AJP - Cell Physiology, Vol 257, Issue 1 C58-C64, Copyright © 1989 by American Physiological Society
Kinetics of Na-Li exchange in high and low K sheep red blood cells
K. H. Ryu, N. C. Adragna and P. K. Lauf
Department of Physiology, Wright State University School of Medicine, Dayton, Ohio 45401-0927.
The kinetic parameters and transport mechanism of Na-Li exchange were
studied in both low K (LK) and high K (HK) sheep red blood cells with
cellular Na [( Na]i) and Li concentrations [( Li]i) adjusted by the
nystatin technique (Nature New Biol. 244: 47-49, 1973 and J. Physiol. Lond.
283: 177-196, 1978). Maximum velocities (Vm) for Li fluxes and
half-activation constants (K1/2) for Li and Na of the Na-Li exchanger were
determined. The K1/2 values for both Li and Na appeared to be similar in
both cell types, although they were about two to three times lower on the
inside than on the outside of the membrane. Furthermore, the K1/2 values
for Li were at least an order of magnitude smaller than those for Na,
suggesting substantial affinity differences for these two cations. The Vm
values for Li fluxes, on the other hand, appear to be lower in HK than in
LK cells. When Na and Li fluxes were measured simultaneously, a trans
stimulatory effect by Na on Li fluxes was observed. From measurements of Li
influx at different concentrations of external Li and different [Na]i, the
ratio of the apparent Vm to the apparent external Li affinity was
calculated to be independent of [Na]i for both types of sheep red blood
cells. Similar trans effects of external Na were observed on Li efflux at
varying [Li]i. These results are expected for a system operating by a
"ping-pong" mechanism.
