Ventromedial Hypothalamic Glucokinase Is an Important Mediator of the Counterregulatory Response to Insulin-Induced Hypoglycemia

Abstract

OBJECTIVE-The counterregulatory response to insulin-induced hypoglycemia is mediated by the ventromedial hypothalamus (VMH), which contains specialized glucosensing neurons, many of which use glucokinase (GK) as the rate-limiting step in glucose's regulation of neuronal activity. Since conditions associated with increased VMH GK expression are associated with a blunted counterregulatory response, we tested the hypothesis that increasing VMH GK activity would similarly attenuate, while decreasing GK activity would enhance the counterregulatory response to insulin-induced hypoglycemia. RESEARCH DESIGN AND METHODS-The counterregulatory response to insulin-induced hypoglycemia was evaluated in Sprague-Dawley rats after bilateral VMH injections of 1) a GK activator drug (compound A) to increase VMH GK activity, 2) low-dose alloxan (4 g) to acutely inhibit GK activity, 3) highdose alloxan (24 g), or 4) an adenovirus expressing GK short hairpin RNA (shRNA) to chronically reduce GK expression and activity. RESULTS-CompoundA increased VMH GK activity sixfold in vitro and reduced the epinephrine, norepinephrine, and glucagon responses to insulin-induced hypoglycemia by 40 -62% when injected into the VMH in vivo. On the other hand, acute and chronic reductions of VMH GK mRNA or activity had a lesser and more selective effect on increasing primarily the epinephrine response to insulin-induced hypoglycemia by 23-50%.CONCLUSIONS-These studies suggest that VMH GK activity is an important regulator of the counterregulatory response to insulin-induced hypoglycemia and that a drug that specifically inhibited the rise in hypothalamic GK activity after insulininduced hypoglycemia might improve the dampened counterregulatory response seen in tightly controlled diabetic subjects. Diabetes 57:1371-1379, 2008 H ypoglycemia is a major complication of insulin therapy for diabetes, and the incidence of hypoglycemia has increased as clinicians have attempted to maintain tighter control of blood glucose levels in diabetic subjects (1). During hypoglycemia, declining glucose levels are detected by peripheral glucosensors and by specialized glucosensing neurons within select areas of the brain (2). A subset of these glucosensing neurons reside in the ventromedial hypothalamus (VMH), a brain area that includes the arcuate hypothalamic nucleus (ARC) and ventromedial hypothalamic nucleus (VMN). Many previous studies have confirmed the importance of the VMH in mediating the counterregulatory response to hypoglycemia (3,4) and suggest that the glucosensing neurons in this area might be critical regulators of these responses.Glucokinase (GK) (hexokinase IV) is a likely candidate as a regulator of glucosensing in VMH neurons (5-10). In vitro, inhibiting GK activity with either RNA interference (RNAi) or with a variety of drugs reduces the sensitivity of VMN glucosensing neurons. On the other hand, pharmacological activation with Compound A increases their sensitivity to glucose (5-8,10). Also, pharmacological inhibition of GK activity with alloxan i...

Gastric Bypass Reduces Symptoms and Hormonal Responses in Hypoglycemia

“…This indicates that glucose sensing and possibly glucose utilization in the brain have been augmented, but the underlying mechanisms need to be further characterized. Glucokinase activity in ventromedial hypothalamus modulates the counterregulatory responses, and an increased activity attenuates hormonal responses to hypoglycemia (33). An increase in gene expression for glucokinase, GLUT2, and GLP-1 receptor in the hypothalamus has been reported in food-restricted rats (34).…”

Glucokinase activity in the arcuate nucleus regulates glucose intake

“…All compounds were administered in a 1 μl volume over a period of 1 minute. Doses were based on previous experiments (18,65,66). Data are presented as food intake and glucose intake relative to those in corresponding rats treated with control during the crossover study.…”

“…In the hypothalamus, glucokinase is expressed in regions, including the arcuate nucleus, ventromedial nucleus (VMN), paraventricular nucleus (PVN), and lateral hypothalamic area (LHA), which regulate energy homeostasis and are part of a CNS glucose-sensing system (14,16,17). In the VMN, glucokinase is involved in mediating the hormonal counterregulatory responses to hypoglycemia and does not regulate energy homeostasis (18,19). While a role for hypothalamic glucokinase in regulating energy homeostasis has been proposed, it has never been confirmed (12,14).…”

Mechanisms of Hypoglycemia-Associated Autonomic Failure in Diabetes

“…Studies of the cellular and molecular biology and pathophysiology of CNS control of glucose counterregulation in rodent models have focused largely, but not exclusively, on the ventromedial hypothalamus. Potential mechanisms 32,33,[63][64][65] include decreased glucose sensing by glucose-excited or glucose-inhibited neurons in the hypothalamus, elsewhere in the brain, and in the periphery; decreased activation of AMP kinase; increased glucokinase activity; loss of a decrease in the counterregulatory inhibitor γ-aminobutyric acid; loss of an increase in the counterregulatory stimulator glutamine; increased urocortin release; and reduced actions of insulin on the brain.…”