High-protein diets augment albuminuria in rats with Heymann nephritis by angiotensin II-dependent and -independent mechanisms.

Urinary albumin excretion (U(alb)V) increases following dietary protein augmentation (DPA) in nephrotic humans and rats. Angiotensin-converting enzyme inhibitors (ACEI) blunt, but do not entirely prevent, increased U(alb)V at doses that reduce blood pressure and entirely block the pressor effect of exogenously administered angiotensin I (Ang-I), suggesting that angiotensin II (Ang-II) might not mediate the effect of DPA on U(alb)V. We determined the effect of losartan (Los), a specific Ang-II receptor antagonist, and compared its effect to that of enalapril (En), an ACEI, on DPA-induced increase in U(alb)V in rats with passive Heymann nephritis (HN). When Los was administered to HN rats for 48 h prior to DPA from 8.5 to 40% casein. U(alb)V increased in an identical fashion in treated and untreated rats, even though Los caused hypotension and prevented the pressor effect of infused Ang-II. Only on day 6 after DPA did U(alb)V decrease. We then measured the effect of duration of pretreatment with Los on Ang-II binding to isolated glomeruli. Maximal inhibition of Ang-II binding required treatment with Los for 6 days. We then pretreated HN rats with either En or Los for 6 days prior to DPA. In contrast to administration of Los for 2 days prior to DPA, pretreatment with either Los or En for 6 days entirely prevented any increase in U(alb)V. We then increased dietary NaCl from 0.2% to 2% (HS) to determine whether En or Los would modulate U(alb)V after DPA when Ang-II activity was suppressed. En reduced the DPA-mediated increase in U(alb)V regardless of dietary NaCl, while Los was effective only in when dietary NaCl was reduced (0.2%), suggesting that under these conditions ACEI reduces U(alb)V by a mechanism that is independent of inhibition of Ang-II and that high protein diets augment U(alb)V by both Ang-II-independent and Ang-II-dependent mechanisms.

Non-iron mediated alteration in hepatic transferrin gene expression in the nephrotic rat.

Both transferrin and the iron it carries are lost in the urine in the nephrotic syndrome. Patients may develop hypochromic microcytic anemia and synthesis of transferrin, a protein regulated in large part by iron availability, is increased. Transferrin synthesis has also been reported to be increased in liver slices from rats with hereditary analbuminemia, and their plasma transferrin levels are increased, suggesting that transferrin synthesis may be stimulated by processes other than iron depletion in this hypoalbuminemic condition. Transferrin metabolism was studied in rats with Heymann nephritis (HN), in a strain of Sprague-Dawley (SD) rats with hereditary analbuminemia [Nagase analbuminemic rats (NAR)], and in normal SD rats. Plasma transferrin concentration and mass was decreased significantly in HN, but increased in NAR. Transferrin synthesis was increased both in NAR (measured either as the disappearance of [125I] labeled transferrin or as the incorporation of [3H] phenylalanine) and in HN (incorporation of [3H] phenylalanine). The fractional rate of transferrin catabolism was unchanged in NAR. Thus transferrin mass was increased in NAR entirely as a consequence of increased synthesis. Transferrin and albumin synthesis correlated with one another in both HN and SD (P < 0.001). Transferrin mRNA was increased in both HN and NAR and was unaffected by administration of iron to HN. Hepatic transferrin and albumin mRNA levels were also correlated positively in HN and SD, suggesting that increased hepatic synthesis of both proteins might be responding to the same stimuli. Transferrin gene transcription was increased in both HN and NAR and was unaffected by administration of iron to HN. Transferrin mRNA was not increased in the testis in either HN or NAR, suggesting that augmentation in transferrin gene expression is driven by a non-iron dependent process and is confined to the liver.

Interstitial exclusion of IgG in rat tissues estimated by continuous infusion.

