[Studies on the mechanism of placental transport of L-glutamate (the effect of K+ in microvillous vesicles on L-glutamate uptake)]

Abstract

The effect of potassium ion and membrane potential on the uptake of L-glutamate in microvilli (brush border) vesicles prepared from human term placenta was studied using a rapid filtration technique. The uptake of L-glutamate into microvilli vesicles is Na+ electrochemical gradient dependent and pre-equilibration of the vesicles with K+ stimulates L-glutamic acid uptake. Imposition of a K+ gradient (K+ in greater than K+ out) enhances Na+-dependent L-glutamate uptake. Changes in membrane potential due to the imposition of anion replacement markedly affect Na+ dependent L-glutamate uptake only in the presence of K+. However, this effect is not significant when changes in membrane potential incur following the imposition of valinomycin induced by K+ diffusion potential. The data indicate that Na+-dependent L-glutamate transport can be additionally energized by a K+ gradient. Furthermore K+ renders Na+-dependent L-glutamate transport sensitive to changes in the transmembrane potential difference.