The Journal of General Physiology, Vol 80, 149-168, Copyright © 1982 by The Rockefeller University Press
Cotransport of lithium and potassium in human red cells
M Canessa, I Bize, N Adragna and D Tosteson
This paper reports the presence of human red cells of an additional
ouabain-insensitive transport pathway for lithium ions, the Li-K
cotransport. Several kinds of observations support this conclusion. Cells
loaded to contain only K, Na, or Li do not exhibit furosemide- sensitive
efflux. Simultaneous presence of K and Li on the same side of the membrane
mutually stimulates furosemide-sensitive Li and K fluxes from that side.
Cells loaded with both Na and Li exhibit no furosemide- sensitive Li
efflux. Thus, Li can apparently replace Na but not K on the outward Na-K
cotransport system in human red cells. Furthermore, Lio, like Ko, inhibits
outward Na-K cotransport. Additional proof for coupled Li-K cotransport is
provided by the observation that an outwardly directed K electrochemical
potential gradient can drive net outwardly directed K electrochemical
potential gradient can drive net outward Li movement against its gradient.
There are several differences between Li-K cotransport and Li-Na
countertransport. The cotransport system has an apparent affinity for Li
that is about one-half that for Na and 30 times lower than the
counter-transport system. Furosemide and chloride replacement inhibit
cotransport but do not affect countertransport. The PCMBS loading procedure
irreversibly inhibits countertransport but not cotransport. Furthermore,
the two systems can apparently function at maximal rates simultaneously.
Present evidence, than, indicates that the two pathways can be separated
operationally as two different systems.