Nutrition and somatomedin. I. Effect of fasting and refeeding on serum somatomedin activity and cartilage growth activity in rats.

The relationship between nutrition, somatomedin, and growth was studied in young male rats which were fasted for 72 h, and then refed. Cartilage growth activity was measured by the incorporation of SO4 by costal cartilage incubated in vitro. Serum somatomedin activity was measured by the ability of sera to stimulate the incorporation of SO4 in vitro by cartilage from hypophysectomized rats or normal young pigs. Cartilage growth activity was unchanged after 24 h of fasting, but decreased significantly to hypopituitary levels after 72 h. The decrease in cartilage growth activity could not be attributed to a decrease in the ability of cartilage to respond to somatomedin, since fasted rat cartilage incubated in vitro had significantly greater stimulation from the addition of normal rat serum (normal somatomedin) than from the addition of hypophysectomized rat serum (low somatomedin). A decrease in serum somatomedin preceded the decrease in growth activity; somatomedin was significantly decreased after 24 h of fasting, and fell to hypopituitary levels after 72 h of fasting. The decrease in somatomedin could not be attributed to growth hormone deficiency, since treatment with bGH did not prevent the fall in somatomedin. The direct addition of fasted rat serum to normal rat serum did not inhibit the somatomedin activity of the normal serum. The refeeding of 72h-fasted rats was associated with an increase in serum somatomedin which was significant after 6 h, and comparable to control levels after 24 h of refeeding. The increase in somatomedin was followed by an increase in cartilage growth activity, which was significant after 12 h, and comparable to control levels after 48 h of refeeding. We conclude that adequate food intake appears to be required for the maintenance of normal levels of somatomedin; the processing of nutrients may modulate the generation of somatomedin.