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+. Thapsigargin (Fig. 1), a naturally occurring sesquiterpene lactone, promotes tumorigenesis in mouse skin (2) but does MATERIALS AND METHODSMeasurements of [Ca2+J1. Hepatocytes were prepared by perfusion of collagenase (type I; Sigma) (16) through livers from fed 250-g male rats and were kept in a Krebs-Henseleit buffer (119 mM NaCl/4.7 mM KCI/1.1 mM KH2PO4/1.2 mM MgSO4/25 mM NaHCO3, buffered with CO2 to pH 7.4) containing 1.0 mM Ca2+. The cells were stored at 3.0 x 106 per ml and continuously gassed with 02/CO2, 19:1 (vol/vol).After 5 min of incubation at 37°C, 10 uM indo-1 acetoxymethyl ester (Molecular Probes) was added and the incubation continued for a further 30 min. After centrifugation at 50 x g for 2 min, the indo-1-loaded hepatocytes were resuspended at 3.0 x 106 per ml in Krebs-Henseleit buffer including 1.0 mM CaC12 or 1.0 mM EGTA. The tTo whom reprint requests should be addressed. 2466The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

1. The effect of Li+ on the agonist-dependent metabolism of [3H]inositol has been studied in rat brain, rat parotid and the insect salivary gland. 2. When brain or parotid slices were incubated in the presence of [3H]inositol, Li+ was found to amplify the ability of agonists such as carbachol, phenylephrine, histamine, 5-hydroxytryptamine and Substance P to elevate the amount of label appearing in the inositol phosphates. 3. A different approach was used with the insect salivary gland, which was prelabelled with [3H]inositol. After washing out the label, the subsequent release of [3H]inositol induced by 5-hydroxytryptamine was greatly decreased by Li+. During Li+ treatment there was a large accumulation of [3H]inositol 1-phosphate. 4. This ability of Li+ to greatly amplify the agonist-dependent accumulation of myo-inositol 1-phosphate offers a novel technique for identifying those receptors that function by hydrolysing phosphatidylinositol. 5. The therapeutic action of Li+ may be explained by this inhibition of myo-inositol 1-phosphatase, which lowers the level of myo-inositol and could lead to a decrease in the concentration of phosphatidylinositol, especially in those neurons that are being stimulated excessively. This alteration in phosphatidylinositol metabolism may serve to reset the sensitivity of those multifunctional receptors that generate second messengers such as Ca2+, cyclic GMP and the prostaglandins.

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