Effects of furosemide on the renal functions of the unanesthetized newborn rat

In neonates pharmacokinetics of furosemide is very slow, as compared to adult individuals, because it is eliminated through glomerular filtration and tubular secretion, mechanisms that are not fully developed in the newborn mammal. In addition, Henle's loop, the main site of action of this diuretic, is shorter in the neonate than in the adult animal. The aim of this study was to measure the response to furosemide in the unanesthetized newborn rat and to compare it with that of the adult. Furosemide (0.5, 1, 1.5, 2, 5 or 10 mg/kg body weight. i.p.) or vehicle were administered to newborn and adult rats and the effects on sodium, potassium and water balance were assessed. Despite the physiological characteristics of the neonatal kidney, furosemide induced a more marked sodium excretion and decrease in free water clearance in the newborn than in the adult rat. In contrast, urinary potassium losses elicited by furosemide were higher in the adult than in the newborn rat at similar doses. At both ages, the effects of the diuretic were dose-dependent. Plasma sodium decreased, whereas plasma potassium and hematocrit increased in the newborn rats, after the highest doses of furosemide. In the adult rat changes were less marked. Our results suggest that the neonatal kidney is more sensitive to equivalent doses of furosemide than the adult kidney.