Facilitated Transport of Lactate by Rabbit Corneal Endothelium

Abstract

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 factate concentration; (2) L-lactate produced a faster maximal H + influx and had a higher affinity for the transporter ( V max = 1·6 mM min -1, apparent K1/2 = 31 mM) than its optical isomer, d -Lactate ( V max = 0·9 mM min -1, apparent K 1/2 = 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.