A simplified method for simultaneously determining countertransport and cotransport in human erythrocytes.

Both sodium countertransport and sodium-potassium cotransport are altered in erythrocytes from some hypertensive subjects and their relatives. Lithium can substitute for sodium in both of these transport mechanisms; they can then be monitored as sodium-lithium countertransport and lithium-potassium cotransport. Using erythrocytes loaded with lithium, we can determine both transport systems simultaneously by monitoring the rate of lithium efflux into three media: (1) NaCl, (2) MgCl2 and (3) MgCl2 with furosemide. The difference between the effluxes into NaCl and MgCl2 is the sodium-lithium countertransport; the difference between the effluxes into MgCl2 with and without the cotransport inhibitor furosemide is the lithium-potassium cotransport. At the intracellular Li concentrations used in these experiments, lithium-potassium cotransport is a linear function of the Li+ concentration and can be expressed by the equation for a first order reaction. The rate constant can be calculated by dividing the lithium-potassium cotransport by the intracellular lithium concentration and correlates well (r = 0.80, n = 30) with sodium-potassium cotransport measured by Dagher and Garay's method. The simultaneous measurement of countertransport and cotransport requires much less time, effort and material than measuring the two transports separately.