Influence of fetal extracellular volume contraction on renal reabsorption of bicarbonate in fetal lambs.

In order to determine the renal threshold for bicarbonate in the fetal lamb and factors that may influence renal reabsorption of bicarbonate in the fetus, 18 acute fetal lamb preparations (110-140 days of gestation) were studied. In the first series of experiments involving nine fetuses, the renal bicarbonate threshold of normal fetuses varied from 12.0 to 23.5 mM/liter with a mean value of 17.7 +/- 1.37 mM/liter. This is significantly lower (P less than 0.005) than the value measured in five adult sheep of 28.7 +/- 1.68 mM/liter. There was a significant and positive correlation between the fetal plasma bicarbonate at threshold level and the fetal kidney weight, as well as fetal age. In a second series of experiments, the excretion of bicarbonate and sodium was studied in nine fetuses before and after dehydration by peritoneal dialysis. After peritoneal dialysis there was a significant decrease in urinary pH (P less than 0.025), bicarbonate excretion (P less than 0.001), sodium excretion (P less than 0.001), fractional excretion of sodium (P less than 0.01). When glucose was replaced by mannitol in the peritoneal dialysis fluid the effects on bicarbonate reabsorption paralleled those when glucose was present in dialysis fluid. It was also shown that when glucose was given intravenously to the fetus, up to a plasma concentration of 200 mg/100 ml, there was no effect on the fetal renal reabsorption of bicarbonate. These data indicate that the low threshold for bicarbonate reabsorption by the fetal kidney is not due to a limited capacity to increase bicarbonate or sodium reabsorption and suggest that the fetal kidney is able to respond to volume depletion by increasing its reabsorption in bicarbonate and electrolytes.