Influence of vitamin D3 status, phenobarbital, and diphenylhydantoin treatment on the plasma 25-hydroxyvitamin D3 concentrations in the rat

Effect of theophylline on calcium metabolism and circulating vitamin D metabolites

Theophylline has been shown to induce the hepatic microsomal enzyme system. These same enzymes increase the metabolism of vitamin D and 25-hydroxyvitamin D when induced by chronic barbiturate or phenytoin administration. To assess the long-term effects of theophylline on vitamin D and calcium metabolism, young rats were treated for 4 weeks with constant subcutaneous theophylline infusions. Theophylline-treated animals had a significantly increased urinary calcium excretion (p less than 0.0001), a significantly decreased total body calcium per gram body weight (p less than 0.05), and significantly decreased serum 25-hydroxy-vitamin D concentrations (p less than 0.002) when compared to control animals. These alterations in the concentration of 25-hydroxyvitamin D may impair the ability to increase 1,25-dihydroxyvitamin D-dependent intestinal calcium absorption to compensate for excessive urinary calcium losses. These data suggest that theophylline promotes skeletal calcium loss, and its use may be a risk factor for the development of osteopenia in humans.

Hepatic handling of vitamin D3 in micronodular cirrhosis: a structure-function study in the rat

The response to vitamin D3 (D3) was studied in a model of micronodular cirrhosis induced by CCl4. The uptake and C-25 hydroxylation of D3 were then studied in isolated-perfused liver preparations. CCl4-treated rats had a significantly lower fractional hepatic D3 uptake than controls; they also had lower 25-hydroxyvitamin D3 (25(OH)D3) concentrations in both liver and perfusate following 150 min of perfusion. CCl4 induced a wide spectrum of hepatic morphologic changes ranging from mild to large collagen infiltration, but micronodular cirrhosis was present in more than 90% of the animals. Histomorphometric analysis of the liver indicated an overall highly significant increase in the volume density (Vv) of collagen infiltration, and a reduction in the Vv normal hepatocytes following CCl4. Linear relationships were also observed between the Vv normal hepatocytes and the liver, perfusate, and total 25(OH)D3, while the 25(OH)D3 production decreased in a logarithmic fashion as the collagen infiltration of the liver parenchyma increased. These data show that the overall production of 25(OH)D3 is decreased in micronodular cirrhosis; they also indicate, however, that the D3-25 hydroxylase seems to stay unimpaired in the remaining hepatocytes of the diseased liver, and that the Vv normal hepatocytes constitute one of the major determinants of the 25(OH)D3 production by the cirrhotic rat liver.

Effect of increasing doses of phenytoin on the plasma 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D concentrations

The circulating 25-hydroxyvitamin D [25(OH)D] and 1,25-dihydroxyvitamin D [1,25-(OH)2D] concentrations were studied in a patient receiving increasing doses of phenytoin. The plasma 1,25(OH)2D concentrations were independent of the dose of phenytoin administered, as well as of the drug plasma concentrations. The plasma 25(OH)D concentrations were, on the other hand, increased by low phenytoin concentration but rapidly declined when the dose of phenytoin was increased and/or as the length of time of exposure to the drug increased. A linear relationship (R = 0.9651, P less than 0.05) was found between the plasma 25(OH)D concentrations and the dose/plasma phenytoin concentration ratio, suggesting that chronic phenytoin administration may have a dose-related effect on the circulating 25(OH)D concentrations.

Interrelationships between vitamin D3 and 25-hydroxyvitamin D3 during chronic ethanol administration in the rat

Vitamin D [D] depleted female Sprague-Dawley rats were fed for a period of 4 wk a D deficient diet containing 36% of total calories as ethanol while control animals received an isocaloric regimen where ethanol was substituted for by dextrins. In conjunction with the ethanol feeding 92 I.U. of [14C]-vitamin D3 [(14C)-D3] were administered by intragastric gavage 3 times 1 wk for 3 2/3 wk. At the end of the experiment, [14C]-D3 and [14C]-25-hydroxyvitamin D3 [(14C)-25(OH)D3] concentrations were analyzed in plasma, liver, striated muscle and adipose tissue. Body reserves in unchanged [14C]-D3 were significantly reduced by ethanol treatment as seen by 24%, 26%, and 59% lower plasma (p less than 0.02), muscle (p less than 0.001) and adipose tissue (p less than 0.001) [14C]-D3 concentrations in ethanol-treated compared to control rats. In contrast total plasma and liver [14C]-25(OH)D3 content were increased by 30% (p less than 0.05) and 55% (p less than 0.001) respectively. This increased liver and plasma [14C]-25(OH)D3 following ethanol treatment was not accompanied by a proportional [14C]-25(OH)D3 incorporation into muscle and adipose tissue. These results suggest that during steady state conditions 25(OH)D3 production is increased during chronic ethanol administration while the body pool in unchanged D3 is significantly lowered. These results also point out that in the rat plasma 25(OH)D concentrations are not a reliable guide for the determination of vitamin D status during chronic ethanol administration.