Facilitated transport of lactate by rabbit corneal endothelium

Proton coupled lactate transport across the rabbit corneal endothelium was studied using a pH-sensitive intracellular fluorescent probe. Functional indications that lactate transport is carrier-mediated and coupled to H+ at the apical endothelium (aqueous humor facing) that were found were: (1) proton influx was a saturable function of lactate concentration; (2) L-lactate produced a faster maximal H+ influx and had a higher affinity for the transporter (Vmax = 1.6 mM min-1, apparent K1/2 = 31 mM) than its optical isomer, D-Lactate (Vmax = 0.9 mM min-1, apparent K1/2 = 59 mM); (3) the lactate-induced acidification was inhibitable by apical mersalyl acid. These results are consistent with the presence of an apical lac-:H+ cotransporter. On basolateral addition of lactate to a de-epithelialized cornea, the endothelial cells transiently acidified by 0.05 units, but then alkalinized by 0.07, 0.02 units over baseline. The basolateral lactate-induced acidification was inhibited by 75% by mersalyl acid, while the net alkalinization observed under control conditions was unaffected. The alkalinization was Na(+)-dependent yet amiloride-insensitive. Alkalinization on addition of basolateral lactate could also be inhibited by apical mersalyl. These results suggest that the acidification represents lactate influx through basolateral Lac-:H+ cotransport while the net alkalinization is due to basolateral Na(+)-dependent lactate uptake and lactate efflux through apical Lac-:H+ cotransport. These facilitated transport mechanisms could provide for the efficient removal of lactate from the highly glycolytic cornea.