The control of sodium excretion with normal and reduced nephron populations. The pre-eminence of third factor

The Na/K-ATPase Signaling and SGLT2 Inhibitor-Mediated Cardiorenal Protection: A Crossed Road?

In different large-scale clinic outcome trials, sodium (Na⁺)/glucose co-transporter 2 (SGLT2) inhibitors showed profound cardiac- and renal-protective effects, making them revolutionary treatments for heart failure and kidney disease. Different theories are proposed according to the emerging protective effects other than the original purpose of glucose-lowering in diabetic patients. As the ATP-dependent primary ion transporter providing the Na⁺ gradient to drive other Na⁺-dependent transporters, the possible role of the sodium–potassium adenosine triphosphatase (Na/K-ATPase) as the primary ion transporter and its signaling function is not explored.

Endogenous digitalis-like factors: an overview of the history

The sodium pump is a ubiquitous cell surface enzyme, a Na, K ATPase, which maintains ion gradients between cells and the extracellular fluid (ECF). The extracellular domain of this enzyme contains a highly conserved binding site, a receptor for a plant derived family of compounds, the digitalis glycosides. These compounds inhibit the enzyme and are used in the treatment of congestive heart failure and certain cardiac arrhythmias. The highly conserved nature of this enzyme and its digitalis receptor led to early suggestions that endogenous regulators might exist. Recent examination of this hypothesis emerged from research in two separate areas: the regulation of ECF volume by a natriuretic hormone (NH), and the regulation of peripheral vascular resistance by a circulating inhibitor of vascular Na, K ATPase. These two areas merged with the hypothesis that NH and the vascular Na, K ATPase inhibitor were in fact the same entity, and that it played a causative role in the pathophysiology of certain types of hypertension. The possibility that multiple endogenous digitalis-like factors (EDLFs) exist emerged from efforts to characterize the circulating enzyme inhibitory activity. In this review, the development of this field from its beginnings is traced, the current status of the structure of EDLFs is briefly discussed, and areas for future development are suggested.

Natriuretic hormone: the ultimate determinant of the preservation of external sodium balance

The present manuscript focuses on a putative natriuretic hormone. It includes the history of a long-term search for the pure molecule, ranging from partial purification to synthesis. It includes a description of seven different bioassay systems used, a resume of the sequential steps in purification, and a summary of a series of experimental protocols employed in the effort to define the biologic properties of the inhibitor of sodium (Na) transport. Two closely related molecules were purified and synthesized. Both are xanthurenic acid derivatives (xanthurenic acid 8-O-β-D-glucoside and xanthurenic acid 8-O-sulfate). It is concluded that one or both of these two low molecular weight compounds (MW: 368 and 284) meet many of the criteria for the final modulator of Na excretion.

The central mechanism underlying hypertension: a review of the roles of sodium ions, epithelial sodium channels, the renin-angiotensin-aldosterone system, oxidative stress and endogenous digitalis in the brain

The central nervous system has a key role in regulating the circulatory system by modulating the sympathetic and parasympathetic nervous systems, pituitary hormone release, and the baroreceptor reflex. Digoxin- and ouabain-like immunoreactive materials were found >20 years ago in the hypothalamic nuclei. These factors appeared to localize to the paraventricular and supraoptic nuclei and the nerve fibers at the circumventricular organs and supposed to affect electrolyte balance and blood pressure. The turnover rate of these materials increases with increasing sodium intake. As intracerebroventricular injection of ouabain increases blood pressure via sympathetic activation, an endogenous digitalis-like factor (EDLF) was thought to regulate cardiovascular system-related functions in the brain, particularly after sodium loading. Experiments conducted mainly in rats revealed that the mechanism of action of ouabain in the brain involves sodium ions, epithelial sodium channels (ENaCs) and the renin-angiotensin-aldosterone system (RAAS), all of which are affected by sodium loading. Rats fed a high-sodium diet develop elevated sodium levels in their cerebrospinal fluid, which activates ENaCs. Activated ENaCs and/or increased intracellular sodium in neurons activate the RAAS; this releases EDLF in the brain, activating the sympathetic nervous system. The RAAS promotes oxidative stress in the brain, further activating the RAAS and augmenting sympathetic outflow. Angiotensin II and aldosterone of peripheral origin act in the brain to activate this cascade, increasing sympathetic outflow and leading to hypertension. Thus, the brain Na(+)-ENaC-RAAS-EDLF axis activates sympathetic outflow and has a crucial role in essential and secondary hypertension. This report provides an overview of the central mechanism underlying hypertension and discusses the use of antihypertensive agents.

