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.
Morton D. Sarver Center for Cornea and Contact Lens Research, University of California, School of Optometry, Berkeley 94720.
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Bonanno
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D004728:Endothelium, Corneal / Q000378:metabolism*; D005456:Fluorescent Dyes; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D007773:Lactates / Q000378:metabolism*; D008634:Mersalyl / Q000494:pharmacology; D011817:Rabbits; D012964:Sodium / Q000378:metabolism
D005456:Fluorescent Dyes; D007773:Lactates; D008634:Mersalyl; D012964:Sodium