Abstract
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.
Highlights
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
The rate of glycerol release was a linear function of the mean fat cell size (Fig. 1) when adipose tissue was incubated under basal conditions or in the presence of noradrenaline (NA) or isoprenaline (ISNA)
< 0.001) or in the presence of NA ( r = +0.58, P < 0.01), the maximal tissue level of cAMP was significantly correlated with the fat cell size
Summary
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. 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. It is well established that the rate of lipolysis by human adipose tissue is strongly dependent on the adipocyte size so that large fat cells hydrolyze triglycerides more rapidly than do small cells [1,2,3,4,5,6,7,8] This relationship has been demonstrated both in the basal state and after stimulation with catecholamines, which are the only hormones with pronounced lipolytic activity in human adipose tissue [9]. Since cyclic AMP (CAMP)mediates the action of lipolytic hormones (lo),this study was undertaken to find out whether or not the relationship between the rate of lipolysis and
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