Effect of potassium concentration on bicarbonate reabsorption in the rabbit proximal convoluted tubule

The direct effects of acute changes in K+ concentration on HCO-3 (JnettCO2) and volume reabsorption (Jv) were examined in isolated perfused rabbit proximal convoluted tubules (PCT). Increasing ambient K+ concentration from 5 to 8 mM did not change JnettCO2 (94.5 +/- 16.1 vs. 98.8 +/- 17.7 pmol X mm-1 X min-1) or Jv (1.27 +/- 0.15 vs. 1.24 +/- 0.16 nl X mm-1 X min-1). In contrast, reducing ambient K+ concentration from 5 to 2 mM inhibited JnettCO2 by 22% and Jv by 29%. Reducing luminal K+ concentration from 5 to 0 mM with constant bath K+ concentration at 5 mM did not affect JnettCO2 or Jv. Further reductions in bath K+ concentration to 0.5 and 0 mM showed a similar dependence of both fluxes on K+ concentration. Half-maximum inhibition of JnettCO2 was obtained at 1.1 mM ambient K+ concentration and of Jv at 0.85 mM. At zero bath K+ concentration JnettCO2 was 6.6 +/- 2.5 pmol X mm-1 X min-1 and Jv was 0.03 +/- 0.04 nl X mm-1 X min-1. To determine whether this rate of acidification was significantly different from zero, we examined the ability of the PCT to generate tCO2 concentration gradients with zero bath K+ concentration at slow perfusion rates. The tCO2 concentration gradient generated (0.94 mM) was not different from that found when the perfusate was inserted directly into the collection pipette in the absence of a tubule (0.71 mM). These data are consistent with the view that HCO-3 reabsorption is totally dependent on the Na+-K+-ATPase pump system.     

Effects of lysine on bicarbonate and fluid absorption in the rat proximal tubule

The effects of lysine on bicarbonate and fluid reabsorption in the rat proximal tubule were studied by luminal and capillary perfusion in situ. The proximal tubule and peritubular capillaries were perfused with bicarbonate Ringer solution containing [14C]inulin. The rate of bicarbonate reabsorption (JHCO3) was estimated to be 124 +/- 9.5 peq.min-1.mm-1 using a pH membrane glass electrode. The rate of net fluid reabsorption (Jv) was 2.6 +/- 0.21 nl.min-1.mm-1. When 10 mM L-lysine was added to the luminal perfusate, a 35% reduction in JHCO3 and no change in Jv were observed. Increase of L-lysine concentration in the luminal perfusate to 20 mM did not reduce JHCO3 further nor did it influence Jv.l When 10 mM L-lysine was added to the capillary perfusate, a 13% reduction in JHCO3 was observed (NS). Increase of lysine concentration in the capillary perfusate to 20 mM significantly reduced JHCO3 by 26% (P less than 0.01). There was no significant change in Jv under both conditions. The effect of L-lysine in the lumen was related to its reabsorption kinetics, D-Lysine, which was not reabsorbed significantly, did not affect bicarbonate reabsorption in the proximal tubule. These results indicate that the inhibitory effect of L-lysine is related to the entry of lysine into the cell from the lumen.     

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.     

Evidence for bicarbonate-dependent magnesium reabsorption

During ethacrynic acid administration about 50% of the filtered load of magnesium is reabsorbed. To examine whether the remaining component of magnesium reabsorption is bicarbonate-dependent, i.e. varies with factors known to alter passive reabsorption, experiments were performed in anesthetized dogs. During ethacrynic acid administration MgCl2 infusion raised the plasma concentration of magnesium (PMg) from 0.64 +/- 0.05 to 3.06 +/- 0.27 mM and doubled magnesium reabsorption. The infusion of acetazolamide at high PMg reduced bicarbonate reabsorption by 41 +/- 3% and magnesium reabsorption by 31 +/- 16%. When plasma pH was reduced to 7.04 +/- 0.02 and increased to 7.83 +/- 0.02 by altering PCO2 at a constant plasma bicarbonate concentration of 31.2 +/- 0.8 mM, magnesium and bicarbonate reabsorption were correlated (r = 0.82). The infusion of mannitol, which acts by reducing passive solute transport without affecting bicarbonate reabsorption, halved magnesium reabsorption. By combining mannitol and acetazolamide infusions, only 6 +/- 4% of the filtered magnesium was still reabsorbed. These results indicate that the reabsorption of magnesium remaining after the infusion of ethacrynic acid and after raising PMg varies with changes in PCO2 and is inhibited by the infusion of acetazolamide and mannitol as expected for bicarbonate-dependent passive reabsorption.     

