The effect of insulin on in vivo cerebral glucose concentrations and ratesof glucose transport/metabolism in humans

The continuous delivery of glucose to the brain is critically important to the maintenance of normal metabolic function. However, elucidation of the h ormonal regulation of in vivo cerebral glucose metabolism in humans has bee n limited by the lack of direct, noninvasive methods with which to measure brain glucose. In this study, we sought to directly examine the effect of i nsulin on glucose concentrations and rates of glucose transport/metabolism in human brain using H-1-magnetic resonance spectroscopy at 4 Tesla. Seven subjects participated in paired hyperglycemic (16.3 +/- 0.3 mmol/I.) clamp studies performed with and without insulin. Brain glucose remained constant throughout (5.3 +/- 0.3 mu mol/g wet wt when serum insulin = 16 +/- 7 pmol /l vs. 5.5 +/- 0.3 mu mol/g wet wt when serum insulin = 668 +/- 81 pmol/l, P = NS). Glucose concentrations in gray matter-rich occipital cortex and wh ite matter-rich periventricular tissue were then simultaneously measured in clamps, where plasma glucose ranged from 4.4 to 24.5 mmol/I and insulin wa s infused at 0.5 mU . kg(-1) . min(-1). The relationship between plasma and brain glucose was linear in both regions. Reversible Michaelis-Menten kine tics fit these data best, and no differences were found in the kinetic cons tants calculated for each region. These data support the hypothesis that th e majority of cerebral glucose uptake/metabolism is an insulin-independent process in humans.