The sodium pump and cardiotonic steroids-induced signal transduction protein kinases and calcium-signaling microdomain in regulation of transporter trafficking

The Na/K-ATPase was discovered as an energy transducing ion pump. A major difference between the Na/K-ATPase and other P-type ATPases is its ability to bind a group of chemicals called cardiotonic steroids (CTS). The plant-derived CTS such as digoxin are valuable drugs for the management of cardiac diseases, whereas ouabain and marinobufagenin (MBG) have been identified as a new class of endogenous hormones. Recent studies have demonstrated that the endogenous CTS are important regulators of renal Na(+) excretion and blood pressure. The Na/K-ATPase is not only an ion pump, but also an important receptor that can transduce the ligand-like effect of CTS on intracellular protein kinases and Ca(2+) signaling. Significantly, these CTS-provoked signaling events are capable of reducing the surface expression of apical NHE3 (Na/H exchanger isoform 3) and basolateral Na/K-ATPase in renal proximal tubular cells. These findings suggest that endogenous CTS may play an important role in regulation of tubular Na(+) excretion under physiological conditions; conversely, a defect at either the receptor level (Na/K-ATPase) or receptor-effector coupling would reduce the ability of renal proximal tubular cells to excrete Na(+), thus culminating/resulting in salt-sensitive hypertension.

Central role for the cardiotonic steroid marinobufagenin in the pathogenesis of experimental uremic cardiomyopathy

Patients with chronic renal failure develop a "uremic" cardiomyopathy characterized by diastolic dysfunction, cardiac hypertrophy, and systemic oxidant stress. Patients with chronic renal failure are also known to have increases in the circulating concentrations of the cardiotonic steroid marinobufagenin (MBG). On this background, we hypothesized that elevations in circulating MBG may be involved in the cardiomyopathy. First, we observed that administration of MBG (10 microg/kg per day) for 4 weeks caused comparable increases in plasma MBG as partial nephrectomy at 4 weeks. MBG infusion caused increases in conscious blood pressure, cardiac weight, and the time constant for left ventricular relaxation similar to partial nephrectomy. Decreases in the expression of the cardiac sarcoplasmic reticulum ATPase, cardiac fibrosis, and systemic oxidant stress were observed with both MBG infusion and partial nephrectomy. Next, rats were actively immunized against a MBG-BSA conjugate or BSA control, and partial nephrectomy was subsequently performed. Immunization against MBG attenuated the cardiac hypertrophy, impairment of diastolic function, cardiac fibrosis, and systemic oxidant stress seen with partial nephrectomy without a significant effect on conscious blood pressure. These data suggest that the increased concentrations of MBG are important in the cardiac disease and oxidant stress state seen with renal failure.

Salt loading induces redistribution of the plasmalemmal Na/K-ATPase in proximal tubule cells

BACKGROUND

We have reported that digitalis-like substances (cardiotonic steroids), including marinobufagenin (MBG), induce endocytosis of the plasmalemmal Na/K-ATPase in LLC-PK1 cells. The current report addresses the potential relevance of plasmalemmal Na/K-ATPase redistribution to in vivo salt handling.

METHODS

Male Sprague-Dawley rats were given 1 week of a high salt (4.0% NaCl) or normal salt (0.4% NaCl) diet. Urinary sodium excretion, as well as MBG excretion, was monitored, and proximal tubules were isolated using a Percoll gradient method. Tubular (86)Rb uptake, Na/K-ATPase enzymatic activity, and Na/K-ATPase alpha1 subunit density were determined.

RESULTS

The high salt diet increased urinary sodium (17.8 +/- 1.8 vs. 2.5 +/- 0.3 mEq/day, P < 0.01) and MBG excretion (104 +/- 12 vs. 26 +/- 4 pmol/day), and decreased proximal tubular (86)Rb uptake (0.44 +/- 0.07 vs. 1.00 +/- 0.10, P < 0.01) and Na/K-ATPase enzymatic activity (5.1 +/- 1.1 vs. 9.9 +/- 1.6 micromol/mg pr/hr, P < 0.01) relative to the normal diet. Proximal tubular Na/K-ATPase alpha1 protein density was decreased in the plasmalemma fraction but increased in both early and late endosomes following the high salt diet. In rats fed a high salt diet, anti-MBG antibody caused a 60% reduction in urinary sodium excretion, substantial increases in proximal tubule (86)Rb uptake, and Na/K-ATPase enzymatic activity, as well as significant decreases in the early and late endosomal Na/K-ATPase alpha1 protein content.

