1. The efflux of 14C-Iabelled 5,5-dimethyl-2,4-oxazolidinedione (DMO) from the myocardium of the rabbit has been studied. The perfusate pH was 7·38.

2. The effluent curve is complex and appears to be the resultant of movement between at least four compartments.

3. The first two probably represent intravascular and extracellular compartments; the last two have smaller rate constants and may represent intracellular spaces. Intracellular pH (pH1) calculated from the effluent curve was 7·23 ± 0·05. pH1 estimated from the steady-state distribution volume of DMO was 7·28 ± 0·02.

4. The existence of two intracellular compartments suggests that DMO is not homogeneously distributed in the myocardium. It is suggested that the apparent greater buffering capacity of cardiac than skeletal muscle can be explained by the greater number and volume of mitochondria in the myocardium, compartmentalization of DMO and assumptions inherent in the DMO method for the measurement of pH1.

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