Relationship between the tissue level of cyclic AMP and the fat cell size of human adipose tissue

The relationship between mean fat cell size, maximal tissue cyclic AMP concentration, and glycerol release was investigated in human subcutaneous adipose tissue incubated in vitro with or without isoprenaline or noradrenaline added at maximal effective concentrations. Basal and stimulated glycerol release and cyclic AMP concentration were each related to the fat cell size. Whether or not the phosphodiesterase inhibitor theophylline was present in the incubation system, basal and noradrenaline-induced cyclic AMP levels were significantly correlated with the fat cell size. The noradrenaline-induced cyclic AMP levels resulted in twice as rapid glycerol release as could be expected from the basal ratio between glycerol release and cyclic AMP. Furthermore, both basal and noradrenaline-induced glycerol release in relation to the cyclic AMP levels were more rapid in enlarge fat cells. It is concluded that basal and catecholamine-induced production of cyclic AMP is related to the fat cell size and that a quantitative relationship exists between rate of lipolysis and maximal tissue levels of cyclic AMP in human adipose tissue. Basal and noradrenaline-induced lipolysis are probably regulated by different mechanisms and the lipolytic sensitivity to cyclic AMP seems increased in large fat cells.