All metabolites in the body are in a state of constant flux, and changes in their levels may reflect either increased rate of production or decreased clearance, or both. Stable isotope technology can be used in physiological studies to determine mechanisms mediating changes in metabolites. For example, the decline in blood glucose following administration of insulin could be due to either increased uptake into peripheral tissues, or (as is the case) by suppression of hepatic glucose output. Many metabolites can be labeled by changing one of their constituent atoms to a different stable isotope e.g. [6,6]-2H2 glucose contains two deuterium atoms in the sixth carbon position. When administered in vivo, these labeled molecules are metabolized alongside their endogenous counterparts, but changes in their concentration can be measured conveniently. When stable isotopes are used for in vivo metabolic studies, tracer enrichment is principally measured using either gas chromatography mass spectrometry or isotope ratio mass spectrometry.
The hyperinsulinaemic euglycaemic clamp is a method of measuring systemic insulin sensitivity. With this technique, plasma insulin concentrations are raised above basal levels by a continuous insulin infusion, and plasma glucose concentrations are clamped at a constant level by coadministration of glucose. Under these conditions, infusion and and sequential measurement of [6,6]-2H2 glucose enables hepatic glucose output and insulin-mediated glucose disposal to be measured.
This presentation will describe human, in vivo studies using stable isotope-labelled metabolites to investigate glucose, lipid and protein metabolism, demonstrating the utility of such methodologies in clinical research. Examples will include steady state and non-steady state measurements, and will illustrate how human studies may yield different information from animal models.