CONCLUSION

These data suggest that redistribution of the proximal tubule Na/K-ATPase in response to endogenous cardiotonic steroids plays an important role in renal adaptation to salt loading.

Ouabain induces endocytosis of plasmalemmal Na/K-ATPase in LLC-PK1 cells by a clathrin-dependent mechanism

BACKGROUND

We have demonstrated that ouabain causes dose- and time-dependent decreases in (86)Rb uptake in porcine proximal tubular (LLC-PK1) cells. The present study addresses the molecular mechanisms involved in this process.

METHODS

Studies were performed with cultured LLC-PK1 and Src family kinase deficient (SYF) cells.

RESULTS

We found that 50 nmol/L ouabain applied to the basal, but not apical, aspect for 12 hours caused decreases in the plasmalemmal Na/K-ATPase. This loss of plasmalemmal Na/K-ATPase reverses completely within 12 to 24 hours after removal of ouabain. Ouabain also increased the Na/K-ATPase content in both early and late endosomes, activated phosphatidylinositol 3-kinase (PI(3)K), and also caused a translocation of some Na/K-ATPase to the nucleus. Immunofluorescence demonstrated that the Na/K-ATPase colocalized with clathrin both before and after exposure to ouabain, and immunoprecipitation experiments confirmed that ouabain stimulated interactions among the Na/K-ATPase, adaptor protein-2 (AP-2), and clathrin. Potassium (K) depletion, chlorpromazine, or PI(3)K inhibition all significantly attenuated this ouabain-induced endocytosis. Inhibition of the ouabain-activated signaling process through Src by 4-Amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) significantly attenuated ouabain-induced endocytosis. Moreover, experiments performed in SYF cells demonstrated that ouabain induced increases in the endocytosis of the Na/K-ATPase when Src was reconstituted (SYF+), but not in the Src-deficient (SYF-) cells.

CONCLUSION

These data demonstrate that ouabain stimulates a clathrin-dependent endocytosis pathway that translocates the Na/K-ATPase to intracellular compartments, thus suggesting a potential role of endocytosis in ouabain-induced signal transduction as well as proximal tubule sodium handling.

Atrial natriuretic peptide in dialysis patients under various conditions of volume homeostasis

Atrial natriuretic peptide (ANP) and plasma renin activity (PRA) were studied in 19 patients with end-stage renal disease (ESRD) under haemodialysis (HD). On the basis of clinical findings, patients were divided into three groups: group A, 6 patients, of mean age 41 +/- 15 years, without heart failure and in need of ultrafiltration (658 +/- 282 ml h-1); group B, 6 patients, of mean age 54 +/- 15 years, without heart failure under isovolaemic HD; group C, 7 patients, of mean age 60 +/- 3 years, with heart failure (NYHA III-IV) and in need of ultrafiltration (607 +/- 120 ml h-1). The highest predialysis ANP levels were found in group C (1534 +/- 471 pg ml-1) followed by group A (476 +/- 168 pg ml-1) and group B (236 +/- 138 pg ml-1) (normal range 62 +/- 27 pg ml-1). Systolic and diastolic blood pressure and heart rate did not correlate with ANP levels in either of the groups. However, iso-osmotic reduction of the body weight by ultrafiltration was correlated with decreasing ANP levels during HD (for groups A and C, r = 0.88 and 0.98, respectively). Isovolaemic HD did not alter ANP concentrations (group B). All patients received a volume bolus at the end of HD, and they responded with an instant increase in ANP concentration, which was most pronounced in patients with concomitant heart failure. PRA was not significantly correlated with ANP levels during HD. In conclusion, the results of this study indicate that there is a sensitive response of ANP levels to changes in body fluid status in ESRD.

