Oxygen requirement of bicarbonate-dependent sodium reabsorption in the dog kidney

The ratio between changes in sodium reabsorption and renal oxygen consumption (Na/O2) was measured in anesthetized dogs at high plasma bicarbonate concentration (32 +/- 1 mM); ethacrynic acid was infused continuously to prevent variations in transcellular NaCl reabsorption when sodium reabsorption was altered by varying plasma PCO2 and glomerular filtration rate (GFR). At high plasma PCO2 (110 mmHg) sodium reabsorption varied in proportion to GRF between 50 and 125% of control GFR (glomerulotubular balance). By reducing PCO2 to 20 mmHg, sodium reabsorption was reduced by 50-60% at constant GFR. The Na/O2 ratio was not significantly different during the two procedures and averaged 48 +/- 2. The ratio between changes in NaHCO3 reabsorption and oxygen consumption averaged 17 +/- 1, which is not significantly different from the Na/O2 ratio of Na-K-ATPase-dependent sodium transport. We propose that NaHCO3 is admitted to the cell by Na+/H+ exchange and that sodium is actively transported by Na-K-ATPase across the peritubular cell membrane; NaHCO3 provides the osmotic force for paracellular reabsorption of water and NaCl (bicarbonate-dependent reabsorption) without additional energy requirement.     

Renal sodium reabsorption and oxygen consumption after unilateral splanchnicotomy in the dog

Summary Experiments were carried out on pentobarbital-anaesthetized dogs previously subjected to unilateral splanchnicotomy. Renal blood flow (RBF), glomerular filtration rate (GFR), tubular reabsorption of sodium (TRF_Na), and oxygen consumption (Q _{O 2}) were determined in normal state, following isotonic volume expansion and during furosemide administration (0.5–1.5 mg/kg bw.). Denervation diuresis and natriuresis occurred under all experimental conditions. TRF_Na/Q _{O 2} was 31.4 mEq/mMol for intact and 27.7 mEq/mMol for denervated kidneys in the normal and isotonic volume expanded dogs. These values decreased significantly to 16.8 mEq/mMol and 18.3 mEq/mMol, respectively, upon furosemide loading. No difference between intact and denervated kidneys was observed in either group. TRF_Na, RBF, and GFR were significantly correlated toQ _{O 2} with no difference between intact and splanchnicotomized sides. An inhibitory effect of renal denervation on active sodium transport in the proximal tubule is suggested.     

Carbonic anhydrase-dependent bicarbonate reabsorption in the rat proximal tubule.

The extent to which bicarbonate reabsorption in the rat proximal convoluted tubule depends on carbonic anhydrase has been examined by in vivo microperfusion and the measurement of total CO2 concentration by microcalorimetry. Tubules were perfused with an ultrafiltrate-like solution at 13 nl/min, and volume reabsorptive rate (JV) was measured using [14C]inulin. Addition of either 800 or 100 microM acetazolamide to the perfusion solution completely inhibited the reabsorption of total CO2. The control total CO2 reabsorptive rate (JtCO2) was 147 +/- 23 pmol/mm.min, and acetazolamide reduced JtCO2 to -3 +/- 5 pmol/mm.min. Acetazolamide reduced JV by 65% from a control of 2.3 +/- 0.4 to 0.8 +/- 0.1 nl/mm.min. The dose-response curve for acetazolamide showed that the I50 for inhibition of JtCO2 was 4 microM. The inactive congener of acetazolamide, t-butyl acetazolamide, did not reduce JV or inhibit bicarbonate reabsorption, indicating that the effect of acetazolamide on JtCO2 was specific for carbonic anhydrase inhibition. Since bicarbonate reabsorption was completely blocked by carbonic anhydrase inhibition, there is no need to postulate either carbonic acid recycling or carbonic anhydrase-independent bicarbonate reabsorption.     

