Effect of hormones on lipolysis and esterification of free fatty acids during incubation of adipose tissue in vitro

Using a new approach based on simultaneous measurements of net changes in glycerol and free fatty acids, the rates of lipolysis and of fatty acid esterification were estimated in rat epididymal fat pads under various conditions in vitro. Glycerol release, taken as a measure of the rate of lipolysis, was stimulated by epinephrine, norepinephrine, glucagon, adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), and growth hormone (GH). When lipolysis was stimulated by epinephrine, glucagon, ACTH, or TSH, the calculated rate of esterification was also increased, although to a lesser extent, of course. GH, which produced a relatively small increase in the rate of lipolysis, produced no detectable increase in rate of esterification. The stimulatory effect of epinephrine, ACTH, and GH on lipolysis was inhibited by 10−2m NaF but there was no detectable inhibition of the basal glycerol release from tissues incubated in the absence of hormones. Data are presented suggesting that small amounts of epinephrine (0.3 μg), ACTH (0.12 U), or glucagon (15 μg) are largely inactivated during 30 min of incubation with a fat pad under the conditions employed. The effects of GH and TSH, on the other hand, persisted without diminution for at least 60 min, consonant with the view that the effects of GH on lipolysis in adipose tissue are due to TSH contained in it rather than to ACTH.