Glomerulotubular balance during renal sympathetic stimulation

In volume-expanded dogs receiving ethacrynic acid, a linear relationship, glomerulotubular balance (GTB), applies between the remaining sodium reabsorption and the glomerular filtration rate (GFR) during mechanical aortic constriction. To examine whether GTB applies during sympathetic stimulation, the GFR was progressively reduced by 70-75% in anaesthetized dogs by renal nerve stimulation, intrarenal norepinephrine infusion or by selective stimulation of alpha-adrenoceptors by intrarenal methoxamine infusion. Linear relationships (GTB) were obtained (r greater than 0.9). Reabsorption was not different during the various kinds of sympathetic stimulation, but less than during aortic constriction; the largest difference in NaCl reabsorption at comparable GFR amounted to 10-15% and was obtained 30-40% below control GFR, whereas inhibition of NaHCO3 reabsorption was uncertain. To inhibit NaHCO3 reabsorption and associated NaCl reabsorption in the proximal tubules, acetazolamide (30 mg kg-1) was administered instead of ethacrynic acid. No difference in reabsorption was observed at comparable GFR during norepinephrine infusion and mechanical aortic constriction. Hence, GTB applies during sympathetic stimulation. Compared with data obtained during aortic constriction, alpha-adrenergic stimulation reduces proximal reabsorption of NaCl and, possibly, NaHCO3 and exerts no effect on distal transcellular NaCl reabsorption.     

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.     

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.     

How bicarbonate loading inhibits tubular reabsorption of NaCI in dog kidneys

During continuous infusion of ethacrynic acid in dogs, changes in glomerular filtration rate (GFR) and P_co2 at constant plasma bicarbonate concentration (P_HCO3) alter bicarbonate and chloride reabsorption in a ratio of 1: 2. This ratio did not apply when P_HCO3 was raised by bicarbonate loading in 11 anaesthetized volume-expanded dogs. A rise in P_HCO3 from 30 to 54 mM at constant P_co2 and GFR reduced sodium reabsorption during ethacrynic acid infusion from 3586 ± 725 to 2449 ± 403 μmol 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 P_co2, the inhibitory effect on chloride reabsorption was halved. At constant plasma pH 7.5 a rise in P_HCO3 from 20 to 30 mM reduced chloride reabsorption by 20%. A further 30% inhibition was caused by raising P_HCO3 from 30 to 54 mM. Bicarbonate reabsorption was highest at P_HCO3 54 suggesting a large capacity for bicarbonate reabsorption if P_HCO3 is raised at constant plasma pH 7.5. Water and NaCI reabsorption remaining during ethacrynic acid infusion is almost equally inhibited by alkalosis and by an osmotic effect of unreabsorbed NaHCO₃.     

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.     

Glomerular filtraton rate and PCO2 as determinants of lithium reabsorption

To examine whether lithium reabsorption varies in proportion to the bicarbonate-dependent reabsorption of water and chloride, reabsorption was altered by varying P_CO2 and glomerular filtration rate (GFR) in volume-expanded, anesthetized dogs during ethacrynic acid infusion. At constant GFR and plasma bicarbonate concentration, lithium, bicarbonate, chloride and water reabsorption were inversely related to plasma pH during variations in PCO2. Lithium and bicarbonate reabsorption varied by 9±1% and chloride reabsorption by 7±1% as plasma pH was altered by 0.1 unit from plasma pH 7.5. Calculation of reabsorbate concentrations indicated that lithium was reabsorbed as readily as water (reflection coefficient=0). During mechanical constriction of the suprarenal aorta, GFR was reduced at constant plasma pH. Bicarbonate reabsorption fell more than chloride, water and lithium reabsorption. Lithium reabsorption was not significantly reduced until GFR was reduced by 35%. In stop-flow studies during ouabain infusion, urinary lithium concentrations were reduced below plasma concentrations. This is compatible with passive diffusion of lithium along a lumen-positive potential exceeding 10 mV in the diluting segment. Thus, lithium reabsorption behaved as expected for bicarbonate-dependent paracellular reabsorption during variations in P_CO2; when GFR is reduced, an additional component of lithium reabsorption is disclosed.     