Interstitial exclusion, defined as the fraction of interstitial fluid volume inaccessible to a solute, was evaluated for immunoglobulin G (IgG) in selected tissues of rats by a method previously applied to serum albumin (29). IgG distribution volumes were also measured for intestine. 125I-labeled rat IgG was infused for 5 or 7 days (n = 4 rats each) with an implanted osmotic pump (Alzet). At the termination of infusion, the rat was anesthetized, nephrectomized, and injected with 51Cr-labeled EDTA (4 h) to label total extracellular fluid volume and 131I-labeled bovine IgG (5 min) to label plasma volume. Samples of skin, muscle, and tendon were assayed for total and extractable tracer activity. Interstitial fluid from these tissues was sampled postmortem with nylon wicks for assay of 125I-labeled IgG and endogenous albumin and IgG. Exclusion of IgG was calculated from the difference between extravascular 125I-labeled IgG and 51Cr-labeled EDTA distribution volumes. In contrast to our previous experience with tracer albumin, 125I-labeled IgG was not fully extractable from minced skin, muscle, or tendon by isotonic saline; only 71-83% was recovered under conditions that eluted 92-96% of tracer albumin and 94-99% of tracer EDTA. We conclude that approximately 20% of extravascular 125I-labeled IgG in these tissues is sequestered or bound in the interstitium. Calculation of IgG fractional exclusion from extractable tracer yielded the following values (means +/- SE, n = 8 rats): leg muscles 0.37 +/- 0.09, leg skin 0.44 +/- 0.03, back skin 0.36 +/- 0.04, tail skin 0.40 +/- 0.08, and tail tendon 0.55 +/- 0.04.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma IgG pool is not defended from urinary loss in nephrotic syndrome.

In the nephrotic syndrome, the plasma level of some proteins of hepatic origin is partially defended by an increase in their synthetic rate. The plasma levels of several liver-derived proteins, as well as immunoglobulin G (IgG) are increased in one condition where plasma albumin concentration and oncotic pressure (II) are reduced, i.e., hereditary analbuminemia. To determine whether the urinary loss of IgG, a protein derived from the immune system, is compensated for by an increased synthetic rate, we measured IgG synthesis in normal Sprague-Dawley rats (SD); two models of the nephrotic syndrome: Heymann nephritis (HN) and Adriamycin treatment; and in Nagase analbuminemic rats (NAR), a model of decreased II without urinary protein loss. Plasma IgG and total IgG mass were significantly reduced in both HN and Adriamycin, yet IgG synthesis was nearly identical in HN, Adriamycin, and SD. In contrast, plasma and total IgG and IgG synthesis were all markedly increased in NAR. We derived a pathogen-free colony of NAR by Caesarean section and found that plasma IgG was not increased in pathogen-free NAR, despite reduced II. Thus, unlike proteins of hepatic origin (e.g., albumin) where synthesis increases following urinary loss, no compensatory increase in IgG synthesis occurs. Increased plasma IgG as well as IgG synthesis in the NAR is not a compensatory response to the absence of albumin or reduction in II, but rather is due to subclinical infection. Profound hypogammaglobulinemia of nephrotic syndrome occurs in part because no compensatory synthetic mechanisms balance urinary loss, and alteration in plasma II does not modulate IgG synthesis.

Ineffectiveness of dietary protein augmentation in the management of the nephrotic syndrome.

The nephrotic syndrome is a consequence of altered permselectivity of the glomerular basement membrane resulting in urinary losses of albumin and other serum proteins. Although dietary protein augmentation increases albumin synthesis, it has not been shown to increase serum albumin or muscle protein. Dietary protein was increased from 8.5% to 21% in pair-fed rats with Heymann nephritis and resulted in an increase both in albumin synthesis and urinary albumin excretion, but not in serum albumin concentration or in total albumin pools. The increase in dietary protein was 8 times greater than the resulting increase in urinary protein excretion, but nearly all of the additional ingested protein was catabolized to urea and excreted in the urine rather than used to augment growth. Dietary supplementation with protein has no obvious beneficial effect on nutritional status of nephrotic rats.

Branched-chain amino acids augment neither albuminuria nor albumin synthesis in nephrotic rats.

