EFFECT OF SH-REAGENT THIMEROSAL ON CA2+ HOMEOSTASIS IN THYMOCYTES

A sulfhydryl reagent thimerosal at concentrations of 5-10-mu-M has been found to induce a variety of changes in ion transport in rat thymocytes. In particular, the concentration of intracellular calcium ([Ca2+]i) increased about tenfold from the basal level. The response of [Ca2+]i to thimerosal displayed a two-stage time course, with the main [Ca2+]i rise during the second stage, Evidence has been obtained for the depletion of intracellular Ca2+ pools in thimerosal-treated cells. However, Ca2+ mobilization from the intracellular stores did not contribute significantly to [Ca2+]i rise and the observed [Ca2+]i response arose primarily from an increased Ca2+ influx. Thimerosal elicited permeability not only for Ca2+ but for Mn2 and Ni2+ as well. This permeability for divalent cations was responsible for the major second stage of [Ca2+]i response to thimerosal. The Ca2+-transporting system that mediates the first stage of the [Ca2+]i response was not inhibited by either Mn2+ or Ni2+ and did not transfer heavy metal ions. We failed to obtain any evidence on thimerosal-induced inhibition of plasma membrane Ca2+-ATPase. The disturbance of [Ca2+]i homeostasis in thimerosal-treated cells is attributed to the induction of Ca2+ influx rather than inhibition of Ca2+-ATPase.