Ascorbic acid disposition kinetics in the plasma and tissues of calves.

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.