Investigation of Tubular Handling of Bicarbonate in Man A NEW APPROACH UTILIZING STABLE CARBON ISOTOPE FRACTIONATION (original) (raw)
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Bicarbonate kinetics in Indian males
Journal of Biosciences, 2006
Measurement of rates ofin vivo substrate oxidation such as that of glucose, fatty acids and amino acids, are based on tracer (14C or13C) data, and often depend on the isotopic content of expired CO2. The recovery of tracer-labelled CO2 generated from the oxidation of13C labelled substrates may not be 100% over short term. This can lead to underestimation of oxidation rate of substrates, and consequently a correction for the incomplete recovery of tracer has to be applied by the determination of the recovery of13CO2 in the breath during tracer bicarbonate infusions. We have studied the recovery of tracer-labelled bicarbonate using a bolus administration model, and further characterized kinetics of bicarbonate using a three-compartment model, to assess which compartmental fluxes changed during the change from a fasted state to fed state. Recovery of bicarbonate was lower at 69% and 67% (fasted and fed state) than the value of 71% and 74% found during earlier longer term of continuous infusions. During feeding, there was a 20-fold increase in the flux of bicarbonate between the central compartment and the compartment that was equivalent to the viscera. This study shows that the difference between the fasted and fed state recovery of tracer bicarbonate similar to that obtained with continuous infusions, and that bicarbonate fluxes show large changes between different compartments in the body depending on metabolic state.
1983
alternative mechanisms have been proposed for tubular reabsorption of bicarbonate: (a) H' secretion and CO2 reabsorption and (b) direct reabsorption of HCO3. In an attempt to differentiate between the two mechanisms, the present study utilized the natural abundance of stable carbon isotopes ('3C, '2C) in the urinary total CO2. This novel methodology used mass spectrometric analysis of 13C/12C ratios in urinary total CO2 under normal conditions and during acetazolamide treatment. Blood and respiratory CO2 were analyzed to yield reference values. The results demonstrate that alkaline urine is preferentially enriched with 13C relative to the blood. It is suggested that this fractionation results from reaction out of isotopic equilibrium in which HCOS converts to CO2 during the reabsorption process in the distal nephron. The presence of carbonic anhydrase in the proximal nephron results in rapid isotopic exchange between CO2 and HCO-and keeps them in isotopic equilibrium. The ratio of urinary 13C/12C increases strikingly after acetazolamide administration and consequent inhibition of carbonic anhydrase in the proximal tubule. Although it is possible that in the latter case high HCO3 generates the CO2 (ampholyte effect), the isotope fractionation indicates that CO2 rather than HCOis reabsorbed. In contrast, at low urinary pH and total CO2 values, the carbon isotope composition approaches that of blood
Influence of the bicarbonate pool and on the occurrence of 13CO2 in exhaled air
European Journal of Applied Physiology and Occupational Physiology, 1991
In 13CO 2 breath tests, based on 13C: 1zc ratio measurements, the appearance of 13C in exhaled COz was monitored after the administration of a 13C-labelled compound. Independently of the substrate used, the existence of a bicarbonate pool into which the CO2 produced enters before being exhaled, imposes a delay on the appearance of changes in the 13C: 12C ratio. To estimate the nature and magnitude of this delay, we applied a two-compartment model to describe the kinetics of the body bicarbonate pool and we evaluated the a3C: 12C ratio of CO2 entering that pool from the measured a3C: a2C ratio in the exhaled COz after an oral intake of "naturally labelled" x3C-glucose. Our results demonstrated that discrepancies between total and exogenous glucose oxidation in relation to the peak occurrence time, as well as the absolute quantities, could be adequately explained by the interference of the bicarbonate stores.
