Biochemical study on bone formation in rats reared under hot environment

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It is usually found that cattle growth is extremely retarded in tropical region, i.e. body weight cannot reach 300kg aged 7 or more years old. The causes of retarded body growth may be complicated ones such as genolytic character, low nutritional plane of animal body and hot environment. A series of this experiment focussed on the effect of hot environment on bone growth in rats.It was found in a previous experiment that bone in rats reared at high temperature was thinner and lighter than the bone in rats reared at comfortable temperature. However the change of bone formation in hot environment has not been studied in detail. The experiment was to elucidate the effect of hot environment on bone formation using tetracycline injection method and bone matrix implantation method.In experiment 1, 16 rats were divided into 4 groups ; (1) saline injection at 24゜C (2) <T_3> injection at 24゜C (3) saline injection at 34゜C (4) <T_3> injection at 34゜C. Tetracycline was injected on day 7 and 10 after the beginning of experiment. In experiment 2, 24 rats were divided into 2 groups, temperate environment group (24゜C) and hot environment group (34゜C). Every rat was intramuscularly implanted demineralyzed bone matrix at the beginning of experiment. These matrixes were taken out on day 7, 14 and 21 and exmined histologically and biochemically.The result that bone apposition was reduced in periosteal coincided with the observation of reduced growth of bone width in rats. Alp-ase activities were reduced in the bone of rat reared under hot environment. It was suggested that hot environment disturbed the movement of bone matrix into the diaphsis of bone. <T_3> injection recovered Alp-ase activities in the bone and increased bone apposition in periosteal. Therefore, the reduction of thyroxine secretion might be one of main causes of changed bone formation in rats reared under hot environment.