Effect of insulin and nutrition on serum levels of somatomedin A in the rat.

The serum levels of somatomedin A, as measured by radioreceptor assay, were significantly reduced in rats 2 days after the administration of streptozotocin. The mean decrease was 45.4 +/- 2.9% of the initial values. In rats treated with insulin, blood glucose levels and glycosuria decreased, and serum somatomedin A returned to 108.3% +/- 11.7% of the initial values by the sixth day of treatment. In untreated diabetic rats, serum somatomedin A decreased progressively to 23.4 +/- 4.4% 8 days after streptozotocin administration. The total caloric intakes in the treated and nontreated diabetic rats were similar, suggesting that the low levels of somatomedin A in diabetic rats may be due to lack of insulin. A significant correlation was observed between serum somatomedin A values and body weight (r = 0.90) or the urinary glucose (r = -0.84) or blood glucose levels (r = -0.67). When the diabetic insulin-treated rats were fed a low protein diet, there was no increase in serum somatomedin A. Inhibitory factors in serum which interfere in the bioassay for somatomedin had no effect in our radioreceptor assay.