1989
Using continuous microperfusion techniques, we studied the load dependence of bicarbonate reabsorption along cortical distal tubules of the rat kidney and their bicarbonate permeability. Net bicarbonate transport was evaluated from changes in tracer inulin concentrations and total CO2 measurements by microcalorimetry. Bicarbonate permeability was estimated from the flux of total CO2 along known electrochemical gradients into bicarbonate-and chloride-free perfusion solution containing l0'-M acetazolamide. Transepithelial potential differences were measured with conventional glass microelectrodes. Significant net bicarbonate reabsorption occurred at luminal bicarbonate levels from 5 to 25 mM, and at perfusion rates from 5 to 30 nl/min. Bicarbonate reabsorption increased in a load-dependent manner, both during increments in luminal bicarbonate concentration or perfusion rate, reaching saturation at a load of 250 pmol/min with a maximal reabsorption rate of approximately 75 pmol/min-mm. Rate of bicarbonate reabsorption was flow dependent at luminal concentrations of 10 but not at 25 mM. During chronic metabolic alkalosis, maximal rates of reabsorption were significantly reduced to 33 pmol/min. mm. The bicarbonate permeability was 2.32±0.13 X l0-5 cm/s in control rats, and 2.65±0.26 X 10-1 cm/s in volume-expanded rats. Our data indicate that at physiological bicarbonate concentrations in the distal tubule passive bicarbonate fluxes account for only 16-21% of net fluxes. At high luminal bicarbonate concentrations, passive bicarbonate reabsorption contributes moderately to net reabsorption of this anion. Distal Bicarbonate Transport 931 J. Clin. Invest.
1993
Bicarbonate transport was studied in vivo by separate microperfusion experiments of early and late distal tubules. Total CO2 was measured by microcalorimetry and fluid absorption by 3H-inulin. Significant bicarbonate absorption was observed in all experimental conditions. Bicarbonate transport was loaddependent upon increasing the luminal bicarbonate concentration from 15 to 50 mM in both early and late distal tubule segments and remained constant at higher concentrations at a maximum rate of 100-110 pmol/min per mm. At low lumen bicarbonate concentrations (15 mM), higher rates of bicarbonate absorption were observed in early (32.9±4.57 pmol/min per mm) as compared to late distal tubules (10.7±3.1 pmol/ min per mm). Amiloride and ethyl-isopropylamiloride both inhibited early but not late distal tubule bicarbonate absorption whereas acetazolamide blocked bicarbonate transport in both tubule segments. Fluid absorption was significantly reduced in both tubule segments by amiloride but only in early distal tubules by ethyl-isopropylamiloride. Substitution of lumen chloride by gluconate increased bicarbonate absorption in late but not in early distal tubules. Bafilomycin Al, an inhibitor of H-ATPase, inhibited late and also early distal tubule bicarbonate absorption, the latter at higher concentration. After 8 d on a low K diet, bicarbonate absorption increased significantly in both early and late distal tubules. Schering compound 28080, a potent H-K ATPase inhibitor, completely blocked this increment of bicarbonate absorption in late but not in early distal tubule. The data suggest bicarbonate absorption via Na+-H+ exchange and H-ATPase in early, but only by amiloride-insensitive H' secretion (H-ATPase) in late distal tubules. The study also provides evidence for activation of K+-H+ exchange in late distal tubules of K depleted rats. Indirect evidence implies a component of chloride-dependent bicarbonate secretion in late distal tubules and suggests that net bicarbonate transport at this site results from bidirectional bicarbonate movement. (J. Clin.
Extracorporeal Co2 Removal with Hemodialysis (ECBicCO2R): How to Make up for the Bicarbonate Loss?
The International Journal of Artificial Organs, 1991
Hemodialysis is a powerful tool for extracorporeal CO2 removal, because CO2 can be eliminated both as gas and as bicarbonate with blood flow rates as low as 10-15 mI/kg/min. An unsolved problem remains, however: how to make up for the bicarbonate loss. In an animal model we investigated three methods of realkalinisation: a) indirect alkalinisation with salts of organic anions (acetate, lactate, citrate, pyruvate, fumarate, succinate, malate) b) direct realkalinisation with hydroxyl ions (NaOH) c) direct alkalinisation with TRIS as “CO2-buffer”. a) The decrease of pulmonary CO2 elimination depended on metabolism: acetate and lactate were metabolized at a rate of 1.8-3.5 mmol/min, thus allowing a steady-state elimination of 40-75 mmol CO2/min (25-40% of CO2 production). The other organic acids were not metabolized sufficiently to achieve a measurable reduction of pulmonary CO2 elimination. CO2 removal was quantitatively the same as during routine acetate hemodialysis and could not be ...
