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

Sensational site: the sodium pump ouabain-binding site and its ligands

Cardiotonic steroids (CTS), used by certain insects, toads, and rats for protection from predators, became, thanks to Withering's trailblazing 1785 monograph, the mainstay of heart failure (HF) therapy. In the 1950s and 1960s, we learned that the CTS receptor was part of the sodium pump (NKA) and that the Na+/Ca2+ exchanger was critical for the acute cardiotonic effect of digoxin- and ouabain-related CTS. This "settled" view was upended by seven revolutionary observations. First, subnanomolar ouabain sometimes stimulates NKA while higher concentrations are invariably inhibitory. Second, endogenous ouabain (EO) was discovered in the human circulation. Third, in the DIG clinical trial, digoxin only marginally improved outcomes in patients with HF. Fourth, cloning of NKA in 1985 revealed multiple NKA α and β subunit isoforms that, in the rodent, differ in their sensitivities to CTS. Fifth, the NKA is a cation pump and a hormone receptor/signal transducer. EO binding to NKA activates, in a ligand- and cell-specific manner, several protein kinase and Ca2+-dependent signaling cascades that have widespread physiological effects and can contribute to hypertension and HF pathogenesis. Sixth, all CTS are not equivalent, e.g., ouabain induces hypertension in rodents while digoxin is antihypertensinogenic ("biased signaling"). Seventh, most common rodent hypertension models require a highly ouabain-sensitive α2 NKA and the elevated blood pressure is alleviated by EO immunoneutralization. These numerous phenomena are enabled by NKA's intricate structure. We have just begun to understand the endocrine role of the endogenous ligands and the broad impact of the ouabain-binding site on physiology and pathophysiology.

The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na+ pump endocrine system

Two prescient 1953 publications set the stage for the elucidation of a novel endocrine system: Schatzmann's report that cardiotonic steroids (CTSs) are all Na⁺ pump inhibitors, and Szent-Gyorgi's suggestion that there is an endogenous "missing screw" in heart failure that CTSs like digoxin may replace. In 1977 I postulated that an endogenous Na⁺ pump inhibitor acts as a natriuretic hormone and simultaneously elevates blood pressure (BP) in salt-dependent hypertension. This hypothesis was based on the idea that excess renal salt retention promoted the secretion of a CTS-like hormone that inhibits renal Na⁺ pumps and salt reabsorption. The hormone also inhibits arterial Na⁺ pumps, elevates myocyte Na⁺ and promotes Na/Ca exchanger-mediated Ca²⁺ gain. This enhances vasoconstriction and arterial tone-the hallmark of hypertension. Here I describe how those ideas led to the discovery that the CTS-like hormone is endogenous ouabain (EO), a key factor in the pathogenesis of hypertension and heart failure. Seminal observations that underlie the still-emerging picture of the EO-Na⁺ pump endocrine system in the physiology and pathophysiology of multiple organ systems are summarized. Milestones include: 1) cloning the Na⁺ pump isoforms and physiological studies of mutated pumps in mice; 2) discovery that Na⁺ pumps are also EO-triggered signaling molecules; 3) demonstration that ouabain, but not digoxin, is hypertensinogenic; 4) elucidation of EO's roles in kidney development and cardiovascular and renal physiology and pathophysiology; 5) discovery of "brain ouabain", a component of a novel hypothalamic neuromodulatory pathway; and 6) finding that EO and its brain receptors modulate behavior and learning.

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.

Identification of xanthurenic acid 8-O-beta-D-glucoside and xanthurenic acid 8-O-sulfate as human natriuretic hormones

Hormonal regulation of salt excretion and water balance by the kidneys is well documented. Before 1961, it was widely believed that the glomerular filtration rate and the steroid hormone aldosterone controlled sodium balance in the body. In 1961, deWardener et al. [de Wardener HE, Mills IH, Clapham WF, Hayter CJ (1961) Clin Sci 21:249-258] showed that when these two variables were controlled, the kidney was still able to increase sodium excretion in response to a salt load. Several lines of evidence argued for a small-molecule signal as a definitive modulator of sodium excretion by the kidney. However, the chemical nature of the suspected natriuretic agent remained unknown. Here we report the identification and natriuretic activity of two closely related small molecules isolated from human urine, xanthurenic acid 8-O-beta-d-glucoside and xanthurenic acid 8-O-sulfate. The two compounds were partially purified by activity-guided fractionation and subsequently identified by using NMR spectroscopic analyses of enriched active fractions. Both compounds caused substantial and sustained (1- to 2-h) natriuresis in rats and no or minimal concomitant potassium excretion. We believe these compounds constitute a class of kidney hormones that also could influence sodium transport in nonkidney tissues given that these tryptophan metabolites presumably represent evolutionarily old structures.

An endogenous positive inotropic factor (EPIF) from porcine heart: its effects on sarcoplasmic reticular (SR) Ca2+ metabolism

We have isolated an endogenous positive inotropic factor (EPIF) from porcine left heart ventricular tissue, which demonstrated to have only weak digitalis-like properties including the inhibition of myocardial Na+,K(+)-ATPase. EPIF completely lacks digitalis-like toxicity such as after-contractions in larger doses. In our recent studies, we have demonstrated that EPIF produces a decrease in the amplitude of the post-rest rapid cooling contracture which indicated that EPIF may release Ca2+ from the sarcoplasmic reticulum. In the present study, the effects of EPIF were investigated on the Ca2+ uptake and release properties of SR enriched membrane vesicles from rat heart. At pH 6.8 and in the presence of oxalate, EPIF dose-dependently inhibited the ATP-dependent uptake of Ca2+ by SR vesicles. Concentrations as low as 25 ul (in 1 mL uptake medium) of EPIF caused a 45-47% reduction in the uptake of Ca2+ within 3-4 min. Increases in EPIF concentration to 50 ul/mL caused additional reduction of only 15-20% in the uptake of Ca2+. Concentrations of 25 ul/mL of EPIF had little or no effects on passive release of actively loaded Ca2+ in SR vesicles. On doubling the concentrations to 50 ul/mL EPIF, however, enhanced the release of Ca2+ by 25-28% during 1-2 min and 44-48% after 4 min of incubation of Ca2+ loaded vesicles in the release medium. Relatively smaller effects of EPIF on Ca2+ release implies that EPIF may mainly lower the uptake of Ca2+ in SR. This reduced uptake of Ca2+ may be explained by the EPIF-induced inhibition of Ca2+ pump.

