THIMEROSAL INDUCES CALCIUM MOBILIZATION, FRUCTOSE 2,6-BISPHOSPHATE SYNTHESIS AND CYTOPLASMIC ALKALINIZATION IN RAT THYMUS LYMPHOCYTES

The effect of thimerosal on intracellular calcium ([Ca2+]i), pH (pH(i)) and fructose 2,6-bisphosphate (Fru 2,6-P2) in thymus lymphocytes was investigated. The effect of thimerosal on cell growth was also examined. Thimerosal produced a dose-dependent increase in [Ca2+]i, pH(i) and in the level of fructose 2,6-bisphosphate. Thimerosal was, however, unable to produce cell proliferation and inhibited [3H]thymidine incorporation when cells were challenged with PHA and costimulator. In the absence of external calcium, thimerosal produced only a slight increase in [Ca2+]i. In Na+-containing buffer, thimerosal induced an initial acidification (0.05 +/- 0.01 pH units), followed by an alkalinization of 0.08 pH units/min, whereas in Na+-free media, pH(i) decreased 0.2 +/- 0.02 units and this acidification was maintained for more than 40 min. When external calcium was removed the initial acidification was unchanged and no further increase in pH(i) was observed. Polymyxin B, an inhibitor of protein kinase C, did not modify the initial thimerosal-induced acidification although pH returned to basal levels after 10 min. It was concluded that alkalinization induced by thimerosal is probably due to activation of the Na+/H+ exchanger and that changes in internal Ca2+, pH and metabolic rate are not sufficient to induce cellular proliferation. The mechanism by which thimerosal inhibits thymocyte proliferation remains to be clarified.