Both albuminuria (UalbV) and albumin synthesis (AlbSyn) are modulated by dietary protein in nephrotic rats, but the agent(s) linking diet to altered UalbV and AlbSyn is unknown. Others have reported that branched-chain amino acids (BCAA) cause neither increased renal blood flow nor glomerular filtration rate (GFR) normally induced by dietary protein nor increased blood glucagon thought to be necessary for protein-mediated effects on renal hemodynamics. The effect of BCAA on UalbV is unknown. Because BCAA increase AlbSyn in tissue culture and after a fast, it is possible that feeding BCAA may increase AlbSyn but not UalbV in nephrosis. Nephrotic rats were fed either 8.5% casein (LP); 21% casein (NP); 8.5% casein supplemented with valine, leucine, and isoleucine to the total amount provided by a 21% casein diet (2.37%) (LBC); or 8.5% casein plus 12.5% BCAA providing a diet isonitrogenous to 21% casein (HBC). UalbV and AlbSyn were significantly greater in NP compared with LP, LBC, or HBC and were the same in the latter three groups. Glucagon was infused into nephrotic rats fed 8.5% casein either subcutaneously or intraperitoneally in quantities sufficient to increase plasma levels to over 10 times control but had no effect on UalbV. The ability of dietary protein to increase AlbSyn or UalbV is not a result of total alpha-amino nitrogen intake but is a result of the specific amino acid composition of the diet and must result entirely from the effect of one or more non-BCAA. Increased blood glucagon alone has no effect on UalbV.

Metabolism of albumin and immunoglobulins in the nephrotic syndrome.

The nephrotic syndrome is characterized by the increased urinary excretion of albumin and of other serum proteins of intermediate molecular weight accompanied by a decrease in their serum concentration. Albumin synthesis is increased at the level of mRNA synthesis in response to decreased serum oncotic pressure. However, the magnitude of these responses is dependent upon dietary protein and is insufficient to normalize serum albumin. The absolute rate of albumin catabolism is decreased, serving to normalize serum albumin stores, but in contrast to what occurs in other hypoalbuminemic states, the fractional rate of albumin catabolism is increased. This observation is consistent with a hypothesis that increased renal catabolism contributes to total albumin catabolism in nephrosis. Like albumin, IgG is lost in the urine, its serum concentration is decreased, and the fractional rate of its catabolism is increased, suggesting that the kidney contributes to IgG catabolism in the presence of proteinuria. IgG synthesis responds in a variable fashion in the nephrotic syndrome, and may be decreased, thus contributing to its reduced serum concentration. In contrast, the serum concentration of the high-molecular-weight immunoglobulin IgM is increased, as is the serum concentration of a variety of high-molecular-weight liver-derived proteins. It is unknown by what mechanism serum IgM concentration is increased, but this response serves to defend serum protein concentration and oncotic pressure.

"Salt-sensitive" essential hypertension in men. Is the sodium ion alone important?

We investigated whether the anionic component of an orally administered sodium salt can influence the salt's capacity to increase blood pressure. In five men with essential hypertension in whom blood pressure was normal with restriction of dietary sodium chloride to 10 mmol per day (0.23 g of sodium per day), oral administration of sodium chloride for seven days, 240 mmol per day (5.52 g of sodium per day), induced significant increases in systolic and diastolic blood pressures, of 16 +/- 2 and 8 +/- 2 mm Hg (mean +/- SEM), respectively (P less than 0.05). An equimolar amount of sodium given as sodium citrate induced no change in blood pressure. Replacing supplemental sodium chloride with an equimolar amount of sodium as sodium citrate abolished the increase in blood pressure induced by sodium chloride. Both salts induced substantial and comparable sodium retention, weight gain, and suppression of plasma renin activity and plasma aldosterone, but supplemental sodium chloride increased plasma volume and urinary excretion of calcium, whereas sodium citrate did not. These preliminary findings demonstrate that the anionic component of an orally administered sodium salt can influence the ability of that salt to increase blood pressure, possibly by determining whether the salt induces an increase in plasma volume. Our observations in a small group of men with salt-sensitive hypertension will require confirmation in larger numbers of patients of both sexes.

Coordinately increased lysozymuria and lysosomal enzymuria induced by maleic acid.

