EFFECTS OF CHANGES IN COLLOID OSMOTIC-PRESSURE ON EXCRETION OF SODIUMBY THE OVINE FETAL KIDNEY

To find out if the gestation-dependent increase In fetal oncotic press ure is responsible for the gestation-dependent increase in the capacit y of the fetal proximal tubule to reabsorb sodium, the effects on rena l function of increases in oncotic pressure were studied in 8 volume-e xpanded chronically catheterized fetal sheep aged 128+/-3 (s.e.) days. Fetal extracellular volume was expanded by infusion of 65+/-10.8 (s.e .) mL kg(-1) estimated body weight of 0.15 M saline. This caused a dec rease in fetal plasma protein concentrations (P < 0.01); fetal oncotic pressure decreased (P < 0.05). A diuresis and natriuresis occurred, w hich was due not to an increase in glomerular filtration rate but to a decrease in the fraction of the filtered sodium load reabsorbed by th e proximal tubule (P < 0.05) and a decrease in the fraction of distall y delivered sodium reabsorbed (P < 0.01). Fetal plasma protein concent rations were then increased to greater than control levels (P < 0.01) by infusion of maternal plasma (28+/-1.6mL kg(-1)); oncotic pressure w as greater than after saline expansion (P < 0.05) and similar to contr ol. The fraction of the filtered sodium load reabsorbed by the proxima l tubule remained depressed (P < 0.01) relative to control, as did the fraction of distally delivered sodium that was reabsorbed (P < 0.01). Thus the natriuresis and diuresis continued. There was, however, a sm all effect of oncotic pressure on proximal fractional sodium reabsorpt ion that was unmasked by multiple regression analysis. Obviously, this effect was not sufficient to override other effects of volume expansi on on fetal proximal tubular function. Therefore, the reduction in fet al proximal fractional sodium reabsorption in volume expansion was not due solely to a fall in fetal oncotic pressure. Furthermore, since in fusion of maternal plasma caused a rise in fetal plasma protein concen trations that was similar to the increase that would occur between 128 and 148 days gestation, it is unlikely that any gestation-dependent i ncrease in proximal fractional sodium reabsorption is due solely to th e increase in fetal plasma protein concentrations and hence oncotic pr essure.