Effect of age on the pharmacokinetics of antipyrine in calves.

Single doses of 15 mg kg-1 antipyrine were given intravenously to 10 female calves of the black and white breed at one, two, four, six, eight and 12 weeks of age, and the concentrations of antipyrine, 4-hydroxyantipyrine (4-OHA), 3-hydroxymethyl-antipyrine (HMA) and norantipyrine (NORA) were measured in plasma and urine by high performance liquid chromatography. The first three months of life were characterised by a steady decrease in the apparent volume of distribution (aVd) and half-life (t0.5) of antipyrine. The systemic clearance (CIs) of antipyrine per unit bodyweight increased significantly between one and 12 weeks of age. Age did not influence the excretion of HMA and NORA in urine, but the excretion of 4-OHA by 12-week-old calves was significantly greater than by one-week-old calves. There was an age-related change in the partial clearances of the antipyrine metabolites when expressed per unit bodyweight.