Coupling of NaHCO3 and NaCl reabsorption in dog kidneys during changes in plasma PCO2.

To study the relationship between proximal tubular reabsorption of bicarbonate, sodium, and chloride, the effects of changes in plasma PCO2 were examined in anesthetized dogs. Distal tubular reabsorption was inhibited by ethacrynic acid; plasma bicarbonate concentration was kept constant at 33.4 +/- 0.3 mM; glomerular filtration rate (GFR) was varied over a wide range to examine glomerulotubular balance (constant fractional reabsorption). Hypercapnia (PCO2, 112.0 +/- 2.5 mmHg) increased bicarbonate reabsorption by about 30%, and hypocapnia (PCO2, 19.8 +/- 0.6 mmHg) decreased reabsorption of bicarbonate by more than 50% and altered reabsorption of sodium, chloride, and bicarbonate in the molar ratios 2.7:1.6:1, respectively. During hypercapnia the range of glomerulotubular balance was extended to a GFR 125% of control. During hypocapnia glomerulotubular balance was present only at GFR below 50% of control; reabsorption of bicarbonate sodium, and chloride was constant at GFR exceeding 50% of control. During metabolic acidosis hypercapnia had no significant effect on reabsorption of bicarbonate, sodium, and chloride. These observations support the hypothesis that bicarbonate reabsorption is the main driving force for osmotic reabsorption of water and NaCl in the proximal tubules.     

The effect of loop diuretics on fluid reabsorption from the rat proximal convoluted tubule

The effects of the 'loop diuretics' bumetanide, furosemide and piretanide on fluid reabsorption from the rat proximal convoluted tubule have been tested to assess whether a Na(+)-Cl- cotransport mechanism can support fluid reabsorption from this nephron segment. Proximal convoluted tubules were perfused in vivo at 25 nl min-1 with chloride Ringer solutions containing [3H]inulin. Fluid reabsorptive rate (Jv) was 2.05 +/- 0.07 nl mm-1 min-1 (n = 115) from control chloride Ringer. Bumetanide at 10(-6) and 10(-5) M reduced Jv by about 40%. Bumetanide at 10(-7) M, furosemide (10(-4) and 10(-5) M) and piretanide (10(-4) and 10(-5) M) had no effect on Jv. At 10(-3) M, furosemide and piretanide reduced Jv by about 45%. The results show that luminal application of loop diuretics lowers fluid reabsorption from the proximal convoluted tubule and the order of potency for inhibiting reabsorption is bumetanide greater than furosemide identical to piretanide. The specificity of the loop diuretics at their effective concentrations needs to be confirmed, however, before attribution of a role for Na(+)-Cl- co-transport in the support of proximal tubule fluid reabsorption.     

Measurement of myocardial PCO2 with a microelectrode: its relation to coronary sinus PCO2.

A micro-PCO2 electrode, with dimensions of 1 x 10 mm, and a 63% response time of 14 s was inserted into the left ventricular myocardium of the pentobarbital-anesthetized dog. Continuous recordings were made of myocardial PCO2 (PmCO2), arterial PCO2 (PaCO2), and coronary sinus PCO2 (CSPCO2) during variation of respiratory rate. PmCO2 and CSPCO2 were compared at varying coronary flow. PmCO2 was similar to and closely followed changes in CSPCO2. The difference between PmCO2 and CSPCO2 was -0.52 +/- 3.63 (SD) mmHg, and PmCO2 exceeded PaCO2 by 20.69 +/- 5.12 mmHg. After coronary occlusion, PmCO2, rose promptly, but CSPCO2 was only slightly elevated until the occlusion was released, when a CO2 efflux into the coronary sinus occurred. It is concluded that the electrode measures extracellular PCO2 and that extracellular and myocardial PCO2 are essentially equal. PmCO2 rises rapidly following coronary occlusion.     

