Modification of leucine transport across bovine pigment epithelium by metabolic stress.

The transport of leucine in the apical-to-basal (retina to choroid) direction across the isolated bovine retinal pigment epithelium is mediated predominantly by the L amino transport system at low carrier (10 microns) concentrations. There is no evidence of an active or facilitated transport system operating in the opposite direction. The identification of the L system is based on the lack of sodium dependence, specific inhibition of leucine transport by 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH), and the demonstration of trans-stimulation. Lysine, glutamate, and 2-methylaminoisobutyric acid (MeAIB) did not provide any competitive inhibition. Ouabain and iodoacetate were also ineffective in modifying leucine transport. The transport mediated by the L system was markedly temperature sensitive, whereas no temperature dependence was apparent in the transport of leucine in the basal-to-apical direction (choroid to retina). When treated with dinitrophenol (DNP), the transport of leucine in the apical-to-basal direction was greatly enhanced, but no effect was observed on the leucine movement in the opposite direction. Azide and rotenone had an effect similar to DNP, as did reducing the partial pressure of O2 to less than 40 Torr. The enhancement of transport appeared to be mediated by the activation of an ancillary system, since it was susceptible to different classes of metabolic and competitive inhibitors as well as the observed ionic dependency. After DNP treatment, the transport of leucine was inhibited by lysine and BCH, revealed a sodium dependence, and could be inhibited by iodoacetate. The characteristics of the enhanced transport appear to be similar to those of the recently described G system(s) of amino acid transport.