Renal response of the new-born dog to a saline load: the role of intrarenal blood flow distribution

1. The renal response to an intravenous saline load was studied in thirty new-born mongrel dogs aged 1-30 days and in seven adult dogs.2. After the infusion of isotonic saline at a rate of 2.0 ml. min(-1) kg(-1) for 15 min and then at a rate of 0.5 ml. min(-1) kg(-1) for an additional 105 min, the puppies excreted a significantly smaller fraction of the infused sodium than did the adults (P < 0.01).3. Both puppies and adults increased their glomerular filtration rates after the saline load. However, adults excreted a larger fraction of their filtered sodium than did the puppies (P < 0.01).4. There was no correlation between age and the ability to respond to a saline load during the first month of life.5. Intrarenal blood flow distribution was measured by the radioactive microsphere technique. After saline infusion in the puppy there was an increase in outer cortical blood flow but inner cortical blood flow remained relatively constant. The result was a decrease in the ratio of inner to outer cortical blood flow (IC/OC ratio). In contrast the IC/OC ratio tended to increase in the adult dog after saline expansion.6. There was no correlation between the magnitude of change of sodium excretion and the change of intrarenal blood flow distribution in the puppy.7. These results confirm that the natriuretic response of the new-born dog kidney is less efficient than that of the adult dog. This is due primarily to the failure of the puppy kidney to decrease fractional sodium reabsorption. Although the new-born dog alters intrarenal blood flow distribution in response to saline loading this alteration does not appear to play a significant role in sodium excretion.     

Hemodynamic and excretory response of the neonatal canine kidney to acute volume expansion.

It is generally recognized that developing animals retain sodium due to an enhanced reabsorption in distal tubule segements, even when the amount administered is in excess of their needs. This study was designed to test the relationship between this relative inability to dispose of a saline load and the functional characteristics of the kidney during postnatal maturation. In addition, we explored the role played by some of the factors known to affect natriuresis in the adult subject. Measurements of sodium excretion, glomerular filtration rate (GFR), and renal blood flow (RBF) and its intrarenal distribution were made in three age groups of puppies and in adult dogs. During expansion the GFR rose rapidly and to a similar extent at all ages, but it fell thereafter, the rate of decline being much slower in adult than in developing animals (P less than 0.001). RBF and its intrarenal distribution were not altered by volume expansion. The degree of natriuresis did not reflect either the age-related or the expansion-induced changes in GFR. Fractional and absolute sodium excretion were substantially higher in 2-wk-old puppies than in either 1- or 3-wk-old animals (P less than 0.002). These findings demonstrate that the blunted renal response of the maturing animal to saline loading is due to the persistence of an enhanced tubular reabsorption rather than to a limitation in glomerular filtration.     

Factors affecting the maturation of renal PAH extraction in the new-born dog

1. The maturation of renal para-amino-hippurate extraction (E(PAH)) was studied in thirty-nine mongrel new-born dogs aged 2-29 days. E(PAH) was also measured in five adult dogs for comparison.2. There was no apparent change of PAH extraction during the first month of life but E(PAH) was lower in the puppy than in the adult. Under mannitol diuresis E(PAH) averaged 0.49 for the puppies and 0.83 for the adult dogs (P < 0.001). E(PAH) averaged 0.48 for the puppies under nondiuretic conditions, a value not significantly different from that observed during mannitol diuresis. There was no correlation of E(PAH) with mean arterial blood pressure.3. Inulin extraction (E(In)) remained constant during the first month of life and was the same in the new-born and adult dog suggesting that preglomerular shunting of plasma away from functioning nephrons was the same for both the puppy and adult dog.4. E(PAH) did not change in the puppy when plasma concentrations varied from trace (using [(3)H]PAH) to 0.25 mM indicating that excessive PAH load is not a factor in the low E(PAH) of the puppy.5. Diffusion of PAH from the red blood cell was excluded as an explanation for the low E(PAH) since diffusion was the same for both the adult and new-born dog.     

Effect of infusion of pharmacologic amounts of vasopressin on renal electrolyte excretion.

