Cation fluxes and volume regulation by human lymphocytes

Abstract

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 ⁴²K⁺ efflux and ⁴²K⁺ influx were stimulated severalfold, but the increase in ⁴²K⁺ efflux exceeded the influx, resulting in a net loss of 20% of the lymphocyte K⁺. The increase in ⁴²K⁺ 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 ⁴²K⁺ influx by 76% and 123% respectively. Hypotonic shock stimulated ²²Na⁺ 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 ⁴⁵Ca²⁺ uptake was not significantly changed by hypotonicity. Although volume regulation was abolished in a hypotonic high K medium, ⁴²K⁺ 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.