Loop of Henle bicarbonate accumulation in vivo in the rat (original) (raw)
Related papers
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.
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.
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.
Pfl�gers Archiv European Journal of Physiology, 1984
The membrane potential response of proximal tubular cells to changing HCO3 concentrations was measured in micro-puncture experiments on rat kidney in vivo. No significant effect was noticed when luminal bicarbonate concentration was changed. Changing peritubular HCOg by substitution with C1-resulted in conspicuous membrane potential transients, which reached peak values after I00-200 ms and decayed towards near control with time constants of ~ 2 s. The polarity of the potential changes and the dependence of the initial potential deflections on the logarithm of HCO3 concentration suggest a high conductance of the peritubular cell membrane for HCO3 buffer, but not for CI-, SO 2-, or isethionate. At constant pH, tricot was estimated to amount to ~0.68. At constant Pco2, tHco; was even greater because of an additional effect of OH-or respectively H + gradients across the cell membrane. The secondary repolarization may be explained by passive net movements of K § and HCO~ across the peritubular cell membrane, which result in a readjustment of intracellular HCO3 to the altered peritubular HCO3 concentration. Application of carbonic anhydrase inhibitors in the tubular lumen reduced the initial potential response by one half and doubled the repolarization time constant. The same effect occurred instantaneously when the inhibitor was applied together with the HCO3 concentration step-in the peritubular perfusate. This observation demonstrates that membrane bound carbonic anhydrase is somehow involved in passive rheogenic bicarbonate transfer across the peritubular cell membrane, and suggests that HCO; permeation might occur in form of CO2 and OH-(or H § in opposite direction).
1987
We studied two groups of rats acutely loaded with bicarbonate, control rats on a standard diet and rats kept on a K-free diet for 3 wk. Compared with controls, K-depleted rats had reduced fractional excretion of bicarbonate despite their elevated filtered bicarbonate load. Distal bicarbonate reabsorption increased in K-depleted rats. In the presence of almost identical early distal bicarbonate loads (481±40 pmol/min in controls and 444±50 pmol/min in K depletion), distal bicarbonate reabsorption was significantly enhanced in K depletion (247±17 pmol/min) as compared with controls (179±18 pmol/min). These values are significantly different from each other, and both are severalfold higher than bicarbonate reabsorption in nonloaded conditions. In conclusion, (a) distal bicarbonate reabsorption is load dependent, and (b) distal bicarbonate reabsorption is stimulated in K depletion.
Journal of Membrane Biology, 1979
Fluid transport and net fluxes of Na, K, Cl and HCO3 by guinea pig gallbladder were investigatedin vitro. A perfused gallbladder preparation was devised to simultaneously study unidirectional fluxes of22Na and36Cl. The net Cl flux exceeded the net Na flux during fluid absorption in the presence of HCO3. This Cl excess was counter-balanced by a net HCO3 secretion: a HCO3−Cl exchange. PGE1 reversed the direction of fluid transport and abolished the net Cl flux. The magnitude of the HCO3 secretion remained unchanged, but shifted from a HCO3−Cl exchange to a net secretion of NaHCO3 and KHCO3. Furosemide inhibited both the HCO3−Cl exchange and HCO3 secretion after PGE1 without influencing fluid absorption. Ouabain inhibited the HCO3−Cl exchange as well as fluid absorption; only the effect on the HCO3 secretion was entirely reversible. Secreted HCO3 appeared not to be derived from metabolic sources since HCO3 secretion was abolished in a HCO3-free bathing medium. HCO3 secretion was also dependent on the Na concentration of the bathing fluid. Three lines of evidence are presented in favor of an active HCO3 secretion in guinea pig gallbladder. HCO3 is secreted against: (i) a chemical gradient, (ii) an electrical gradient and (iii) the direction of fluid movement under control conditions.
Model of bicarbonate secretion by resting frog stomach fundus mucosa I. Transepithelial measurements
Pfl�gers Archiv European Journal of Physiology, 1994
In the present in vitro experiments on gastric fundus mucosa of Rana esculenta we try to define the mechanism of alkaline secretion that is observed in summer frogs in the resting stomach (blockage of HC1 secretion by ranitidine, 10 -5 tool/l). The transepithelial voltage and the rate of alkalinization (ASR) of an unbuffered gastric lumen perfusate was measured as a function of serosal (and mucosal) fluid composition. ASR was high (0.88+S.E. 0.09 gEq-cm-2.h -1, n=l 1) during serosal bath perfusion with HCO3--Ringer solution, decreased slightly to 0.50_+0.07 gEq.cm-2-h -1 (n=6) in HCO3--free HEPES-buffered Ringer solution of the same pH, and decreased to approximately 20% when carbonic anhydrase was inhibited by acetazolamide. While replacement of mucosal or serosal C1-did not -within 1 h -significantly alter ASR, replacement of serosal Na + in the presence or absence of HCO 3-strongly reduced ASR, and a similar reduction was observed after serosal application of the anion transport inhibitor DIDS (4,4-diisothiocyanatostilbene-2,2-disulphonate, 2.10-4mol/1), the metabolic poison rotenone (10 -2 tool/l), the uncoupler dinitrophenol (10 -4 tool/l), and the Na + pump inhibitor ouabain (10-4mol/1), while serosal amiloride (10-4mol/l) had no effect. These data can be accounted for by a model of alkaline secretion that consists of basolateral HCO 3-uptake from the serosal fluid into the cell via a DIDS-inhibitable Na+(HCO3-)n-cotransporter and HCO 3secretion from the cell to the gastric lumen via an anionic conductance pathway. Microelectrode experiments on oxyntopeptic cells reported in the subsequent paper suggest that these cells may also be involved in the resting state alkaline secretion.