Expression of growth hormone, growth hormone releasing hormone, and somatostatin genes is unperturbed in the streptozotocin-induced diabetic rat

Ghrelin improves growth hormone responses to growth hormone-releasing hormone in a streptozotocin-diabetic model of delayed onset

GH secretion is markedly altered in diabetes mellitus (DM) in both rats and humans, albeit in opposite directions. In the rat, diabetes suppresses pulsatile GH secretion, especially high amplitude pulses, and decreases GH responses to secretagogue, depending inversely on severity of metabolic alteration. In the present study, we wanted to address the GH responses to GHRH and low doses of ghrelin in a streptozotocin (STZ) model of diabetes characterized by the delayed onset of the metabolic alterations. We have shown that the administration of high doses of STZ (90 mg/kg in 0.01 M solution of chloride-sodium, ip) to five-day-old pups (n5-STZ) can induce the appearance of a characteristic diabetic syndrome in adult age, the diabetic triad, with elevated plasma glucose levels: polyuria, polydipsia, hyperphagia, and reduced body weight gain. At the age of 3 months, in these n5-STZ male and female rats the GH responses to GHRH (1 microg/kg) and GHRH combined with ghrelin (1+3 microg/kg) had diminished both in punctual times and in the area under the curve (AUC). However, the combined administration of GHRH and ghrelin, being the more potent stimulus, elicited a synergistic GH response. Thus, male and female rats with delayed onset diabetes displayed an altered GH response to GHRH, although the combined administration of GHRH and ghrelin was able to restore the GH secretion with a synergistic effect.

GH responses to GHRH and GHRP-6 in Streptozotocin (STZ)-diabetic rats

GH responses to GHRH, the physiologic hypothalamic stimulus, and GHRP-6, a synthetic hexapeptide that binds the Ghrelin receptor, were studied in rats treated with streptozotocin (STZ), an experimental model of diabetes. Sprague-Dawley male rats received a single injection either of STZ (70 mg/Kg in 0.01 M SSC, i.p.) or of the vehicle (0.01 M SSC). GH responses were challenged with two different doses of GHRH (1 and 10 microg/kg) or GHRP-6 (3 and 30 microg/kg) and with a combination of both at low (1 + 3 microg/kg) or high (10 + 30 microg/kg) doses, respectively. We observed a dose-dependent effect for GH responses to GHRH both in STZ-treated rats and in controls. However, we could not find significant differences between STZ-rats and controls. GH responses to GHRP-6 occurred in a dose-dependent manner in STZ-rats, but not in controls. GH responses to GHRP-6 in both groups were clearly lower than those elicited by GHRH. GH responses to 30 microg/Kg of GHRP-6 were significantly greater in STZ-rats than in controls (AUC: 3549.9 +/- 1001.4 vs. 2046.4 +/- 711.7; p<0.05). The combined administration of GHRH plus GHRP-6 was the most potent stimuli for GH in both groups. The administration of doses in the lower range (1 + 3 microg/Kg, GHRH + GHRP-6 respectively) induced a great peak of GH in STZ-rats and in control rats, revealing a synergistic effect of GHRH and GHRP-6 in both groups. When the higher doses were administered (10 + 30 microg/kg), GH levels in time 5, and AUC were significantly higher in control rats. In addition, a negative correlation between WT (weight tendency) values and GH responses, represented as AUC, could be established in STZ-rats (r2=-0.566, p=0.004 for GHRH; r2=-0.412, p=0.028 for GHRP-6). Thus, the more negative the values of WT were, the more severe the metabolic alteration and, therefore, the higher the GH response to GHRH and GHRHP-6. In conclusion, our results do not support the existence of a functional hypothalamic hypertone of SS in diabetic rats, as GH responses were not usually reduced in STZ-rats, except when both secretagogues were administered together at the higher doses. Besides, GH responses to GHRH and GHRP-6 were inversely correlated with the severity of the metabolic alteration in STZ-rats, meaning that worse glycaemic control promoted higher GH secretion. These results resemble those found in humans, where GH responses to secretagogues are increased in type-1 diabetes and depend on hyperglycaemia, and are representative of not well-controlled insulin-dependent diabetic status.

