Effect of streptozotocin diabetes and insulin replacement on growth hormone in rats

The effects of insulin deprivation on the growth rate, plasma and pituitary growth hormone (GH) and GH synthesis were investigated in male Wistar rats. Diabetes was induced by administration of streptozotocin (STZ), 7 mg/100 g bw, and plasma and pituitary GH levels were measured by means of a specific radioimmunoassay. GH synthesis was determined in pituitaries by the in vitro incorporation of [3H] leucine into specific immunoprecipitates. The body weight and the pituitary GH content of normally developing rats showed an almost linear increase throughout the observation period, whereas diabetic rats stopped growing immediately after receiving STZ, and remained smaller than age-paired controls. Pituitary GH content remained within the control range through the 5 days following STZ administration and thereafter decreased reaching 10% of control values by the 30th day. Furthermore, pituitaries from diabetic rats incorporated [3H] leucine into r-GH at a greatly reduced rate, which could explain the diminished r-GH storage in pituitaries of diabetic rats. Plasma GH concentrations remained within the normal range for 10 days after STZ, thereafter plasma GH were markedly reduced. Insulin treatment prevented the metabolic changes, and restored normal levels of plasma and pituitary GH in diabetic rats. These findings indicate that diabetes, in rat, is characterized by an inhibitory effect on GH secretion, probably via a diminished GH synthesis by the pituitary gland.