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

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.