Sixty clinically well infants of various gestational ages (27 to 40 weeks) were studied from 24-40 hours after birth to evaluate glomerular filtration rate and renal excretion rate of sodium at various stages of fetal maturation. Creatinine clearance was directly related to gestational as (r = 0.643). Fractional sodium excretion was inversely related to gestational age (r = -0.755). The renal functions of small for gestational age infants were similar to those of full-term infants whose birth weights were appropriate for gestational age. The data showed that the glomerular functions of an infant below 32 weeks of gestation were more predominant than the tubular function resulting in a greater fractional sodium excretion rate and higher urinary Na loss in infants of this gestational age, when compared with the more mature infants.

SummaryThirteen chronic fetal lamb preparations between 95 and 142 days of gestation (term 145-150 days), and 10 newborn lambs were studied before and after the acute (1-2 min) infusion of furosemide (2 mg/kg). The baseline to peak plasma renin activity (PRA) response to furosemide increased from A 3.0 & 1.3 ng/ml/hr (M and SEM) and 95-106 days of gestation to A 18.4 +: 4.0 ( P < 0.01) at 123-142 days and A 33.6 + 6.5 (P < 0.001) in the newborn.Baseline plasma aldosterone concentrations were similar in the fetus and pregnant ewe; aldosterone levels were higher in the newborn lamb than in the nonpregnant ewe. The newborn plasma aldosterone response to furosemide via the endogenous reninangiotensin system was A 17.1 2 4.2 ng/dl ( P < 0.01); the fetal lamb plasma aldosterone level did not increase. The results indicate that the renin-angiotensin system cannot be stimulated by furosemide under 106 days of gestation; the response after 110 days increases with gestational age. Aldosterone concentrations in the fetal lamb are probably maintained primarily by the pregnant ewe and do not increase in response to endogenous renin stimulation as in the newborn. SpeculationFetal renin responsiveness increases with gestational age, while fetal aldosterone secretion is unresponsive to stimulation by the renin-angiotensin system. Therefore, the fetal adrenal cortex is the limiting factor in the pituitary-renal-adrenal axis before term.The newborn lamb has high baseline PRA and aldosterone concentrations (15), and the renin-angiotensin-aldosterone system responds promptly to stimulation by furosemide in the newborn lamb. The renin-angiotensin system in the chronic fetal lamb preparation after 110 days of gestation responds to stimulation by furosemide (18) blood volume reduction (13, 17) or aortic constriction (17). Alexander et al. (1) were unable to increase fetal plasma aldosterone levels in three acute fetal sheep preparations by ACTH or exogenous angiotensin I1 infusion and were unable to depress plasma aldosterone with dexamethasone. Although the acute fetal sheep preparation is stressed by surgery, these results suggest that the fetal lamb adrenal gland is unresponsive to aldosterone stimulation or suppression, while the kidney is capable of renin secretion. The present study was conducted to determine in the chronic nonstressed fetal sheep preparation 1) the pattern of baseline PRA and aldosterone levels during the last trimester, and 2) when the system components will respond to stimulation. MATERIALS AND METHODSThirteen chronically catheterized fetal lamb preparations between 95 days of gestation and term and 10 newborn lambs less than 1 wk of age were studied. Pregnant ewes were obtained from a local source and maintained in the vivarium before surgery. Rompun (0.1-0.2 ml), a tranquilizer, was administered to the ewe 9 before uterotomy and fetal catheterization; epidural anesthesia (5 ml of 2% xylocaine) was employed for the surgical procedure. The uterus was exposed through a small midline abdominal incision and the ...

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