Osmolality, bicarbonate concentration, and water reabsorption in proximal tubule of the dog nephron

Abstract

Micropuncture and microanalytical techniques were used to study the effect of antidiuresis and water diuresis on osmolality, bicarbonate concentration, and water reabsorption in the proximal tubule of the dog nephron. Samples collected during antidiuresis and water diuresis remained isotonic to plasma throughout the first 50% of the proximal convoluted tubule. Mean bicarbonate concentrations of 16 mEq/liter and 17 mEq/liter were found in the middle third of the tubule during antidiuresis and water diuresis, respectively. These values were slightly less than the plasma concentration of 22 mEq/liter. Proximal tubular fluid samples for inulin concentration were collected during antidiuresis, water diuresis, and during vasopressin infusion in water-loaded dogs. A mean tubular fluid to plasma (TF/P) inulin ratio of 2.3 was found in the middle third of the proximal tubule during antidiuresis. This value is significantly different ( P < 0.01) from a mean of 1.6 in the same portion of the tubule during water diuresis. Vasopressin administration to hydrated dogs returned the TF/P inulin ratio in the middle third of the proximal tubule to 2.0. These results suggest that vasopressin stimulated Na reabsorption in the proximal tubule of the dog nephron.

Factors modifying renal tubular bicarbonate reabsorption in the dog

“…Therefore, despite the fact that some fresh, more alkaline glomerular filtrate continues to enter the proximal convolutions ofoccluded nephrons (Taylor & Ullmann, 1961), the reduction in HC03-concentration must have been achieved while the fluid was arrested in the proximal tubules, and cannot have been due to distal tubular activity during the final passage of the proximal contents through the terminal part of the system. This demonstrates that, in the dog, proximal as well as distal intratubular HC03-concentration can fall below the plasma level, and is in accord with the results calculated by Clapp, Watson & Berliner (1963) from pH measurements on proximal fluid obtained by micropuncture.…”

“…Proximal tubular fluid was found to be more acid than the plasma in most recent micropuncture studies, but these were carried out on animals whose plasma HC08-level was 24 m-equiv/l., or less (Gottschalk et al 1960; 9ELISABETH ULLMATN Giebisch et al 1960;Litchfield & Bott, 1962;Clapp et al 1963). In the present work, by contrast, the dogs' plasma HC03-was raised above 24 mequiv/l., and a drop of 'proximal' pH and HC03-concentration below the plasma level was only seen if water reabsorption from the tubules was restricted by mannitol.…”

Mechanism of glomerulotubular balance. I. Effect of aortic constriction and elevated ureteropelvic pressure on glomerular filtration rate, fractional reabsorption, transit time, and tubular size in the proximal tubule of the rat.

“…This, however, does not occur. In fact, the only available micropuncture data (30) comparing proximal reabsorption in water diuresis and antidiuresis suggest that water diuresis (and presumably distal distension) depresses rather than enhances proximal reabsorption. It seems unlikely, therefore, that glomerulotubular balance is mediated by an internal feedback system.…”

Potassium reabsorption in the proximal tubule of the dog nephron.

“…Polyethylene catheters were also inserted into the foreleg veins and the femoral vein or artery for the administration of fluids and collection of blood. The left kidney (experimental) was exposed through a flank incision and prepared for micropuncture as previously reported (6). Methods of tubular fluid collection and identification were also similar to those previously reported (6).…”

“…In these experiments, tubular fluid samples were collected for the determination of bicarbonate concentration as well as potassium and inulin. The tubule was entered first with a quinhydrone microelectrode, and 0.005 to 0.001 1,l of tubular fluid was collected directly into the electrode for the determination of pH as previously described (6). The tubule was then reentered through the same puncture site, and at least 0.05 4Al of tubular fluid was collected for the determination of potassium and inulin concentrations.…”