Influence of hematocrit on whole blood glucose levels: new evidence from marine mammals.

The water content and glucose concentration in the whole blood of marine mammals were found to be correlated to red blood cell concentration. Because hematocrit (Hct) undergoes significant periodic shifts in these mammals during periods of apnea and/or diving, the measured values of whole blood glucose change due to alterations in Hct, independent of shifts in metabolite regulatory pathways. In contrast to humans, where red blood cell and plasma glucose concentrations are equivalent, in most other mammalian species red blood cell glucose concentration is much lower than that in plasma. Therefore, as Hct increases, the whole blood concentration of glucose must decrease in a linear manner, depending on the magnitude of the Hct change and the concentration of glucose in the blood cells. This study demonstrates that water and metabolite distribution must be considered when interpreting whole blood metabolite data. The results suggest that plasma or serum analysis of metabolites would avoid distribution problems. This is especially critical in species where Hct varies during the course of the experiment, as in exercising or diving mammals.