Use of Model-Based Compartmental Analysis to Study Effects of 2,3,7,8-Tetrachlorodibenzo-p-dioxin on Vitamin A Kinetics in Rats

Abstract

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a highly toxic, widespread environmental contaminant that has dramatic adverse effects on the metabolism of vitamin A. We used model-based compartmental analysis to investigate sites and quantitative impacts of TCDD on vitamin A kinetics in rats given on oral loading dose of TCDD in oil (3.5 micrograms/kg) followed by weekly maintenance doses (0.7 microgram/kg) or oil only. [3H]Retinol in its plasma transport complex (experiment 1) of lymph containing chylomicrons labeled mainly with [3H]retinyl esters (experiment 2) were administered i.v., and tracer kinetics in plasma, liver, carcass, urine, and feces were measured for up to 42 days. TCDD treatment caused significant reductions in liver vitamin A levels and significant changes in tracer kinetics and tracer excretion. A four-compartment model was used to fit tracer data for experiment 1; for experiment 2, compartments were added to describe the metabolism of newly absorbed vitamin A. The compartmental models predict that TCDD caused a slight delay in plasma clearance (via an increased recycling to plasma), and in liver processing, of chylomicron-derived vitamin A. Models for both experiments predict that TCDD exposure did not affect the fractional uptake of plasma retinol from the rapidly turning-over extravascular pool, but it doubled the fractional transfer of recycled retinol from slowly turning-over pools of vitamin A to plasma. The residence time for vitamin A was reduced by 70% in TCDD-treated rats, transfer into urine and feces was tripled, and vitamin A utilization rates were significantly increased. Since our results do not indicate that retinol esterification is inhibited, we hypothesize that some of the significant effects of TCDD on vitamin A metabolism result from increased catabolism and mobilization of vitamin A from slowly turning-over pools (especially the liver).

Model-based compartmental analysis indicates a reduced mobilization of hepatic vitamin A during inflammation in rats

“…Aliquots of freeze-dried livers were saponified using a modified method described previously (16). Triplicate weighed aliquots of liver (?0.15 g) were combined with 1.0 ml of ethanol containing 0.1% pyrogallol as an antioxidant.…”

“…In previous studies, model-based compartmental analysis has been fruitfully applied to describe and quantify the dynamics of whole-body VA metabolism and to understand the impact of various treatments on VA kinetics (12)(13)(14)(15)(16)(17). A major observation from this type of analysis has been the extensive recycling of VA (i.e., retinol and retinyl esters) before it is irreversibly lost (18).…”

Tissue distribution of retinoids in common dolphins Delphinus delphis

“…This indicates that the pollutants mobilize retinoid storage forms, which is followed by an increase in their degradation and renal elimination through the urine (Kelley et al 1998(Kelley et al , 2000. Study on the disruption of retinoids induced by pollutants in tissues other than plasma in marine mammals is restricted to the work of Nyman et al (2003).…”

Vitamin A Kinetics in Neonatal Rats vs. Adult Rats: Comparisons from Model-Based Compartmental Analysis,

“…The methods of tracer kinetics and model-based compartmental analysis are applied to VA kinetic data in adults for >30 y (15,18,19,22,32,33). These studies contribute much to our understanding of VA metabolism and kinetics in adult animals and humans, including prediction of extensive recycling of retinol among plasma, liver, and extrahepatic tissues before its irreversible loss and estimates of the transit, turnover, storage, and utilization of retinol (19,22,(33)(34)(35)(36).…”