In Vivo and In Vitro Quantification of Glucose Kinetics: From Bedside to Bench

Like other substrates, plasma glucose is in a dynamic state of constant turnover (i.e., rates of glucose appearance [R(a) glucose] into and disappearance [R(d) glucose] from the plasma) while staying within a narrow range of normal concentrations, a physiological priority. Persistent imbalance of glucose turnover leads to elevations (i.e., hyperglycemia, R(a)>R(d)) or falls (i.e., hypoglycemia, R(a)<R(d)) in the pool size, leading to clinical conditions such as diabetes. Endogenous R(a) glucose is divided into hepatic glucose production via glycogenolysis and gluconeogenesis (GNG) and renal GNG. On the other hand, R(d) glucose, the summed rate of glucose uptake by tissues/organs, involves various intracellular metabolic pathways including glycolysis, the tricarboxylic acid (TCA) cycle, and oxidation at varying rates depending on the metabolic status. Despite the dynamic nature of glucose metabolism, metabolic studies typically rely on measurements of static, snapshot information such as the abundance of mRNAs and proteins and (in)activation of implicated signaling networks without determining actual flux rates. In this review, we will discuss the importance of obtaining kinetic information, basic principles of stable isotope tracer methodology, calculations of in vivo glucose kinetics, and assessments of metabolic flux in experimental models in vivo and in vitro.