The change of disposition kinetics for creatinine and urea accompanied by growth in mice

In order to clarify the change of disposition kinetics accompanied by growth, the whole-blood levels and whole-body autoradiography following intravenous administration of creatinine and urea, which are considered to pass through the water-filled pores of biological membranes easily, were investigated in 1-day-old, 1-week-old, 3-week-old and 8-week-old mice. For both creatinine and urea, the total body clearance was considerably lower in 1-day-old and 1-week-old mice than in more aged mice. Relatively lower renal clearance (i.e. rather incomplete renal function) in 1-day-old and 1-week-old mice was considered to be the major reason for this observation. Moreover, the lower metabolic clearance also seemed to be the cause in the case of urea, because the expiratory excretion of 14CO2 following the intravenous administration of 14C-urea was almost negligible in 1-day-old and 1-week-old mice in contrast to more aged (16-21%). The whole-body autoradiograms obtained following intravenous administration of 14C-creatinine indicated that creatinine was more rapidly transferred from blood to muscle in 1-day-old and 1-week-old mice, especially in 1-day-old mice, than in more aged mice. This might be caused by the difference in muscular blood flow rate or muscular cell membrane permeability to creatinine.