Anatomically specific changes in the expression of somatostatin, growth hormone-releasing hormone and growth hormone receptor mRNA in diabetic rats

Growth hormone (GH) secretion is altered in poorly controlled diabetic animals. However, modifications in the hypothalamic neuropeptides that control GH secretion, somatostatin and GH-releasing hormone (GHRH), as well as changes in the sensitivity of the hypothalamus and pituitary to the feedback effects of GH, are less clear. We have used RNase protection assays and in-situ hybridization to address whether the mRNA expression of GH, somatostatin and GHRH, as well as of the GH receptor (GHR) in the hypothalamus and anterior pituitary, are altered in streptozotocin-induced diabetic rats. After induction of diabetes, rats were treated with insulin twice daily for 3 weeks to obtain either poorly controlled (mean plasma glucose >300 mg/dl) or well-controlled diabetic rats. Although no significant change in pituitary GH mRNA expression was found, the hypothalamic expression of GHRH and somatostatin mRNA was reduced in poorly-controlled diabetic rats and returned to control values with normalisation of plasma glucose concentrations (P<0.0001 and P<0.002, respectively). Somatostatin mRNA expression was reduced only in the central portion of the periventricular nucleus, with no change being seen in the other areas of the periventricular nucleus or in the arcuate, suprachiasmatic or paraventricular nuclei. A significant decline in GHRH mRNA expression was observed in both the arcuate nucleus and ventromedial hypothalamus. Anterior pituitary GHR mRNA expression was significantly reduced in both well and poorly-controlled diabetic rats, while there was no change in the hypothalamus. To examine whether the evolution time of the diabetes influences these parameters, in a subsequent experiment, diabetic rats received no insulin for 2 months. A significant decline in GHRH and somatostatin mRNA expression was also observed in these rats. In addition, pituitary GH mRNA expression declined significantly in long-term diabetic rats. These results demonstrate that: (1) the expression of both GHRH and somatostatin declines specifically in anatomical areas involved in anterior pituitary hormone control; (2) GHR mRNA expression is decreased in the pituitary of diabetic rats, but not in the hypothalamus, and does not return to control values with normalisation of mean blood glucose concentrations; and (3) the evolution time of the diabetes is important for detecting some changes, including the decrease in pituitary GH mRNA expression.

Hypothalamic mediation of reduced GH secretion in diabetic rats: evidence for reduced cholinergic inhibition of somatostatin release

The Goto-Kakizaki (GK) rat is a new model of diabetes mellitus and in this study we have characterized the diabetic and growth hormone (GH) secretory status of male GK rats at 6 and 16 weeks of age. We have also investigated the role of endogenous somatostatin (SS) and cholinergic manipulation on the GH responses to GH-releasing hormone (GHRH). GK rats were non-obese with significant fasting hyperglycaemia, hyperinsulinaemia and absent insulin responses to IV glucose. The GH response to GHRH was reduced at 16 weeks compared with normal, age-matched Wistar rats but no differences were observed at 6 weeks. Pretreatment of older rats (16 weeks) with anti-somatostatin antibodies (SS-Ab) significantly increased GH responses to GHRH in both normal and GK groups. Cholinergic augmentation with pyridostigmine (PD) reversed the blunted GH responses to GHRH in older GK rats but had no effect in the normal or young (6 weeks) GK rats. These results indicate that SS release mediates the blunted GH response to GHRH in GK rats and that reduced hypothalamic cholinergic signalling to the somatostatinergic neurone may mediate the increase in SS release. This view is supported by the results from in vitro studies in which cholinergic muscarinic blockade with pirenzepine (PIR) caused dose-related stimulation of SS release from normal rat hypothalami but was without effect on GK rat hypothalami. The cause of this alteration in hypothalamic function is, at present, unknown.

Effects of streptozotocin-induced diabetes on lymphocyte POMC and growth hormone gene expression in the rat

Diabetes in the rat is associated with a change in the profiles of several neuroendocrine hormones resulting in poor growth and decreased immune function. Since lymphocytes can also serve as a source of neuroendocrine hormones, we have examined whether the change in hormone profiles are accompanied by an impairment of lymphocyte GH and POMC gene expression in the immune system. Diabetes was induced by the administration of streptozotocin (STZ; 10 mg/100 g body weight) and 3 days later GH and ACTH protein and mRNA were determined. The results show a modest diminution of GH RNA in the spleen of diabetic animals whereas the expression of POMC mRNA and ACTH by the thymus was enhanced. The expression of POMC in the spleen appeared unaltered while the increase of POMC RNA in the thymus was evident after the first day of STZ treatment. STZ had no direct effect on GH or POMC expression in the spleen or thymus cells in vitro. Insulin does not appear to be involved in the expression of lymphocyte GH or POMC. The administration of insulin to the diabetic animals had no significant effect on the expression of GH or POMC by the immune cells. In addition, lymphocytes do not appear to serve as a source of insulin or are the expression of genes for lymphocyte GH or ACTH altered by insulin in vitro. Taken together, the findings are the first to report on the expression of neuroendocrine genes in lymphocytes during diabetes. The mechanism for the inhibition of GH and stimulation of POMC expression by lymphocytes in diabetic animals is unknown, but it is tempting to speculate an important role in the development of the autoimmunity that characterizes this complex disease.