Gestational changes in renal responsiveness to cortisol in the ovine fetus

The ontogenetic renal responsiveness to exogenous cortisol was examined in the chronically cannulated ovine fetus. The contribution of effects at proximal and distal tubule of the kidney were studied also. Cortisol (81.5 micrograms/h) was infused into immature ovine fetuses (mean gestational age -113.9 days) on five occasions and increased blood cortisol from 0.8 +/- 0.5 to 21.3 +/- 6.2 nmol/liter. This dose of cortisol produced a highly significant diuresis and natriuresis, in part due to an increase in GFR and in part due to a significant decrease in proximal tubular reabsorption of sodium. Cortisol (107.2 +/- 4.7 micrograms/h) was infused into mature fetuses (mean gestational age 133.4 days) and produced an increase in blood cortisol concentration from 11.4 +/- 5.6 to 33.7 +/- 6.8 nmol/liter. No natriuresis or diuresis was seen in the mature fetuses. Cortisol caused a significant depression of proximal tubular sodium reabsorption in mature fetuses, but this extra load was reabsorbed in the distal tubule in these fetuses. The inability of the premature or very low birth wt baby to maintain normal sodium balance on a standard salt intake may be due, at least in part, to a "fetal" renal response to the high plasma cortisol concentrations found in such babies. As the kidney matures it becomes capable of increasing distal tubular sodium reabsorption to compensate for any increased distal tubular fluid delivery.