Atrial natriuretic peptide and sodium homeostasis in chronic renal failure

In order to evaluate the possible role of vasoactive hormones in the mechanism of exaggerated sodium loss due to reduced renal mass we measured plasma concentration of atrial natriuretic peptide (ANP), aldosterone, plasma renin activity (PRA), plasma noradrenaline, and dopamine, in 12 children with advanced chronic renal failure (mean CIn 17.8 +/- 2.6, mean +/- SEM, CPAH 93.5 +/- 17 ml/min per 1.73 m2, FENa 7.0 +/- 0.95%). No patient had clinical signs of volume overload. Plasma concentrations of ANP were not significantly different from those of healthy age-matched controls (29.2 +/- 7.2 vs 23.2 +/- 3.1 fmol/ml) and did not correlate with urinary sodium excretion. Plasma concentrations of aldosterone, PRA and noradrenaline, were also within the physiological range, while plasma dopamine levels were elevated (260 +/- 36 vs 98 +/- 11 pg/ml, less than 0.001). Our data do not support the notion that ANP or the renin-aldosterone axis play a major role in the adaptation of remaining nephrons to maintain long-term sodium balance in normotensive children with chronic renal failure.

Erythrocyte Na,K pump activity and arterial hypertension in uremic dialyzed patients

We have evaluated in 26 uremic patients [21 on hemodialysis, 5 on continuous ambulatory peritoneal dialysis (CAPD)], 11 normotensive, and 15 hypertensive (MAP greater than 110 mm Hg) patients the following properties: a) erythrocyte (RBC) Na concentration [Nai] and ouabain-sensitive and -resistant Na effluxes; b) the effect of uremic sera on ouabain-sensitive Na efflux in normal RBC; c) serum digoxin-like immunoreactivity; d) cardiac index and total peripheral resistance. In 19 healthy subjects a) and c) were also evaluated. RBC Na,K pump activity was lower in uremic patients than in normal subjects (P less than 0.0005), and lower in hypertensive (P less than 0.02) than in normotensive patients. Serum from uremic patients inhibited ouabain-sensitive Na efflux in normal RBC, the inhibition being correlated with both the rate constant for ouabain-sensitive Na efflux (r = -0.67; P less than 0.005) and [Nai] (r = 0.43; P less than 0.05) of RBC of patients from whom the serum was obtained. Inhibition of ouabain-sensitive Na efflux was significantly higher with serum from hypertensive than from normotensive patients (P less than 0.05). Serum digoxin-like immunoreactivity was present in all uremic patients (0.402 +/- 0.054 ng/ml in normotensive and 0.428 +/- 0.040 ng/ml in hypertensive, P = ns), while it was not detectable in normal subjects. Hypertensive patients had peripheral resistance significantly higher than normotensive (P less than 0.05), while cardiac index was similar in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial content and plasma levels of atrial natriuretic peptides in rats with chronic renal failure

The possible role of atrial natriuretic peptides (ANP) for the adaptive changes in renal Na excretion during chronic renal failure was studied in 5/6 nephrectomized (NX) rats maintained on a normal (100 mmol/kg) and a high (800 mmol/kg) Na diet. Atrial content of natriuretic substances was determined by bioassay and plasma ANP by radioimmunoassay. Nephrectomized rats showed a twofold increase in plasma ANP irrespective of their Na intake. Atrial ANP content was increased by high Na diet but unchanged by NX. Nephrectomized rats maintained on high Na diet showed partial depletion of atrial ANP stores. There were no significant changes in the volume fraction of atrial granules determined. The results suggest that ANP is involved in the regulation of renal Na excretion during chronic renal failure and acute Na loading; other mechanisms are probably involved in the adaption to chronic Na loading.

Atrial natriuretic factor determinations and chronic sodium homeostasis

To examine the physiological role of atrial natriuretic factor (ANF) in the maintenance of sodium homeostasis under various conditions, we performed experiments in rats across a wide range of sodium intake, in rats with chronic renal insufficiency at extremes of sodium intake, and in rats given desoxycorticosterone acetate. After three weeks of a very low sodium diet, regular diet, or regular diet plus 1% saline as drinking water, no difference in plasma values of immunoreactive atrial natriuretic factor (IR-ANF) were identified, while rats at the lowest level of sodium intake had elevated atrial values. Normal rats, and rats with 5/6 nephrectomy has plasma values of IR-ANF which were no different irrespective of their sodium intake, nor were atrial values in these rats different. Although mineralocorticoid "escape" could be documented by changes in urine sodium excretion, neither plasma nor atrial IR-ANF values showed differences either at 24 or 72 hr after "escape". The data are consistent with previous observations that ANF serves the purpose of affecting rapid adjustments to large alterations in circulating fluid volume. Chronic high sodium intake, adaptation to renal insufficiency, and adjustment to the effect of mineralocorticoid do not appear to be associated with increased circulating plasma concentrations of IR-ANF.

