Determination of Brain Extracellular Glucose in Vivo with an Implanted Biosensor

Abstract

A number of approaches have been used for estimating the extracellular (ECF) concentration of glucose in the brain which have yielded very different results. Calculations from the glucose content of whole brain give a value of 2–4 mM (1,2). More recently microdialysis has been used (3). However, although microdialysis samples the extracellular compartment of the brain, the concentration in the dialysate is not equal to the concentration in the ECF, but depends on the conditions of mass transfer. In vitro the main diffusion barrier is the dialysis membrane, while in vivo there are a number of additional factors which affect recovery. One of the techniques for determining the ECF concentration with microdialysis is the zero net flux (ZNF) method of Lonnroth, first used for the determination of tissue glucose concentration (4). Different concentrations of glucose are infused; at concentrations below the ECF level glucose is lost from the tissue, at concentrations above the ECF level glucose is taken up by the tissue. By plotting influx again the difference between influx and efflux the point of ZNF, which is the true extracellular concentration, can be calculated by regression analysis. A value of ca. 0.5 mM has been reported using this technique.