Adipose tissue species differences

Phospholipids in liver, kidney, muscle, and other tissues. Organ meats such as liver, kidney, and muscles are a major source of dietary phospholipids. The reader is referred to Kuksis (16) for the distribution of various phospholipid classes in the liver, kidney, muscles (heart and skeletal), spleen, lung, blood cells, bile, and adipose tissue of different animal species. Compositional data of fatty acids for these tissues and fluids are also given. 

In 1967 Rudman and Di Girolamo published a comprehensive comparative study of the physiology of adipose tissue which clearly shows that there are remarkable diflFerences between the adipose tissues from different mammalian species. There is less information in the review regarding the adipose tissue of birds. This class of vertebrates, however, is of interest because birds share with mammals the common property of having a well-developed subcutaneous adipose tissue and that of being homeotherms. 

Table II, Some Physiologic Characteristics of Adipose Tissue of Different Vertebrate Species,...

Current available information does not permit definitive conclusions on the nature, specificity, and mechanism of action of the protein cofactor (s) of lipoprotein lipase. It is verj difiicult to correlate the observations described above (summarized in Table 10) since the enzyme preparations used were not pure or well characterized, and were derived from various sources. For instance, two species of lipoprotein lipase have been reported to exist in rat adipose tissue (G4), and major differences between enzymes of liver and adipose tissue have been noted (G16). Also, the nature of the apoprotein preparations employed as protein cofactor (s) of lipoprotein lipase has not been clearly specified in all the studies contaminated materials may account for the spurious results observed. At present, it is not known how apoproteins such as apo Glu, apo Ala, and apo Ser could exhibit their activator or inhibitor activity on lipoprotein lipase. If these different apoproteins indeed prove to be cofactors for lipoprotein lipase, the nature of the lipid-protein specificity must be established and thus the role played by carbohydrates, since some of these apoproteins are glycoproteins. 

A simple explanation of the failure of previous studies to detect any differences could be as a result of these studies not evaluating the relevant markers for health. Four of the markers (except lymphocyte proliferative capacity) can be assessed non-invasively (sleep, accumulation of adipose tissue) or in blood samples (IgA, protein oxidation), and are therefore suitable for use in human studies. In addition, all the markers can be assessed on a range of different animal species. 

The blood levels of 1,1,1-trichloroethane in human subjects were lower following exposure to 350 ppm [1910 mg/m ] (approximately 2 mg/L) (Nolan et al., 1984) than those found in rats and mice following exposure to 150 ppm [820 mg/m ] (9.6 mg/L and 12.6 mg/L, respectively) (Schumann et al., 1982b). The species differences between humans and rats are probably the result of a lower 1,1,1-trichloroethane blood air partition coefficient and greater adipose tissue volume in humans (Dallas et al., 1989). 

As mentioned above, the process of biotransformation of persistent CACs has been substantiated by the finding of metabolites in laboratory and wildlife species. In particular for PCBs, the presence of both the hydroxylated congeners in blood as well as the lipophilic methylsulfonyl-PCBs in adipose, liver and lung tissue of different species has proved that metabolism proceeds according to the mechanisms outlined above. Similarly, hydroxy metabolites of PCDFs have been found in rodents.73... 

Early research on lipolytic enzymes in cows milk suggested that at least two major lipases were present a plasma lipase in the skim portion and a membrane lipase associated with the milk fat globule membrane (Tarassuk and Frankel, 1957) while later research indicated that there might be up to six different molecular species with lipase activity (Downey and Andrews, 1969). However, work by Korn (1962) showed that milk contained a lipoprotein lipase (EC 3.1.1.34) (LPL) with properties very similar to those of post-heparin plasma, adipose tissue and heart LPLs, particularly the enhancement of its activity on emulsified triglycerides by blood serum lipoproteins. It is now accepted that LPL is the major, if not the only, lipase in cows milk. Its properties have been reviewed by Olivecrona et al. (2003). 

In 1979, Saleh et al. studied the metabolism of B7-515 (P-32) and technical toxaphene in six mammals and chicken. Both were metabolized the least in chicken and the most in monkeys [175] B7-515 (P-32) was readily degraded, and three metabolites were formed. Two of them were B6-923 and B6-913, which were excreted with the feces [175]. Also, the octachlorobornanes B8-806/B8-809 (P-42) were metabolized by all test species, and highest degradation rates were found for monkeys [175]. A second feeding study of primates was conducted by Andrews et al. [202], B8-1413 (P-26), B9-1679 (P-50), B8-2229 (P-44), and B9-1025 (P-62) were identified as the most abundant toxaphene congeners in both blood and adipose tissue [202]. In blood an equilibrium level of approximately 40 ppb was reached after 10 weeks and in adipose tissue levels of ca. 4000 ppb after 15-20 weeks [202]. Different abundance ratios were found for GC/ECD and GC/NICI-MS, and GC/NICI-MS was considered as less suitable for this study [202]. 

All three subtypes of P-ARs are expressed in the heart (73). Despite the existence of species-related differences (reviewed in ref. 74), prARs are the predominant form of ARs. The positive chronotropic and inotropic response of the heart to catecholamine stimulation is mediated almost exclusively by prARs (75-77). Coupling of (32-ARs to cardiac contractility is less defined and species related, showing a positive effect in human hearts (77) but not affecting contractility in the mouse (75). Better defined is the role of P2-ARs in the regulation of vascular tone and blood pressure (78). P3-ARs, atypical P-ARs, are expressed in the adipose tissue, where they mediate lipolysis and thermogenesis (79,80), and in smooth muscle cells, where they mediate vasorelaxation (81). 

The enzymes have molecular weights ranging from about 47 kDa (mouse and rhesus monkey) to 65 kDa (human), and can be found in liver, kidney, small intestine, heart, muscle, lung and other respiratory tissues, adipose tissue, CNS, and blood. Within the blood, CES have been found both in plasma as well as within leukocytes. Species differences in their presence in neuronal tissue and capillary endothelial cells of the CNS have led to some equivocation about their contribution to the blood—brain barrier. That is, the extent to which these enzymes might limit access of drugs into the brain by hydrolyzing them before they can diffuse from the blood into the brain isn t aU that clear. CES are localized both in the cytosol, endoplasmic reticulum (ER), and in lysosomes. 

The results of our in vivo and in vitro experiments, together with other observations in the literature, strongly suggest the existence of different patterns of hormonal control of adipose tissue lipolysis among the species studied. 

In their excellent study on the comparative physiology of adipose tissue, Rudman and Di Girolamo have also noted remarkable differences between the rat and other species. The study includes an examination of the lipolytic effects of other hormones, in particular a number of pituitary peptides. Of great importance is the critical analysis made by the authors of the limitations of the experimental data. These limitations should be clearly kept in mind. 

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