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Articles by R. S Sherwin
Total Records ( 13 ) for R. S Sherwin
  W Zhu , D Czyzyk , S. A Paranjape , L Zhou , A Horblitt , G Szabo , M. R Seashore , R. S Sherwin and O. Chan
 

Local delivery of glucose into a critical glucose-sensing region within the brain, the ventromedial hypothalamus (VMH), can suppress glucose counterregulatory responses to systemic hypoglycemia. Here, we investigated whether this suppression was accomplished through changes in GABA output in the VMH. Sprague-Dawley rats had catheters and guide cannulas implanted. Eight to ten days later, microdialysis-microinjection probes were inserted into the VMH, and they were dialyzed with varying concentrations of glucose from 0 to 100 mM. Two groups of rats were microdialyzed with 100 mM glucose and microinjected with either the KATP channel opener diazoxide or a GABAA receptor antagonist. These animals were then subjected to a hyperinsulinemic-hypoglycemic glucose clamp. As expected, perfusion of glucose into the VMH suppressed the counterregulatory responses. Extracellular VMH GABA levels positively correlated with the concentration of glucose in the perfusate. In turn, extracellular GABA concentrations in the VMH were inversely related to the degree of counterregulatory hormone release. Of note, microinjection of either diazoxide or the GABAA receptor antagonist reversed the suppressive effects of VMH glucose delivery on counterregulatory responses. Some GABAergic neurons in the VMH respond to changes in local glucose concentration. Glucose in the VMH dose-dependently stimulates GABA release, and this in turn dose-dependently suppresses the glucagon and epinephrine responses to hypoglycemia. These data suggest that during hypoglycemia a decrease in glucose concentration within the VMH may provide an important signal that rapidly inactivates VMH GABAergic neurons, reducing inhibitory GABAergic tone, which in turn enhances the counterregulatory responses to hypoglycemia.

  X Fan , Y Ding , S Brown , L Zhou , M Shaw , M. C Vella , H Cheng , E. C McNay , R. S Sherwin and R. J. McCrimmon
  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 ± 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 ± SE, area under the curve over time (AUC/t) 144 ± 43 vs. 50 ± 11 ng·l–1·min–1; P < 0.05] and epinephrine [4.27 ± 0.96 vs. 1.06 ± 0.26 nmol·l–1·min–1; P < 0.05] responses in RH-BB rats, and amplified the glucagon [151 ± 22 vs. 85 ± 22 ng·l–1·min–1; P < 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.
  L Jiang , R. I Herzog , G. F Mason , R. A de Graaf , D. L Rothman , R. S Sherwin and K. L. Behar
  OBJECTIVE

The objective of this study was to characterize the changes in brain metabolism caused by antecedent recurrent hypoglycemia under euglycemic and hypoglycemic conditions in a rat model and to test the hypothesis that recurrent hypoglycemia changes the brain's capacity to utilize different energy substrates.

RESEARCH DESIGN AND METHODS

Rats exposed to recurrent insulin-induced hypoglycemia for 3 days (3dRH rats) and untreated controls were subject to the following protocols: [2-13C]acetate infusion under euglycemic conditions (n = 8), [1-13C]glucose and unlabeled acetate coinfusion under euglycemic conditions (n = 8), and [2-13C]acetate infusion during a hyperinsulinemic-hypoglycemic clamp (n = 8). In vivo nuclear magnetic resonance spectroscopy was used to monitor the rise of13C-labeling in brain metabolites for the calculation of brain metabolic fluxes using a neuron-astrocyte model.

RESULTS

At euglycemia, antecedent recurrent hypoglycemia increased whole-brain glucose metabolism by 43 ± 4% (P < 0.01 vs. controls), largely due to higher glucose utilization in neurons. Although acetate metabolism remained the same, control and 3dRH animals showed a distinctly different response to acute hypoglycemia: controls decreased pyruvate dehydrogenase (PDH) flux in astrocytes by 64 ± 20% (P = 0.01), whereas it increased by 37 ± 3% in neurons (P = 0.01). The 3dRH animals decreased PDH flux in both compartments (–75 ± 20% in astrocytes, P < 0.001, and –36 ± 4% in neurons, P = 0.005). Thus, acute hypoglycemia reduced total brain tricarboxylic acid cycle activity in 3dRH animals (–37 ± 4%, P = 0.001), but not in controls.

CONCLUSIONS

Our findings suggest that after antecedent hypoglycemia, glucose utilization is increased at euglycemia and decreased after acute hypoglycemia, which was not the case in controls. These findings may help to identify better methods of preserving brain function and reducing injury during acute hypoglycemia.

 
 
 
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