Role of the circulation in measurement of lactate turnover rate

Previous studies have shown that venous lactate specific activity during arterial tracer lactate infusion differs from arterial lactate specific activity during systemic venous tracer lactate infusion. We performed paired experiments on chronically catheterized rabbits to compare left ventricular (LV) infusion with femoral venous (FV) infusion of L-[U-¹⁴C]lactate. Blood was sampled from both the femoral artery (FA) and right ventricle (RV) during both modes of infusion. The mean lactate specific activity measured for each combination (infusion site, sampling site) was (FV,FA) 4,380 ± 452, (FV,RV) 4,370 ± 471, (LV,FA) 4,364 ± 239, and (LV,RV) 3,325 ± 240 (SE) dpm/μmol. Lactate turnover calculated from the specific activity in the (LV,RV) mode was significantly higher than from the other three modes (P < 0.001). Models of lactate turnover are discussed demonstrating that the (FV,FA) and analogous modes of infusion sampling measure the turnover rate of lactate molecules that cycle through the circulation. This estimate of turnover is less than the turnover rate by the whole organism to the extent that some produced lactate is metabolized locally without entering the general circulation. The turnover calculated by the (LV,RV) mode overestimates the turnover of circulating lactate and relates to whole body lactate turnover in a complex manner.