Comparison between changes in membrane potential induced by CA²⁺ mobilizing agents in lymphocytes and mast cells

We studied membrane potential changes accompanying Ca²⁺ signals in two different cellular models, human lymphocytes and rat mast cells. The cells were stimulated with agents known to induce both Ca²⁺ release from internal stores and influx of extracellular Ca²⁺, namely thapsigargin and ionomycin for lymphocytes and compound 48/80 for mast cells. The fluorescent dye bis-oxonol was used to determine the membrane potential and Fura-2 to monitor Ca²⁺ concentration. Thapsigargin induced a hyperpolarization in lymphocytes, temporally correlated with the increase in intracellular Ca²⁺ concentration. This hyperpolarizalion is due to the activation of a K⁺ conductance and it has two phases: a first phase independent on external Ca²⁺ and a second one blocked in a Ca²⁺ free medium. High doses of ionomycin induce a Ca⁺²-dependent depolarization attributed to a massive influx of external Ca⁺². Stimulation of mast cells with compound 48/80 induces a fast hyperpolarization as well as a big increase in intracellular Ca²⁺ level. Besides different time courses, this hyperpolarization differs from that induced by thapsigargin in lymphocytes in other two aspects: it is carried mainly by a Cl^- current and it is completely inhibited in the absence of extracellular Ca²⁺.