Renal bicarbonate reabsorption in the rat. I. Effects of hypokalemia and carbonic anhydrase
1986
Free-flow micropuncture studies were carried out on superficial rat proximal and distal tubules to assess the participation of different nephron segments in bicarbonate transport. Particular emphasis was placed on the role of the distal tubule, and microcalorimetric methods used to quantitate bicarbonate reabsorption. Experiments were carried out in control conditions, during dietary potassium withdrawal, and after acute intravenous infusions of carbonic anhydrase. We observed highly significant net bicarbonate reabsorption in normal acid-base conditions as evidenced by the maintenance of significant bicarbonate concentration gradients in the presence of vigorous fluid absorption. Distal bicarbonate reabsorption persisted in hypokalemic alkalosis and even steeper transepithelial concentration gradients of bicarbonate were maintained. Enhancement of net bicarbonate reabsorption followed the acute intravenous administration of carbonic anhydrase but was limited to the nephron segments between the late proximal and early distal tubule. The latter observation is consistent with a disequilibrium pH along the proximal straight tubule (S3 segment), the thick ascending limb of Henle, and/or the early distal tubule.
Determination of urinary bicarbonate with the Henderson-Hasselbalch equation
Pediatric Nephrology, 2004
The determination of urinary bicarbonate with the Henderson-Hasselbalch equation was compared using two methods: (1) correcting the pK in every urine sample according to ionic strength and using the solubility constant of CO 2 in urine (a=0.0309) and (2) using a fixed pK value (6.1) and a CO 2 solubility constant of 0.0301, which we use to calculate blood bicarbonate. Nine patients were studied and 29 determinations were performed. A high correlation was found between the methods (r=0.99). Bicarbonate calculated with corrected pK was 24.3€6.6 mEq/l (95% confidence interval 11.4-37.2) and bicarbonate calculated with pK fixed at 6.1 was 25.6€6.6 mEq/l (95% confidence interval 12.7-38.5). For each urine sample, the D bicarbonate was calculated as the difference between the bicarbonate obtained with pK at 6.1 minus that obtained with the corrected pK (mean 1.25, standard error 0.83, P=0.15). This indicates that the difference between the methods was not significant. No difference was found whether pK was corrected or fixed (6.1). Therefore, our results suggest that it is valid to take the value shown by the equipment for blood gas determination as the urinary bicarbonate value. This would allow the rapid and accurate determination of urinary bicarbonate in patients with hyperchloremic acidosis, especially those with renal tubular acidosis.
Tissue sequestration of C-labelled bicarbonate [HCO3−] in fed and fasted young sheep
Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 1999
Carbon dioxide entry rates (CER) based on isotopic activities in either expired air or blood following a 24-h intravenous infusion of [ 13 C]-and [ 14 C] sodium bicarbonate were compared with CO 2 production quantified by respiration hood in young sheep (28-30 kg) either fed (three animals) or fasted (three animals). CO 2 production increased with intake (5.2 vs 10.3 mol/day; PB 0.001) as did CER values based on expired air (9.9 vs 18.6 mol/day; PB0.001) or blood (7.5 vs 16.5 mol/day; P B0.001). The differences between air and blood CER values were significant (PB0.001). There were no differences, however, when data were compared between [ 13 C] and [ 14 C] measurements. How much of these differences could be attributed to sequestration of label in body tissues was examined at the end of the infusion. The highest specific radioactivities (dpm/g dry matter) in acid-fast tissue material were observed for the more metabolically active tissues, liver, jejunum and kidney, with the lowest values for fat and muscle. When tissue mass was taken into account, however, the largest proportions of the dose sequestered were in bone muscle, skin and fat with significantly more retained for the former three (PB 0.01) during fasting. Separately, losses as urinary urea were also quantified. Total measured sequestration of label only accounted for approximately 24-44% of the difference between CER and CO 2 production.