AJP - Regulatory, Integrative and Comparative Physiology, Vol 257, Issue 1 180-R188, Copyright © 1989 by American Physiological Society
Na-dependent L-glutamate transport by eel intestinal BBMV: role of K+ and Cl-
P. M. Romano, G. A. Ahearn and C. Storelli
Dipartimento di Biologia, Universita di Lecce, Italy.
L-[3H]glutamate uptake into eel (Anguilla anguilla) intestinal brush-border
membrane vesicles (BBMV) was a sigmoidal function of extravesicular Na,
suggesting that two or more cations accompanied the amino acid during
transport. L-[3H]glutamate influx illustrated the following kinetic
constants: apparent membrane binding affinity (Kapp) = 0.80 +/- 0.12 mM;
influx velocity (Jmax) = 2.61 +/- 0.31 nmol.mg protein-1.min-1; and
permeability coefficient (P) = 0.65 +/- 0.10 microliters.mg protein-1.
min-1. Results from the imposition of diffusion potentials across vesicle
membranes using K-valinomycin or H-carbonyl-cyanide
p-chloromethoxyphenylhydrazone suggested that Na-dependent L-glutamate
transport was sensitive to transmembrane electrical potential difference.
Extravesicular aspartate was a competitive inhibitor of L-[3H]glutamate
influx [inhibitory constant (Ki) = 0.28 +/- 0.04 mM]. Intravesicular K and
extravesicular Cl ions enhanced maximal amino acid influx and transient
L-glutamate accumulation against a concentration gradient (overshoot).
Intravesicular K reduced the Kapp of the membrane to L-glutamate, whereas
extravesicular Cl increased L-glutamate Jmax. A model for L-[3H]glutamate
transport is suggested involving the cotransport of at least two Na and one
L-glutamate that is activated by one intravesicular K ion and at least two
extravesicular Cl ions.
