Glucose metabolism, in adipose tissue

Effects on glucose metabolism in adipose tissue and muscle 218... 

EFFECTS ON GLUCOSE METABOLISM IN ADIPOSE TISSUE AND MUSCLE... 

Fig. 31.15. Glucose metabolism in various tissues. A. Effect of insulin on glycogen synthesis and degradation and on VLDL synthesis in the liver. B. Glucose metabolism in resting muscle in the fed state. The transport of glucose into cells and the synthesis of glycogen are stimulated by insulin. C. Glucose metabolism in adipose tissue in the fed state. FA = fatty acids DHAP = dihydroxyacetone phosphate. FA = fatty acids TG = triacylglycerols -I- = stimulated by insulin — = inhibited by insulin.

Tea contains polyphenols such as catechins and its polymeric theafravins, the latter in black tea. Tea components serve as anti-carcinogens (13), and antimutagens (14) among others. Recently, it has reported that green tea extracts reduce the risk of obesity in mice and rats (15,16). In addition, oolong tea is traditionally reported to have anti-obesity and hypolipidaemic actions (17). There is, therefore, a possibility that tea extracts modulate glucose metabolism in adipose tissue. 

Figure 25-14 Mechanism of insulin action. Binding of insulin to the extracellular a-subunit of the insulin receptor induces autophosphorylation of the -subunit of the receptor and phosphorylation of selected intracellular proteins, such as She and the IRS family,These latter phosphoproteins interact with other targets, thereby activating phosphorylation cascades, which result in glucose uptake (in adipose tissue and skeletal muscle), glucose metabolism, synthesis (of glycogen, iipid, and proteins), enhanced gene expression, cell growth, and differentiation, p, protein phosphorylation aPKC, atypical protein kinase C, See text for details.

It is effective in lowering blood sugar and FFA levels in intact, adrenalectomized and hypophysectomized rats. This compound is metabolized to 2-carboxy-pyrazine, which is responsible for its action. 39 This is substantiated by the finding that pyrazinamide does not increase glucose oxidation or inhibit FFA release from adipose tissue in vitro whereas 2-carboxypyrazine exerts both actions. Hypoglycemic activity is attributed to the ability of 2-carboxypyrazine to increase glucose utilization in adipose tissue. 39... 

Insulin is a peptide hormone, secreted by the pancreas, that regulates glucose metabolism in the body. Insufficient production of insulin or failure of insulin to stimulate target sites in liver, muscle, and adipose tissue leads to the serious metabolic disorder known as diabetes mellitus. Diabetes afflicts millions of people worldwide. Diabetic individuals typically exhibit high levels of glucose in the blood, but insulin injection therapy allows diabetic individuals to maintain normal levels of blood glucose. 

In adipose tissue, the effect of the decrease in insulin and increase in glucagon results in inhibition of lipo-genesis, inactivation of lipoprotein lipase, and activation of hormone-sensitive lipase (Chapter 25). This leads to release of increased amounts of glycerol (a substrate for gluconeogenesis in the liver) and free fatty acids, which are used by skeletal muscle and liver as their preferred metabolic fuels, so sparing glucose. 

Fat is the major way we store energy. Fat is stored principally in adipose tissue and can be stored in virtually unlimited amounts. Fat is metabolized through (1 oxidation to acetyl-CoA and then through the TCA cycle, where it s burned completely to C02 to make ATP. Fat cannot be used to make carbohydrate because acetyl-CoA cannot be converted directly to precursors of glucose without losing its carbon atoms first (you ll see what this really means later). 

The metabolism of VLDL is very similar to that of chylomicrons, the major difference being that VLDL are assembled in hepatocytes to transport triglyceride containing fatty acids newly synthesized from excess glucose, or retrieved from the chylomicron remnants, to adipose tissue and musde. ApoB-100 is added in the hepatocytes to mediate release into the blood. Like chylomicrons, VLDL acquire apoC-II and apoE from HDL in the blood, and are metabolized by lipoprotein lipase in adipose tissue and musde. 

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