Membrane potential changes associated with calcium signals in human lymphocytes and rat mast cells

Human lymphocytes and rat mast cells, two non-excitable cellular models, we re used to investigate membrane potential changes accompanying Ca2+ signals . Cells were stimulated with agents known to induce both Ca2+ release from internal stores and influx of extracellular Ca2+, namely thapsigargin, iono mycin and compound 48/80. Thapsigargin and ionomycin were used to activate lymphocytes, while compound 48/80 was used to stimulate mast cells. Membran e potential changes and Ca2+ concentration were monitored with the fluoresc ent dyes bis-oxonol and fura-2, respectively. In lymphocytes, thapsigargin induced a hyperpolarization temporally correlated with the increase in resp ectively. In lymphocytes, thapsigargin induced a intracellular Ca2+ concent ration. This hyperpolarization is due to activation of a K+ conductance whi ch consists of two phases, a first phase independent on external Ca2+ and a second one blocked in a Ca2+-free medium. Ionomycin induced a Ca2+-depende nt depolarization attributed to a massive influx of external Ca2+. On the o ther hand, stimulation of mast cells with compound 48/80 produced a fast hy perpolarization and an increase in intracellular Ca2+ levels. Besides diffe rent time-courses, this hyperpolarization differs from that induced by thap sigargin in lymphocytes in two aspects, it is mainly due to a Cl--entry cur rent and exit of K+ and it is completely inhibited in the absence of extrac ellular Ca2+. Compound 48/80-induced histamine release is not related to me mbrane potential changes.