The effect of varying the amount of unlabelled 5,5-dimethyloxazolidine-2,4-dione (DMO) in the measurement of rat hepatic intracellular pH using (14C)DMO.

1. Hepatic intracellular pH (pH,) was measured in intact rats by distribution of 5,5dimethyloxazolidine-2,4-dione (DMO), using [14C]DM0 and a range of doses of unlabelled DMO, previously neutralized with sodium hydroxide. 2. Mean calculated pHi fell from 7-27 to 7-17 as the dose of added unlabelled DMO was increased from zero to 0.19 mmol/kg (25 mg/kg) and thereafter remained constant up to and including the highest dose group (1.55 mmol/kg; 200 mg/kg). 3. Control observations showed that the change in calculated pH, with DMO dose could have been related to the consequential sodium load, rather than to the presence of a saturable process determining the distribution of DMO. 4. Reasons are given why it is nevertheless desirable to add carrier DMO when employing [14C]DM0 for the measurement of hepatic pHi in the rat.