Adipose tissue - continued

Adipose tissue continues to dump free fatty acids into the circulation until its supplies are exhausted. 

The rate of mitochondrial oxidations and ATP synthesis is continually adjusted to the needs of the cell (see reviews by Brand and Murphy 1987 Brown, 1992). Physical activity and the nutritional and endocrine states determine which substrates are oxidized by skeletal muscle. Insulin increases the utilization of glucose by promoting its uptake by muscle and by decreasing the availability of free long-chain fatty acids, and of acetoacetate and 3-hydroxybutyrate formed by fatty acid oxidation in the liver, secondary to decreased lipolysis in adipose tissue. Product inhibition of pyruvate dehydrogenase by NADH and acetyl-CoA formed by fatty acid oxidation decreases glucose oxidation in muscle. 

Concern for the continued widespread use of chlordane centers on its ability to cause liver cancer in domestic mice. Other adverse effects in mammals, such as elevated tissue residues and growth inhibition, were frequently associated with diets containing between 0.76 and 5.0 mg chlordane/kg feed. Metabolism of technical chlordane by mammals results primarily in oxychlordane, a metabolite that is about 20 times more toxic than the parent compound and the most persistent metabolite stored in adipose tissues. Chlordane interactions with other agricultural chemicals produced significant biological effects in warm-blooded organisms, indicating a need for additional research on this subject. 

Fatty acids are oxidized in several tissues, including liver, muscle, and adipose tissue, by the pathway of P-oxidation. Neither erythrocytes nor brain can use fatty acids, and so continue to rely on glucose during normal periods of fastii. Erythrocytes lack mitochondria, and fatty acids do not cross the blood-hrain barrier efficiently. 

Figure 7.9 The degradation of triaq/lglycerol in adipose tissue to fatty acids and glycerol. The figure indicates the progressive release of fatly acids and the types of fatty acid that are usually present at each position and, therefore, released from each position as the triacylglycerol molecule. Sat. - Saturated. A lipase that is not regulated by hormones is also present is adipose tissue. It is continually active. Its role is described below.

In the ebb phase, there is increased activity of the sympathetic nervous system and increased plasma levels of adrenaline and glucocorticoids but a decreased level of insulin. This results in mobilisation of glycogen in the liver and triacylglycerol in adipose tissue, so that the levels of two major fuels in the blood, glucose and long-chain fatty acids, are increased. This is, effectively, the stress response to trauma. These changes continue and are extended into the flow phase as the immune cells are activated and secrete the proinflammatory cytokines that further stimulate the mobilisation of fuel stores (Table 18.2). Thus the sequence is trauma increased endocrine hormone levels increased immune response increased levels of cytokines metabolic responses. 

No studies were located regarding quantitative absorption of PBBs in animals after inhalation exposure to PBBs. However, increased bromine concentrations were found in the liver and adipose tissue of rats exposed continuously to a commercial mixture of octabromobiphenyl for 15 weeks, suggesting that absorption had occurred (Waritz et al. 1977). 

Beta-3 Receptors. Although it was originally thought that only two subtypes of beta receptors existed, we now know that a third subtype, the beta-3 receptor, plays a functional role in certain tissues. In particular, beta-3 receptors are located on adipose tissue, and stimulation of beta-3 receptors increases lipol-ysis (see Table 18-2).9 In addition, beta-3 receptors are also found on the heart and some smooth muscles, and the functional significance of beta-3 receptors at these locations continues to be investigated.5,23... 

The 1991 National Research Council report Monitoring Human Tissues for Toxic Substances recommended that any new program to assay chemical concentrations in tissues of the U.S. population be based primarily on analysis of blood. The use of blood permits sampling of a wider sector of the population, better comparison of exposed populations with national averages, repeat sampling of persons who have high tissue concentrations, and opportunities to follow chemical clearance with time. The 1991 report also advised analysis of adipose tissue (especially for persistent pesticides) that would provide continuity with previous studies and confirmation that a survey based on blood also detects important tissue residues of persistent chemicals (NRC 1991). 

TG are derived directly from the diet and secreted from the intestines (primarily by way of the lymph) as CM and TRL or synthesized into VLDL in the liver. The net transport of TG is therefore from the intestines and the liver to skeletal and cardiac muscle or to adipose tissue for storage. Cholesterol is used for membrane synthesis and steroid production and is primarily synthesized in extrahepatic tissues. It is continuously transported between the liver, intestines, and extrahepatic tissues, but the net transport of cholesterol is from the extrahepatic tissues to the liver and intestines from where it is eliminated. 

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