Consequently, the number of mitochondria can participate in different processes. It must be said that the above-mentioned systems of warm-blooded animals are most likely intended to maintain tissue metabolism, particularly on the cellular and mitochondrial level. The above mentioned facts show that the efficiency of many energetic processes in an organism is high. To suppose that the endothermy of an organism is due to the heat formation at the expense continually and low efficiency flow of energetic processes is a little grounded point of view.
Against this point of view comes the fact of the existence of cold-blooded animals, in the organism of which the energetic processes are also continually proceeding. However, they have relatively lower metabolism intensity than in warm-blooded animals. These assumptions allow us to think that the warm-bloodedness can be caused by a special system of heat production that has arisen in these organisms during the evolution process. In consideration of close relationship of subcellular mechanisms with thermogenesis, a number of studies have been carried out in this direction using biochemical, biophysical and genetic approaches.
In general, experimental works were carried out at the mitochondrial and cellular levels, and in some works using endo- and ectothermic organisms. One of the important approaches in this direction is study of the proton leakage process of mitochondria caused by increased permeability of their inner membrane. Proton leakage is caused by specific membrane proteins-uncouplers uncoupler proteins-UCPs localized in internal mitochondrial membranes. Dissemination of the proton gradient is a source of biological heat, providing heat production of endothermic organism. It is possible to admit the existence of a part of respiration of mitochondria independent of the membrane potential.
In particular, high uncoupling respiration is realized in mitochondria of brown fat at oxidation of various substrates. Earlier high uncoupling respiration was found during NADH oxidation in mitochondria of skeletal muscles 67 —74 it was found uncoupled ATPase, which may be near to the problem of thermogenesis. A comparison was carried out in the phylogeny of different groups of animals.
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In reptiles it has been found that proton leakage in hepatocytes and their mitochondria is lower than in mammals, although the difference is insignificant. Therefore it was studied the possibility that the ANT anion-translocator is plays the important role in tissues of thermogenesis. Thus, at comparison of endo-and ectothermic organisms, it has been shown that the value of proton conduction is well correlated with the amount of mitochondrial anion carriers, in particular, with the amount of adenine nucleotide-translocase ANT. It is suppose that the ANT does not determine the entire basal proton leakage resting metabolism , since neither carboxyatractyloside inhibitor of ANT nor GDP inhibitor of UCPs significantly suppress the proton leakage process, so participation of other mitochondrial anion-translocators is also possible.
These representations are substantiated by an experimental comparison of the proton conductivity of hepatocytes in mammalian and reptilian. In the latter, the rate of oxygen consumption is times weaker, which correlated with a 4-fold lower level of ANT in reptiles. Therefore, the amount of ANT in the mitochondria of mammalian hepatocytes is considered to be an important modulator of thermogenesis at the cellular level.
It is desirable to continue studies in this regard but this contribution does not explain the difference of metabolism between endo- and ectothermic organisms, differing in the range of 10 times or higher in standard metabolism. It has been demonstrated a participation of the UCPs in thermoregulation, obesity, diseases, aging, as well as in the regulation of the superoxide level. In our opinion, it is not sufficient for the heat production necessary to maintain the endothermy of the organism.
Another option for solving the problem of biological thermogenesis is carried out at the mitochondrial and cellular levels using other variations of the methodological approaches, which allowed considering the mitochondrial path of thermogenesis from other positions. In these studies, isolated mitochondria from different tissues of endo-and ectothermic animals were used.
To briefly outline the general picture of the research we describe here the results obtained on mitochondria and rat heart cells cardiocytes. In this series of investigation existence in tissues two types of mitochondria is discussed; one of them couple with the ATP synthesis, second type is not couple und fulfill thermogenic function.
Specific features of the oxidation of the first three substrates were interesting. Thus, at succinate the rate of respiration occurred at an increased rate of V 4. This widely known fact is interpreted by us as a sign of its oxidation simultaneously in two types of mitochondria, coupled and uncoupled with ATP synthesis.
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Such explanation was confirmed by an analysis of the nature of the oxidation of other substrates. Consequently, their oxidation occurs along the main mitochondrial respiratory chain. Obtained results allowed to postulate that in the studied preparations of tissues contain not only phosphorylating, but also non-phosphorylating noncoupled mitochondria. The biological function of noncoupling respiration may be heat production, as previously shown a weak expression of this form of respiration in cardiac and liver mitochondria of cold-blooded frog.
In our model of respiration 84 there is a group of uncoupled mitochondria, along with a group of phosphorylating ones, which makes it possible to explain the features of the oxidation of different substrates in the suspension of mitochondria and cells. In conclusion, it can be said that the study of thermogenesis problem in endothermic organisms has a complex history.
How could nature transfer the animal's organism from cold-blooded to warm-blooded? How could it raise the level of basic metabolism to ten or more times? More possibly, an important transformation occurred in evolution at the level of mitochondria of tissues a creation of proton leakage system and in non-coupled mitochondria along with phosphorylating ones in the tissues of mammals and birds.
It should be said that our concept of uncoupling respiration of mitochondria differs from the literary one. However, it is possible to find a common side for the last two views, which talk about proton leakage and uncoupled respiration of mitochondria.
In our opinion, the common side for them is proton leakage. Our view postulates the existence of two types of mitochondria. One of them synthesizes ATP, while the other type has a high membrane permeability for many substrates and carries out proton leakage during oxidation of different substrates. According to our hypothesis the UCPs may be located in second type of mitochondria. Unfortunately, our published scheme 84 has not been discussed so far in the modern literature. In our opinion, testing our results by other researchers could help to discuss ideas and solve a number of difficulties that arise in this direction.
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This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and build upon your work non-commercially. Withdrawal Policies Publication Ethics. International Journal of. Review Article Volume 3 Issue 6. Effectiveness of the respiratory chain functioning and thermogenesis For a long time among the researchers there was a very popular point of view about low efficiency of energetic reactions in tissues as source of thermo production.
The role of protein biosynthesis in thermogenesis It has been investigated high activity of protein synthesis in warm-blooded organisms as the main factor of energy expenditure. Proton leak, uncoupling respiration of mitochondria and thermogenesis In consideration of close relationship of subcellular mechanisms with thermogenesis, a number of studies have been carried out in this direction using biochemical, biophysical and genetic approaches. Moberly YR. The metabolic responses of the common Iguana, to walking and diving.
Comp Biochem Physiol. Energetic cost of running. Scaling of the energetic cost of running to body size in mammals.
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