Insulin-stimulated plasma membranes from rat adipocytes: their physiological and physicochemical properties

Abstract

Using methods previously described, we have isolated plasma membranes from insulin-treated adipocytes of the rat epididymal fat pad. Such membranes showed an accelerated uptake and release of d-glucose when compared with similar preparations from cells not exposed to the hormone. The only change observed was in the rate of d-glucose uptake; at equilibrium, d-glucose space was identical in both preparations. Vigorous alkaline hydrolysis of the insulin resulted in loss of its typical effect on both intact cells and derived plasma membranes. Anti-insulin antiserum, potent enough to inhibit the effect of the hormone when both were added to intact cells, did not prevent the insulin effect when added to plasma membranes prepared from insulin-treated cells. Addition of insulin directly to plasma membranes was without effect; exposure of cells prior to rupture was required. Studies of infrared spectra, native membrane protein fluorescence, and fluorescence of 8-anilino-1-naphthalene sulfonate added to the membranes, showed no differences between control plasma membranes and those prepared from insulin-treated cells. We conclude that: (1) plasma membranes can be prepared from insulin-treated fat cells which retain an enhanced glucose transport; (2) the effect of insulin on glucose transport does not involve large scale changes in the structure of the plasma membrane; (3) the insulin unresponsiveness of isolated plasma membranes, as well as the resistance to anti-insulin serum of membranes prepared from insulin-treated cells, appears to result from an uncoupling of insulin binding from glucose transport, the basis of which requires further exploration.