Recent studies have documented that 1,25 dihydroxyvitamin D3 exerts cellular effects on a variety of tissues. Rapid actions of the vitamin D metabolite on the intestine and hepatocyte raise the possibility that the mechanism is independent of genome activation, perhaps attributable to a direct interaction with membranes. 1,25 dihydroxyvitamin D3 induces the deacylation of hepatocyte membrane phosphatidylinositol resulting in increased levels of glycerophosphorylinositol and presumably arachidonic acid within 5 minutes. In addition, 1,25 dihydroxyvitamin D3 rapidly increases hepatocyte cytosolic calcium, an effect which appears to be due to the mobilization of intracellular calcium stores and appears to be mediated by a deacylation product of phosphatidylinositol. These effects of 1,25 dihydroxyvitamin D3 on phosphatidylinositol and cytosolic calcium are blocked by inhibitors of phospholipase A activity. Comparable biochemical effects of similar magnitude mediate epinephrine's action on platelets. The proposed research will determine the relationship of these rapid, presumably membrane-mediated actions of 1,25 dihydroxyvitamin D3 to the effects of the metabolite on hepatocyte 25 hydroxyvitamin D synthesis. The effects of the metabolite on glycogen phosphorylase, an enzyme activated by increased cytosolic calcium, will also be investigated. Inhibitors of phospholipase A, the metabolically inactive 1 beta, 25 dihydroxyvitamin D3, and isolated hepatic membranes will be employed to further elucidate the nature of the membrane actions of 1,25 dihydroxyvitamin D3. The actions of the vitamin on Na+/H+ exchange as a modulator of phospholipase A activity will be investigated. The specificity of the vitamin's actions on inositol phospholipids will be examined as well as the relationship of increased phospholipid deacylation to hepatocyte prostaglandin production.
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