Effect of intraluminal bicarbonate and chloride on fluid absorption by the rat renal proximal tubule

How bicarbonate loading inhibits tubular reabsorption of NaCl in dog kidneys

During continuous infusion of ethacrynic acid in dogs, changes in glomerular filtration rate (GFR) and PCO2 at constant plasma bicarbonate concentration (PHCO3) alter bicarbonate and chloride reabsorption in a ratio of 1:2. This ratio did not apply when PHCO3 was raised by bicarbonate loading in 11 anaesthetized volume-expanded dogs. A rise in PHCO3 from 30 to 54 mM at constant PCO2 and GFR reduced sodium reabsorption during ethacrynic acid infusion from 3586 +/- 725 to 2449 +/- 403 mumol min-1. Bicarbonate and chloride reabsorption were reduced in a ratio of 1:10. When plasma pH was restored from 7.8 to 7.5 by raising PCO2, the inhibitory effect on chloride reabsorption was halved. At constant plasma pH 7.5 a rise in PHCO3 from 20 to 30 mM reduced chloride reabsorption by 20%. A further 30% inhibition was caused by raising PHCO3 from 30 to 54 mM. Bicarbonate reabsorption was highest at PHCO3 54 mM, suggesting a large capacity for bicarbonate reabsorption if PHCO3 is raised at constant plasma pH 7.5. Water and NaCl reabsorption remaining during ethacrynic acid infusion is almost equally inhibited by alkalosis and by an osmotic effect of unreabsorbed NaHCO3.

Evidence for an osmotic effect of glucose in the in vivo rat proximal tubule

A series of micropuncture experiments were performed to assess whether phlorizin inhibition of proximal tubular glucose reabsorption inhibits proximal tubular fluid reabsorption due to a change in the transepithelial osmotic gradient for glucose. End proximal tubular filtrate reabsorption was measured, control vs experimental, in the same rat. The effects of infusing phlorizin in one group were compared to those of graded small infusions of mannitol in a second group of rats. In the phlorizin series tubular fluid and peripheral plasma were analyzed for glucose. Net renal sodium and renal oxygen consumption were measured in these same rats. In the mannitol series tubular fluid and peripheral plasma mannitol concentrations were determined. The results show that phlorizin inhibits proximal tubular fluid and glucose reabsorption without changing renal oxygen consumption. Mannitol inhibits proximal fluid reabsorption per unit transepithelial concentration difference to the same extent as does glucose during phlorizin infusion. We interpret these data as most consistent with the conclusion that the reduction in proximal tubular fluid reabsorption during phlorizin infusion is primarily due to the resultant change in the effective transepithelial osmotic gradient for glucose. Normally this gradient favors net fluid reabsorption. During phlorizin it favors retention of fluid in the proximal tubular lumen.

Acute renal failure--an integrative discussion of morphologic and functional findings

The ultrastructural alterations at the nephron established in animal experiments, were also confirmed, by means of an electron-microscopic examination, in eight cases of human acute renal failure (ARF). Special consideration was given in this study to single cell alterations, particularly in proximal tubular cells, with emphasis being placed on alterations due to single cell damage in the region of the renal fluid compartments. The ultrastructural alterations of the tubular cells in ARF, suggest serious impairment of the cellular capacity for electrolyte transport and metabolic processes. The shunt paths between the tubular fluid compartment and the functional interstitium, arising from necrosis of the tubular cells or dissolution of the gap or tight junctions, were discussed in terms of their significance for the directional, active transport processes of the tubular cells for sodium chloride and the passive water flow. The morphologic findings were reviewed in light of recent findings on cellular membrane processes and electrolyte transport. A reinterpretation of the morphologic and functional findings in ARF is suggested. This takes into consideration single cell function and the integrity of the renal fluid compartments.

Correlation between fluid reabsorption and proximal tubule ultrastructure during development of the rat kidney

Parallel functional and ultrastructural studies were performed in maturing rats in order to elucidate factors determining the development of proximal tubular fluid reabsorption. Three groups of hydropenic animals, which were 22 to 24, 28 to 32 and 40 to 45 days old, were studied. Nephron function was evaluated at the single nephron level by micropuncture technique. The ultrastructure of the developing proximal tubules was analysed by morphometric techniques following fixation of single nephrons. Kidney weight, proximal convoluted tubule length and diameter increased during postnatal development. SNGFR increased from 2.98 to 8.57 and to 20.5 nl/min in respective group of rats whereas proximal tubular fluid reabsorption Jv (a) increased from 0.15 to 0.22 and 0.34 micron3.micron-2.s-1. Parallel to the functional development the relative area of lateral and basal cell membrane increased, resulting in a constant relationship between net fluid reabsorption and the lateral and basal cell membrane area during the fourth postnatal week and then only a slight increase in this relation during the further development. The results suggest that net fluid transport during hydropenia is determined by the amount of available lateral and basal cell membranes where the transporting enzyme for sodium is located.

Comparison of chloride concentration and osmolality in proximal tubular fluid, peritubular capillary plasma and systemic plasma in the rat

1. Chloride concentration and osmolalities were compared in consecutively collected samples of proximal tubular fluid, peritubular capillary plasma and systemic plasma. 2. Mean chloride concentrations (m-mole/l) were 141.3+/-2.6 in tubular fluid, 114.8+/-1.7 in peritubular capillary plasma and 119.4+/-1.8 in systemic plasma. 3. Mean osmolalities (m-osmole/kg H2O) were 297+/-2.2 in tubular fluid, 293+/-2.4 in peritubular capillary plasma and 299+/-1.8 in systemic plasma. 4. These differences are discussed in relation to the anatomical and functional organization of the peritubular capillaries and renal tubules.