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Activation of the Sodium Pump Blocks the Growth Hormone-Induced Increase in Cytosolic Free Calcium in Rat Adipocytes¹

Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655

Address all correspondence and requests for reprints to: Dr. H. Maurice Goodman, Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655. E-mail: maurice.goodman{at}umassmed.edu

GH promptly increases cytosolic free calcium ([Ca²⁺]_i) in freshly isolated rat adipocytes. Adipocytes deprived of GH for 3 h or longer are incapable of increasing [Ca²⁺]_i in response to GH, but instead respond in an insulin-like manner. Insulin blocks the GH-induced increase in [Ca²⁺]_i in GH-replete cells and stimulates the sodium pump (i.e. Na⁺/K⁺-ATPase), thereby hyperpolarizing the cell membrane. Blockade of the Na⁺/K⁺-ATPase with 100 µM ouabain reversed these effects of insulin and enabled GH to increase [Ca²⁺]_i in GH-deprived adipocytes. Both insulin and GH activated the sodium pump in GH-deprived adipocytes, as indicated by increased uptake of ⁸⁶Rb⁺. Decreasing availability of intracellular Na⁺ by blockade of Na⁺/K⁺/2Cl^- symporters or Na⁺/H⁺ antiporters abolished the effects of both hormones on ⁸⁶Rb⁺ uptake and enabled both GH and insulin to increase [Ca²⁺]_i in GH-deprived adipocytes. The data suggest that hormonal stimulation of Na⁺/K⁺-ATPase activity interferes with activation of voltage-sensitive calcium channels by either membrane hyperpolarization or some unknown interaction between the sodium pump and calcium channels.

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