Differential responses of the growth hormone axis in two rat models of streptozotocin-induced insulinopenic diabetes

The impact of streptozotocin (STZ)-induced, insulinopenic diabetes on the GH axis of rats and mice differs from study to study, where this variation may be related to the induction scheme, severity of the diabetes and/or the genetic background of the animal model used. In order to begin differentiate between these possibilities, we compared the effects of two different STZ induction schemes on the GH axis of male Sprague-Dawley rats: (1) a single high-dose injection of STZ (HI STZ, 80 mg/kg, i.p.), which results in rapid chemical destruction of the pancreatic beta-cells, and (2) multiple low-dose injections of STZ (LO STZ, 20 mg/kg for 5 consecutive days, i.p.), which results in a gradual, autoimmune destruction of beta-cells. STZ-treated animals were killed after 3 weeks of hyperglycemia (>400 mg/dl), and in both paradigms circulating insulin levels were reduced to <40% of vehicle-treated controls. HI STZ-treated rats lost weight, while body weights of LO STZ-treated animals gradually increased over time, similar to vehicle-treated controls. As previously reported, HI STZ resulted in a decrease in circulating GH and IGF-I levels which was associated with a rise in hypothalamic neuropeptide Y (NPY) mRNA (355% of vehicle-treated controls) and a fall in GH-releasing hormone (GHRH) mRNA (45% of vehicle-treated controls) levels. Changes in hypothalamic neuropeptide expression were reflected by an increase in immunoreactive NPY within the arcuate and paraventricular nuclei and a decrease in GHRH immunoreactivity in the arcuate nucleus, as assessed by immunohistochemistry. Consistent with the decline in circulating GH and hypothalamic GHRH, pituitary GH mRNA levels of HI STZ-treated rats were 58% of controls. However, pituitary receptor mRNA levels for GHRH and ghrelin increased and those for somatostatin (sst2, sst3 and sst5) decreased following HI STZ treatment. The impact of LO STZ treatment on the GH axis differed from that observed following HI STZ treatment, despite comparable changes in circulating glucose and insulin. Specifically, LO STZ treatment did suppress circulating IGF-I levels to the same extent as HI STZ treatment; however, the impact on hypothalamic NPY mRNA levels was less dramatic (158% of vehicle-treated controls) where NPY immunoreactivity was increased only within the paraventricular nucleus. Also, there were no changes in circulating GH, hypothalamic GHRH or pituitary receptor expression following LO STZ treatment, with the exception that pituitary sst3 mRNA levels were suppressed compared with vehicle-treated controls. Taken together these results clearly demonstrate that insulinopenia, hyperglycemia and reduced circulating IGF-I levels are not the primary mediators of hypothalamic and pituitary changes in the GH axis of rats following HI STZ treatment. Changes in the GH axis of HI STZ-treated rats were accompanied by weight loss, and these changes are strikingly similar to those observed in the fasted rat, which suggests that factors associated with the catabolic state are critical in modifying the GH axis following STZ-induced diabetes.