Ascorbic acid disposition kinetics in the plasma and tissues of calves

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Abstract
Kinetic plasma disposition parameters and tissue distribution of ascorbic acid (AA) and dihydroascorbic acid (DHA) were determined in newborn calves. After a radiolabeled AA intravenous administration, the plasma clearance (Cl) was low (40.8 ± 9.5 ml ⋅ kg−1 ⋅ h−1), the steady-state volume of distribution (Vss) was very high (8.9 ± 2.2 l/kg), and the AA mean residence time (MRT) was long (230 ± 85 h). After administration of a 3-g dose of AA, the Cl was high (450 ± 146 ml ⋅ kg−1 ⋅ h−1), the Vss was low (0.658 ± 0.236 l/kg), and the MRT was short (1.49 ± 0.41 h), indicating a strong nonlinearity of AA disposition in calves and the impossibility of preventing scurvy with the use of a loading AA dose. Nonlinearity was explained by the saturation of both kidney reabsorption and tissue uptake. The estimated AA body pool size was 23.1 ± 6.8 mg/kg. On the basis of a compartmental analysis and actual tissue concentration measurements, it is suggested that the lung (19% of the pool) constitutes a low-capacity but rapidly mobilized pool able to cover an acute need for AA, whereas muscle and liver (40 and 33% of the pool, respectively) are high-capacity AA pools, but slowly mobilized and involved in covering the calf’s long-term AA requirements. The average daily AA entry rate over the first 7 days of life was 3.43 ± 1.16 mg/kg, and it is suggested that the calf is able to synthesize AA at an early stage.