A multicompartmental model of vitamin A kinetics in rats with marginal liver vitamin A stores

A linear, first-order, constant-coefficient multicompartmental model is presented which describes the dynamics of [3H]retinol turnover in adult rats with normal plasma retinol concentrations but low liver stores (less than 100 micrograms of retinol equivalents). To fit plasma and tissue (liver, kidney, and rest of carcass) tracer and tracee data, eight physiological compartments were required in the model: two in plasma (proposed to correspond to the retinol transport complex, and retinyl esters in plasma lipoproteins) and two each in liver, kidneys, and other extrahepatic tissues. Extensive recycling of retinol among plasma, liver, and the rest of carcass was also required. The model predicted that 44% of whole body vitamin A (143 micrograms) was in extrahepatic tissues. The vitamin A utilization rate (system disposal rate) was 6.9 micrograms of retinol equivalents/day. The system residence time (mean sojourn time) for vitamin A was 21 days, and the fractional catabolic rate for the system was 5%/day. The mean transit time (turnover time) for vitamin A in its plasma retinol transport complex was 0.078 days (1.9 hr); the residence time was 0.98 day, versus 11 days in the liver, 9 days in carcass, and 0.54 days in kidneys. The model predicted that, of the plasma turnover, 48% recycled to the liver and 52% to extrahepatic tissues. The liver retinol secretion rate was 48 micrograms/day, more than half of which was from recycled plasma retinol. Since the plasma retinol turnover rate (87 micrograms/day) was 13 times the system disposal rate, the data suggest that this is a high response system in which changes in the dynamics of recycling of retinol allow for rapid adjustment in vitamin A distribution in response to changes in nutritional, metabolic, or physiological conditions; and in which plasma retinol levels are controlled homeokinetically by changes in hepatic and extrahepatic recycling of holo retinol-binding protein.     

Changes in rat liver mitochondrial lipids in vitamin A deficiency

The alterations in the lipid profiles of rat liver mitochondria due to vitamin A deficiency were studied. The amount of total lipids and phospholipids were decreased with a concomitant increase in triglycerides and cholesterol levels in mitochondria, isolated from vitamin A-deficient animals. Of particular significance was the observation that the content of lysolecithin, a potent cytolytic agent, was increased. An analysis of individual fatty acids showed that the percentage of polyunsaturated fatty acids was decreased significantly in vitamin A deficiency. Further, mitochondria from vitamin A-deficient animals, when incubated in 0.1 M Tris-HCl buffer (pH 7.4)in vitro, produced increased amounts of malondialdehyde and lipofuchsin pigments indicating increased susceptibility of the mitochondrial membrane to peroxidative damage. These results suggest a possible role of vitamin A in the prevention of the decomposition of structural lipids.     

Studies on metabolism of vitamin A. The effect of the stage of vitamin A deficiency on sulphate activation in rat liver

1. The synthesis of ;active sulphate' in rat liver was studied at various stages of vitamin A deficiency, with the corresponding pair-fed controls. 2. The activity was significantly decreased even at the onset of the deficiency, and at the acute stage there was further loss. 3. Only at the earlier stages of the deficiency was the addition of retinol, in vitro, fully effective in restoring the lost activity; retinoic acid was partially active. No such restoration was possible at the acute deficiency stage.