Low-dose acetazolamide reduces the hypoxic ventilatory response in the anesthetized cat

Low intravenous dose acetazolamide causes a decrease in steady-state CO(2) sensitivity of both the peripheral and central chemoreflex loops. The effect, however, on the steady-state hypoxic response is unknown. In the present study, we measured the effect of 4 mg x kg(-1) acetazolamide (i.v.) on the isocapnic steady-state hypoxic response in anesthetized cats. Before and after acetazolamide administration, the eucapnic steady-state hypoxic response in these animals was measured by varying inspiratory P(O2) levels to achieve steady-state Pa(O2) levels between hyperoxia Pa(O2) approximately 55 kPa, approximately 412 mmHg) and hypoxia (Pa(O2) approximately 7 kPa, approximately 53 mmHg). The hypoxic ventilatory response was described by the exponential function V(I) = G exp (-DP(o2) + A with an overall hypoxic sensitivity G, a shape parameter D and ventilation during hyperoxia A. Acetazolamide significantly reduced G from 3.057 +/- 1.616 to 1.573 +/- 0.8361 min(-1) (mean +/- S D). Parameter A increased from 0.903 +/- 0.257 to 1.193 +/- 0.321 min(-1), while D remained unchanged. The decrease in overall hypoxic sensitivity by acetazolamide is probably mediated by an inhibitory effect on the carotid bodies and may have clinical significance in the treatment of sleep apneas, particularly those cases that are associated with an increased ventilatory sensitivity to oxygen and/or carbon dioxide.     

CO2 along the proximal tubules in the rat kidney.

The proximal intratubular pH of the rat kidney was measured in vivo with an antimony electrode system. PCO2 and bicarbonate concentration of the proximal tubular fluid were determined with an ultramicro equilibration system. The tubular fluid to plasma inulin concentration ratio was evaluated by a microscope fluorometric method. The acid-base parameters and the inulin concentrations were determined under control conditions and during acetazolamide treatment. The intratubular PCO2 was higher than the PCO2 of the systemic arterial blood under control conditions and the difference in PCO2 was increased during acetazolamide treatment. In acetazolamide treated rats the rate of fractional bicarbonate reabsorption was decreased in the early part of the proximal tubule, while it was of about the same in the middle and late parts as compared with control rats. The total bicarbonate reabsorption in the proximal tubule was reduced by 50% due to the carbonic anhydrase inhibition. It seems possible that the bicarbonate is still reabsorbed as CO2 after carbonic anhydrase inhibition, as hydrogen ion secretion is not totally stopped by this treatment. The increase in intratubular PCO2 after acetazolamide treatment is assumed to be due to an inhibition of the carbonic anhydrase facilitating effect on outward diffusion of CO2 from the tubular lumen across the cell wall.     

Comparison of acidification parameters in superficial and deep nephrons of the rat

The purpose of this study was to determine and compare pH, PCO2, and fractional bicarbonate delivery in both superficial and juxtamedullary nephrons by microelectrode techniques and microcalorimetry in the rat in vivo in order to define more clearly the role of deeper nephron segments in urinary acidification. Values for pH and total CO2 concentration ([tCO2]) at the bend of Henle's loop (LOH) (7.39 +/- 0.04 units and 20.5 +/- 1.5 mM) were significantly greater and the PCO2 was significantly less (36.6 +/- 1.5 mmHg) than values for these same parameters in the superficial late proximal tubule (LPT) (6.78 +/- 0.03 units, 8.1 +/- 1.2 mM, and 63.2 +/- 1.0 mmHg, P less than 0.001). The fraction of filtered bicarbonate delivered to the LPT and LOH did not differ, however (12.2 +/- 2.5 vs. 9.0 +/- 0.8%). The pH and PCO2 values in the late distal tubule (6.59 +/- 0.04 units and 64.0 +/- 1.3 mmHg) were significantly greater than at the base (6.24 +/- 0.07 units and 34.5 +/- 1.5 mmHg) and tip (6.12 +/- 0.03 units and 35.2 +/- 1.2 mmHg) of the papillary collecting duct. The [tCO2] in the LOH and an adjacent vasa recta was compared and did not differ significantly (20.5 +/- 1.5 vs. 21.2 +/- 1.3 mM, P greater than 0.05). In summary, we have demonstrated significant alkalinization of tubule fluid in the deep LOH as a result of water abstraction and CO2 diffusion from the nephron. Our results suggest that a spontaneous disequilibrium pH may not exist in the LOH. Furthermore, similar values for [tCO2] in vasa recta and the LOH suggest that passive HCO-3 reabsorption in the thin ascending limb of Henle would be unlikely and does not contribute to the "loop" component of bicarbonate reabsorption.     

