Characterization of the adipocyte ghost (Na+,K+) pump. Insights into the insulin regulation of the adipocyte (Na+,K+) pump.
The (Na+,K+) ATPase in plasma membranes isolated from rat adipocytes is insensitive to insulin (Lytton J., Lin, J.C., and Guidotti, G. (1985) J. Biol. Chem. 260, 1177-1184). For this reason, the characteristics of the (Na+,K+) pump in adipocyte ghosts, prepared by hypotonic lysis of adipocytes (Rodbell, M. (1967) J. Biol. Chem. 242, 5744-5750), were studied. Herein it is demonstrated that the (Na+,K+) pump in ghosts is identical to that described in isolated plasma membranes, sharing the following characteristics: 1) the Ki values for ouabain are 1.3 x 10(-7) M and 4.5 x 10(-5) M for the alpha 2 and alpha 1 isozymes, respectively; 2) the K0.5 values for sodium are 11.4 +/- 1.6 and 7.2 +/- 3.8 mM for the alpha 2 and alpha 1 isozymes, respectively; 3) both forms of the (Na+,K+) pump are insensitive to insulin stimulation, presumably because the activities are already maximal. The ghosts are not in an insulin-stimulated state because the activity of the glucose transporter is not increased as it is in ghosts prepared from insulin-treated cells. In addition, presented evidence demonstrates that ghost internal sodium concentration, [Na+]i, is very sensitive to changes in the activity of the (Na+,K+) pump. If the [Na+]i, of adipocytes is also very sensitive to the activity of the (Na+,K+) pump, the mechanism of insulin stimulation of the adipocyte (Na+,K+) pump requires reexamination.
McGill
D L
DL
Department of Biochemistry and Molecular Biology, Harvard University, Cambridge, Massachusetts 02138.
D000273:Adipose Tissue / Q000378:metabolism* / Q000502:physiology; D000818:Animals; D001693:Biological Transport, Active; D002462:Cell Membrane / Q000378:metabolism / Q000502:physiology; D007328:Insulin / Q000502:physiology*; D007527:Isoenzymes / Q000378:metabolism; D008297:Male; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism
D007328:Insulin; D007527:Isoenzymes; D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium