Adipose tissue lipolysis, glucagon-induced

The regulation of fat metabolism is relatively simple. During fasting, the rising glucagon levels inactivate fatty acid synthesis at the level of acetyl-CoA carboxylase and induce the lipolysis of triglycerides in the adipose tissue by stimulation of a hormone-sensitive lipase. This hormone-sensitive lipase is activated by glucagon and epinephrine (via a cAMP mechanism). This releases fatty acids into the blood. These are transported to the various tissues, where they are used. 

The initial event in the utilization of fat as an energy source is the hydrolysis of triacylglycerols by lipases, an event referred to as lipolysis. The lipase of adipose tissue are activated on treatment of these cells with the hormones epinephrine, norepinephrine, glucagon, and adrenocorticotropic hormone. In adipose cells, these hormones trigger 7TM receptors that activate adenylate cyclase (Section 15,1.3 ). The increased level of cyclic AMP then stimulates protein kinase A, -which activates the lipases by phosphorylating them. Thus, epinephrine, norepinephrine, glucagon, and adrenocorticotropic hormone induce lipolysis (Figure 22.6). In contrast, insulin inhibits lipolysis. The released fatty acids are not soluble in blood plasma, and so, on release, serum albumin binds the fatty acids and serves as a carrier. By these means, free fatty acids are made accessible as a fuel in other tissues. 

Campbell, R.M. Scanes, CG. (1987). Growth hormorre inhibition of glucagon- and cAMP-induced lipolysis by chicken adipose tissue in vitro. Proc. Soc. Exp. Biol Med., 184, 456-60. 

The in vitro lipolytic effects of glucagon and epinephrine were tested in tissues from three avian (goose, duck and owl) and three mammalian species (dog, rat, rabbit). In addition, the effects of insulin and nicotinic acid on the glucagon-induced lipolysis were also tested. The concentrations of the various compounds tested were those known to produce maximal responses in rat epididymal adipose tissue. The experiments provide a comparison of the responses of the adipose tissue from various species with each other, and with that elicited in the rat adipose tissue by hormone concentrations capable of producing the maximal response in the latter tissue (see ref. 36). 

Lipolytic Effects of Glucagon and Epinephrine and the Effects of Insulin and Nicotinic Acid on Glucagon-induced Lipolysis Adipose tissue in vitro from various mammals and birds ... 

ASPECTS OF GLUCAGON-INDUCED LIPOLYSIS IN ADIPOSE TISSUE. 

Hormone induced cAMP generation in adipose tissue is very repid. One minute after addition of epinephrine to perfusate. Ho and Neng found elevated values (6). Lipolysis is stimulated to maximal rates within about 30 to 60 seconds after addition of glucagon to adipose tissue in vitro (7). This must mean that the hormone quickly equilibrates with its presumptive receptor sites. 

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