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      Thapsigargin, a tumor promoter, discharges intracellular Ca2+ stores by specific inhibition of the endoplasmic reticulum Ca2(+)-ATPase.

      Proceedings of the National Academy of Sciences of the United States of America
      Animals, Biological Transport, drug effects, Calcium, metabolism, Calcium-Transporting ATPases, antagonists & inhibitors, Carcinogens, pharmacology, Cell Membrane, Cells, Cultured, Endoplasmic Reticulum, Fluorescent Dyes, Guanosine Triphosphate, Humans, Indoles, Inositol 1,4,5-Trisphosphate, Kinetics, Liver, Male, Microsomes, Liver, Plants, Medicinal, Rats, Terpenes, Thapsigargin

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          Abstract

          Thapsigargin, a tumor-promoting sesquiterpene lactone, discharges intracellular Ca2+ in rat hepatocytes, as it does in many vertebrate cell types. It appears to act intracellularly, as incubation of isolated rat liver microsomes with thapsigargin induces a rapid, dose-dependent release of stored Ca2+. The thapsigargin-releasable pool of microsomal Ca2+ includes the pools sensitive to inositol 1,4,5-trisphosphate and GTP. Thapsigargin pretreatment of microsomes blocks subsequent loading with 45Ca2+, suggesting that its target is the ATP-dependent Ca2+ pump of endoplasmic reticulum. This hypothesis is strongly supported by the demonstration that thapsigargin causes a rapid inhibition of the Ca2(+)-activated ATPase activity of rat liver microsomes, with an identical dose dependence to that seen in whole cell or isolated microsome Ca2+ discharge. The inhibition of the endoplasmic reticulum isoform of the Ca2(+)-ATPase is highly selective, as thapsigargin has little or no effect on the Ca2(+)-ATPases of hepatocyte or erythrocyte plasma membrane or of cardiac or skeletal muscle sarcoplasmic reticulum. These results suggest that thapsigargin increases the concentration of cytosolic free Ca2+ in sensitive cells by an acute and highly specific arrest of the endoplasmic reticulum Ca2+ pump, followed by a rapid Ca2+ leak from at least two pharmacologically distinct Ca2+ stores. The implications of this mechanism of action for the application of thapsigargin in the analysis of Ca2+ homeostasis and possible forms of Ca2+ control are discussed.

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