Pharmacological Characterizations of Adrenergic Receptors in Human Adipocytes

Abstract

Three types of adrenergic receptors, beta, alpha-1, and alpha-2, were identified in human adipocytes, isolated from properitoneal adipose tissue, using both the binding of radioactive ligands and the effects of adrenergic agents on receptor-specific biochemical responses. Adrenergic binding studies showed the following results: [³H]dihydroalprenolol binding (beta adrenergic) B_max 280 fmol/mg protein, K_D 0.38 nM; [³H]para-aminoclonidine binding (alpha-2 adrenergic) B_max 166 fmol/mg protein, K_D 0.49 nM; [³H]WB 4101 binding (alpha-1 adrenergic) B_max 303 fmol/mg protein, K_D 0.86 nM. In adipocytes from subcutaneous adipose tissue, [³H]dihydroergocryptine binding indicated the presence of alpha-2 but not alpha-1 receptors. Beta and alpha-2 adrenergic receptors appeared to be positively and negatively coupled to adenylate cyclase, respectively. Cells or cell membranes were incubated with epinephrine (10 μM) alone and in combination with the antagonists yohimbine (alpha-2) and prazosin (alpha-1). Epinephrine alone prompted a modest increase in adenylate cyclase activity, cyclic AMP, and glycerol release, an index of lipolysis. Yohimbine (0.1 μM) greatly enhanced these actions whereas prazosin was without effect. The beta agonist, isoproterenol, stimulated glycerol release, whereas the alpha-2 agonist, clonidine, inhibited lipolysis and cyclic AMP accumulation. To assess further alpha-1 receptors, cells were incubated with [³²P]phosphate and epinephrine (10 μM) alone and in combination with prazosin and yohimbine. Epinephrine alone caused a three- to fourfold increase in ³²P incorporation into phosphatidylinositol. Prazosin (0.1 μM) blocked this action whereas yohimbine (0.1 μM) was without effect. Thus, in a homogeneous cell preparation, the human adipocyte appears to have three different adrenergic receptors, each of which is coupled to a distinct biochemical response.