Changes in chemoreflex characteristics following acute carbonic anhydrase inhibition in humans at rest

The effect of carbonic anhydrase (CA) inhibition with acetazolamide (ACZ, 10 mg kg(-1) I.V.) on the peripheral and central chemosensitivity and breathing pattern was investigated in four women and three men aged 25 +/- 3 years using a modified version of Read's rebreathing technique. Subjects were exposed to dynamic increases in CO2 in hypoxic and hyperoxic backgrounds during control conditions and following acute CA inhibition. All manoeuvres were repeated twice and averaged for data analysis. The central chemoreflex sensitivities, estimated from the slopes of the ventilatory response to CO2 during hyperoxic rebreathing, increased following acute CA inhibition (control vs. ACZ treatment: 1.87 +/- 0.66 vs. 4.07 +/- 1.03 l x min(-1) (mmHg CO2)(-1), P < 0.05). The increased slope was reflected by an increase in the rate of rise of tidal volume and breathing frequency. Furthermore with ACZ, there was a left-ward shift of the ventilation vs. end-tidal PCO2 curve during hyperoxic hypercapnia but not hypoxic hypercapnia. The peripheral chemoreflex sensitivity was isolated by subtracting the hyperoxic slope (central only) from the hypoxic slope (central and peripheral). Following ACZ administration, the peripheral chemosensitivity was blunted (control vs. ACZ treatment: 3.66 +/- 0.92 vs. 1.33 +/- 0.46 l x min(-1) (mmHg CO2)(-1), P < 0.05). In conclusion, acute CA inhibition enhanced the central chemosensitivity to CO2 but diminished the peripheral chemosensitivity.     

Disturbance of CO2 elimination in the lungs by carbonic anhydrase inhibition

Carbonic anhydrase in the red blood cell and in the pulmonary endothelium facilitates the elimination of CO2 in the lungs. Although a carbonic anhydrase inhibitor, such as acetazolamide which is frequently used in patients with glaucoma or with metabolic alkalosis, is known to impair the CO2 elimination in the lungs, the dose-response curve of CO2 elimination with acetazolamide has not been well documented in CO2 homeostasis. In the present study, the effects of inhibited carbonic anhydrase were tested in 8 anesthetized dogs; various dosages of acetazolamide were used. When the administered clinical dosage of acetazolamide increased from 5 to 20 mg/kg, PaCO2, PVCO2, arterial-alveolar PCO2 difference (a-ADCO2), and physiological VD/VT ratio increased progressively to 52.0 +/- 2.1 Torr, 58.0 +/- 3.0 Torr, 23.4 +/- 1.2 Torr, and by 19.2 +/- 1.8% (S.E.) respectively, whereas inhibition rate of red blood cell carbonic anhydrase (RCA) activity increased progressively to 73.1 +/- 2.1% (S.E.). On the other hand, PACO2 decreased to 27.1 +/- 1.8 Torr (S.E.) upon the first injection of 5 mg/kg of acetazolamide, but PACO2 did not change further upon 3 additional 5 mg/kg injections. Mixed venous-arterial PCO2 difference [V-a)PCO2), VCO2, and anatomical VD/VT ratio were unchanged by the administration of any doses of acetazolamide.     

Effects of Changes in pH and PCO2 on Wall Tension in Isolated Rat Intrapulmonary Arteries

We examined mean ( S.E.M.) changes in wall tension in isolated rat intrapulmonary arteries on switching from control conditions (pH 7.38 +/- 0.01; PCO2, 34.4 +/- 0.5 mmHg) to hypercapnic acidosis (pH change, -0.24 +/- 0.01; PCO2 change, +27.5 +/- 0.9 mmHg), isohydric hypercapnia (pH change, -0.02 +/- 0.01; PCO2 change, +28.5 +/- 0.8 mmHg) and normocapnic acidosis (pH change, -0.24 +/- 0.01; PCO2 change, -0.5 +/- 0.3). Arteries were submaximally preconstricted with prostaglandin F2 and changes in tension are expressed as a percentage of the 80 mM KCl-induced contraction (%Po). Mean changes in wall tension on switching to hypercapnic acidosis (+4.4 +/- 3.7 %Po), isohydric hypercapnia (+1.9 +/- 2.2 %Po) and normocapnic acidosis (-1.5 +/- 1.9 %Po) were not significantly different from the change observed on switching to control conditions (+3.5 +/- 1.1 %Po), and were unaltered by endothelial removal. In isolated carotid preparations, the change in tension in isohydric hypercapnia (-6.8 +/- 7.1 %Po) was not significantly different from that observed in control switches (+8.6 +/- 3.2 %Po). Significant reductions in tension (P < 0.001) were observed in hypercapnic (-42.9 +/- 7.8 %Po) and normocapnic acidosis (-36.4 +/- 9.0 %Po). These data suggest that intrapulmonary arteries are resistant to the vasodilator effects of extracellular acidosis observed in systemic (carotid) vessels.     

