LACTATE TRANSPORT MECHANISMS AT APICAL AND BASOLATERAL MEMBRANES OF BOVINE RETINAL-PIGMENT EPITHELIUM

The isolated bovine retinal pigment epithelium actively transports lac tate from the apical to the basal bath. Net short-circuit [C-14]lactat e flux in 20 mM lactate was 0.46 +/- 0.09 mu eq.cm(-2).h(-1) (n = 8). In open circuit, with a physiological lactate gradient, net [C-14]lact ate flux was 0.66-1.31 mu eq.cm(-2).h(-1) (n = 3). Lactate in the apic al bath caused intracellular acidifications that were saturable, appar ently stereospecific, and reduced in magnitude by several H-lactate co transport inhibitors. In the basal bath, lactate caused intracellular alkalinizations that were dependent on the presence of Na. In short ci rcuit, 20 mM lactate in both baths reversed the direction of net trans epithelial Na-22 transport from secretion to absorption, suggesting th e presence of basolateral Na-lactate cotransport moving lactate out of the cells. Outwardly directed Na-lactate cotransport requires a lacta te:Na stoichiometry > 1.4:1, consistent with the coupled movement of N a, lactate, and net negative charge across the basolateral membrane. I ntracellular microelectrode recordings showed that basal lactate hyper polarized and apical lactate depolarized the basolateral membrane. For lactate absorption, this is a novel arrangement of membrane proteins: luminal H-lactate cotransport and serosal electrogenic Na:(n)lactate cotransport. Lactate transport across the retinal pigment epithelium m ay play an important role in regulating retinal metabolism and subreti nal space volume and composition.