Cation fluxes and volume regulation by human lymphocytes

The ionic basis of volume regulation by human peripheral blood lymphocytes in hypotonic Tyrode's medium has been studied. The intracellular water space of lymphocytes increased to a maximum after 1 min in 0.68 × isotonic Tyrode's but returned to the isotonic value by 20 min at 37°C. During this phase of volume regulation (1–20 min) both 42 K + efflux and 42 K + influx were stimulated severalfold, but the increase in 42 K + efflux exceeded the influx, resulting in a net loss of 20% of the lymphocyte K + . The increase in 42 K + efflux during the phase of cell shrinkage was unaffected by ouabain or by quinidine. Hypotonicity increased both the ouabain‐sensitive (active) and ouabain‐insensitive components of 42 K + influx by 76% and 123% respectively. Hypotonic shock stimulated 22 Na + influx by only 25%, but cell Na + content was unchanged at 1 min and even decreased after 20 min. Thus active K + influx and Na + extrusion is increased by hypotonicity, but greater pumping cannot explain the net decrease in cell cations that leads to volume regulation. The 45 Ca 2+ uptake was not significantly changed by hypotonicity. Although volume regulation was abolished in a hypotonic high K medium, 42 K + efflux was still stimulated 2‐fold by the reduction in tonicity. These findings support the hypothesis that volume regulation in hypotonic media occurs largely by a passive loss of cell K + , which results from a selective increase in membrane permeability to this ion. The increase in K + permeability in hypotonic media is observed even in the absence of volume regulation by the cell.