Properties of the macula densa mechanism for renin release in the dog

To study the macula densa mechanism for renin release, both the macula densa and the haemodynamic mechanisms were activated in anaesthetized dogs with denervated kidneys, either by renal arterial constriction to a renal arterial pressure (RAP) of 52 +/- 2 mmHg or by ureteral occlusion to a ureteral pressure of 95-105 mmHg, 20-25 mmHg below RAP. Renal arterial constriction increased renin release from 0.3 +/- 0.2 to 16 +/- 4 micrograms AI min-1. At low RAP, renin release was subsequently reduced to 7 +/- 3 micrograms AI min-1 when sodium excretion was raised far above control values by plasma volume expansion and acetazolamide infusion. Ethacrynic acid (3 mg kg-1 body wt.) restored renin release to pre-expansion values, and a large dose (25 mg kg-1 body wt.) prevented renin release from falling even after unclamping the artery. During ureteral occlusion with stopped glomerular filtration, plasma volume expansion, acetazolamide and ethacrynic acid infusion did not alter renin release. On the other hand, beta-adrenergic stimulation by isoproterenol raised renin release equally (by 30-40 micrograms AI min-1) before and after plasma volume expansion, during both renal arterial constriction and ureteral occlusion. Indomethacin (10 mg kg-1 body wt.) abolished renin release induced by ethacrynic acid infusion and ureteral occlusion. We conclude that the macula densa mechanism for renin release is inactivated by high NaCl reabsorption during plasma volume expansion and acetazolamide infusion, reactivated by inhibition of NaCl reabsorption with ethacrynic acid and completely inhibited by indomethacin. The degree of activation does not influence the renin release induced by beta-adrenergic stimulation.     

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.     

Dependence of cell pH and buffer capacity on the extracellular acid-base change in the skeletal muscle of bullfrog

Intracellular pH was measured with single- or double-barreled liquid ion-exchanger microelectrodes in the bullfrog sartorius muscle perfused in vitro. A neutral carrier ligand was used for pH sensor of microelectrodes. Average slopes of the single-barreled microelectrodes were -56.4 +/- 1.34 mV/pH (n = 30) and the double-barreled -52.6 +/- 1.34 mV/pH (n = 65). While changing acid-base parameters of bathing media (pHe from 6.7 to 8.4, PCO2 from 3.7 to 37 mmHg, and HCO3- concentrations from 5 to 75 mM), paired muscle cell pH (pHi) and membrane potential (EM) values were determined at 23 degrees C. In control conditions (pHe = 7.6, HCO3- = 15 mM, PCO2 = 11 mmHg), pHi and EM (n = 20) averaged 6.99 +/- 0.04 (S.E.) and -69.2 +/- 2.2 mV, respectively. A negative correlation was observed between pHi and EM (correlation coefficient r = -0.564, p less than 0.002). The change in EM per unit pH change was approximately -30mV, indicating that the H+ distribution across the cell membrane only incompletely obeys the Donnan rule. The pHi varied more or less with pHe. Namely, changes in pHe and PCO2 at constant HCO3- produced relatively large changes in pHi, but elevation of pHe and HCO3- at constant PCO2 produced relatively minor rise in pHi. The stability of pHi or the size of buffer capacity were proportional to external HCO3- concentrations. These data suggested that a transmembrane distribution of buffer pairs depends largely on non-ionic diffusion of CO2-HCO3- buffer system and partly on ion fluxes of HCO3- or H+.     

