EFFECT OF GROWTH HORMONE AND NUTRITION ON THE LEVEL OF SOMATOMEDIN A IN THE RAT

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Abstract
Somatomedin A was determined by radioreceptor assay of serum from rats with various conditions. The mean value of somatomedin A in 28 day old male rats was 6.2 ± 0.44 U/ml. Following hypophysectomy, serum somatomedin A decreased with a half-life of approximately 6 h. After a single ip injection of human growth hormone into hypophysectomized rats, the levels of somatomedin A increased within 4 h and reached the maximal level between 8 and 24 h. There was a dose-dependent increase of somatomedin A at 24 h after injection of hGH when the injected dose was between 3.2 and 80 μg. Serum somatomedin A was significantly decreased after 24 h of fasting to the same levels as after hypophysectomy. However the changes in serum rGH between fasting rats and fed rats did not show any significant differences. After different intakes of diets with different composition for 32 days, the mean levels of serum somatomedin A were 14.5 ± 2.2, 6.22 ± 0.4, 2.5 ± 0.3 and 13.0 ± 1.1 U/ml when control-, high protein-, high carbohydrate-low protein- and high fat diets were given respectively. There was a positive correlation between the per cent increase in weight and the serum level of somatomedin A. These results indicate that not only growth hormone but also adequate food intake is required for the generation of somatomedin.