OBJECTIVEThe response of ventromedial hypothalamic (VMH) glucose-inhibited neurons to decreased glucose is impaired under conditions where the counterregulatory response (CRR) to hypoglycemia is impaired (e.g., recurrent hypoglycemia). This suggests a role for glucose-inhibited neurons in the CRR. We recently showed that decreased glucose increases nitric oxide (NO) production in cultured VMH glucose-inhibited neurons. These in vitro data led us to hypothesize that NO release from VMH glucose-inhibited neurons is critical for the CRR.RESEARCH DESIGN AND METHODSThe CRR was evaluated in rats and mice in response to acute insulin-induced hypoglycemia and hypoglycemic clamps after modulation of brain NO signaling. The glucose sensitivity of ventromedial nucleus glucose-inhibited neurons was also assessed.RESULTSHypoglycemia increased hypothalamic constitutive NO synthase (NOS) activity and neuronal NOS (nNOS) but not endothelial NOS (eNOS) phosphorylation in rats. Intracerebroventricular and VMH injection of the nonselective NOS inhibitor NG-monomethyl-l-arginine (l-NMMA) slowed the recovery to euglycemia after hypoglycemia. VMH l-NMMA injection also increased the glucose infusion rate (GIR) and decreased epinephrine secretion during hyperinsulinemic/hypoglycemic clamp in rats. The GIR required to maintain the hypoglycemic plateau was higher in nNOS knockout than wild-type or eNOS knockout mice. Finally, VMH glucose-inhibited neurons were virtually absent in nNOS knockout mice.CONCLUSIONSWe conclude that VMH NO production is necessary for glucose sensing in glucose-inhibited neurons and full generation of the CRR to hypoglycemia. These data suggest that potentiating NO signaling may improve the defective CRR resulting from recurrent hypoglycemia in patients using intensive insulin therapy.

Mechanisms to coordinately regulate energy expenditure and glucose uptake into muscle and fat cells are not well described. Insulin stimulates glucose uptake in part by causing site-specific endoproteolytic cleavage of TUG, which mobilizes GLUT4 glucose transporters to the cell surface. Here, we show that the TUG C-terminal cleavage product enters the nucleus, binds the transcriptional regulators PGC-1a and PPARg, and increases oxidative metabolism and thermogenic protein expression. Muscle-specific genetic manipulation of this pathway impacts whole-body energy expenditure, independent of glucose uptake. The PPARg2 Pro12Ala polymorphism, which reduces diabetes risk, enhances TUG binding. The TUG cleavage product stabilizes PGC-1a and is itself susceptible to an Ate1 arginyltransferase -dependent degradation mechanism; binding of the TUG product confers Ate1-dependent stability upon PGC-1a. We conclude that TUG cleavage coordinates energy expenditure with glucose uptake, that this pathway may contribute to the thermic effect of food, and that its attenuation may be important in obesity..

-In nondiabetic rodents, AMP-activated protein kinase (AMPK) plays a role in the glucose-sensing mechanism used by the ventromedial hypothalamus (VMH), a key brain region involved in the detection of hypoglycemia. However, AMPK is regulated by both hyper-and hypoglycemia, so whether AMPK plays a similar role in type 1 diabetes (T1DM) is unknown. To address this issue, we used four groups of chronically catheterized male diabetic BB rats, a rodent model of autoimmune T1DM with established insulin-requiring diabetes (40 Ϯ 4 pmol/l basal c-peptide). Two groups were subjected to 3 days of recurrent hypoglycemia (RH), while the other two groups were kept hyperglycemic [chronic hyperglycemia (CH)]. All groups subsequently underwent hyperinsulinemic hypoglycemic clamp studies on day 4 in conjunction with VMH microinjection with either saline (control) or AICAR (5-aminoimidazole-4-carboxamide) to activate AMPK. Compared with controls, local VMH application of AICAR during hypoglycemia amplified both glucagon [means Ϯ SE, area under the curve over time (AUC/t) 144 Ϯ 43 vs. 50 Ϯ 11 ng ⅐ l Ϫ1 ⅐ min Ϫ1 ; P Ͻ 0.05] and epinephrine [4.27 Ϯ 0.96 vs. 1.06 Ϯ 0.26 nmol ⅐ l Ϫ1 ⅐ min Ϫ1 ; P Ͻ 0.05] responses in RH-BB rats, and amplified the glucagon [151 Ϯ 22 vs. 85 Ϯ 22 ng ⅐ l Ϫ1 ⅐ min Ϫ1 ; P Ͻ 0.05] response in CH-BB rats. We conclude that VMH AMPK also plays a role in glucose-sensing during hypoglycemia in a rodent model of T1DM. Moreover, our data suggest that it may be possible to partially restore the hypoglycemia-specific glucagon secretory defect characteristic of T1DM through manipulation of VMH AMPK. epinephrine; glucagon; ventromedial hypothalamus; adeno-associated viral vector HYPOGLYCEMIA REMAINS THE MAJOR limiting factor to intensive insulin therapy in type 1 diabetes (T1DM) (9). Individuals with T1DM are particularly prone to hypoglycemia both because of their need for exogenous and unregulated insulin therapy and because of defects in the normal physiological counterregulatory response to acute hypoglycemia (29). Developing therapies to reduce the frequency or severity of hypoglycemia will require a greater understanding of the physiological mechanisms underlying hypoglycemia detection and the impact of T1DM on these mechanisms.Falling glucose levels are detected within discrete regions of the brain (3-5, 14, 40), and periphery (11). Of these, the ventromedial hypothalamus (VMH) is thought to play a key role in the detection and integration of hypoglycemia signals and subsequent triggering of a counterregulatory defense response (3-5, 13, 30, 31, 33, 42, 47). Emerging evidence supports a key role for the serine/threonine kinase AMPactivated protein kinase (AMPK) in the sensing of hypoglycemia within the VMH. Local activation of AMPK in the VMH with 5-aminoimidazole-4-carboxamide (AICAR) amplified the glucose counterregulatory response to hypoglycemia in normal Sprague-Dawley rats (31) and reversed impaired hormonal counterregulatory responses in normal rats exposed to recurrent hypoglycemia (RH) (30). Moreover, sele...

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