Mechanism of osmotic diuresis studied by infusion of NaHCO3 and mannitol in dogs

To examine whether mannitol and NaHCO3 are equally potent inhibitors of proximal tubular fluid reabsorption, experiments were performed in 10 anaesthetized volume-expanded dogs during continuous infusion of ethacrynic acid. At plasma pH 7.5, a rise in plasma osmolality of 40 mosmol kg-1 reduced the remaining tubular fluid reabsorption in five dogs by 14 +/- 3% during NaHCO3 infusion and by 28 +/- 1% during mannitol infusion. Bicarbonate reabsorption increased by 25 +/- 5% during NaHCO3 infusion and decreased by 14 +/- 1% during mannitol infusion. At equal rates of bicarbonate reabsorption the inhibitory effects on tubular fluid and NaCl reabsorption were slightly less during mannitol than during NaHCO3 infusion. In five other dogs studied at constant plasma concentration of sodium, changes in bicarbonate reabsorption were avoided by raising plasma pH to 7.7 during NaHCO3 infusion and by reducing plasma pH to 7.4 during mannitol infusion. Tubular fluid reabsorption was reduced 32 +/- 4% by NaHCO3 and 34 +/- 4% by mannitol infusion, indicating equal inhibitory effects. The mechanism may be that the osmotic force for paracellular reabsorption of water and NaCl across the tight junction is equally reduced by equiosmolal increments in the NaHCO3 and mannitol concentration of the proximal tubular fluid.     

Effect of sodium nitrate loading on electrolyte transport by the renal tubule.

Effects of sodium nitrate were compared with sodium chloride loading on transport of electrolytes by the nephron. Maximal levels of free water clearance/clomerular filtration rate (CH2O/GFR) averaged 8.4% with nitrate loading and 14.4% with saline loading. Since ethacrynic acid and chlorothiazide exert their major natriuretic effect in the distal nephron, the increment in Na ad Cl reabsorbed beyond the proximal tubule. The administration of these agents resulted in an increase in fractional sodium excretion (CNa/GFR) of 21.1%, urinary sodium excretion (UNaV) of 1,126 mueq/min, and urinary chloride excretion (UClV) of 848 mueq/min during nitrate loading compared with an increase in CNa/GFR of 37.6%, UNaV of 2,362 mueq/min, and UClV of 2,397 mueq/min during saline loading. The smaller diuretic-induced increment in Na and Cl excretion in the nitrate studies suggests, as do the hydrated studies, that less Cl and Na are reabsorbed in the distal nephron during nitrate than saline loading. At every level of UNaV, fractional bicarbonate reabsorption was higher, urine pH was lower, and urinary potassium excretion (UKV) was higher in the nitrate studies. Thus, compared with saline loading, sodium nitrate decreases chloride and sodium reabsorption in the distal nephron. The higher hydrogen and potassium secretion in the nitrate studies may be consequent to the decreased ability of the distal nephron to reabsorb chloride.     

Effect of parathyroid hormone secretion on sodium reabsorption by the proximal tubule.

To determine if an increase in the endogenous secretion of parathyroid hormone could decrease sodium reabsorption by the proximal tubule, the ionized calcium concentration of blood perfusing the parathyroid gland of eight unilaterally thyroid parathyroidectomized dogs (TPTX) was reduced by infusion of an isotonic sodium citrate plus sodium chloride solution into the blood supply of the parathyroid gland. The fractional clearance of phosphate increased significantly (+9.3 +/- 2.8 ml/min per 100 ml GFR), while fractional sodium reabsorption by the proximal tubule decreased (-.06 +/- .02; P less than .025). In seven normal control dogs that received isotonic sodium chloride infusion, neither fractional sodium reabsorption by the proximal tubule nor the fractional clearance of phosphate was significantly altered. In five bilaterally TPTX dogs that received a sodium citrate plus sodium chloride infusion, sodium reabsorption by the proximal tubule was not significantly altered. There were no significant changes in glomerular filtration rate or renal plasma flow in any of these groups. The data demonstrate that alterations in endogenous parathyroid hormone secretion can play a significant role in the regulation of sodium reabsorption by the proximal tubule.     

