Some aspects of the biochemical basis of metabolic adaptation

Abstract

1. In cold-acclimated animals the free amino acids in body fluids decrease, while the bound amino acids and the total protein within cells and tissues increase. 2. A parallel increase in RNA content is noticed, along with an increase in the rate of protein synthesis. A number of dehydrogenases and other enzymes as well as cytochrome c increase during cold acclimation. 3. There is a shift in the carbohydrate metabolism, resulting in the predominance of the HMP pathway. Ascorbic acid level and tissue glycogen increase during cold acclimation. 4. Cold acclimation furthermore results in an increase of calcium, potassium, and sodium in blood and body fluids while magnesium, sulphate and chloride show a significant decrease. These changes may result in increased muscle metabolism permitting increased activity of the organism. 5. The unsaturation of lipids (iodine number) increases, indicating preferential mobilization of saturated triglycerides in cold acclimation. Lipase activity increases greatly. Phospholipids, which help in regard to the structural integrity of mitochondria, also show an increase. Ketone bodies and cholesterol increase significantly. 6. Factors contained in body fluids or nerve tissue extracts of acclimated earthworms, scorpions or fish have direct effect on the respiration of tissues of normal organisms in vitro (Rao 1962, Rao & Saroja 1963). The sterol fraction of such extracts appears to contain the active principle. 7. It is indicated that hormone or hormone-like substances released into the "milieu interieur", during acclimation, trigger and control a series of changes leading to the biochemical alterations summarized above. 8. Similar changes, but in the opposite direction, occur in slowing down metabolism during the aestivation of the snail Pila globosa. The switching down of metabolic rate, in this case as well, is triggered by a sterol fraction obtainable from the central nervous system. 9. Considering the biochemical mechanisms in acclimation to be similar in different organisms, three possible modes of triggering and control are suggested, namely, direct effects of temperature, effects of the nervous system and, most important of all, regulation through the release of hormones or hormone-like substances.