INSULIN-INDUCED TRANSLOCATION OF NA-K+-ATPASE SUBUNITS TO THE PLASMA-MEMBRANE IS MUSCLE-FIBER TYPE-SPECIFIC()

We have previously shown that an acute insulin treatment induces redis tribution of the alpha(2)- and beta(1)-isoforms of the Na+-K+-ATPase f rom intracellular membranes to plasma membranes detected on subcellula r fractionation of mixed muscles and immunoblotting with isoform-speci fic antibodies (H. S. Hundal et al. J. Biol. Chem. 267: 5040-5043, 199 2). In the present study we give both biochemical and morphological ev idence that this insulin effect is operative in muscles composed mostl y of oxidative (red) fibers but not in muscles composed mostly of glyc olytic (white) fibers. The redistribution of the Na+-K+-ATPase alpha(2 )- and beta(1)-isoforms after insulin injection was detected in membra nes isolated from red muscles (soleus, red gastrocnemius, red rectus f emoris, and red vastus lateralis) but not in membranes from white musc les (white gastrocnemius, tensor fasciae latae, white rectus femoris, and white vastus lateralis). After insulin injection, the potassium-de pendent 3-O-methylfluorescein phosphatase activity of the enzyme was h igher by 22% in the plasma membrane-enriched fraction and lower by 15% in the internal membrane fraction isolated from red but not from whit e muscles. Quantitative immunoelectron microscopy of ultrathin muscle cryosections showed that in vivo insulin stimulation augmented the den sity of Na+-K+-ATPase alpha(2)- and beta(1)-isoforms at the plasma mem brane of soleus muscle by 80 and 124%, respectively, with no change in white gastrocnemius muscle. The effect of insulin to increase the con tent of Na+-K+-ATPase alpha(2)-and beta(1)-subunits in isolated plasma membranes was still observed when glycemia was prevented from droppin g by using hyperinsulinemic-euglycemic clamps. We conclude that the in sulin-induced redistribution of the alpha(2)- and beta(1)-isoforms of the Na+-K+-ATPase from an intracellular pool to the plasma membrane is restricted to oxidative fiber-type skeletal muscles. This may be rela ted to the selective expression of beta(1)-subunits in these fibers an d implies that the beta(2)-subunit, typical of glycolytic muscles, doe s not sustain translocation of alpha(2) beta(2)-complexes.