Inhibition of transcellular NaCl reabsorption in dog kidneys during hypercalcemia

Reduced concentrating and diluting capacity of the kidney in acute and chronic hypercalcemia may partly be due to inhibition of transcellular sodium reabsorption (RNa) in the thick ascending limb of Henle's loop. To examine this hypothesis, local heat production and RNa were measured during normo- and hypercalcemia at comparable glomerular filtration rate (GFR) in volume expanded, anesthetized dogs. Changes in proximal RNa which might occur during CaCl2 infusion, were minimized by infusing acetazolamide (75 mg/kg body wt iv). When ultrafiltrable calcium was increased from 1.12 +/- 0.09 to 2.95 +/- 0.10 mmol/l, cortical heat production was unchanged, whereas outer medullary heat production fell by 32 +/- 4%. RNa was reduced by 32 +/- 6%. Bicarbonate reabsorption did not change but calcium reabsorption and potassium excretion increased significantly. The potassium content of cortex and outer medulla increased during hypercalcemia, whereas ouabain, an inhibitor of Na+, K+-ATPase reduces the potassium content. We conclude that hypercalcemia does not inhibit transcellular RNa in the diluting segment by a direct effect on the Na+, K+-ATPase or the mitochondria, but by interfering with the coupled NaCl transport across the luminal cell membrane.     

Renal Na-K-ATPase activity during saline infusion in the rabbit.

During saline infusion, sodium reabsorption (RNa) in the diluting segment (thick ascending limb of Henle's loop) increases acutely. The mechanism for this higher pumping rate of outer medullary Na-K-ATPase is unknown. Following left-sided nephrectomy, immediate i.v. infusion of hypertonic saline increased RNa in the remaining whole right kidney by 28 +/- 14% (p less than 0.05). Na-K-ATPase activity in outer medulla was raised by (delta) 23 +/- 4% above the left kidney (p less than 0.05), whereas cortical activity was unchanged. The mechanism for this increase in Na-K-ATPase activity was explored. The catalytic rate per enzyme did not differ in the two kidneys and equalled 5 340 min-1. The increase was therefore due to higher tissue concentration of active enzyme. The response was fully developed during continuous infusion within 20 min, and of equal magnitude whether protein synthesis had been inhibited by cycloheximide (delta = 23 +/- 7%) or stimulated by unilateral nephrectomy 6 days earlier combined with saline infusion for 2 h (delta = 34 +/- 10%). Thus, during hypertonic saline infusion, the increased RNa in the outer medulla was partly accounted for by the activation of latent Na-K-ATPase. High delivery of sodium to the diluting segment for more than 20 min during hypertrophy caused no further activity change.     

Evidence for bicarbonate-dependent lithium reabsorption in dog kidneys

To examine whether lithium is reabsorbed along a transcellular or a paracellular route, experiments were performed in anesthetized volume-expanded dogs under conditions of constant glomerular filtration rate (GFR). Ouabain, in doses inhibiting about 80 % of Na,K-ATPase, and ethacrynic acid, another inhibitor of transcellular NaCl reabsorption, did not inhibit lithium or bicarbonate reabsorption. Lithium reabsorption increased in proportion to plasma concentration of lithium (P_Li) up to 12 mM, suggesting a passive transport of lithium. During ouabain administration acetazolamide halved bicarbonate reabsorption, the main driving force for paracellular reabsorption, and halved the reabsorption of lithium. The reabsorbate concentration of lithium, calculated from data obtained before and after acetazolamide infusion, was almost equal to P_Li. Mannitol, which reduces paracellular osmotic transport without affecting bicarbonate reabsorption, reduced lithium and chloride reabsorption in the same proportion as acetazolamide (r=0.87). Combined acetazolamide and mannitol administration reduced fractional lithium reabsorption to 0.09±0.02. These data indicate that lithium is not actively transported but reabsorbed passively along a paracellular route by osmotic forces provided by transcellular NaHCO₃ reabsorption.     

Dopamine-induced dissociation between renal metabolic rate and sodium reabsorption.

The stimulatory effect of dopamine on renal energy metabolism and its relationship to changes in tubular sodium reabsorption and plasma concentration of free fatty acids (FFA) were examined in anesthetized dogs. Dopamine infused intravenously at 25 mug/kg body wt-min for 30-60 min increased renal oxygen consumption (Rvo2) by 28 +/- 3%; glomerular filtration rate rose from 37 +/- 3 to 40 +/- 2 ml/min without significant changes in sodium excretion. Plasma FFA increased about 6 times. Total-body metabolic rate increased to 152 +/- 7% and fell to 119 +/- 5% of control after normalizing plasma FFA by beta-pyridylcarbinol; Rvo2 remained unchanged. Cortical and outer medullary heat accumulation rated increased to 137 +/- 6 and 133 +/- 10% after 1 h and to 163 +/- 18 and 179 +/- 26% of control after 2 h of dopamine infusion without further changes in sodium reabsorption. Furosemide reduced cortical and outer medullary metabolic rates as much as in control experiments (14 +/- 8 and 69 +/- 7%, respectively). Hence, dopamine exerts a renal calorigenic effect which cannot be accounted for by increased sodium reabsorption or attributed to increased supply of FFA.     

