Diabetes mellitus

Endocrinology

Insulin resistance

Glucose tolerance test

Medicine

Molecular Metabolism

OBJECTIVE: The glucose tolerance test (GTT) is a widely used assay in preclinical research to interrogate glucose metabolism, but there is no standardised way by which glucose is administered. The aim of this study was to directly compare the effect of the route of glucose administration on glucose and insulin kinetics during a GTT in mice. METHODS: A GTT was performed in lean male and female mice and obese male mice and glucose was administered by either the oral or intraperitoneal (I.P.) route. Frequent samples were taken during the GTT to provide a full time-course of the insulin and glucose excursions. In another cohort of lean male mice, plasma concentrations of insulin, c-peptide and incretin hormones were measured at early timepoints after glucose administration. A stable-isotope labelled GTT was then performed to delineate the contribution of exogenous and endogenous glucose to glycaemia during a GTT, comparing both methods of glucose administration. Finally, we present a method to easily measure insulin from small volumes of blood during a GTT by directly assaying whole-blood insulin by ELISA and show a good concordance between whole-blood and plasma insulin measurements. RESULTS: We report that I.P. glucose administration results in an elevated blood glucose excursion and a largely absent elevation in blood insulin and plasma incretin hormones when compared to oral administration. Utilising stable-isotope labelled glucose, we demonstrate that the difference in glucose excursion between the two routes of administration is mainly due to a lack of suppression of glucose production in I.P. injected mice. Additionally, rates of exogenous glucose appearance into circulation are different between lean and obese mice after I.P., but not after oral glucose administration. CONCLUSION: Reflecting on these data, we suggest that careful consideration be given to the route of glucose administration when planning a GTT procedure in mice and suggest that in most circumstances the oral route of glucose administration should be preferred over the I.P. route to avoid possible artifacts originating from a non-physiological route.

Comparative analysis of oral and intraperitoneal glucose tolerance tests in mice