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 Γ isotonic Tyrode's but returned to the isotonic value by 20 min at 37Β°C. During this phase of volume regulation (1β20 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 ouabainβsensitive (active) and ouabainβinsensitive 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 2βfold 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.