Role of Calcium Ions in Rapid Effects of L-Thyroxine on Phosphoinositide Metabolism in Rat Liver Cells

O. A. Krasilnikova, N. S. Kavok, and N. A. Babenko^*

Institute of Biology, Kharkov National University, pl. Svobody 4, Kharkov 61077, Ukraine; E-mail: babenko@univer.kharkov.ua

^* To whom correspondence should be addressed.

Received July 9, 2002

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The role of calcium ions in the L-thyroxine-induced initiation of hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP₂) and also the course of releasing individual fractions of inositol phosphates and diacylglycerides (DAG) were studied in liver cells during early stages of the hormone effect. L-Thyroxine stimulated a rapid hydrolysis in hepatocytes of PtdInsP₂ labeled with [¹⁴C]linoleic acid and [³H]inositol mediated by phosphoinositide-specific phospholipase C. This was associated with accumulation of [¹⁴C]DAG, total inositol phosphates, [³H]inositol 1,4,5-trisphosphate (Ins1,4,5P₃) and [³H]inositol 1,4-bisphosphate (Ins1,4P₂). Elimination of calcium ions from the incubation medium of hepatocytes did not abolish the effect of thyroxine on the accumulation of [¹⁴C]DAG and total [³H]inositol phosphates. Preincubation of liver cells with TMB-8 increased the stimulatory effect of L-thyroxine on the accumulation of [¹⁴C]DAG. During the incubation of hepatocytes in the presence of the hormone the content of ¹⁴C-labeled fatty acids did not change. The L-thyroxine-induced accumulation of [³H]Ins1,4,5P₃ and [³H]Ins1,4P₂ did not depend on the presence of calcium ions in the incubation medium of the cells.

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KEY WORDS: thyroxine, hepatocytes, phospholipase C, phosphatidylinositol 4,5-bisphosphate, diacylglycerol, inositol 1,4,5-trisphosphate, calcium

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