Adipose tissue mammals

Additional roles traditionally recognised for white adipose tissue include thermal insulation (as in the blubber of sea mammals) and mechanical protection to internal organs. However, in recent years it has become apparent that the function of white adipose tissue is much more extensive. This follows from the discovery that it is a major endocrine organ, secreting a diverse... 

Fatty acids are synthesized by an extramitochondrial system, which is responsible for the complete synthesis of palmitate from acetyl-CoA in the cytosol. In the rat, the pathway is well represented in adipose tissue and liver, whereas in humans adipose tissue may not be an important site, and liver has only low activity. In birds, lipogenesis is confined to the liver, where it is particularly important in providing lipids for egg formation. In most mammals, glucose is the primary substrate for lipogenesis, but in ruminants it is acetate, the main fuel molecule produced by the diet. Critical diseases of the pathway have not been reported in humans. However, inhibition of lipogenesis occurs in type 1 (insulin-de-pendent) diabetes mellitus, and variations in its activity may affect the nature and extent of obesity. 

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. 

The main depot for dioxin accumulation in mammals is the liver and adipose tissue. When dioxin is fed to rodents with fodder, about 10% of the dose comes in the liver. Should this toxic agent be applied on rat skin, 13% of the dose is absorbed in blood already in 5 hours, and 10% remains on skin surface. 

Analytical methods exist for measuring heptachlor, heptachlor epoxide, and/or their metabolites in various tissues (including adipose tissue), blood, human milk, urine, and feces. The common method used is gas chromatography (GC) coupled with electron capture detection (ECD) followed by identification using GC/mass spectrometry (MS). Since evidence indicates that heptachlor is metabolized to heptachlor epoxide in mammals, exposure to heptachlor is usually measured by determining levels of heptachlor epoxide in biological media. A summary of the detection methods used for various biological media is presented in Table 6-1. 

This subject is discussed in Chapter 15, but a brief summary is given here, as an introduction to fat metabolism. The three major components of a human diet are meat, fish and plants, and the fats associated with each component can be different. Meat usually comprises the muscle of mammals and birds but it is always associated with fat. Even apparently lean meat may have a high fat content a 250 g beef steak may contain 60 g fat in addition to its 80 g protein. Some of this will be adipose tissue between muscles the remainder is triacylglycerol (TAG) within the fibre. In addition, milk and other dairy products contain significant amounts of fat about a quarter of the fat in the average UK diet comes from dairy produce. 

BDE 17, 28, 47, 66, 85, 99, 100, 153, 154 183 Adipose tissues from marine mammals, chicken and trout Adipose Tissues, chicken and trout MSPD with silica gel/anhydrous sodium sulfate powder, purification thorug GPC extraction with 400 mL of 1 1 (v/v) acetone/hexane mixture Gas Chromatography (VF-5MS Eactor Eour, Varian) IT-MS 0.07-1.3 pg (instrumental limit of detection) [42]... 

There is a remarkable and instructive exception to the general rule that respiration slows when the ATP supply is adequate. Most newborn mammals, including humans, have a type of adipose tissue called brown fat in which fuel oxidation serves not to produce ATP but to generate heat to keep the newborn warm. This specialized adipose tissue is brown because of the presence of large numbers of mitochondria and thus large amounts of... 

Levels of POPs in marine mammals and birds liver are generally one to three orders of magnitudes higher than in fish liver which, in turn, are four orders of magnitude lower than human adipose tissues (Liang et al., 1999 Dietz et al., 2000). Mammals, such as smooth-coated otters (Lutro-gale perspicillata), and over 100 species of birds have been recorded in the... 

TCDD and OCDD in the Arctic than in sub-Arctic areas is thought to be transpolar movement of aerosols from combustion-related sources originating in Eurasia (Norstrom et al. 1990). CDDs and CDFs were determined in caribou tissue samples from 7 herds across the Canadian Arctic (Hebert et al. 1996). In contrast to marine mammals, concentrations for caribou were extremely low, sub-ng/kg (lipid basis), for all congeners except OCDD and 1,2,3,7,8-PeCDD in one herd. OCDD was found in most of the samples at concentrations ranging from < 0.2 ng/kg in fat to 4.7 ng/kg in adipose tissue. The one pooled liver sample analyzed from the Yukon had an OCDD concentration of 11 ng/kg (lipid basis). 

TCDD was detected in adipose tissue samples of two herds in the eastern Canadian Arctic at levels of 0.73 and 0.14 ng/kg, but was not detected in tissue samples from other herds at detections limits as low as 0.03 ng/kg (lipid basis). CDF levels were sub-ng/kg in all cases. TEQs were dominated by non-ortho substituted PCBs in all cases, and ranged from 0.33 ng/kg to 3.29 ng/kg in adipose tissue. The authors concluded that caribou tissues are therefore less contaminated than tissues from marine mammals. 

Brown adipose tissue is most prevalent in neonatal mammals, but it is now thought to persist in many adult forms as well, including adult humans (for review, see Ricquier and Bouillaud, 2000). Small mammals such as rodents rely heavily on BAT thermogenesis as adults. The high surface-to-volume ratios of small mammals appear to favor the persistence of this mode of heat generation throughout life. 

In considering the distribution of particular lipids, it must be emphasized that there is wide variation in the lipid composition of various cells (see section entitled An Excursion into the Complexities of Phospholipids Found in Certain Cells Defining the Problem ). Of the three classes of lipids depicted in Figure 1-1, triglycerides (triacylglycerols) form the chief lipid constituent of adipose tissue in the mammal and also are found in plasma. 

Liver and adipose tissue are the major storage compartments for CACs. Ratios between liver and adipose tissue distribution are species dependent and follow the order rodents > birds > humans > fish.30 In marine mammals this ratio is determined by their relatively large adipose tissue compartment, with large storage capacities. CAC metabolites may bind to specific target proteins, e.g. in blood, kidney or lung. 

PDP is stimulated by calcium and magnesium. Like PDK, PDP is a dimer, but it is more loosely bound than the kinase to the complex. Mammals have two PDP isoforms, each of which contains both regulatory and catalytic subunits. PDP1 is highly expressed in cardiac and, to a lesser degree, skeletal muscle. PDP2 is present in both oxidative (heart, kidney) and lipogenic (liver, adipose) tissues. 

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