Sodium transport inhibitor in proximal tubular urine during acute volume expansion.

Harvested proximal tubular fluid from mannitolsaline expanded rats caused a 50% inhibition of transepithelial sodium concentration difference when compared to an artificially prepared test solution used in the same and nonexpanded animals. Because of the methodology employed, none of the usual factors known to affect sodium reabsorption by the kidney could have been responsible for these changes. The factor responsible acts from the luminal side, is an inhibitor and has a short duration of action.