Stimulation by thimerosal of histamine-induced Ca2+ release in intact HeLa cells seen with aequorin targeted to the endoplasmic reticulum

The oxidizing thiol reagent, thimerosal, has been shown to activate reversibly the inositol 1,4,5-trisphosphate (InsP(3)) receptor in several cell types. We have studied here the effects of thimerosal by monitoring the [Ca2+] inside the endoplasmic reticulum (ER) of intact HeLa cells with targeted aequorin.; We show that thimerosal produced little effects on the ER-Ca2+-pump and only slightly increased the ER-Ca2+-leak in intact cells. Instead, thimerosal increased the sensitivity to histamine of ER-Ca2+-release by about two orders of magnitude, made the response much more prolonged at saturating histamine concentrations and enhanced both cytosolic and mitochondrial [Ca2+] responses to histamine. Moreover, inhibition of ER-Ca2+ release by cytosolic [Ca2+] microdomains was fully preserved and sensitive to BAPTA-loading, and histamine-induced Ca2+ release remained quantal in the presence of both thimerosal and intracellular BAPTA. The effects of thimerosal were reversible in the presence of dithiotreitol, suggesting the possible presence of a physiological redox regulatory mechanism. However, in permeabilized cells thimerosal potentiated InsP(3)- induced Ca2+ release but oxidized glutathione had no effect. In addition, thimerosal increased the [Ca2+](ER) steady-state level in permeabilized cells. Thimerosal partially inhibited also plasma membrane Ca2+ extrusion and increased Ca2+ (Mn2+) entry through the plasma membrane, both phenomena contributing to increase the steady-state cytosolic [Ca2+]. Thimerosal-induced Ca2+ entry was additive to that induced by emptying of the ER, suggesting that store-operated Ca2+ channels may not be involved. These results provide new insights on the mechanisms of activation and inactivation of InsP(3) receptors. (C) 2001 Harcourt Publishers Ltd