Early Kinetic Studies Of Vitamin A Metabolism

about 26 days for the labeled dose to equilibrate with the total body vitamin A pool and that body pool sizes ranged from 1.10 to 3.07 mmol. They found that vitamin A utilization rate decreased during depletion, providing early support for the idea that the vitamin is conserved in the face of low vitamin A intake. Adding to this idea, Hicks et al. (1984) later reported that excretion of labeled vitamin A metabolites into bile of rats fed increasing levels of vitamin A and dosed with [3H]retinyl acetate was constant when liver vitamin A levels were low (up to 112 nmol/g) and then increased rapidly (by eightfold) to a plateau at 490 nmol/g.

Autopsy studies in the late 1960s and early 1970s (reviewed by Sauberlich et al., 1974) documented the importance of the liver in vitamin A storage, and thus indirect methods for estimating liver vitamin A were sought. Rietz et al. (1973, 1974) developed an isotopic method for estimating the body vitamin A pool in rats. Rats were given an intravenous dose of [3H]vitamin A, and specific radioactivity (cpm/IU vitamin A) in plasma, and radioactivity in liver, was determined. Assuming that the dose had equilibrated with body vitamin A pools, liver vitamin A was calculated as radioactivity in liver divided by plasma vitamin A specific activity. Estimates compared well with fluorometric determinations of liver vitamin A. Over the years, isotope dilution methods for estimating vitamin A stores have been further developed. Results from modern isotope dilution studies, in which plasma data are collected following an oral dose of stable isotope-labeled vitamin A, are providing important information about vitamin A status in various populations and the efficacy of vitamin A interventions in improving vitamin A status (for review see Furr et al., 2005).

The work of Rietz et al. (1973) corroborated the conclusions of Sewell et al. (1967), which indicated that vitamin A stores are in a dynamic state. In Sewell et al.'s study, rats received an oral dose of tritium-labeled retinyl acetate. Feces and urine were collected for 5 days, and blood, liver, and kidneys were obtained at the time of killing (5-45 days after dosing). When the log of liver radioactivity was plotted against time, the decline followed a single exponential with a turnover time of 82 days. Since liver vitamin A concentrations remained constant while radioactivity decreased, the authors suggested that there is a dynamic exchange of vitamin A among blood, liver, and vitamin A-requiring tissues. As discussed subsequently, this is a feature of the vitamin A system that has been documented and quantified by modeling.

Sundaresan (1977) studied the rate of metabolism of retinol in vitamin A-deficient rats dosed intermittently with RA. [14C]Retinol was injected intraperitoneally, and radioactivity in urine and feces was monitored for 10 weeks. Blood and livers were also collected at the time of killing. A plot of the log of daily urinary radioactivity versus time after dosing showed three phases which Sundaresan interpreted to indicate (1) initial rapid metabolism and excretion of newly absorbed vitamin A, (2) normal physiological metabolism, and (3) vitamin A metabolism after liver reserves were exhausted. In conjunction with work by Carney et al. (1976) in vitamin A-depleted rats, this study provides early support for the existence of two distinct pools of vitamin A in the liver: one includes newly absorbed vitamin A and is small and rapidly turning-over, the other contains stored vitamin A (Anonymous, 1977).

In the early 1980s, our laboratory collaborated with Underwood's group in a study designed to use kinetics to estimate vitamin A disposal rate (utilization rate) in rats (Lewis et al., 1981). Control and vitamin A-deficient rats received a dose of [3H]retinol-labeled plasma, and tritium kinetics in plasma were monitored for 48 h. Data on plasma tracer concentration versus time after dosing were plotted semi-logarithmically but did not follow the expected single exponential (i.e., a straight line), indicating either that plasma retinol was kinetically heterogeneous or that retinol was recycling to plasma before irreversible utilization. This work led both to the realization that vitamin A metabolism is more complex than had been previously believed and to further studies in our laboratory which used kinetic methods and compartmental analysis to describe and quantitate whole-body and organ-level vitamin A dynamics. Before reviewing those studies, we will present some background on compartmental analysis.

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