The incorporation of [³²P]P_i into phosphatidylinositol by rat fat-cells was markedly increased in the presence of adrenaline. Phosphatidic acid labelling was also increased, but to a lesser extent. These effects are due to α₁-adrenergic stimulation since they were unaffected by propranolol, blocked by α-blockers in the potency order prazosin«phentolamine<yohimbine and mimicked by methoxamine. The α-adrenergic stimulation of phosphatidylinositol labelling did not require extracellular Ca²⁺, which supports the hypothesis that an increased turnover of phosphatidylinositol is involved in α-adrenergic activation of Ca²⁺ entry. Insulin and the ionophore A23187 gave a small increase in ³²P labelling of phosphatidylinositol in Ca²⁺-free medium containing 1mm-EGTA. The increases due to insulin or ionophore A23187 were abolished if 2.5mm-Ca²⁺ was added to medium containing EGTA. However, the increases in labelling of phosphatidylinositol due to α-adrenergic amines were still evident in medium containing EGTA and Ca²⁺. Lipolytic agents such as corticotropin, dibutyryl cyclic AMP, adrenaline in the presence of phentolamine and isoproterenol decreased [³²P]P_i incorporation into phosphatidylinositol, phosphatidylethanolamine and phosphatidic acid. This inhibitory effect may be secondary to accumulation of intracellular unesterified fatty acids, since it was decreased by incubating fewer cells in medium with 6 rather than 3% albumin and was restored by the addition of oleate to the medium. The incorporation of [³²P]P_i into phosphatidylcholine was unaffected by lipolytic agents. The data suggest that there is an inhibition of the synthesis of certain phospholipids in the presence of lipolytic agents, which may be secondary to intracellular accumulation of unesterified fatty acids.

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.