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Regulation by Growth Hormone-Releasing Hormone and Somatostatin of a Na⁺ Current in the Primary Cultured Rat Somatotroph¹

Department of Physiology I, Nippon Medical School, Sendagi 1, Bunkyo Tokyo 113, Japan

Address all correspondence and requests for reprints to: M. Kato, Department of Physiology I, Nippon Medical School, Sendagi 1, Bunkyo Tokyo 113, Japan. E-mail: mkato{at}nms.ac.jp

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.

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