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-hydroxymethylantipyrine (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 (aV(d)) and half-life (t(0.5)) of antipyrine. The systemic clearance (Cl-5) 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.