During the acute renal tubular dysfunction of Fanconi syndrome and type 2 renal tubular acidosis (FS/RTA2) induced by maleic acid in the unanesthetized dog, we observed: 30 minutes after the onset of FS/RTA2, the urinary excretion of lysosomal enzymes, N-acetyl-beta-glucosaminidase (NAG), beta-glucuronidase (beta-gluc) and beta-galactosidase (beta-galac), increased simultaneously with the anticipated increase in renal clearance of lysozyme; the severities of all these hyperenzymurias increased rapidly, progressively, and in parallel, all reaching a peak some 60 to 80 minutes after their onset; thereafter, while the FS/RTA2 continued undiminished in severity, the severity of the hyperenzymurias decreased rapidly, greatly, progressively, and in parallel; and sodium phosphate loading strikingly attenuated the FS/RTA2 and the hyperenzymurias. Thus, the maleic acid-induced FS/RTA2 is attended by an acute reversible-complex derangement in the renal tubular processing of proteins that: affects not only lysozyme which is normally filtered, but also NAG and other lysosomal enzymes, which are not; and is to some extent functionally separable from that of FS/RTA2. The findings suggest that the derangements in renal processing of lysozyme and lysosomal enzymes are linked, and that a phosphate-dependent metabolic abnormality in the proximal tubule can participate in the pathogenesis of both these derangements and the FS/RTA2.

Liquid-chromatographic measurements of inosine, hypoxanthine, and xanthine in studies of fructose-induced degradation of adenine nucleotides in humans and rats.

We applied a sensitive, precise liquid-chromatographic method of analysis for inosine, hypoxanthine, and xanthine to the study of fructose metabolism in humans and in rats. In the rat, intravenous loading with fructose induced, within minutes, substantial increases in the concentrations of inosine, hypoxanthine, and xanthine in plasma and urine. In plasma, these concentrations peaked after 5 min, then practically disappeared within 10 min. As expected, the fructose-induced increase in hypoxanthine was greatly amplified by pretreating the rats with allopurinol, an inhibitor of xanthine oxidase. In a healthy human subject, intravenous administration of fructose also induced prompt, substantial, and rapidly reversing increases in the concentrations of these metabolites of adenine nucleotides in plasma. The finding that fructose induced almost-immediate increases in the plasma concentrations of inosine, hypoxanthine, and xanthine is consistent with previous studies in rats, in which parenteral administration of fructose induced almost-immediate decreases of total adenine nucleotides (ATP + ADP + AMP) in the liver, and increased concentrations of uric acid and allantoin in the plasma.

Phosphate loading attenuates renal tubular dysfunction induced by maleic acid in the dog.

The metabolic pathogenesis of the complex renal tubular dysfunction of type II renal tubular acidosis and Fanconi's syndrome (RTA II/FS) acutely induced by maleic acid could depend on the occurrence of a positive feedback loop in cells of the proximal renal tubule: impaired mitochondrial oxidation----increased glucose uptake----increased formation and concentration of phosphorylated glycolytic intermediates----limitation on availability of cellular inorganic phosphate----more severely impaired mitochondrial oxidative metabolism. To test this hypothesis we intravenously administered maleic acid both alone and after initiating intravenously administered neutral sodium phosphate, sodium sulfate, or sodium chloride to 10 unanesthetized trained female dogs undergoing water diuresis. We made the following observations: 1) Administration of maleic acid alone predictably induced dose-dependent increments in urine flow (V) and in renal clearance of HCO3-, Na+, K+, and alpha-aminonitrogen and a pronounced increase in the renal clearance and excretion of citrate. 2) Prior phosphate loading, which increased the plasma concentration of phosphate from 2.5 +/- 0.20 to 11.3 +/- 2 mg/dl: a) attenuated the increment in renal clearance of HCO3- by one-half even though the filtered load of bicarbonate was higher by 37%, owing to the higher values of both GFR and plasma bicarbonate concentration that obtained with phosphate loading; b) prevented the increment in renal clearance and excretion of alpha-aminonitrogen; c) significantly attenuated the increments in V and renal clearance of K+; but d) did not affect the increment in renal clearance and excretion of citrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of the kaliuresis after acute unilateral nephrectomy in the dog.