Aqueous vasopressin was infused to bicarbonate- and glucose-loaded dogs and to nonloaded antidiuretic dogs in doses of 50 mU/kg per min or 50 mU/kg per h. Both doses caused a marked increase in sodium, chloride, and water excretion. The larger dose raised the fractional excretion (sodium clearance (C-Na)/glomerular filtration rate (GFR) times 100) of these ions from 2% or less to in excess of 20%. Blocking the pressor effects of these doses of vasopressin with sodium nitroprusside did not alter the marked natriuretic and chloriuretic effect. The maximal rate of bicarbonate and glucose reabsorption was not depressed by vasopressin infusion; fractional phosphate excretion, however, was markedly increased. Inhibiting distal hydrogen ion secretion by inducing selective aldosterone deficiency failed to uncover a vasopressin-induced inhibition of proximal bicarbonate reabsorption that might have been masked by increased distal bicarbonate reabsorption. There was no significant change in GFR, renal plasma flow, or filtration fraction. The distribution of cortical renal blood flow (measured by the radioactive microsphere technique) shifted toward the inner cortex after vasopressin administration. Vasopressin, in pharmacologic doses, is a potent diuretic that most likely exerts this effect by directly inhibiting sodium reabsorption at a point in the nephron distal to the proximal tubule.     

Lithium-induced uremia in rats — a new model of chronic renal failure

Three groups of new-born rats were studied: Group Li/Li treated with Li for 16 weeks, group Li/C treated for 8 weeks followed by 8 weeks without Li, and Group C/C 16 weeks old controls. Both Li-treated groups showed severe reduction of renal function, particularly group Li/Li, where the mean GFR was reduced by 80%. Plasma urea, creatinine, and osmolality were increased, blood hemoglobin and hematocrit were reduced, whereas plasma Na, K, and standard bicarbonate were unchanged. Na clearance was maintained and fractional Na excretion thus increased. Fractional Li excretion was also increased, indicating inhibition of proximal tubular salt and water reabsorption. Renal concentrating ability was markedly reduced. When Li was withdrawn, plasma urea levels remained unchanged or continued to rise, and the concentrating defect persisted. The results demonstrate that Li administration to new-born rats causes irreversible chronic renal failure which may progress even in the absence of Li. This model of chronic renal failure has several characteristics in common with chronic renal failure in humans.     

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.     

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.     

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.     

Attenuation of urinary sodium excretion during cold-air exposure in trained athletes.

To clarify whether an increase in urinary sodium (Na) excretion during cold-air exposure is attenuated in trained athletes, we analyzed the urinary and hormonal responses to cold-air exposure at 15 degrees C in trained (TR, n = 9) and untrained men (UT, n = 9). During 15 degrees C exposure in the UT group, the urinary Na excretion (UNaV), fractional excretion of Na and urinary Na to K ratio (UNa/K) increased significantly (p<0.01-0.05), but there were no variations in creatinine clearance or the filtered Na load. In the TR group, however, no significant changes were demonstrated in these parameters. The amount of increase in plasma noradrenaline and of decrease in plasma volume were greater, and the ADH and adrenaline responses were smaller in the TR group than those found in the UT group during 15 degrees C air exposure. A significant positive correlation was demonstrated between the increase of UNaV and both the relative changes of UNa/K and mean blood pressure. These results indicated that natriuresis during cold-air exposure was induced by the decrease in tubular reabsorption of Na, and that natriuresis was attenuated in trained athletes.     

Effect of splanchnicotomy on the renal excretion ofd-glucose in the anaesthetized dog

Summary The effects of acute intravenousd-glucose (G) loading were studied on anaesthetized, unilaterally splanchnicotomized (renal denervation) dogs. Glomerular filtration rate (GFR) was generally not different on the innervated and denervated side, while urine flow (V), sodium excretion (U_NaV) and urinary excretion (U_GV) of glucose on the splanchnicotomized side were significantly increased at any plasma G concentration. Tubular reabsorption (T_G) as well as Tm of G in denervated kidneys was considerably depressed. In a series of experiments with moderately elevated plasma glucose level glucosuria on the sympathectomized side was found that seems to be the consequence of a lower threshold for G in denervated kidneys. The positive correlation between the tubular reabsorption of Na and G was not influenced by renal denervation.     

Abnormal relationship between sodium excretion and hypertension in spontaneously hypertensive rats

Summary Inulin clearance, single nephron glomerular filtration rate (SNGFR), Na and K excretion were studied following an acute saline infusion in spontaneously hypertensive (Okamoto strain) rats (SH). 1. Hypertonic saline load: experiments were performed in adult and young SH rats. As compared to control normotensive Wistar rats (NT), the sodium excretion rose much less following the load in SH. During the 75 min following the beginning of the load, adult SH excreted 15% (NT: 58%) and young SH 9% (NT=38%) of the sodium load (P<0.01 in both cases). Cin were similar in SH and NT during control period, and a similar increase was observed following the load. The superficial to juxtamedullary SNGFR ratio was 0.80±0.05 in 5 non diuretic adult SH, a value not different from that found in normal rats, and 0.87±0.07 in 3 salt loaded SH, indicating that no significant intrarenal GFR redistribution occurs in SH following an acute hypertonic saline load. 2. Following an isotonic saline load, the results were not different: the percentage of the sodium load excreted was 44±5% and 10±2% in NT and SH adult rats respectively.The possibility that the observed defect in sodium excretion of SH rats involves distal tubular function is suggested by the pattern of evolution in Na and K excretion.This work was partially presented at the Symposium on Current problems of Hypertension, 1973, Mainz (Germany).Maître de Recherches INSERM.     

