AJP - Renal Physiology, Vol 248, Issue 6 869-F875, Copyright © 1985 by American Physiological Society
Cation dependence of renal outer cortical brush border membrane L-glutamate transport
Y. Fukuhara and R. J. Turner
The cation dependence of L-glutamate transport in renal outer cortical
brush border membrane vesicles was studied. Both cis sodium and trans
potassium were required for optimal L-glutamate flux. Relative to
simultaneous sodium (out greater than in) and potassium (in greater than
out) gradient conditions, flux was reduced 50-fold by potassium removal and
more than 100-fold by sodium removal. The effect of potassium removal in
this preparation was markedly larger than that observed in other renal
brush border membrane preparations. No other monovalent cation tested was
effective in replacing sodium. However, Rb+ and Cs+ and to a lesser degree
NH+4 were found to be effective potassium substitutes. Kinetic analysis of
the Na/K-dependent component of L-glutamate flux indicated a single
transport system of Km = 13 microM and Vmax = 1.3 nmol X min-1 X mg
protein-1. Studies of the dependence of L-glutamate flux on potassium and
sodium concentrations yielded Hill coefficients of 0.9 and 1.9,
respectively, consistent with involvement of one potassium ion and two or
more sodium ions in the L-glutamate transport event. Efflux studies
indicated that sodium and potassium act at different steps in the transport
cycle, sodium to facilitate the translocation of the glutamate-carrier
complex, and potassium to facilitate the return of the unloaded carrier. A
model for renal Na/K-dependent L-glutamate transport is suggested on the
basis of these results.
