A simple technique for the measurement of ouabain-sensitive sodium transport in red cells

A new, simple and non-radioactive technique is described for the measurement of ouabain-sensitive sodium transport in red cells, which is an alternative to the more complex standard technique using radioactive sodium. The new technique is based on the observation that when excess ouabain is added to blood, the sodium content of the red cells rises, due to complete inhibition of ouabain-sensitive sodium efflux while influx remains unchanged. The rate of increase of red cell sodium is therefore a measure of ouabain-sensitive sodium efflux (Eos). This estimate of Eos, when expressed as a fraction of the original red cell sodium concentration, gives a measure of the ouabain-sensitive, efflux rate constant (ERCos). In healthy persons, ERCos obtained with this new technique agrees closely with that obtained with the standard radioactive technique. In hypokalaemic patients the ERCos estimated by the new technique was less than that obtained with the standard technique, but this difference disappeared as the patients became normokalaemic. We suggest that this difference arises because the new technique measures only net ouabain-sensitive sodium transport whereas the standard technique also measures the ouabain-sensitive Na : Na exchange which is present in patients with hypokalaemia.