Role of aldosterone on renal sodium and potassium excretion during fetal life and newborn period.

The renal response to endogenous aldosterone production at different times during gestation and immediately after birth, as well as factors controlling aldosterone secretion during fetal life have been studied in 34 lamb fetuses (106-142 days of gestation, term 145 days) and in 6 newborn lambs (3-8 days postnatal age). It was demonstrated that the increase in fetal plasma aldosterone concentrations correlated with the increase in fetal plasma renin activity (PRA) (r = 0.67, p < 0.001). Moreover, it was fond that the slope of the regression line between plasma aldosterone and PRA during fetal life was significantly steeper (p < 0.05) than after birth. Significant correlation coefficients were found between the fetal urinary to plasma ratios (U/P) of sodium over potassium [(U/P)Na+/(U/P)K+] and either fetal plasma aldosterone (r = -0.53, p < 0.005) or fetal age (r = -0.52, p < 0.005). Moreover, in 7 out of 9 fetuses over 125 days, frank secretion of potassium by the fetal tubular cells was demonstrated; such a secretion was present in only 1 of the 15 fetuses between 101 and 125 days of gestation. Finally, it was demonstrated that the increase in mean arterial blood pressure during fetal life and after birth is not a major factor in the increase in fetal glomerular filtration rate during gestation or in the adaptation of glomerular filtration rate to extrauterine life.