Influence of urinary sodium excretion on the clinical assessment of renal tubular calcium reabsorption in hypercalcaemic man.

The relation between urinary sodium excretion (NaE) and renal tubular calcium reabsorption (TmCa/GFR) was assessed in patients with hypercalcaemia associated with malignancy and primary hyperparathyroidism. On acute saline loading of seven normally hydrated patients with primary hyperparathyroidism and five patients with malignancy, raised values of TmCa/GFR were reduced to normal in most cases, in association with increases in NaE. The reduction in TmCa/GFR, which occurred, may have been due to a reduction in proximal tubular calcium reabsorption associated with sodium: this would have obscured the effect of humorally mediated increases in distal tubular calcium reabsorption, which are stimulated either by parathyroid hormone or by a putative humoral mediator in hypercalcaemia of malignancy. In patients who were normally hydrated NaE and TmCa/GFR were not significantly correlated. When data were included from patients who were dehydrated and from those undergoing acute saline loading, significant inverse correlations between NaE and TmCa/GFR were observed both in primary hyperparathyroidism (r = -0.49; p less than 0.02) and malignancy (r = -0.60; p less than 0.001). In clinical practice changes in TmCa/GFR associated with sodium seem to be of minor importance under normal circumstances, but they become evident at the upper and lower extremes of urinary sodium excretion. In clinical studies of renal calcium handling urinary sodium excretion must also be assessed, as interpreting TmCa/GFR data is difficult in states of excessive sodium loading or depletion.     

Neural control of renal excretory function during behavioral stress in conscious dogs

The renal and neural mechanisms underlying the excretory response to behavioral stress (aversive conditioning) were examined in 30 conscious dogs. Twenty-one dogs decreased urine flow more than 20% during stress, whereas 9 dogs showed less than a 10% decrease. In 11 of the 21 renal-reactive dogs, decreases in urine flow (42%) and sodium excretion (45%) were associated with unchanged glomerular filtration (-1.5%; GFR; inulin clearance) and effective renal blood flow (-4%; RBF; p-aminohippurate clearance). In the other 10 renal-reactive dogs, similar declines in urine flow (54%) and sodium excretion (52%) occurred with decreases in GFR (24%) and RBF (27%). Among all 30 dogs, greater increases in cardiovascular activity during stress were associated with greater decreases in renal excretion. Surgical renal denervation abolished the excretory response to stress in 4 of 5 dogs. These findings suggest that excretory responses in most dogs are mediated 1) primarily via increased tubular reabsorption rather than decreased GFR, 2) via central integration with cardiovascular responses, and 3) via the renal nerves.     

Maleate-induced bicarbonaturia in the dog: a carbonic anhydrase-independene effect.

Studies were performed to characterize the renal effects of maleate in anesthetized dogs. Following the intravenous administration of maleate or maleic acid (50 mg/kg), mean fractional bicarbonate excretion (CHCO3/GFR) rose to as high as 26%. Na, K, and phosphate excretion also increased markedly, whereas C1 excretion remained low. An initial transient fall in urinary pH from 6.53 to 6.13 contrasted sharply with the rapid alkalinization of the urine induced by acetazolamide administration. During saline expansion CHCO3/GFR rose from 4 to 37% after maleate administration, whereas Cl excretion did not change significantly. During continuous carbonic anhydrase inhibition with acetazolamide, maleate administration resulted in a further rise in CHCO3/GFR from 22 to 35%. Whereas CPO4/GFR increased only from 1 to 3% during acetazolamide administration, this ratio reached 75% following the addition of maleate. Fumarate, the transisomer of maleate, and malonate, a well-known inhibitor of Krebs cycle, failed to affect bicarbonate excretion. This study demonstrates that maleate inhibits the fraction of bicarbonate reabsorption uncatalyzed by carbonic anhydrase. Impaired anionic reabsorption of bicarbonate or accelerated passive backflux of this ion into proximal tubular lumen are the two mechanisms that best explain the bicarbonaturia induced by maleate.     

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.     

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.     

Effect of splanchnicotomy on the renal excretion of para-aminohippuric acid in the anaesthetized dog

Summary Renal excretion of para-aminohippuric acid (PAH) was studied during PAH loading on unilaterally splanchnicotomized (denervated) anaesthetized dogs. Urine flow, sodium excretion of denervated kidneys were significantly increased. Below a plasma concentration of 20 mg% there were no differences between intact and denervated kidneys in urinary excretion and in calculated tubular transport of PAH. However, maximum secretion rate on the splanchnicotomized side was significantly decreased (innervated: 34.8, denervated: 25.2 mg/100 ml GFR, respectively). Although both Na reabsorption and PAH secretion are impaired by denervation, the exact mechanism of action of renal sympathectomy is not elucidated as yet.     

The secretion of organic acids and bases by the ovine fetal kidney

Excretion of organic acids and bases was studied in twelve fetal sheep aged 120-140 days. There was no significant plasma protein binding of the organic anion, p-aminohippurate (PAH), nor of the organic cation, [14C]tetraethylammonium (TEA). There was a significant amount of acetyl-PAH (20 +/- 3%) in fetal urine but none could be detected in fetal plasma. The fractional excretion of unconjugated PAH was less than one, i.e. there was net reabsorption of 31.7 +/- 3.9% of the filtered load of unconjugated PAH. Since there was no acetyl-PAH in fetal plasma it is concluded that all acetyl-PAH in fetal urine occurred as a result of metabolism of PAH and secretion of the metabolite into the tubular lumen. The rate of excretion of acetyl-PAH in fetal urine varied from 0 to 14.0 micrograms min-1. Thus unconjugated PAH is filtered and there is net reabsorption; in addition, PAH is metabolized and enters the urine via tubular mechanisms. The fractional excretion of PAH was unaffected by I.V. administration of penicillin either acutely or chronically. The clearance of [14C]TEA was significantly greater than the glomerular filtration rate (GFR). The mean fractional excretion of [14C]TEA was 5.4 +/- 0.17. Thus 80.7 +/- 0.63% of the excreted TEA was secreted. The clearance of TEA was related to body weight (P less than 0.001) but the fractional excretion of TEA declined with gestation age, probably because GFR increased at a greater rate than the rate at which the secretory pathways increase their activity. It is concluded that those pathways that excrete organic anions like PAH into the urine mature much later (probably after birth) than those pathways responsible for the tubular secretion of organic bases.     

Bicarbonate reabsorption in chronic renal failure studies in man and the rat

Bicarbonate reabsorption during bicarbonate loading was measured in rats and in patients with chronic renal failure (CRF) and in normal human subjects. Bicarbonate reabsorption expressed as Tm/GFR was not significantly different between normal and CRF rats despite the fact that fractional chloride excretion was significantly greater (an indication of volume expansion) in CRF rats than in normals. The ratio of absolute bicarbonate reabsorption/absolute sodium reabsorption, a parameter which corrects for the influence of volume on bicarbonate reabsorption, was significantly higher in CRF rats than in normals. In patients with CRF, bicarbonate reabsorption expressed either as Tm/GFR or as the ratio of absolute bicarbonate reabsorption/absolute sodium reabsorption was not different from that of the normal subjects. In patients with CRF, glomerular filtration rate (GFR) varied between 3 and 47 ml/min. The ratio of absolute bicarbonate reabsorption/absolute sodium reabsorption was inversely and significantly related to GFR, i.e. as nephron mass decreases sodium bicarbonate is preferentially reabsorbed over sodium chloride. These data demonstrate that in CRF bicarbonate reabsorption is enhanced relative to that of sodium and chloride in man.     

Effects of extracellular volume expansion on the tubular reabsorption of glucose

Summary Clearance and intratubular injections of (¹⁴C) glucose were performed in glucose loaded rats, during control (C) and extracellular fluid volume expansion (VE) to 10% of body weight. VE resulted in a significant decrease in hematocrit from 47.50±1.06 to 38.80±1.14% and plasma protein from 6.23±0.25 to 4.13±0.21 gm/100ml. Glomerular filtration rate (GFR) increased by 51% from 1.06±0.07 to 1.60±0.35 ml/min. Fractional excretion of sodium increased significantly from 0.42±0.07 to 12.58±1.25%. Maximal glucose reabsorption (TmG) was unchanged from 3.47±0.42 to 3.29±0.41 mg/min. However, TmG/GFR decreased significantly from 3.14±0.22 to 1.94±0.21 mg/ml GFR. As compared to C, VE resulted in a significant increase in (¹⁴C) glucose recovery after injection into the early and late proximal tubules, from 63±3 to 81±2% and 82±2 to 88±1% respectively. After distal tubular injections (¹⁴C) glucose recovery was complete in both C and VE; early distal injection 97±1 vs 98±1%, late distal injection 98±1 vs 99±1%. These results indicate an inhibitory effect of VE on fractional glucose reabsorption in the superficial nephron. There is no evidence for glucose reabsorption in the superficial distal nephron during C and VE.