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

Plasma potassium concentration as a determinant of proximal tubular NaCl and NaHCO3 reabsorption in dog kidneys

To examine whether an acute increase in plasma potassium concentration ([K]p) may inhibit proximal tubular transport, clearance studies were performed in seven anaesthetized, volume expanded dogs treated with amiloride (1 mg kg-1 body wt) to block tubular potassium secretion, and with bumetanide (30 micrograms kg-1 body wt) to inhibit NaCl reabsorption in Henle's loop. As [K]p was raised in steps from 2.6 +/- 0.2 to 7.9 +/- 0.2 mM, bicarbonate, chloride, and sodium reabsorption decreased by 232 +/- 56, 520 +/- 59 and 958 +/- 112 mumol min-1, respectively, at constant glomerular filtration rate (GFR). On average, the molar ratio between the inhibitory effects on bicarbonate and chloride reabsorption were 1:2.2-2.4. Reabsorption was calculated at GFR 100 ml min-1: (reabsorption 100/GFR (mmol min-1). It was inversely correlated to ln [K]p with r = -0.82 for bicarbonate and with r = -0.89 for chloride. Fractional potassium reabsorption remained constant at 0.31 +/- 0.03. Administration of acetazolamide (100 mg kg-1 body wt) in eight dogs at [K]p 8 mM reduced fractional reabsorption of bicarbonate, chloride and sodium as much as in previous studies on normokalaemic dogs. We conclude that acute elevation of [K]p inhibits NaHCO3 transport and passive proximal tubular NaCl reabsorption. This inhibition is not related to changes in potassium secretion and carbonic anhydrase activity, but may be secondary to depolarization of the basolateral membrane.

Low oxygen cost of carbonic anhydrase-dependent sodium reabsorption in the dog kidney

To examine the oxygen requirement of carbonic anhydrase-dependent sodium reabsorption in the proximal tubule, 18 anaesthetized dogs were studied under conditions of saturated distal NaCl reabsorption; the latter was accomplished by volume expansion (all groups) combined with infusion of loop diuretics (groups 1 and 3). Acetazolamide reduced HCO3- reabsorption by 602 +/- 32 mumol min-1 (55%, group 1) and by 777 +/- 103 mumol min-1 (66%, group 2). This was accompanied with a reduction in sodium reabsorption and oxygen consumption in a molar delta Na/delta O2 ratio of about 45 in both groups of dogs. The delta HCO3/delta O2 ratio averaged 16 +/- 1, which was not significantly different from the theoretical value of 18 expected for transcellular sodium transport by Na+, K+-ATPase. Mannitol (group 3) reduced NaCl reabsorption by 37 +/- 2% without affecting NaHCO3 reabsorption or oxygen consumption significantly. We conclude that carbonic anhydrase-dependent NaCl reabsorption in the proximal tubules is passive, and that NaHCO3 reabsorption is the only important active sodium transport which is sensitive to inhibition of carbonic anhydrase.

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.

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.

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.

Hypernatremia inhibits NaHCO3 reabsorption and associated NaCl reabsorption in dogs

To examine the effect of selective rise of plasma NaCl concentration (hypernatremia) on NaHCO3 reabsorption and associated NaCl reabsorption remaining during continuous ethacrynic acid infusion, hypertonic NaCl solution was infused in three groups of anesthetized volume-expanded dogs. In six dogs examined at constant hematocrit and plasma pH, bicarbonate and water reabsorptions were inversely related to PNa and reduced by 37% and 39% respectively by raising PNa from 140 to 200 mM. Chloride reabsorption remained essentially constant until PNa exceeded 170 to 180 mM. At PNa 200 mM, sodium reabsorption was reduced by 22 +/- 6%. In six other dogs, mechanical variations of GFR showed that the inhibitory effects of hypernatremia (PNa 199 +/- 3 mM) were less pronounced at low GFR. After subsequent administration of acetazolamide (30 mg/kg body wt), only 20% of control bicarbonate reabsorption remained and glomerulo-tubular balance was completely abolished. Both hypernatremia and acetazolamide inhibited NaHCO3 and NaCl reabsorption in a molar ratio of about 1:2, as in normonatremic dogs. Finally, experiments in six dogs showed that the inhibitory effects of hypernatremia (PNa 213 +/- 4 mM) were not altered by varying PCO2 and plasma pH. We conclude that hypernatremia inhibits paracellular water and NaCl reabsorption in the proximal tubules by reducing the osmotic force caused by transcellular NaHCO3 reabsorption. A rise in PNa does not stimulate transcellular NaCl reabsorption during distal inhibition by ethacrynic acid.