AJP - Cell Physiology, Vol 257, Issue 5 C940-C947, Copyright © 1989 by American Physiological Society
Modification of leucine transport across bovine pigment epithelium by metabolic stress
E. L. Pautler, C. Tengerdy, J. Beyer and D. Beezley
Department of Physiology, Colorado State University, Fort Collins 80523.
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.
