title: Tributyltin increases cytosolic free Ca2+ concentration in thymocytes by mobilizing intracellular Ca2+, activating a Ca2+ entry pathway, and inhibiting Ca2+ efflux.
abstract: The immunotoxic environmental pollutant tri-n-butyltin (TBT) kills thymocytes by apoptosis through a mechanism that requires an increase in intracellular Ca2+ concentration. The addition of TBT (EC50 = 2 microM) to fura-2-loaded rat thymocytes resulted in a rapid and sustained increase in the cytosolic free Ca2+ concentration ([Ca2+]i) to greater than 1 microM. In nominally Ca(2+)-free medium, TBT slightly but consistently increased thymocyte [Ca2+]i by about 0.11 microM. The subsequent restoration of CaCl2 to the medium resulted in a sustained overshoot in [Ca2+]i; similarly, the addition of MnCl2 produced a rapid decrease in the intracellular fura-2 fluorescence in thymocytes exposed to TBT. The rates of Ca2+ and Mn2+ entry stimulated by TBT were essentially identical to the rates stimulated by 2,5-di-(tert.-butyl)-1,4-benzohydroquinone (tBuBHQ), which has previously been shown to empty the agonist-sensitive endoplasmic reticular Ca2+ store and to stimulate subsequent Ca2+ influx by a capacitative mechanism. The addition of excess [ethylenebis(oxyethylenenitrilo)]tetraacetic acid to thymocytes produced a rapid return to basal [Ca2+]i after tBuBHQ treatment but a similar rapid return to basal [Ca2+]i was not observed after TBT treatment. In addition, TBT produced a marked inhibition of both Ca2+ efflux from the cells and the plasma membrane Ca(2+)-ATPase activity. Also, TBT treatment resulted in a rapid decrease in thymocyte ATP level. Taken together, our results show that TBT increases [Ca2+]i in thymocytes by the combination of intracellular Ca2+ mobilization, stimulation of Ca2+ entry, and inhibition of the Ca2+ efflux process. Furthermore, the ability of TBT to apparently mobilize the tBuBHQ-sensitive intracellular Ca2+ store followed by Ca2+ and Mn2+ entry suggests that the TBT-induced [Ca2+]i increase involves a capacitative type of Ca2+ entry.
authors.affiliation: Department of Toxicology, Karolinska Institutet, Stockholm, Sweden.
authors.forename: S C
authors.initials: SC
authors.lastname: Chow
authors.forename: G E
authors.initials: GE
authors.lastname: Kass
authors.forename: M J
authors.initials: MJ
authors.lastname: McCabe
authors.forename: S
authors.initials: S
authors.lastname: Orrenius
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002451:Cell Compartmentation / Q000187:drug effects; D003593:Cytoplasm / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D013950:Thymus Gland / Q000166:cytology* / Q000187:drug effects; D014220:Trialkyltin Compounds / Q000633:toxicity*
chemical_list: D014220:Trialkyltin Compounds; C011559:tributyltin; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium