section 22.6

Lipid Homeostasis

505

and export are stimulated by insulin and glucose and in-

hibited by catecholamines. Mammary gland lipoprotein

lipase activity is elevated maximally at parturition when

lipid is needed for milk formation. Cardiac lipoprotein li-

pase is elevated during starvation. The lipoproteins trans-

port triacylglycerol and cholesterol (Chapter 20).

Production of Triacylglycerol from Carbohydrate

Processes involved in triacylglycerol storage are presented

in Figure 22-18. When dietary carbohydrate exceeds re-

quirements, it is converted into triacylglycerol within

adipocytes or hepatocytes. The liver daily synthesizes

40-100 g of triacylglycerols, partly from glucose and

partly from free fatty acids present in excess of its energy

needs. Their transport to adipocytes occurs via VLDLs,

whose synthesis, secretion, and fate are analogous to those

of chylomicrons.

Release of Lipid from Adipose Tissue Stores

During fasting or exercise and at times of stress, lipid

is used as a source of energy. Hormone-sensitive lipase

catalyzes the sequential hydrolysis of triacylglycerol to

T A B L E 2 2 -3

Hormonal Regulation o f Adipocyte Lipolysis

Rapid

Slow

Stimulators

Stimulators

Inhibitors

Epinephrine

Glucocorticoids

Insulin

Norepinephrine

Glucagon

ACTH

Secretin

Vasopressin

Growth hormone

Prostaglandin E ,

yield ultimately 3 mol of fatty acids and 1 mol of glycerol.

Hormone-sensitive lipase is regulated by phosphorylation

by cAMP-dependent protein kinase. Listed in Table 22-3

are hormones that affect lipase. Epinephrine, glucagon,

and the other rapid stimulators release free fatty acids

within minutes. The actions of growth hormone and of

glucocorticoids require hours, but their sites of action are

unknown. Insulin and prostaglandin Ei suppress lipolysis

by depressing cAMP levels.

The major activities of adipocytes are summarized in

Figure 22-19. Glucagon, an indicator of the need for

F I G U R E 2 2 - 1 9

Regulation of adipocyte triacylglycerol synthesis and degradation. FA, Fatty acid; PDE, phosphodiesterase.