The function of the proximal and distal tubule was studied in the rhesus monkey during antidiuresis and during the diuresis after furosemide administration (during which extracellular fluid volume was maintained).
In the proximal tubule, fluid to plasma ratios for sodium, potassium, and osmolality approximated unity. During antidiuresis, about 30% of the filtered water remained at the end of the accessible portion of this segment (92% of length). Fluid was hypotonic to plasma throughout the distal tubule. 25% of the filtered water was present in the early distal tubule. Small but significant water reabsorption (about 8% of filtered) occurred in remainder of this segment. Tubule fluid to plasma potassium concentration ratios tended to increase along the distal tubule, and the amount of potassium, relative to the amount filtered, increased from 13% in the early portion of this segment to 26% in the late portion.
After furosemide was administered animals excreted about one-third of the filtered sodium and water. Despite this diuresis, electrolyte and water reabsorption along the proximal tubule did not differ from values obtained in control animals. Osmolality and sodium concentration of fluid from the distal tubule approached those of plasma. 22% of the filtered sodium (twice the control values) reached the distal tubule, whereas the fraction of filtered water remaining was only slightly increased. These findings indicate that, after the administration of this drug, inhibition of sodium reabsorption occurred in the water-impermeable segment of the nephron, rather than in the proximal tubule. After furosemide administration, all tubule fluid to plasma potassium concentration ratios in the distal tubule were equal to or greater than one, suggesting inhibition of active potassium reabsorption at or prior to this site.
Fluid to plasma bicarbonate concentration ratios from the midportion of the proximal tubule were consistently less than one in normal monkeys. After acetazolamide was administered, the bicarbonate concentration of samples of tubule fluid recollected from these same sites was the same as, or higher than in plasma. This fact demonstrates the inhibition of bicarbonate reabsorption in this portion of the tubule.