Regulation by growth hormone-releasing hormone and somatostatin of a Na+ current in the primary cultured rat somatotroph.
The purpose of the present study is to characterize Na+ current activated by GH-releasing hormone (GHRH) and to investigate the effect of somatostatin (SRIF) on that current, because the Na+ current has been suggested to play a pivotal role in the process of GHRH-induced GH secretion. Primary-cultured pituitary somatotrophs were prepared from male Wistar rats. Whole-cell membrane currents were recorded and analyzed by a perforated patch clamp system. To isolate Na+ current, K+ and Ca2+ were replaced with Cs+ and Mg2+, respectively, in the extracellular solution, and cesium aspartate was used for the pipette solution. Furthermore, tetrodotoxin and nifedipine were added to the extracellular solution to eliminate the voltage-gated currents. Under these conditions, GHRH activated a mean inward Na+ current (-1.86 +/- 0.33 pA, mean +/- SE) at potentials between -50 and -20 mV and a smaller current (-0.59 +/- 0.13 pA) at potentials between -100 and -80 mV, which were completely blocked by protein kinase A blocker (H-89). In addition, SRIF (1-10 nM) partially suppressed these Na+ currents, which were not affected by phosphatase inhibitors (okadaic acid and calyculin A). These results suggest that GHRH activates the Na+ current through phosphorylation by protein kinase A and that SRIF partially suppressed this current and that the current was larger at more positive potentials than at more negative potentials.
Kato
M
M
Department of Physiology I, Nippon Medical School, Bunkyo Tokyo, Japan. mkato@nms.ac.jp
Sakuma
Y
Y
D000818:Animals; D002478:Cells, Cultured; D004553:Electric Conductivity; D004791:Enzyme Inhibitors / Q000494:pharmacology; D013006:Growth Hormone / Q000378:metabolism* / Q000494:pharmacology; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D019382:Human Growth Hormone / Q000494:pharmacology; D007546:Isoquinolines / Q000494:pharmacology; D008297:Male; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000502:physiology*; D013004:Somatostatin / Q000494:pharmacology*; D013449:Sulfonamides
D004791:Enzyme Inhibitors; D007546:Isoquinolines; D013449:Sulfonamides; D019382:Human Growth Hormone; D013004:Somatostatin; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D012964:Sodium; C063509:N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide