Transpulmonary lactate shuttle

The shuttling of intermediary metabolites such as lactate through the vasculature contributes to the dynamic energy and biosynthetic needs of tissues. Tracer kinetic studies offer a powerful tool to measure the metabolism of substrates like lactate that are simultaneously taken up from and released into the circulation by organs, but in each circulatory passage, the entire cardiac output traverses the pulmonary parenchyma. To determine whether transpulmonary lactate shuttling affects whole-body lactate kinetics in vivo, we examined the effects of a lactate load (via lactate clamp, LC) and epinephrine (Epi) stimulation on transpulmonary lactate kinetics in an anesthetized rat model using a primed-continuous infusion of [U-(13)C]lactate. Under all conditions studied, control 1.2 (SD 0.7) (Con), LC 1.9 (SD 2.5), and Epi 1.9 (SD 3.5) mg/min net transpulmonary lactate uptake occurred. Compared with Con, a lactate load via LC significantly increased mixed central venous ([v]) [1.9 mM (SD 0.5) vs. 4.7 (SD 0.4)] and arterial ([a]) [1.6 mM (SD 0.4) vs. 4.1 (SD 0.6)] lactate concentrations (P < 0.05). Transpulmonary lactate gradient ([v] - [a]) was highest during the lactate clamp condition [0.6 mM (SD 0.7)] and lowest during Epi [0.2 mM (SD 0.5)] stimulation (P < 0.05). Tracer measured lactate fractional extractions were similar for control, 16.6% (SD 15.3), and lactate clamp, 8.2% (SD 15.3) conditions, but negative during Epi stimulation, -25.3% (SD 45.5) when there occurred a transpulmonary production, the conversion of mixed central venous pyruvate to arterial lactate. Further, isotopic equilibration between L and P occurred following tracer lactate infusion, but depending on compartment (v or a) and physiological stimulus, [L]/[P] concentration and isotopic enrichment ratios ranged widely. We conclude that pulmonary arterial-vein concentration difference measurements across the lungs provide an incomplete, and perhaps misleading picture of parenchymal lactate metabolism, especially during epinephrine stimulation.