Up-regulation of insulin receptors in rat liver plasma membranes

Kinetic experiments (uptake versus time) were utilized to examine the effects of occupancy on insulin receptor availability in rat liver plasma membranes in vitro. The following observations were made: 1) at 4 degrees C, a 3-h exposure of membranes to 100 nM native insulin, followed by removal of unbound hormone, resulted in a subsequent decrease of 125I-insulin binding at 4 degrees C. In a similar experiment at 23 degrees C, no decrease of 125I-insulin binding was observed. 2) At 23 degrees C, 131I-insulin (5 nM) was bound to membranes in a slowly reversible manner after a 3-h association. After removal of free hormone, the 131I-insulin-treated membranes displayed similar binding of 125I-insulin (1 nM) relative to controls despite persistent high level occupancy of receptors by 131I-insulin. 3) At 23 degrees C, phospholipase pretreatment of membranes enhanced 125I-insulin uptake (approximately 40%). Phospholipase-digested membranes exposed to 100 nM native insulin for 3 h bound more 125I-insulin (approximately 40%) than did nondigested membranes preincubated without native insulin. The results allowed speculation that rat liver membranes up-regulated insulin receptors after treatment with insulin and that this was mediated by exposure of cryptic binding sites.