1α,25-dihydroxyvitamin D3-induced increments in hepatocyte cytosolic calcium and lysophosphatidylinositol: Inhibition by pertussis toxin and 1ß,25-dihydroxyvitamin D3

Abstract

1α,25-Dihydroxyvitamin D₃ rapidly increases cytosolic calcium and alters membrane phospholipid metabolism in hepatocytes. To define the causal relationship between these events, we examined the effects of 1α,25-dihydroxyvitamin D₃ on ³²P-labeled lysophosphatidylinositol levels and cytosolic calcium as affected by pertussis toxin and 1ß,25-dihydroxyvitamin D₃, the biologically inactive analog. ³²P-labeled lysophosphatidylinositol was determined by two-dimensional thin-layer chromatography. Cytosolic calcium was measured in cells loaded with quin-2AM. Within 5 min, 1α,25-dihydroxyvitamin D₃ increased hepatocyte cytosolic calcium by 31% (p < 0.05) and ³²P-labeled lysophosphatidylinositol by 38% (p < 0.05). Pertussis toxin inhibited the hormone-induced rise in cytosolic calcium but not the increase in ³²P-labeled lysophosphatidylinositol. Exposure to exogenous lysophosphatidylinositol for 5 min increased cytosolic calcium by 40% (p < 0.05), an effect that was also inhibited by pertussis toxin. 1ß,25-Dihydroxyvitamin D₃ had no effect on either hepatocyte cytosolic calcium or ³²P-labeled lysophosphatidylinositol but prevented the 1α,25-dihydroxyvitamin D₃-induced increments. The results suggest that a G protein sensitive to pertussis toxin is required for the transduction of the lysophosphatidylinositol signal but not the generation of the signal. The ability of 1ß,25-dihydroxyvitamin D₃ to inhibit the 1α,25-dihydroxyvitamin D₃-induced changes in phospholipids suggests that the epimer may compete with 1α,25-dihydroxyvitamin D₃ for an initiating receptor.