Studies on desensitization of adrenergic receptors of human adipocytes

The studies described here were undertaken to determine whether or not desensitization of human adipocyte beta and alpha-2 adrenergic receptors could be demonstrated. Cells, isolated from peritoneal adipose tissue obtained from patients undergoing elective abdominal surgery, were preincubated for 3 hr in buffer alone or in the presence of isoproterenol, 10-5M. Cells in both sets of flasks were then washed and exposed to isoproterenol for 1/2 hr; cyclic AMP was then measured as an end point of beta receptor activation. Cells which had had no prior exposure to isoproterenol responded significantly greater to isoproterenol than did cells that had had prior exposure to the catecholamine, The beta receptor characteristics of cells undergoing beta desensitization were assessed using [3H] dihydroalprenolol. Compared to control cells, adipocytes exposed to isoproterenol had a reduction in Bmax while KD values were the same. Thus desensitization of beta adrenergic receptors of human adipocytes occurs and is associated with down regulation in the number of beta receptors. In comparable studies, preincubation with epinephrine 10-5M did not affect the response of cells to a subsequent exposure to this catecholamine. In alpha-2 receptor binding assays, there was a decreased number of [3H]p-aminoclonidine binding sites, but the level of [3H]yohimbine binding was not altered following the incubation with epinephrine. Thus, desensitization of alpha-2 receptors was not demonstrated.