Relationship between glucose and sodium excretion in the new-born dog

1. The relationship between renal glucose and sodium excretion was studied in thirty-three new-born dogs aged 1-14 days and in ten adult dogs.2. Glucose was infused into the animals at rates sufficient to produce an amount of filtered glucose at least 1.5 times the tubular transport of glucose (saturating glucose load). In both puppies and adults tubular glucose reabsorption at saturating glucose loads varied directly with the glomerular filtration rate (r = 0.54 and 0.73 respectively, P < 0.01 for both).3. In the puppy, as the fraction of filtered sodium excreted (C(Na)/C(In)) increased from 0.05 to 0.45, the ratio, renal tubular glucose transport divided by glomerular filtration rate at saturating glucose loads, (T(G)/GFR)(m), decreased from 3.7 to 1.7 mg/ml. (r = -0.75, P < 0.01). In the adult C(Na)/C(In) was below 0.08 in all experiments and (T(G)/GFR)(m) was within the 95% confidence limits predicted by regression analysis of the data from puppies. Although mean (T(G)/GFR)(m) was greater in the adult than in the puppy (P < 0.01), when puppies had C(Na)/C(In) similar to that for adults, they had (T(G)/GFR)(m) values equivalent to that for the adult.4. There was excellent correlation between glucose excretion and water excretion for both adult and new-born dogs (r = 0.93 and 0.87, respectively). However, for any glucose loss, water loss was greater in the puppy than in the adult (P < 0.01).5. During the control period total sodium excretion (per gram kidney) and C(Na)/C(In) were similar in the new-born and adult dog. However, during glucose loading, the puppies excreted more sodium and had a higher C(Na)/C(In) than did the adult, although glucose excretion was greater in the adult than in the puppy (P < 0.01 for all comparisons).6. Glomerular blood flow, as measured by radioactive microspheres, was redistributed towards inner cortical nephrons during glucose loading in the puppy. There was no such redistribution of glomerular blood flow in the adult.7. Sodium reabsorption beyond the proximal tubule was blocked with ethacrynic acid and chlorothiazide. In the puppy, the increase of C(Na)/C(In) following a glucose load was the same whether the glucose load followed control or distal blockade collections, suggesting that reductions of sodium reabsorption following a glucose load probably came from the proximal tubule. C(Na)/C(In) during glucose loading plus distal blockade was significantly (P < 0.01) higher in the puppy (0.598) than in the adult (0.280), indicating that glucose diuresis produced a greater inhibition of proximal tubular sodium reabsorption in the new-born than in the adult dog.These results support the hypothesis that the high sodium excretion rate during glucose diuresis in the new-born dogs appears to be due to the greater sensitivity of the neonatal proximal tubule to the osmotic effect of glucose. When presented with a glucose osmotic load the new-born dog diminishes net proximal sodium reabsorption more than does the adult and thus depresses tubular glucose reabsorption to a greater extent. The lower values of maximal glucose transport rates found in new-born animals may be related, therefore, to the higher fractional sodium excretion rates during glucose diuresis rather than to a diminished intrinsic glucose transport capacity in the new-born kidney.     

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.     

Renal Na-K-ATPase Activity during Saline Infusion in the Rabbit

During saline infusion, sodium reabsorption (R_Na) in the diluting segment (thick ascending limb of Henle's loop) increases acutely. The mechanism for this higher pumping rate of outer medullary Na-K-ATPase is unknown. Following left-sided nephrectomy, immediate i.v. infusion of hypertonic saline increased R_Na in the remaining whole right kidney by 28 ± 14% (p < 0.05). Na-K-ATPase activity in outer medulla was raised by (A) 23 + 4% above the left kidney (p < 0.05), whereas cortical activity was unchanged. The mechanism for this increase in Na-K-ATPase activity was explored. The catalytic rate per enzyme did not differ in the two kidneys and equalled 5 340 min^-1. The increase was therefore due to higher tissue concentration of active enzyme. The response was fully developed during continuous infusion within 20 min, and of equal magnitude whether protein synthesis had been inhibited by cycloheximide (Δ= 23 ± 7%) or stimulated by unilateral nephrectomy 6 days earlier combined with saline infusion for 2 h (Δ= 34+ 10 %). Thus, during hypertonic saline infusion, the increased R_Na in the outer medulla was partly accounted for by the activation of latent Na-K-ATPase. High delivery of sodium to the diluting segment for more than 20 min during hypertrophy caused no further activity change.     

