Renal proximal tubular buffer-(glycodiazine) transport. Inhomogeneity of local transport rate, dependence on sodium, effect of inhibitors and chronic adaptation

Phosphate transport in the proximal convolution of the rat kidney. III. Effect of extracellular and intracellular pH

Inorganic phosphate (Pi) transport was evaluated using the standing droplet method with simultaneous microperfusion of the peritubular capillaries. To evaluate rather small differences in Pi transport and to eliminate the influence of tubular heterogeneity, the technique of crossed paired samples was applied. 1. In chronic PTX rats changing the luminal or both luminal and peritubular pH by varying the HCO-3-concentration between 4 and 50 mmol/l at constant 5% CO2 had no influence on Pi transport. 2. If, however, bicarbonate was omitted from the perfusate and 2 mmol/l phosphate (pH 7.4) was the only buffer, Pi transport was decreased from the control. It was, however, further reduced when the perfusates were gased with 5% CO2 i.e. the starting pH was 5.6. 3. When the solutions contained HEPES buffer (25 mmol/l), Pi transport at pH 8 was much larger than at pH 6.0. 4. Raising the CO2 pressure from 35 to 70 mm Hg did not change the Pi transport when both perfusates had a HCO-3-concentration of 25 mmol/l. It reduced, however, the Pi transport, when the luminal perfusate had only 4 mmol/l bicarbonate. 5. Lowering the CO2 pressure from 38 to 7.6 mm Hg did hardly change the Pi transport when the luminal perfusate contained 4 mmol/l bicarbonate. It lowered, however, the Pi transport significantly when the luminal perfusate had 2k mmol/l bicarbonate. 6. Acetazolamide, 10-4M, lowered the Pi transport when the luminal perfusate contained 4 or 25 mmol/l bicarbonate. At 4 mmol/l luminal HCO-3, raising the pCO2 to 228 mmol/l depressed Pi transport even more. At 25 mmol/l luminal bicarbonate, raising the pCO2 from 38 to 114 mm Hg reversed the acetazolamide inhibition of the Pi transport almost completely. The data indicate that luminal acidosis and intracellular alkalosis inhibits the transtubular Pi transport. A shift of the intracellular pH to a more alkaline value seems to be responsible for the inhibition of Pi transport by acetazolamide, while omission of buffer from the perfusate inhibits Pi transport by effecting an acidic luminal pH.

Importance of bicarbonate in bile salt independent fraction of bile flow

The bile salt independent fraction (BSIF) of canalicular bile flow from the isolated rat liver perfused with bicarbonate-free perfusate is 50% of that from the liver perfused with bicarbonate-containing perfusate. HCO3-excretion is nearly eliminated and Na+ and Cl- excretion is reduced 50%. Replacement of HCO3- into perfusate increased bile flow by 0.3 microliter/g.min without changing bile acid excretion rate. 5.5-Dimethyl-2,4-oxazolidinedione (DMO) produced a similar effect. DMO was passively distributed between bile and plasma. The data indicate that a bicarbonate transport mechanism is responsible for production of up to 50% of the BSIF. Another weak acid, N-5[5-(2-methoxyethoxy)-2-pyrimidinyl]sulfamoylbenzene (glymidine), was rapidly excreted into bile and increased bile flow by over 2.0 microliter/g.min. Glymidine is probably excreted by an independent organic anion transport mechanism, and any effect on the bicarbonate transport mechanism is obscured. Canaliculus-enriched hepatocyte membrane fractions contained no HCO3-stimulated ATPase activity. Either this enzyme is unimportant in hepatocyte bicarbonate transport or transport occurs across membranes other than the bile canalicular membrane.

