Action of propranolol on lipolysis induced by catecholamine, dibutyryl cyclic AMP and serum

Effect of rat serum on glycerol metabolism in adipose tissue in vitro

It has been proposed that the lipolytic effect of serum is based on the presence of either lipoproteins or catecholamines. To test these hypotheses, pieces of epididymal fat pads from fed rats were incubated in the presence of albumin and glucose for 120 min. The addition of rat serum (5 mul/vial) enhanced the rates of both glycerol release to the media and [U-14C] glycerol utilization by the tissue. Heparin did not alter these parameters or the response produced by serum. VLDL from rat plasma also enhanced glycerol release and utilization for the formation of CO2 and lipids, and heparin significantly augmented these effects. Neither of the conditions studied affected the percentual distribution of 14C-lipid fractions in the tissues. It is known that in similar conditions to those used in the present study, adrenaline produces a decrease in the utilization of glycerol. Thus our findings do not support the proposed hypothesis explaining the fat-mobilizing action of serum, the mechanism of which remains to be determined.

Permissive effects on adipose tissue lipolysis of a plasmatic factor which inhibits prostaglandin biosynthesis

Plasma obtained from fasted rats provoked a dose-dependent stimulation of fat cell glycerol release and was able to inhibit also generation of prostaglandins E2 and F2 alpha from arachidonate, in vitro, over a very similar range of doses. It is proposed, therefore, that a plasmatic fat-mobilizing factor may act as an endogenous inhibitor of adipose tissue prostaglandin biosynthesis, mediating in the acute (long-term not precluded) regulation of adipose tissue lipolysis. The proposed mechanism of permissive effects of the plasmatic factor in reducing the effects of endogenously-generated inhibitory prostaglandins may be implicated in the development of obesity by reduced availability of the factor and thus reduced ability to utilize fat stores.