False in vitro and in vivo elevations of uric acid levels in mouse blood.

Uric acid (UA) levels in mouse blood have been reported to range widely from 0.1 μM to 760 μM. The aim of this study was to demonstrate false in vitro and in vivo elevations of UA levels in mouse blood. Male ICR mice were anesthetized with pentobarbital (breathing mice) or sacrificed with overdose ether (non-breathing mice). Collected blood was dispensed into MiniCollect® tubes and incubated in vitro for 0 or 30 min at room temperature. After separation of plasma or serum, the levels of UA and hypoxanthine were determined using HPLC. From the non-incubated plasma of breathing mice, the true value of UA level in vivo was 13.5±1.4 μM. However, UA levels in mouse blood increased by a factor of 3.9 following incubation in vitro. This "false in vitro elevation" of UA levels in mouse blood after blood sampling was inhibited by allopurinol, a xanthine oxidase inhibitor. Xanthine oxidase was converted to UA in mouse serum from hypoxanthine which was released from blood cells during incubation. Plasma UA levels from non-breathing mice were 19 times higher than those from breathing mice. This "false in vivo elevation" of UA levels before blood sampling was inhibited by pre-treatment with phentolamine, an α-antagonist. Over-anesthesia with ether might induce α-vasoconstriction and ischemia and thus degrade intracellular ATP to UA. For the accurate measurement of UA levels in mouse blood, the false in vitro and in vivo elevations of UA level must be avoided by immediate separation of plasma after blood sampling from anesthetized breathing mice.