1. We describe in this paper a new ouabain-insensitive pathway for Na+ and K+ in human erythrocytes. K+ efflux was measured in cells loaded by the p-chloromercuribenzene-sulphonate (PCMBS) procedure to contain approximately equal amounts of Na+ and K+. K+ efflux was stimulated by external Na+ in the presence of ouabain and frusemide. Na+-stimulated K+ efflux was 0.35 ± 0.12 (mmol h−1 l−1 of cells) in eight normal subjects and 0.64 ± 0.13 in 13 patients with essential hypertension.
2. The Na+-stimulated K+ efflux was not observed in cells loaded in the presence of EGTA. This inhibition by EGTA suggests that K+ efflux is dependent on intracellular calcium. The Ki+-Nao+ countertransport of hypertensive patients was also inhibited by EGTA. The elevated K+—Na+ countertransport of hypertensive patients could be due to a higher intracellular Ca2+ content (Cai2+) or to an increased affinity for Cai2+. The relationship of this pathway to the Gardos effect is not clear since Na+-stimulated K+ efflux occurs without metabolic depletion or inhibition of the Ca2+ pump. As a tentative hypothesis, we relate the Ca2+-dependent downhill movement of K+ and Na+ to the Ca2+-dependent channels described in muscle and nerve, but other hypotheses cannot be excluded at this stage of our research.