enriched by density centrifugation. Membrane voltage and currents were recorded with whole-cell patch-clamp configuration. After 48-h treatment with GHRP-2 (10(-8) M), the percentage of cells with spontaneous action potential was increased (51 vs. 27%) without change of resting potential. This GHRP-2 treatment also increased the amplitude of voltage-gated K+ currents (predominantly transient A-type-like current but also delayed rectifier or K-type-like current) without modification of biophysical kinetics. Down-regulation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate at the time of adding GHRP-2 blocked the increase in K+ currents. Inclusion of calphostin C (PKC inhibitor) but not H(89) (protein kinase A inhibitor) significantly reduced the increase in K+ currents by GHRP-2. Inclusion of actinomycin D (transcription inhibitor) or cycloheximide (protein synthesis inhibitor) abolished the increase in K+ currents. These data indicate that 48-h GHRP-2 treatment increases the density of K+ channels via PKC and channel protein synthesis. Such a modification on K+ channels by GHRP-2 may be partially responsible for the change of somatotrope electrophysiological properties and sensitivity to GHRH stimulation.
Chen
Chen
C
Department of Endocrine Cell Biology, Prince Henry's Institute of Medical Research, Clayton, Victoria 3168, Australia. chen.chen@med.monash.edu.au
D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004594:Electrophysiology; D013006:Growth Hormone / Q000378:metabolism*; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology; D009842:Oligopeptides / Q000494:pharmacology*; D018408:Patch-Clamp Techniques; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D024642:Potassium Channels, Voltage-Gated / Q000187:drug effects*; D011493:Protein Kinase C / Q000378:metabolism; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D012756:Sheep
D009842:Oligopeptides; D024642:Potassium Channels, Voltage-Gated; D011500:Protein Synthesis Inhibitors; D012333:RNA, Messenger; D013006:Growth Hormone; C091874:growth hormone-releasing peptide-2; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C