Intracellular Ca2+ Signaling Induced by Osmotic Shock in Human T Lymphocytes

Abstract

After exposure to hypo-osmotic medium most vertebrate cells are able to reregulate their volume by losing electrolytes (K, Cl) and water, a process called regulatory volume decrease (RVD). The whole process of RVD requires a sensor(s) to detect swelling, a transducer(s) to translate the signal, and effectors that cause electrolyte loss. In many cell types an increase in cytoplasmic calcium (Cai) is the transducer and T lymphocytes were formerly thought to fit this pattern. However, this model was thrown into doubt by experiments on Cai-depleted T cells and by the previous failure to detect Cai changes. In the present study we used Ca fluorescence measurements of fluo-3-loaded normal human T lymphocytes exposed to 60% hypo-osmotic saline in a perfused cuvette. We show that hypo-osmotic shock causes a rapid rise in Cai (averaged over ≈ 104 cells) due to both release of Ca from internal stores and influx. Ca2+ influx occurred at room temperature as well as at 37°C and at a variety of external Ca2+ concentrations (1, 1.5, 2.5 mM). Following hypo-osmotic shock, reexposure to normal osmolarity restored Cai to resting levels. Cell viability and biological responsiveness were not impaired by these osmotic treatments and the subsequent biphasic Cai rise in response to a mitogenic lectin was normal. Using the whole-cell, patch-clamp technique we have isolated an inward cation current that can be carried by Ca2+. Both this current and the Cai rise were blocked by micromolar gadolinium; hence, this current may provide the Ca2+ influx pathway during a hypo-osmotic shock. Finally, these results and recent information on K, Cl, and cation channels in human T cells are incorporated into a model for RVD in these cells.