Interrelationships between sodium clearance, plasma aldosterone, plasma renin activity, renal hemodynamics and blood pressure in renal disease

This study was designed to evaluate the role of aldosterone, glomerular filtration and blood pressure on sodium excretion in renal disease. Sodium clearance (CNa), plasma aldosterone (PA), plasma renin activity (PRA), glomerular filtration rate (GF), paraaminohippurate clearance (CPAH) and blood pressure were measured simultaneously in 19 normal subjects, 38 patients with benign essential hypertension, 3 with renal artery stenosis, 48 with chronic glomerulonephritis, 20 with the nephrotic syndrome, 24 with tubulo-interstitial disease and 21 with a renal homograft. CNa was significantly depressed in patients with the nephrotic syndrome. Mean PA and PRA were increased in renal artery stenosis but within the normal range in other groups. CNa correlated inversely with PA in all groups but one (tubulo-interstitial disease). CNa correlated directly with GF in the nephrotic syndrome and with the mean blood pressure (mBP) in chronic glomerulonephritis and tubulo-interstitial disease. PA correlated directly with PRA and inversely with GF or CPAH in most groups. It is concluded that PA is an important determinant of the basal natriuresis in renal disease with the exception of tubulo-interstitial nephropathies. In the nephrotic syndrome sodium retention is largely determined by the interaction of PA and GF. In chronic nephropathies, but not in benign essential hypertension, the fractional sodium excretion is partly blood pressure-dependent. Impairment of renal function is often accompanied by a rise in PA.

A review of the consequences of fluid and electrolyte shifts in weightlessness

This review describes the renal-endocrine mechanisms related to the early losses of fluid-electrolytes from the body during weightlessness as well as their contribution to longer term adaptation of fluid-electrolyte balance. The hypotheses presented were generated by a systematic analysis of body fluid and renal dynamics observed under conditions of actual and simulated spaceflight. These have increased our understanding of the effects of acute headward fluid shifts on renal excretion, the factors promoting excess sodium excretion and the regulation of extracellular fluid composition.

Hepatic complications

Hepatic dysfunction is one of the most common complications following major surgical intervention, and it presents a wide spectrum of clinical manifestations as discussed. Recent progress in hepatology has provided better knowledge in etiology and pathophysiology of hepatic dysfunction and its sequelae. In addition, modern medical technology has made the diagnosis of liver disease easier and more precise. Underlying liver disease should be disclosed precisely prior to surgery and surgical complication minimized for patients with hepatic impairment. If hepatic complications occur following surgical intervention, the most logical etiology and pathophysiologic explanation should be found, utilizing all the clinical and laboratory data and diagnostic procedures in order to institute proper treatment. Specific complications in hepatic surgery are also discussed. A good understanding of pathophysiologic alterations following hepatic surgery is mandatory together with detailed anatomic knowledge and surgical skill in order to prevent and to treat serious complications.

An inhibitor of sodium transport in the urine of dogs with normal renal function

The urine and serum of chronically uremic patients and dogs contain an inhibitor of sodium transport that reduces short-circuit current (SCC) in the toad bladder and produces natriuresis in the rat. The present studies represent an effort to determine whether the same inhibitor is detectable in urine of normal dogs maintained on a dosium intake varying from 3 to 258 meq/day. Observations were made with and without fludrocortisone. The same Sephadex G-25 gel filtration fraction previously shown to contain the "uremic" inhibitor was tested in both the isolated toad bladder and rat bioassay systems. The fraction from dogs maintained on 258 meq qodium plus 0.2 mg fludrocortisone/day consistently inhibited SCC in the toad bladder and induced a natriuresis in the rat (P less than 0.001). The fraction from dogs on the same sodium intake without fludrocortisone was also natriuretic (P less than 0.01) but did not inhibit SCC significantly. In contrast, the fraction from dogs fed 3 meq sodium with fludrocortisone or 91 meq sodium without fludrocortisone had no significant effect in either assay system. Thus, an inhibitor of sodium transport has been found in the urine of nonuremic dogs. Both the degree of natriuresis in the rat and the degree of inhibition of SCC in the toad bladder correlated with the state of sodium balance which ensued in the dog.

Factors influencing sodium and water excretion in uremic man

Urinary excretion of sodium and water was investigated in patients with chronic end-stage renal disease before and after three different experimental manipulations: reduction in urea solute load by dialysis while extracellular fluid volume (ECFV) was maintained, dialysis without alteration in urea solute load or ECFV and reduction in ECFV without alteration in urea solute load. Sodium and water excretion significantly declined in association with a reduction in both urea solute load and ECFV, but not during a dialysis when reduction on both of these indexes was prevented. The excretory changes occurred in the absence of any alteration in creatinine clearance. The studies suggest that both solute load and the degree of extracellular fluid volume expansion contribute independently to the rate of sodium and water excretion in chronic renal disease. It is concluded that in any given patient the overall excretion of sodium and water is directly influenced by a number of factors including the solute load, the degree of ECFV and the glomerular filtration rate.

The effects of the natriuretic factor from uremic urine on sodium transport, water and electrolyte content, and pyruvate oxidation by the isolated toad bladder

The urine of patients with chronic uremia contains a gel filtration fraction that is natriuretic in the rat. The effects of this fraction on the isolated urinary bladder of the toad were examined in the present studies. When added to the serosal surface of the bladder, a significant and substantial fall in short-circuit current and potential difference was observed. The changes began after a lag period of at least 10 min and continued over a period of 60 min. The decrease in short-circuit current at the end of 1 h averaged 44%. The same fraction from the urine of normal subjects produced no significant change in either short-circuit current or potential difference. When the isolated epithelial cells from the toad bladder were incubated in the presence of the inhibitor, intracellular sodium content increased significantly. There was no change in intracellular water content; hence the intracellular concentration of sodium increased by a mean of 7 meq/liter. The changes in intracellular potassium content and concentration were not satistically significant. When the isolated epithelia were incubated with the uremic factor, there was also a significant decrease in pyruvate utilization in relation to cells from paired hemibladders incubated in the absence of the fraction. The fraction from normal subjects produced no change in either intracellular sodium content or pyruvate oxidation. The results suggest that the inhibitor acts from the serosal surface, inhibits sodium transport across the serosal barrier, and produces a decrease in substrate utilization in association with the change in transepithelial sodium transport.

Micropuncture studies of sodium transport in the remnant kidney of the dog. The effect of graded volume expansion

Proximal and distal tubule micropuncture studies were performed to examine the response to graded extracellular volume (ECV) expansion in 10 normal dogs (stage I), 11 dogs with a unilateral remnant kidney (stage II), and 7 dogs with a remnant kidney after removal of the contralateral kidney (stage III). Before ECV expansion in stage III, there was a suggestive reduction in proximal tubule as well as loop fractional reabsorption of sodium. After ECV expansion to 3% body weight proximal tubule reabsorption was depressed in all groups of animals, while little further inhibition was observed in this segment with additional expansion to 10% body weight. In contrast, the fraction of filtered sodium remaining in the distal tubule rose progressively in all three groups after graded ECV expansion, suggesting that the graded natriuretic response found in the final urine was largely due to a similar response in the loop of Henle rather than that in the proximal tubule. The distal tubule response of the remnant kidney in both stages II and III was greater than that in stage I. These data indicate that although enhanced sodium excretion per nephron in chronic renal failure may be related to uremia, its exaggerated response to ECV expansion is due, at least in part, to certain as yet unidentified intrarenal factors consequent to reduction in functioning renal mass.