Modifications of microvascular filtration capacity in human limbs by training and electrical stimulation

This study investigated whether an increase in microvascular surface area as a result of endurance training, which increases human skeletal muscle capillarity, would translate to greater capacity for fluid filtration compared with strength training, which does not affect capillary supply. Values for filtration capacity, Kf, derived from the slope of calf volume change, Jv, measured by venous occlusion plethysmography, against cuff pressure during a protocol of small cumulative pressure steps, were significantly higher in endurance athletes (5.78 +/- 0.88 mL min(-1) 100 mL(-1) mmHg(-1) x 10(-3), P < 0.05) than controls (3.38 +/- 0.32 mL min(-1) 100 mL(-1) mmHg(-1) x 10(-3) whereas strength-trained athletes had values similar to control (4.08 +/- 0.56 mL min(-1) 100 mL(-1) mmHg(-1) x 10(-3), ns), suggesting that surface area is important. However, when sedentary subjects underwent either a 4-week unilateral dynamic plantarflexion training programme (70% peak power, 20 min day(-1), 5 days week(-1) or a calf muscle electrical stimulation programme (8 Hz, 3 x 20 min day(-1), 5 days week(-1), neither of which caused limb blood flow to alter after training nor would be expected to increase capillarity, only the stimulation group showed a significant increase in Kf (6.68 +/- 0.62 mL min(-1) 100 mL(-1) mmHg(-1) x 10(-3) post-training vs. 3.38 +/- 0.38 pre-training, P < 0.05). This may be because stimulation enhances perfusion preferentially to glycolytic fibres, or maintains high levels of vascular endothelial growth factor (VEGF) or changes lymph clearance.     

Function of proximal tubule carbonic anhydrase defined by selective inhibition

The specific role of luminal carbonic anhydrase in bicarbonate reabsorption by the proximal tubule has not been established because it has been difficult to inhibit selectively the luminal enzyme without simultaneous inhibition of the cytoplasmic enzyme. The present experiments employed in vivo microperfusion, microcalorimetry, and microelectrode techniques to determine the effects of luminal application of a dextran-bound carbonic anhydrase inhibitor (DBI) on bicarbonate reabsorptive rate (JtCO2) and intraluminal pH in the rat proximal convoluted tubule. Tubules were perfused at 20 nl/min with an artificial ultrafiltrate. Aminoethyl dextran (AED) with no enzyme-inhibitor activity was added to the control perfusate, and the effects of the parent inhibitor STZ not bound to dextran were also determined. Both DBI and STZ significantly reduced JtCO2 from 138 +/- 10 pmol X mm-1 X min-1 (control) to 30 +/- 4 and 30 +/- 9, respectively. In contrast to the indistinguishable effects on JtCO2, intraluminal pH measured close to the site of perfusion was 6.80 +/- 0.02 during DBI perfusion, whereas with STZ perfusion the pH was 7.24 +/- 0.04 (P less than 0.001). Using the collected perfusate total CO2 concentration and a renal cortical PCO2 of 60 mmHg, the calculated equilibrium pH for this solution was 7.27. DBI inhibited only luminal carbonic anhydrase, therefore. We conclude that luminal carbonic anhydrase is in functional contact with proximal tubule fluid and is necessary for at least 80% of bicarbonate reabsorption by this segment.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Determinants of peritubular capillary fluid uptake in hydropenia and saline and plasma expansion.

Hydrostatic (HPc) and oncotic (phic) pressures within the peritubular capillary, tubular pressure (Pt), nephron filtration rate, and plasma flow, and proximal fractional and absolute reabsorption (APR) were measured in anesthetized rats during hydropenia and plasma and saline expansion. Net interstitial pressure (phii-HPi) was estimated from subcapsular hydrostatic and oncotic pressures during saline expansion and these data were applied to a mathematical model of peritubular capillary fluid uptake to determine the profile of effective reabsorption pressure (ERP) with distance (x*) alongthe capillary and calculate the peritubular capillary permeability coefficient (LpAr). ERPX* = (PHIC MINUS HPC)X* MINUS (PHII MINUS HPi) and APR = ERPX*LpAr.During saline expansion phii minus HPi was -12.1 plus or minus 0.8 mmHg and ERP,3.8 mm. The LpAr was 0.07 nl/s per g KW per mmHg, and this value was applied to hypropenia and plasma expansion to determine ERP and phii minus HPi. The phiiminus HPi was +6.0 and +5.0 mmHg, respectively, and ERP was 4.1 and 3.5 mmHg.Efective reabsorptive pressure remained positive along x* in all states, and phii minus HPi correlated best changes in phic and poorly with changes in efferent plasma flow. The APR did not correlate with either calculated phii minus HPi or the transepithelial driving pressure, Pt + phii minus HPi.