Catecholamines and Metabolism of Human Adipose Tissue

Abstract. Human omental tissue was incubated in KrebsHenseleit bicarbonate medium containing 3% albumin and glucose with additions of noradrenaline, isopropylnoradrenaline, or theophylline. Glycerol release was measured and taken as the index of lipolytic activity. In the first series of experiments the effects of equimolar concentrations of noradrenaline and isopropylnoradrenaline were compared. At each concentration lsopropylnoradrenaline produced significantly higher glycerol release. Noradrenaline‐stimulated lipolysis was suppressed completely by the beta‐adrenergic blocking agent propranolol added at a concentration of 1 μg per ml of the incubation medium. At this concentration propranolol had no effect on basal glycerol release and that induced by theophylline (10 ‐2 M ). The alpha‐adrenergic blocking agent phentolamine inhibited only in some experiments the glycerol release produced by noradrenaline and this occurred only when the agent was added at a high concentration (500 μg/ml) which also inhibited theophylline (10 ‐2 M ) induced lipolysis. These data indicate that in human omental adipose tissue catecholamine‐induced lipolysis is mediated by beta‐adrenergic receptors. They also suggest that the antilipolytic action of high phentolamine concentrations is localized between the formation of cyclic adenosine‐3′,5′‐monophosphate (3′,5′‐AMP) and the induction of triglyceride lipase activity. In all experiments glycerol release produced by noradrenaline was further increased by addition of small concentrations (0.05–100 μg/ml) of phentolamine to the incubation medium. The stimulation of noradrenalineinduced lipolysis was negatively correlated with the effect of noradrenaline alone on basal lipolysis. These small concentrations of phentolamine did not significantly influence the glycerol release produced by theophylline or isopropylnoradrenaline. The maximal lipolytic responses to noradrenaline plus phentolamine were of similar magnitude to those produced either by theophylline or isopropylnoradrenaline alone. These results were interpreted as indicating that in human omental tissue there exist alpha‐adrenergic receptors which counteract the lipolytic effect of catecholamines by reducing the formation of 3′,5′‐AMP.