The neuroendocrine control of glucose allocation.
Here we propose that glucose metabolism can be understood on the basis of three concept-derived axioms: (I) A hierarchy exists among the glucose-utilizing organs with the brain served first, followed by muscle and fat. (II) Tissue-specific glucose transporters allocate glucose among organs in order to maintain brain glucose concentrations. (III) Exogenous carbohydrate supply compensates for glucose alterations that can temporarily occur in muscle and fat. Derived from the control theory, the simplest solution of allocating supply to 2 organs, e.g. brain and muscle, is a "fishbone"-structured model. We reviewed the literature, searching for neuroendocrine and metabolic mechanisms that can fulfill control functions in such a model: The tissue-specific glucose transporters are differentially regulated. GLUT 1, carrying glucose across the blood-brain-barrier, is independent of insulin. Instead, this trans-endothelial glucose transporter is rather dependent on potent regulators of blood vessel function like vascular endothelial growth factor - a pituitary counterregulatory hormone. GLUT 4, carrying glucose across the membranes of muscle and fat cells, depends on insulin. Thereby, insulin allocates glucose to muscle and fat. The hypothalamus-pituitary-adrenal (HPA) axis, the sympathetic nervous system (SNS), and vascular endothelial growth factor allocate glucose to the brain. Multiple "sensors" (some of which have only recently been identified as ATP sensitive potassium channels) measure glucose or glucose equivalents at various sites of the body: the ventromedial hypothalamus, the lateral hypothalamus, portal vein, pancreatic beta cell, renal tubule, muscle and adipose tissue. Feedback pathways both from the brain and from muscle and fat are involved in regulating glucose allocation and exogenous glucose supply. The main feedback signal from the brain is found to be glucose, that from muscle and fat appears to be leptin. In fact, the literature search revealed two or more biological mechanisms for the function of each component in the model, finding glucose regulation highly redundant. This review focuses on "brain glucose" control. The concept of glucose allocation presented here challenges the common opinion of "blood glucose" being the main parameter controlled. According to the latter opinion, hyperglycemia in the metabolic syndrome is due to a putative defect located within the closed loop including the beta cell, muscle and fat cells. That traditional view leaves some peculiarities of e.g. the metabolic syndrome unexplained. The concept of glucose allocation, however, would predict that weight gain - with abundance of glucose in muscle and fat - increases feedback to the brain (via hyperleptinemia) which in turn results in HPA-axis and SNS overdrive, impaired insulin secretion, and insulin resistance. HPA-axis overdrive would account for metabolic abnormalities such as central adiposity, hyperglycemia, dyslipidemia, and hypertension, that are well known clinical aspects the metabolic syndrome. This novel viewpoint of "brain glucose" control may shed new light on the pathogenesis of the metabolic syndrome and type 2 diabetes.