Abstract
The characteristics of lactate transport in brush-border membrane vesicles isolated from normal human full-term placentas were investigated. Lactate transport in these vesicles was Na+-independent, but was greatly stimulated when the extravesicular pH was made acidic. In the presence of an inwardly directed H+ gradient ([H+]o greater than [H+]i), transient uphill transport of lactate could be demonstrated. This H+ gradient-dependent stimulation was not a result of a H+ diffusion potential. Transport of lactate in the presence of the H+ gradient was not inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid or by furosemide, ruling out the participation of an anion exchanger in placental lactate transport. Many monocarboxylates strongly interacted with the lactate transport system, whereas, with the single exception of succinate, dicarboxylates did not. The monocarboxylates pyruvate and lactate, but not the dicarboxylate succinate, when present inside the vesicles, were able to exert a trans-stimulatory effect on the uptake of radiolabeled lactate. Kinetic analyses provided evidence for a single transport system with a Kt of 4.1 +/- 0.4 mM for lactate and a Vmax of 54.2 +/- 9.9 nmol/mg of protein/30 s. Pyruvate inhibited lactate transport competitively, by reducing the affinity of the system for lactate without altering the maximal velocity. It is concluded that human placental brush-border membranes possess a transport system specific for lactate and other monocarboxylates and that this transport system is Na+-independent and is energized by an inwardly directed H+ gradient. Lactate-H+ symport rather than lactate-OH- antiport appears to be the mechanism of the H+ gradient-dependent lactate transport in these membranes.
Highlights
The characteristics of lactate transport in brush- but rather utilizes lactate as the primary energy substrate to border membrane vesicles isolated from normahl uman generate the needed metabolic energy [6]
Since the uptake rateosf these two forms into the placental brush-border membrane vesicles are expected to be significantly different, we examined the effects of extravesicular pH on lactate uptake
Substrate Specificityof the PlacentaLl actate Transport System-To determine the substrate specificity of the human placental lactate transportsystem, we investigated the ability of various monocarboxylic and dicarboxylic acids to compete with radiolabeled lactate for the uptake process
Summary
The characteristics of lactate transport in brush- but rather utilizes lactate as the primary energy substrate to border membrane vesicles isolated from normahl uman generate the needed metabolic energy [6]. This is a very full-term placentas were investigated. There is evidence that the human placenta is capable of maintaining higher concentrations of lactate in the fetal cirout the participation of an anion exchanger in placentalculation than in the maternal circulation [7] It is lactate transport.Many monocarboxylates strongly in- likely that in humans, as in laboratory animals, lactate is a teracted with the lactate transport system, whereas, crucial metabolic substrate for the developing fetus.
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