Effect of magnesium on passive fluxes of lithium in the human erythrocyte

Correction for the adverse influence of sodium-potassium cotransport on apparent sodium-lithium countertransport activity in human erythrocytes

INTRODUCTION

Erythrocyte sodium-lithium countertransporter (SLC) has traditionally been characterised as the sodium-stimulated lithium efflux from lithium-loaded erythrocytes. Concurrent activity of the sodium-potassium cotransporter (NKCC) can be expected to lead to imprecise estimates of the activity of the SLC. In the present study, we have characterised this methodological problem and have shown that it can be corrected with the inclusion of bumetanide in the physiological salt solution.

METHODS

Lithium efflux was studied in lithium-loaded erythrocytes from 35 healthy, normotensive subjects. Erythrocytes were divided into two identical samples (A and B) and lithium efflux characteristics in both samples studied simultaneously by incubating aliquots from each in 10 media of differing external sodium concentrations. Efflux media employed for A and B were the same except for 0.02 mM bumetanide in the media used in B.

RESULTS

Increased external sodium was associated with increasing lithium efflux both in the absence and presence of bumetanide; efflux rates were consistently lower in media containing bumetanide (P<.05 in all cases). As external sodium increased, bumetanide-sensitive lithium efflux decreased in a manner that correlated inversely with external sodium concentration (r=-.77, P<.01). A small, nonsignificant increase in SLC activity was observed between measurements made under control conditions (median [range] in mmol Li/l RBC h; 0.272 [0.098-0.491]) and those made in the presence of bumetanide (0.286 [0.135-0.650]; P=.064). Bumetanide did not influence maximal rate of turnover or the affinity constant for external sodium. In contrast, the ratio of these variables was lower when determined in the absence than in the presence of bumetanide (5.5 [1.5-14.6] vs. 6.9 [2.8-24.2], respectively; P<.05).

DISCUSSION

This work shows that a component of lithium efflux mediated by the NKCC changes substantially with alterations in external sodium, resulting in a variable contribution of this second transport pathway to apparent SLC activity. To eliminate this variability, bumetanide should be included in all media when studying SLC, and the relationship to external sodium concentration determined.

Kinetics of Na-Li exchange in high and low K sheep red blood cells

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.