Facilitated diffusion of lactic acid in the guinea-pig placenta

Abstract

In the guinea-pig placenta which was artificially perfused on the fetal side while maternal placental blood flow was controlled, the placental transfer per mean transplacental concentration difference (the transfer coefficient TC) was determined for lactate. TC forl-lactate (TC_LL) was compared to that ford-lactate (TC_DL) and measured for various concentrations ofl-lactate, bicarbonate, pyruvate and CO₂. Applying a “closed circuit” perfusion technique,l-lactate and proton concentrations on both sides of the placenta were followed during infusion of HCl and sodiuml-lactate into the fetal circulation. It was found that TC_LL is 3 times TC_DL. TC_LL is depressed by increasing concentrations ofl-lactate while TC for Cl-36 is not. TC_LL is also depressed by 50 mmol·l⁻¹ pyruvate. Concentration changes of glucose do not affect TC_LL. TC_LL rises with the proton concentration, independently of the concomitant changes of the bicarbonate concentration. Transplacentral proton concentration gradients producel-lactate concentration gradients and vice versa. It is concluded that (1) facilitated diffusion ofl-lactate occurs in the placenta and that (2)l-lactate transfer is coupled with proton transfer. Beside the well-known placental transport system for glucose this is the second passive transport system found in a placenta.