Hypotonic dilution of human peripheral blood lymphocytes (PBL) induces large conductive permeabilities for K+ and Cl−, associated with the capacity of the cells to regulate their volumes. When rapid cation leakage is assured by the addition of the ionophore gramicidin, the behavior of the anion conductance pathway can be independently examined. Using this technique it is demonstrated that the volume-induced activation of Cl− transport is triggered at a threshold of ~1.15 × isotonic cell volume. If the volume ofa cell is increased to this level or above, the Cl− transport system is activated, whereas if the volume of a swollen cell is decreased below the threshold value, the Cltransport is inactivated. Activation and inactivation are independent of the relative volume changes and of the actual cellular Na+, K+, or Cl− concentrations, as well as of the changes in membrane potential in PBL. When net salt movement and thus volume change are inhibited by specific blockers of K+ transport (e.g., quinine, or Ca2+ depletion), volume-induced Cl− conductance shows a time-dependent inactivation, with a half-time of 5-8 min. The Clconductance, when activated, appears to involve an all-or-none response. In contrast, volume-induced K+ conductance is a graded response, with the increase in K+ flux being roughly proportional to the hypotonicity-induced increase in cell volume. The data indicate that during lymphocyte volume response in hypotonic media, anion conductance increases by orders of magnitude, exceeding the K+ conductance, so that the rate of the volume decrease (KCI efflux) is determined by a graded alteration in K+ conductance. When the cell volume approaches the isotonic value, it is stabilized by the inactivation of the anion conductance pathway.
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