Transport of protons and lactate in cultured human fetal retinal pigment epithelial cells

Monolayer cultures of human fetal retinal pigment epithelial (RPE) cells we re examined for ultrastructural characteristics and junctional integrity by means of electron microscopy. Intracellular pH (pH(i)) and cell volume cha nges were measured using the fluorescent dye BCECF. The EM studies indicate that the RPE cells preserve in vivo morphology before and after loading wi th BCECF. Monolayer cultures were placed in a perfusion chamber in which th e solution facing the retinal cell membrane could be changed rapidly. Remov al of Na+ or the addition of amiloride caused intracellular acidifications, pH(i) recovery from an NH4+-induced acid load was blocked by sodium remova l or amiloride addition. These results suggest the presence of a Na+-H+ exc hange mechanism in the retinal cell membrane. When Cl- was replaced isotoni cally by lactate or pyruvate the cells acidified. The intracellular acidifi cations were saturable, reversibly reduced with the inhibitor probenecid (2 mM), and the lactate-induced acidifications were reversibly inhibited by e quimolar concentrations of pyruvate. These results indicate the presence of a H+-lactate cotransport mechanism in the retinal membrane. When Cl- was r eplaced by lactate the cells not only acidified, they also swelled. The dat a are compatible with water transport induced by the H+-lactate cotransport er.