Effect of benzolamide on luminal pH in proximal convoluted tubules of the rat kidney

Luminal pH in early and late proximal tubules was recorded continuously with antimony microelectrodes before and during carbonic anhydrase inhibition. Following i.v. application of benzolamide (25 mumol/kg BW), luminal pH decreased almost immediately in early proximal tubules (deltapH--0.42 +/- 0.06SEM), but increased in late proximal tubules (deltapH +0.27 +/- 0.06). Urinary pH increased (deltapH + 1.6 +/- 0.16) after a delay of some 30 s. Similar results, i.e. decrease of pH in early and increase of pH in late proximal tubules, were obtained, when benzolamide containing solutions were microinfused into early proximal tubules or superfused on the nephron surface. In contrast, luminal pH decreased in late proximal tubules, when benzolamide was microinfused into the same nephron segment. The decrease of luminal pH indicates inhibition of luminally active carbonic anhydrase, leading to delayed buffering of secreted hydrogen ions. The increase of luminal pH in late proximal tubules may be attributed to several factors including increased delivery of bicarbonate, impaired bicarbonate exit at the antiluminal membrane and decreased hydrogen ion formation in the tubular cell due to inhibition of cellular carbonic anhydrase.

Effect of bicarbonate and other buffers on choroid plexus Na+/K+pump

The Na+/K+ pump on the apical membrane of the choroid plexus epithelium was found to be sensitive to bicarbonate. Glycodiazine, a non-volatile, lipid soluble buffer with a pK of 5.7, mimicked the effect of bicarbonate, and was transported across the epithelium in the same direction as sodium. These results are explained in terms of a Na+/H+ exchange mechanism on the basal-lateral membrane.

[The function of kidney membranes]

Plasma membranes were isolated from rat kidney and their transport properties for sodium, calcium, protons, phosphate, glucose, lactate, and phenylalanine were investigated. The results demonstrate that the luminal plasma membrane, which is in contact with the primary urine, and the contraluminal plasma membrane, which is in contact with the interstitial fluid, differ in their content of ion-sensitive ATPases and sodium-cotransport systems. This allows conclusions on the mechanism of renal transport in which luminal and contraluminal membranes are arranged functionally in series.

Coupling between proximal tubular transport processes. Studies with ouabain, SITS and HCO3-free solutions

The rate of active transport by the proximal renal tubule of amino acid (L-histidine), sugar (alpha-methyl-D-glycoside), H+ ions (glycodiazine), phosphate and para-aminohippurate was evaluated by measuring the zero net flux concentration difference (deltac) of these substances. In the case of calcium the electrochemical potential difference (delta + zF-CIdeltaphi/RT) was the criterion employed. The rate of isotonic Na+-absorption (JNa) was measured with the shrinking droplet method. The effect of ouabain on the transport of these substances was tested in the golden hamster and the effect of SITS (4-acetamido-4'isothiocyanatostilbene 2,2'-disulfonic acid) was observed in rats. Ouabain (1 mM) applied peritubularly incompletely inhibited JNa (80%), but in combination with acetazolamide (0.2 mM) the inhibition was almost complete (93%). In addition, ouabain inhibited the sodium coupled (secondary active) transport processes of L-histidine, alpha-methyl-D-glycoside, calcium and phosphate by more than 75%. It did not affect H+ (glycodiazine) transport and PAH transport was only slightly affected. When SITS (1 mM) was applied from both sides of the cell it inhibited H+ (glycodiazine) transport by 72% and reduced JNa by 38% when given from only the peritubular cell side. SITS (1 MM), however, had no significant affect on H+ secretion and sodium reabsorption if it was applied from only the luminal side. Furthermore it had no affect on the other transport processes tested, regardless of the cell side to which it was applied. When the HCO-3 buffer or physically related buffers were omitted from the perfusate the absorption of Na+ was reduced by 66%, phosphate by 44%, and L-histidine by 15%. All the other transport processes tested were not significantly affected. The data are consistent with the hypothesis that the active transport processes of histidine, alpha-methyl-D-glycoside and phosphate, which are located in the brush border, are driven by a sodium gradient which is abolished by ouabain. This may also apply to the Na+-Ca2+ countertransport located at the contraluminal cell side. The residual Na+ transport remaining in the presence of ouabain is likely to be passively driven by the continuing H+ transport which probably is driven directly by ATP. SITS seems to inhibit the exit step of HCO-3 from the cell and secondary to that, the luminal H+-Na+ exchange and consequently the Na+ reabsorption. In the absence of HCO-3 buffer in the perfusates the luminal H+-Na+ exchange seems to be affected and the pattern of inhibition of the other transport processes is almost the same as with SITS. The different effects on Pi reabsorption observed under these conditions might be explained by possible variations in intracellular pH.