Effect of renal venous pressure elevation on tubular sodium and water reabsorption in the dog kidney

This study was performed in order to quantify the effects of renal venous pressure (RVP) elevation on absolute and fractional reabsorption rates of sodium and water in proximal and distal segments of the nephron in dog kidneys. Renal blood flow (RBF) was measured electromagnetically. Clearance of [51Cr]EDTA was used as a measure of the rate of glomerular filtration (GFR). GFR, urinary excretion rates of sodium and water, and lithium clearance were used for assessing the absolute and fractional reabsorption rates of sodium and water in the proximal as well as in more distal segments of the nephron. In the kidneys with intact innervation RVP elevation to 19.9 +/- 0.1 mmHg caused significant increases in both absolute (APR) and fractional (FPR) proximal reabsorption rates from 33.4 +/- 4.2 to 38.7 +/- 2.0 ml min-1 and from 0.62 +/- 0.04 to 0.71 +/- 0.04, respectively. These responses were unaffected by acute surgical denervation of the kidneys. In contrast, chronic renal denervation or infusion of phentolamine (5 micrograms kg-1 min-1) into the renal artery eliminated the increase in APR and FPR during RVP elevation to 20 mmHg. Chronic, but not acute renal denervation depleted renal tissue content of adrenaline and noradrenaline. The results suggest that the increase in APR and FPR during RVP elevation is due mainly to local sympathetic reflex mechanisms.     

Bicarbonate and sodium reabsorption by the isolated perfused kidney.

The role of bicarbonate reabsorption and of transtubular chloride gradients in the bulk reabsorption of sodium and water by renal tubules can be tested in the isolated perfused kidney by perfusing with a medium from which bicarbonate has been omitted. Perfusion of the isolated rat kidney with an artificial medium in which bicarbonate is replaced by chloride results in a fall in fractional reabsorption of sodium from 97 to 84%. Stepwise restoration of bicarbonate concentration in the perfusion medium to 25 meq/liter is associated with a parallel recovery of sodium reabsorption to control levels. Inhibition of bicarbonate reabsorption with acetazolamide produces a slightly smaller reduction in sodium reabsorption (97-89%), an effect not seen in the absence of bicarbonate. Acetazolamide greatly increases phosphate excretion and free water clearance in a way consistent with suppression of proximal tubular reabsorption. By contrast, simple omission of bicarbonate from the perfusing medium does not alter phosphaturia or free water clearance. Reabsorption of bicarbonate appears to account for a fraction of sodium reabsorption roughly equivalent to the proportion of sodium associated stoichiometrically with bicarbonate in the glomerule filtrate. The data do not support the hypothesis that the development of a transtubular chloride gradient is critically important for the reabsorption of a large fraction of the glomerular filtrate.     

Energy requirement of sodium reabsorption in the thick ascending limb of Henle's loop in the dog kidney: effects of bumetanide and ouabain

To examine whether sodium reabsorption in the thick ascending limb of Henle's loop (TALH) in the dog kidney has a passive component, the ratios between reductions in sodium reabsorption and oxygen consumption (ΔNa/ΔQo ₂ ratio) were measured by inhibiting tubular transport with bumetanide (30 μg kg^-1) and ouabain (120 ng kg^-1 intrarenally). Clearance studies were performed in volume expanded dogs treated with acetazolamide (100 mg kg^-1) or maleate (400 mg kg^-1). In five acetazolamide-treated dogs, bumetanide gave a ΔNa/ΔQo ₂ ratio of 29.9±2.5, whereas the combination of bumetanide and ouabain gave 19.0±0.6. When ouabain was given before bumetanide, ouabain gave a ΔNa/ΔQo ₂ ratio of 19.2±1.1 and the combination gave 19.9±1.2. In the maleate-treated dogs, bumetanide gave a ΔNa/Qo ₂ ratio 30.3±1.7, and the combination of bumetanide and ouabain gave 27.1±1.5. To localize the metabolic effect of bumetanide and ouabain, local heat production was measured at 18 places in four kidneys with copper-constantan thermocouples. Bumetanide reduced metabolic rate in the outer medulla by 51±4%, and in the cortex by 16±6%. Subsequent infusion of ouabain reduced metabolic rate in the outer medulla by only 9±3%, whereas cortical metabolism was reduced by 33±4%. The results show that bumetanide mainly acts in the outer medullar where TALH is located, whereas the additional effect of ouabain is mainly located in cortical segment of the nephron including the proximal tubules. Bumetanide inhibits the reabsorption of 30 mol sodium for each mole oxygen consumed, which show that for each 18 mol sodium that are transported through the cells in the TALH in dog kidneys, 12 mol (40%) are transported along the paracellular route without additional requirement of energy.