Activity and phosphorylation state of glucose transporters in plasma membranes from insulin-, isoproterenol-, and phorbol ester-treated rat adipose cells

The counterregulatory action of catecholamines on insulin-stimulated glucose transport and its relation to glucose transporter phosphorylation were studied in isolated rat adipose cells. Plasma membranes exhibiting reduced glucose transport activity were prepared as described previously (Joost, H.G., Weber, T.M., Cushman, S.W., and Simpson, I.A. (1986) J. Biol. Chem. 261, 10033-10036) from cells treated with insulin, and subsequently with isoproterenol and adenosine deaminase. In these membranes, transporter affinity for cytochalasin B binding was significantly reduced (K(D) = 133.5 ± 14 versus 89.8 ± 11 nM, means ± S.E.) with no change in number of sites or immunoreactivity of the transporter on Western blots. Reconstituted plasma membrane transport was significantly lower with isoproterenol treatment (0.50 ± 0.12 versus 0.97 ± 0.27 nmol/mg protein/10 s). In contrast, transport activity reconstituted from corresponding intracellular transporters (from low density microsomes) was unchanged (5.4 ± 2.2 versus 6.9 ± 1.2 nmol/mg protein/10 s). Thus, the intrinsic activity change of the transporter produced by catecholamines appears to reflect a structural modification that is confined to the plasma membrane and not recycled into the intracellular compartment. In cells equilibrated with [³²P]phosphate, neither insulin nor isoproterenol induced [³²P]phosphate incorporation into the glucose transporter immunoprecipitated from plasma membranes. Conversely, phorbol 12-myristate 13-acetate stimulated significant incorporation of [³²P]phosphate into the glucose transporter in insulin-stimulated cells without any change in plasma membrane transport activity or transporter concentration. Thus, the phosphorylation state of the glucose transporter does not seem to be involved in either signaling transporter translocation or triggering changes in transporter intrinsic activity.