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G_i3 Mediates Somatostatin-Induced Activation of an Inwardly Rectifying K⁺ Current in Human Growth Hormone-Secreting Adenoma Cells¹

Fourth Department of Internal Medicine (K.T., J.Y.-T., T.F.), University of Tokyo School of Medicine, Tokyo 112, Japan; and Department of Neurosurgery (A.T.), Nippon Medical School, Tokyo 113, Japan

Address all correspondence and requests for reprints to: Koji Takano, M.D., Ph.D., Fourth Department of Internal Medicine, University of Tokyo School of Medicine, 3–28-6 Mejirodai, Bunkyo-ku, Tokyo 112, Japan.

SRIF activates an inwardly rectifying K⁺ current in human GH-secreting adenoma cells. Activation of this K⁺ current induces hyperpolarization of the membrane and abolishment of action potential firing. This mechanism is an essential mechanism for SRIF-induced decrease in intracellular Ca²⁺ concentration and inhibition of GH secretion. The activation of the inwardly rectifying K⁺ current is mediated by a pertussis toxin-sensitive G protein. In this article, the expression of the pertussis toxin-sensitive G protein -subunits in the human GH-secreting adenoma cells were analyzed by RT-PCR, and the G protein transducing the SRIF-induced activation of this inwardly rectifying K⁺ current was investigated. RT-PCR of the messenger RNA from two human GH-secreting adenomas revealed that all G_i1, G_i2, G_i3, and G_o were expressed in these adenomas. Primary cultured cells from these two adenoma cells were investigated under the voltage clamp of the whole-cell mode. Specific antibodies against the carboxyl terminus of G protein -subunits were microinjected into the cells. Microinjection of antibody against the carboxyl terminal sequence of G_i3 attenuated the SRIF-induced activation of the inwardly rectifying K⁺ current, whereas antibody against the common carboxyl terminal sequence of G_i1 and G_i2 did not. These data indicate that the G protein transducing the SRIF-induced activation of the inwardly rectifying K⁺ current is G_i3.

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