Site and magnitude of the tubular inhibitory effect of expanding the extracellular volume in dogs

Ethacrynic acid inhibits energy-requiring transcellular NaCl reabsorption without affecting NaHCO3 reabsorption. Acetazolamide inhibits NaHCO3 and most of the remaining NaCl reabsorption in the proximal tubules (bicarbonate-dependent reabsorption) but raises distal transcellular NaCl reabsorption. After administration of both diuretics, the remaining bicarbonate-dependent and transcellular reabsorptions become constant until glomerular filtration rate (GFR) is almost halved. The inhibitory effect of expanding the extracellular volume (ECV) until plasma volume and GFR increased 30-40% was examined in anesthetized dogs. Examinations at comparable GFR obtained by altering arterial perfusion pressure showed that the inhibitory effect of ECV expansion was attenuated by administering acetazolamide. Ethacrynic acid amplified the inhibitory effect which for sodium and chloride reabsorption amounted to 6-7% of the filtered load at comparable GFR. An inhibitory effect of ECV expansion of bicarbonate reabsorption was disclosed only after raising plasma bicarbonate concentration. Thus, the small inhibitory effect of massive ECV expansion is confined to proximal tubular bicarbonate-dependent reabsorption and is of the same magnitude as previously demonstrated in experiments of similar design by raising plasma pH by only 0.07 unit. Since ouabain inhibits transcellular NaCl reabsorption, a natriuretic hormone is more likely to be an inhibitor of carbonic anhydrase than of Na,K-ATPase.     

Dependency of renal potassium excretion on Na,K-ATPase transport rate

Potassium secretion may depend on the transport rate of Na, K-ATPase in basolateral cell membranes of distal tubular cells. To examine this hypothesis experiments were performed in anaesthetized dogs during inhibition of proximal potassium reabsorption by acetazolamide or mannitol (fractional potassium excretion 1.2-1.4) or additional stimulation of potassium secretion by ethacrynic acid (fractional potassium excretion 2.1). Ouabain in a dose which inhibits 70–80% of the Na, K-ATPase activity reduced fractional potassium excretion to 0.8-0.9 by an effect on distal tubular secretion since potassium transport in the proximal tubules was not affected. Ouabain-sensitive potassium excretion varied in proportion to ouabain-sensitive sodium reabsorption during variation in glomerular nitration rate, even at urinary sodium concentrations exceeding 80 mmol 1^-1. In experiments without ouabain, saline infusion raised potassium excretion and sodium reabsorption until maximal Na, K-ATPase transport rate was reached, as judged from heat production measurements, but not during further increments in urine flow. After inhibition of Na, K-ATPase activity by hypokalaemia, potassium excretion and cortical heat production remained constant over a wide range of urine flow and sodium excretion. We conclude that potassium secretion is dependent on intact Na, K-ATPase activity and is stimulated by sodium delivery to the distal nephron until maximal transport rate of the enzyme is reached.     

Dependency of renal potassium excretion on Na,K-ATPase transport rate

Potassium secretion may depend on the transport rate of Na, K-ATPase in basolateral cell membranes of distal tubular cells. To examine this hypothesis experiments were performed in anaesthetized dogs during inhibition of proximal potassium reabsorption by acetazolamide or mannitol (fractional potassium excretion 1.2 - 1.4) or additional stimulation of potassium secretion by ethacrynic acid (fractional potassium excretion 2.1). Ouabain in a dose which inhibits 70-80% of the Na, K-ATPase activity reduced fractional potassium excretion to 0.8 - 0.9 by an effect on distal tubular secretion since potassium transport in the proximal tubules was not affected. Ouabain-sensitive potassium excretion varied in proportion to ouabain-sensitive sodium reabsorption during variation in glomerular filtration rate, even at urinary sodium concentrations exceeding 80 mmol X 1(-1). In experiments without ouabain, saline infusion raised potassium excretion and sodium reabsorption until maximal Na,K-ATPase transport rate was reached, as judged from heat production measurements, but not during further increments in urine flow. After inhibition of Na,K-ATPase activity by hypokalaemia, potassium excretion and cortical heat production remained constant over a wide range of urine flow and sodium excretion. We conclude that potassium secretion is dependent on intact Na,K-ATPase activity and is stimulated by sodium delivery to the distal nephron until maximal transport rate of the enzyme is reached.