Influence of bicarbonate on parathyroid hormone-induced changes in fluid absorption by the proximal tubule

Segments of the proximal tubule of the rabbit kidney were perfused in vitro in order to examine the influence of bicarbonate on the reduction in fluid absorption that occurs following the acute administration of parathyroid hormone (PTH). Studies were performed using either normal ultrafiltrate as perfusion fluid and normal rabbit serum as bath or low bicarbonate ultrafiltrate as perfusion fluid and low bicarbonate rabbit serum as bath. Low bicarbonate fluids were prepared by replacement of bicarbonate with chloride. In the presence of normal concentrations of bicarbonate, the addition of PTH to the bath (1 U/ml) resulted in a decrease in the fluid absorption rate (Jv) from 1.13 +/- 0.08 to 0.60 +/- 0.04 nl/mm X min (p is less than 0.001) in 23 convoluted segments and from 0.64 +/- 0.05 to 0.46 +/- 0.05 nl/mm - mm (P less than 0.01) in ten straight portions. Simultaneous with the PTH-induced reduction in Jv, the chloride concentration in the collected fluid changed from 119.0 +/- 2.0 to 113.4 +/- 1.1 mEq/liter (P less than 0.01) in the pars convoluta and from 117.7 +/- 0.6 to 114.0 +/- 1.9 mEq/liter (P less than 0.01) in the pars recta. However, there was no change in the net flux of chloride which averaged 42.58 +/- 5.00 pEq/mm - min during the control periods. Additional studies were performed in eight convoluted segments during perfusion on a randomized basis with low bicarbonate fluids as well as during perfusion with fluids having normal levels of bicarbonate. As before, in the presence of normal levels of bicarbonate, PTH reduced Jv from 1.16 +/- 0.15 to 0.68 +/- 0.07 nl/mm - min (P less than 0.001) and the chloride concentration in the collected fluid ([Cl]o) from 118.6 +/- 2.9 to 111.6 +/- 1.3 mEq/liter (P less than 0.005). Substitution of low bicarbonate fluids for normal bicarbonate fluids resulted in a decrease in Jv from 1.16 +/- 0.15 to 0.74 +/- 0.10 nl/mm - min (P less than 0.001). In the presence of low bicarbonate fluids, the addition of PTH resulted in no further decrease in Jv (0.74 +/- 0.10 vs. 0.72 +/- 0.10 nl/mm - min). These data indicate that in the proximal tubule the PTH-induced reduction in fluid absorption may be mediated by changes in bicarbonate absorption.

Role of monovalent ions in the reabsorption of fluid by isolated perfused proximal renal tubules of the rabbit

Proximal convoluted tubules were dissected from rabbit kidneys and perfused in vitro in order to determine the effect of monovalent ions on fluid absorption and transepithelial voltage. Replacement of sodium in the perfusate and bath by lithium, tetramethyl ammonium or choline caused the rate of fluid absorption and voltage to fall to near zero. Replacement of potassium in the bath by sodium had the identical effect. Replacement of chloride by nitrate or perchlorate had comparatively little effect. The results are consistent with the generally held view that active sodium transport (mediated by a Na- and K- activated adenosine triphosphatase) is the primary process responsible for the absorption of the fluid and the voltage. Replacement of bicarbonate in the perfusate and bath by chloride caused the rate of fluid absorption to decrease by 33%. The possible relation between sodium transport and bicarbonate is discussed.

