In Vitro Effects of Glucocorticoid on Glucose Transport in Rat Adipocytes: Evidence of a Post-Receptor Coupling Defect in Insulin Action1

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Abstract
The in vitro effect of glucocorticoid on insulin binding and glucose transport was studied with rat adipocytes. Isolated rat adipocytes were incubated with or without 0.70μg/ml (1.9 μmol) of hydrocortisone in TCM 199 medium at 37°C, 5% CO2/95% air (v/v), pH 7.4, for 2, 4, and 8 h, and then fat cell insulin binding and insulin-stimulated 3-O-methylglucose transport were measured. Hydrocortisone did not affect insulin binding in terms of affinity or receptor number. Glucose transport in the absence of insulin was significantly decreased at the incubation time of 2 h and continued to decrease up to 8 h of incubation with hydrocortisone. Decreased insulin sensitivity of glucose transport (i.e, a right-ward shift of the dose response curve) was also demonstrated after 2 h incubation with hydrocortisone, and the ED50 of insulin was maximally increased at 4 h of incubation (0.53 ng/ml for treated vs 0.22 ng/ml for control cells). Maximal insulin responsiveness was also significantly decreased in treated cells after 8 h incubation with hydrocortisone. When percent maximum glucose transport was expressed relative to receptor-bound insulin, the ED50 values of treated and control cells were 10.5 and 7.2 pg of bound insulin, per 2×105 cells, respectively. Thus, it was evident that glucocorticoid induced a post-receptor coupling defect in the signal transmission of insulin-receptor complex.