We examined the factors regulating potassium excretion (UKV) in anesthetized, mechanically ventilated, mineralocorticoid-injected dogs undergoing acute unilateral nephrectomy (AUN). In control dogs, AUN caused both sodium excretion (UNaV) and UKV to increase in association with an increase in bicarbonate excretion (UHCO3V) but no change in chloride excretion (UClV). In dogs made acutely acidotic by intravenous infusion of dilute HCl, AUN led to comparable increase in UNaV and UKV, but with increased UClV and only a trivial change in UHCO3V. In dogs in which prostaglandin synthesis was inhibited by indomethacin or meclofenamate, AUN led to an increase in UKV but without any increase in UNaV or UHCO3V. The fractional excretion of potassium rose significantly and comparably in all groups. Our results indicate that the post-nephrectomy kaliuresis occurs without change in respiratory acid-base status or mineralocorticoid hormone activity, and can be dissociated from increased UHCO3V and, probably, increased delivery of fluid out of the proximal tubule. They therefore suggest a specific and potent effect of AUN to stimulate UKV through an increase in plasma potassium concentration or through some as yet undetermined mechanism.

Dysfunction of the proximal tubule underlies maleic acid-induced type II renal tubular acidosis.

To investigate whether dysfunction of the proximal tubule underlies maleic acid-(MA) induced type II ("proximal") renal tubular acidosis (RTA II), we intravenously administered either MA or acetazolamide to eight conscious trained dogs undergoing water diuresis and examined the relationship between fractional solute-free water clearance (Ch2o/GFR), a measure of NaCl reabsorption in the post-proximal nephron, and either fractional urine flow (V/GFR), a measure of total solute rejected by the proximal tubule, or the sum of fractional excretion of Cl- and Ch2o/GFR [(Ccl + Ch2o)/GFR], a measure of proximally rejected solute that is potentially reabsorbable by the thick ascending limb. When MA or acetazolamide induced brisk bicarbonaturia at normal plasma bicarbonate concentrations: 1) V/GFR, (Ccl + Ch20)GFR, and Ch2o/GFR increased strikingly; 2) at any increment of Ch2o/GFR ws not; 3) the increments of V/GFR correlated positively with those of fractional excretion of bicarbonate (P less than 0.001); 4) during hyperchloremic acidosis, MA-induced bicarbonaturia was greatly attenuated; the increment in V/GFR was halved and approximated that in Ch20/GFR, which was unchanged; 5) when plasma bicarbonate was abruptly increased, bicarbonaturia increased strikingly and V/GFR increased further but Ch20/GFR and aminoaciduria did not. We conclude that MA induces a reduction in the net rate at which the proximal tubule reabsorbs HCO-3, Na+, and Cl-. This dysfunction underlies RTA II and evokes greatly increased reabsorption of Cl- and Na+ in the post-proximal tubule.

Factors determining electrolyte excretion and renin secretion after closure of an arteriovenous fistula in the dog.

We studied 12 pentobarbital-anesthetized dogs 8 to 21 days after surgical placement of a femoral AV fistula in order to characterize the renal response to subsequent closure and reopening of the fistula. Closure of the fistula resulted in a significant reduction in cardiac output from 3.3 +/- 0.2 L/min to 2.2 +/- 0.1 (p less than 0.001) and heart rate from 163 +/- 7 beats/min to 150 +/- 7 (p less than 0.005), whereas blood pressure did not change significantly. These changes in systemic hemodynamics were associated with significant increases in GFR, renal vascular resistance, and filtration fraction and a significant decrease in RBF. UNaV (16.4 +/- 3.8 microEq/min to 31.3 +/- 6.5), UKV (27.9 +/- 1.6 microEq/min to 47.5 +/- 3.8), and UHCO3V (6.8 +/- 1.4 mumol/min to 25.0 +/- 2.4) increased significantly (p less than 0.005), whereas UCIV was unchanged. Renal denervation blunted the renal hemodynamic and natriuretic consequences of fistula closure, but other electrolyte excretory responses paralleled those in innervated kidneys. In innervated kidneys RSR fell significantly after fistula closure; RSR was low in denervated kidneys and did not change on fistula closure. Reopening the AV fistula restored systemic hemodynamics toward initial control values but produced no further changes in renal hemodynamics or electrolyte excretion. Overall, an inverse relationship between changes in cation excretion and changes in RSR was observed in innervated kidneys. These results demonstrate involvement of the renal nerves as well as an increase in GFR in the increased cation excretion that occurs after AV fistula closure. However, persistently elevated cation excretion after the reopening of the fistula may be due to other mechanisms.