Sodium fluxes were measured in erythrocytes from three species of mammals. Unidirectional fluxes were slowest in swine RBCs (low sodium cells), fastest in dog RBCs (high sodium cells), and between these extremes in ox cells (intermediate level of internal sodium). In addition, efflux and influx in swine cells both correlated positively with intracellular sodium concentration between 12 to 4 µeq/ml. Tracer effluxes in swine and beef cells were separated into three components: active transport, diffusion, and exchange diffusion. The last two also contributed to influx. Transport was greater in swine cells than in beef, while the leak was similar in both. Pump to leak ratios were about 21 for swine and 3 for beef, a difference that probably accounts for the higher cell sodium in the latter. Exchange diffusion was faster in beef cells than in swine resulting in a larger tracer movement in beef. The exchange mechanism was temperature-sensitive, but was not inhibited by strophanthin. The unidirectional fluxes in canine cells were inhibited by low temperature, but they were sensibly unaffected by strophanthin. When placed in magnesium Ringer's solution (inhibits exchange diffusion in beef and swine cells) dog RBCs lost more than half of their internal sodium at a rate approximating the isotope flux in plasma or normal Ringer's solution. It was, however, not possible to separate the total tracer movement into pump, leak, and exchange.

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