Disproportionate inhibition of sodium reabsorption in the unilaterally diseased kidney of dog and man after an acute saline load

Clearance studies were performed on 49 split-bladder dogs with a unilateral pyelonephritic or remnant kidney and three patients with unilateral kidney disease to examine the effects of an acute saline load on the diseased kidney (DK) as opposed to a simultaneously studied, contralateral control kidney (CK), which also served to maintain a nonuremic environment. Before saline loading, base line studies in many of the dogs and the three humans were in agreement with previously published data. However, in dogs with a severe pyelonephritic lesion, a greater difference in DK vs. CK fractional excretion of sodium (FE(Na)) and water was noted, whose magnitude was inversely correlated with the level of glomerular filtration rate (GFR) and maximum urine osmolality of DK compared to CK. An acute saline load (75 ml/kg) resulted in an inhibition of fractional sodium and water reabsorption in the diseased dog kidney which was disproportionately greater than in the simultaneously studied CK, regardless of the type or severity of the lesion. While mean DK GFR for all dogs increased 15% more than CK GFR, failure of FE(Na) to increase after induction of a disproportionate increase in DK GFR with parathyroid hormone suggested that the saline-induced disproportionate increase in GFR was not solely responsible for the exaggerated inhibition of fractional sodium and water reabsorption in the diseased dog kidney. Studies in the three patients after saline loading (25 ml/kg) revealed a similar disproportionate resetting of glomerulotubular balance.Thus, regardless of base line function before expansion, the unilaterally diseased kidney of dog and man possesses unique characteristics in the absence of uremia which render it more reactive to the stimuli produced by acute saline loading. This suggests that the intrarenal environment of the kidney with a reduced nephron population may under some circumstances serve as a determinant of its function.

Inhibition of transepithelial sodium transport in the frog skin by a low molecular weight fraction of uremic serum

An inhibitor of transepithelial sodium transport was found in a low molecular weight fraction obtained from serum of patients with far advanced chronic renal disease. In 18 nondialyzed patients, the mean inhibition of short circuit current (SCC) was 24.9 +/-2.2% (SE). With a comparable fraction from 11 normal subjects. SCC decreased by only 5.3 +/-1.5%. There was significantly greater inhibition with the serum fractions of patients with end stage renal disease being maintained on chronic hemodialysis than in the normal control group; but the degree of inhibition in the dialyzed population was significantly less than that observed in the nondialyzed chronically uremic patients. The inhibition of SCC produced by the serum fractions of a group of seven patients with acute renal failure was not significantly different from the control group despite the presence of high grade uremia in the former. The inhibitory fraction has characteristics identical with the uremic serum fraction which previously has been shown to inhibit p-aminohippurate (PAH) uptake by rabbit kidney cortical slices. With gel filtration through Sephadex G-25, the active fraction appears after the major peaks of substances as small as urea and sodium; hence it may have been retarded on the column. But its ultrafiltration characteristics suggest that its molecular weight may be less than 1000. The inhibitory capability was not destroyed by boiling, freezing, or digestion with chymotrypsin or pronase. Neither methylguanidine nor guanidinosuccinic acid in concentrations well above those present in the serum of uremic patients inhibited sodium transport in the frog skin. The data suggest that there is an inhibitor of sodium transport in the serum of patients with chronic uremia. The role of this material in the regulation of sodium excretion in uremia as well as its possible role as a uremic toxin are subjects of both theoretical and practical interest.

The influence of hypertonic saline infusions upon the fractional reabsorption of urate and other ions in normal and hypertensive man

Renal hemodynamics and the excretion rates of six ions were studied in a group of 24 normotensive and hypertensive subjects during expansion of extracellular fluid volume (ECF) with intravenous infusions of hypertonic saline. In response to the 2.5% saline infusions arterial blood pressure did not change from control values, but glomerular filtration rate increased, and renal vascular resistance decreased.

Accompanying these hemodynamic alterations the urinary excretion rates of sodium, calcium, magnesium, potassium, chloride, and urate increased significantly. Net fractional reabsorption of the six ions fell significantly below control values. The induced changes in net Ca, Mg, K, Cl, and urate reabsorption in all the subjects were directly and significantly related to the simultaneous depression of Na reabsorption.

The data indicate that increased excretion and net tubular rejection of urate accompany depression of tubular sodium reabsorption during hypertonic saline infusions in normal and hypertensive man. The nonspecificity of depressed fractional ion reabsorption during the infusions is compatible with the hypothesis that physical forces which alter fluid uptake by peritubular capillaries determine to a significant extent the natriuretic response of the human nephron to hypertonic saline infusions. Alternatively, if a natriuretic hormone elicited by ECF volume expansion in man accounts for the depressed fractional Na reabsorption, the data imply that directly or indirectly this substance inhibits net fractional reabsorption of a variety of other ions.