Adipose triglyceride mobilization

Starvation elicits mobilization of triglycerides from the adipose tissue and inhibits the endogenic cholesterol synthesis owing to the low activity of hydroxy-methylglutaryl-CoA reductase. The latter process provides the possibility for the active production of ketone bodies in the liver. 

Adipose Adipose tissue is the primary storage facility for fat. Fat is stored in these tissues as an intracellular droplet of insoluble triglyceride. A hormone-sensitive lipase mobilizes triglyceride stores by hydrolysis to free fatty acids. 

Phospholipids are considered to be involved in the transport of triglycerides through die liver, especially during mobilization from adipose tissue, Conditions which could be interpreted as interfering with phosphatidylcholine formation, such as deficiency of choline or its precursors, result in a pronounced increase 111 liver triglycerides. 

The mobilization of the depot fat of adipose tissues requires hydrolysis of the triglycerides. Adrenalin or other lipolytic hormones trigger a... 

After ingestion of food, a small rise in serum insulin suppresses effectively fatty acid mobilization from adipose tissue but has little effect on glucose transport. A single intravenous injection of insulin does not decrease serum triglycerides. However, small amounts of insulin infused for a number of hours result in significant reduction, leading finally to an increase in insulin... 

Nicotinic acid exerts a variety of effects on lipoprotein metabolism (7,16,49). One of its most important actions is the inhibition of lipolysis in adipose tissue. This initial inhibition, like those of previously discussed antihyperlipidemic agents, produces a sequence of events that ultimately result in the lowering of plasma triglycerides and cholesterol. Impaired lipolysis decreases the mobilization of free fatty acids, thus reducing their plasma levels and their delivery to the liver. In turn, this decreases hepatic triglyceride synthesis and results in a decreased production of VLDL. Enhanced clearance of VLDL through stimulation of lipoprotein lipase also has been proposed to contribute to the reduction of plasma VLDL levels. Because LDL is derived from VLDL (Fig. 30.5), the decreased production of VLDL ultimately leads to a decrease in LDL levels. The sequential nature of this process has been clinically demonstrated. The reduction in triglyceride levels occurs within several hours after ... 

Adipose tissue is the major energy store in humans. Synthesis and breakdown of triglycerides are constant events in adipose tissue. Fatty acid and glycerol mobilization is stimulated by activation of hormone-sensitive lipase [9]. 

What signals the need for mobilization of triglycerides from adipose tissue ... 

VLDL is assembled in liver cells from lipid obtained via degradation of lipoproteins, synthesis from dietary carbohydrate and protein, and from fat mobilized from adipose tissue. VLDL is also rich in triglyceride (about 50%) and contains a substantial portion of cholesterol mainly as cholesterol ester. VLDL transports about 15% of the total cholesterol found in the blood. The apohpoproteins present on the surface of VLDL are apo BlOO, Cn and E. VLDL circulates in the blood and is acted upon by lipoprotein lipase in the same manner as for chylomicrons. The resulting VLDL remnants are LDL. 

Further supplies of energy are provided by the breakdown of liver glycogen to replenish the blood glucose and by the hydrolysis of triglycerides to fatty adds and glycerol in both liver and adipose tissue. In this way there is a general mobilization of the body s energy reserves. 

It seems well established that fatty livers produced by starvation, diabetes or the administration of anterior pituitary hormones are primarily due to an increased mobilization of fatty acids from adipose tissue (see chapter III 2) (Barrett et al. 1938 Stetten and Salcedo 1944). The rate of hepatic triglyceride synthesis is directly proportional to the level of free fatty acids in the blood (Feigelson et al. 1961), and increased mobilization will therefore residt in accelerated triglyceride synthesis. 

Adipose tissue triglycerides are mobilized as their split products — fatty acids and glycerol. The identification of free fatty acids as the main form of transport of fat from adipose tissue has opened a completely new scope to our understanding of energy metabolism. The elevation of this minor plasma constituent to the rank of a major metabolite was made acceptable by the detection of its extremely rapid turnover. Half life times of approximately two minutes were reported for plasma free fatty acids. Free fatty acids are burnt in many organs and can supply a considerable portion of the metabohc energy of heart and skeletal muscle. A detailed discussion on the metabolism of free fatty acids has been published (Fredrickson and Gordon 1958). 

Current interest in the study of adipose tissue metabolism has been stimulated by the work of many researchers in various countries. It is not my purpose to give a full account of the historical development of the subject, but a few highlights should be mentioned. Among these (a) the demonstration of the rapid turnover of stored fat " (b) the publication in 1948 of the classic review on the physiology of adipose tissue by Wertheimer and Shapiro (c) the identification of the plasma free fatty acids (FFA) as the form in which the triglycerides stored in the adipose tissue are mobilized and made available as fuel to the body tissues, - and the introduction of the concept of caloric homeostasis . ... 

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