Search Results

Fasting results in a reciprocal shift in hypothalamic neuropeptide Y (NPY) and GH-releasing hormone (GHRH) expression in the adult male rat. It is hypothesized that the fasting-induced rise in NPY is responsible for the GHRH decline and subsequent attenuation of pulsatile GH release. Fasting also leads to a decrease in circulating IGF-I, attributed to both reduced GH release and peripheral GH resistance. Although pituitary GH output is suppressed in the fasted rat, we report herein that pituitary GHRH receptor (GHRH-R) and GH secretagogue receptor (GHS-R) mRNA levels are increased, while pituitary expression of the somatostatin receptor subtype 2 (sst2) and 5 (sst5) is decreased, as determined by real-time reverse transcription (RT)-PCR. A shift in the expression of pituitary receptor subtypes to favor GH synthesis and release may be due, at least in part, to a decline in GH/IGF-I negative feedback. In order to test this hypothesis, we compared hypothalamic and pituitary response to fasting (72 h) in normal male rats and rats with isolated GH deficiency (spontaneous dwarf rats (SDR)). Circulating GH levels were undetectable in SDR, and IGF-I levels were less than 10% of normal controls. Fasting stimulated NPY mRNA levels in SDR; however, the rise in NPY mRNA levels was not accompanied by a fall in GHRH mRNA, as observed in fasted normal rats. In fact, GHRH mRNA levels paradoxically rose in the fasted SDR to 135% of fed controls. At the pituitary level, fasting did not alter sst2 and sst5 mRNA levels in SDR but did stimulate the expression of GHRH-R and GHS-R to 165% and 149% of fed controls, respectively. These results demonstrate that the fasting-induced changes in pituitary expression of sst2 and sst5, but not GHRH-R and GHS-R, are GH/IGF-I dependent. In addition, these results argue against the theory that the negative association of NPY and GHRH expression observed following fasting represents a simple cause-and-effect relationship and suggest that GH, either directly or indirectly, mediates the effects of fasting on hypothalamic GHRH expression.

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 β-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 β-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.

The short heterodimer partner (SHP) (NR0B2) is an orphan nuclear receptor whose function in pancreatic β-cells is unclear. Mitochondrial uncoupling protein (UCP2) in β-cells is upregulated in obesity-related diabetes, causing impaired glucose-stimulated insulin secretion (GSIS). We investigated whether SHP plays a role in UCP2-induced GSIS impairment. We overexpressed SHP in normal islet cells and in islet cells overexpressing UCP2 by an adenovirus-mediated infection technique. We found that SHP overexpression enhanced GSIS in normal islets, and restored GSIS in UCP2-overexpressing islets. SHP overexpression increased the glucose sensitivity of ATP-sensitive K⁺ (K_ATP) channels and enhanced theATP/ADP ratio. A peroxisome proliferator-activated receptor gamma (PPARγ) antagonist, GW9662, did not block the SHP effect on GSIS. SHP overexpression also corrected the impaired sensitivity of UCP2-overexpressing β-cells to methylpyruvate, another energy fuel that bypasses glycolysis and directly enters the Krebs cycle. K_ATP channel inhibition mediated by dihydroxyacetone, which gives reducing equivalents directly to complex II of the electron transport system, was similar in Ad-Null-, Ad-UCP2- and Ad-UCP2+Ad-SHP-infected cells. The mitochondrial metabolic inhibitor sodium azide totally blocked the effect of SHP overexpression on GSIS. These results suggest that SHP positively regulates GSIS in β-cells and restores glucose sensitivity in UCP2-overexpressing β-cells by enhancing mitochondrial glucose metabolism, independent of PPARγ activation.