Active Ca2+ reabsorption in the proximal tubule of the rat kidney. Dependence on sodium- and buffer transport

Using the stop flow microperfusion technique with simultaneous capillary perfusion the rate active Ca2+ reabsorption was evaluated by measuring the static head electrochemical potential difference as well as the permeability of the tubular wall for Ca2+ ions. Under control conditions the active Ca2+ transport was calculated to be 3.35 X 10(-13) mol/cm - s. It declined toward zero if the ambient Na+ was replaced by choline or lithium. Parallel experiments in the golden hamster showed that active Ca2+ transport, vanished completely if active Na+ transport was blocked by ouabain (1 mM). These data indicate that the active Ca2+ reabsorption from the proximal tubule depends on the active reabsorption of Na2+ presumably via a Na+-Ca2+ countertransport at the contraluminal cell membrane. The static head electrochemical potential difference of Ca2+ is the same in late and early proximal tubules. It is also not affected by the presence of acetazolamide (10(-4) M) by the absence of bicarbonate or glycodiazine buffer or by the absence or presence of phosphate (2 mM).

Renal test dyes IV. Intravital Fluorescence microscopy and microphotometry of the tubularly secreted dye sulfonefluorescein

The present study describes for the first time the use of the fluorescent dye sulfonefluorescein for intravital microscopy and microphotometry on the renal surface of rats. With help of this dye tubular secretion can be observed in mammalian kidney in vivo. Tubular secretion can further be quantified with microphotometrical measurements. The molecular structure of sulfonefluorescein is closely related to phenol red, which is known for its tubular secretion. Clearance experiments also show a secretion of sulfonefluorescein. The secretion can be inhibited by probenecid. Ureter ligation causes a strong increase in tubular concentration of the dye. After a temporary ischemia dye accumulation in tubular lumen is greatly reduced.

H in cortical peritubular capillaries of rat kidney

The pH of peritubular capillaries was measured by means of antimony microelectrodes, during their perfusion with mammalian Ringer's solutions at different pH, in control and acetazolamide infused rats. In capillaries perfused with a solution more acid than blood, significant alkalinization was observed at increasing distances from the point of perfusion, while during perfusions with more alkaline solutions, acidification was observed. Plotting the pH change observed per micrometer of distance from the perfusion point against the pH of the perfusing solution, the pH in equilibrium with tubular cells was interpolated. A value of 7.51 +/- 0.01 was found for control rats, significantly higher than the mean arterial blood pH of this group, of 7.39. In acetazolamide infused rats an equilibrium pH of 7.44 +/- 0.02 was found, still higher than the blood pH of 7.34. The slope of these lines was significantly greater in control than in acetazolamide treated rats. This slope was shown to evaluate permeability to the ions responsible for acid-base balance. The present data suggest that peritubular alkalinization is reduced after carbonic anhydrase inhibition due to decreased peritubular permeability to the involved ions, which represents a further site of action of these inhibitors.

Renal phosphate transport: inhomogeneity of local proximal transport rates and sodium dependence

The standing droplet method has been used in combination with the peritibular perfusion of blood capillaries to determine the build up of transtubular concentration differences of phosphate (Piota) in the renal proximal convoluted tubule of parathyroidectomized rats. Electron probe analysis was used to estimate Piota. At zero time both the intraluminal and the contraluminal Piota concentration was 2 mM. The time dependent decrease of the intraluminal Piota concentration was approximately 4 times faster in the early than in the late proximal convoluted tubule. After 45 sec an intraluminal steady state concentration of 0.20 mM Piota was achieved in the early part. In the late part the intraluminal Piota concentration approached a steady statevalue of 0.54 mM at 123 sec. When sodium free solutions were used the intaluminal Piota concentration increased to 2.22 mM in the earlier and to 2.76 mM in the late part. The data indicate that in the proximal convoluted tubule 1. the rate of phosphate reabsorption is greater in the early part than in the later part, and 2. phospate reabsorption might occur as co-transport with Na+ ions.