Biologic and physical characteristics of the non-peptidic, non-digitalis-like natriuretic hormone

At least three independent groups of natriuretic hormones have been isolated over the past ten years. Two, atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP), are proteins and the third is made up of digitalis-like substances (DLS). The present report concerns the isolation, substantial purification and biologic actions of an entirely different natriuretic hormone (NH) which appears to be steroidal in nature and an isomer of cortisone. The source of NH was uremic urine. Purification involved successive chromatographic steps including gel filtration and multiple HPLC runs through C-18 resins. A translucent crystal ultimately was obtained. The product was examined using mass spectroscopy with trimethylsilyl derivatization. Only one compound was identifiable. The characteristics of the molecule include: a molecular weight, 360.4; a molecular formula, C21H28O5; a steroidal nucleus; UV absorption at 220 and 290 nm; and intrinsic fluorescence. The onset of action occurs within minutes both in the rat and, as previously shown, in several in vitro systems including the frog skin, toad bladder, fibroblasts and renal tubular epithelial cells grown in culture and isolated perfused cortical collecting tubules. In contrast to DLS, NH has been previously shown not to cross react with digoxin antibodies. Moreover, when given to intact rats, it produces a profound natriuresis but little or no kaliuresis. In contrast to ANF and BNP the compound is active orally as well as intravenously. It is clearly different from cortisone, based both on its biologic and mass spectroscopic characteristics.

A receptor binding assay for endoxin in human urine

Endoxin is an endogenous substance known to participate in the regulation of the sodium balance and hypertension. Its chemical nature remains elusive. Based on its capacity to specifically bind to Na-K ATPase receptors we describe a receptor assay for its measurement in human urine. The endoxin was extracted by methanol and desalted on a silicagel column with a mixture of chloroform-ethanol. Calibration curves have been established by the transformation of the weight of crude extract in actual content of active material expressed in nmol/l as calculated from Scatchard plots analysis. Normal values are given for subjects on regular salt diet. Changes in endoxin urinary elimination after an oral salt load are outlined.

Comparative effects of ouabain, natriuretic factor and ammonium chloride in the toad urinary bladder

Electrical changes and direct effects on Na-K ATPase activity induced by an endogenous digitalis-like natriuretic factor (NF), NH4Cl and ouabain were studied in toad bladders. NF inhibited the SCC and the Na-K ATPase activity in a similar manner to ouabain, but induced a greater increase in calculated direct current resistance (R) (p less than 0.05). NH4Cl was a weak inhibitor of Na-K ATPase activity, although it produced steeper SCC inhibition slopes than those observed with ouabain or NF (p less than 0.01). The data suggested the same mechanism of action of NF and ouabain on the sodium pump, with an additional effect of the former on apical sodium permeability of the cells and/or closure of paracellular routes leading to an increased tissue resistance. In contrast, the effects of NH4Cl were mostly compatible with intracellular inhibition of apical sodium entry into the cell.

A study of lipid effects on the digoxin immunoassay and on the binding to and activity of Na+/K+-ATPase

The present study has examined the cross-reactivity of fatty acids and mono- and di-glycerides in the fluorescence polarization immunoassay for digoxin. The ability of these compounds to inhibit 86Rb uptake by the erythrocyte as well as their ability to displace 3H-ouabain from membrane-bound dog kidney ATP-ase was also assessed. Some unsaturated fatty acids (palmitoleic, palmitelaidic, oleic, linoleic, linolelaidic, linolenic, gamma-linolenic and arachidonic) were found to cross-react significantly in the digoxin immunoassay and to inhibit 3H-ouabain binding to membrane-bound Na+/K+-ATPase. Of the monoglycerides studied mono-11-eicosenoin was found to cross-react in the digoxin immunoassay, inhibit red cell 86Rb uptake and displace 3H-ouabain from its receptor, membrane-bound Na+/K+-ATPase. Two other monoglycerides, 1-monolinoleoyl and 1-monolinolenoyl glycerides, were able to displace 3H-ouabain from membrane-bound Na+/K+-ATPase, but had no effect on the digoxin immunoassay or on red cell 86Rb uptake.

The pathogenesis of renal sodium retention and ascites formation in Laennec's cirrhosis

This chapter critically reviews our current understanding of the pathogenesis, clinical syndrome, and therapy of the disturbances of renal sodium handling, renal perfusion, and glomerular filtration rate that occur in patients with Laennec's cirrhosis. Avid renal sodium reabsorption, a characteristic feature of cirrhosis, occurs independent of moderate changes in renal function and precedes the onset of ascites. The initiation of sodium retention may be a direct consequence of the hepatic disease process and may also result from defective intravascular filling. In the presence of ascites the most important sodium retaining signal is a defective intravascular volume. The principal effectors of renal sodium retention and vasoconstriction are stimulation of the renin-angiotensin-aldosterone axis and augmentation of renal sympathetic nerve activity. Deficient production of natriuretic hormone(s) and endogenous renal vasodilators, such as prostaglandins and kinins, also contributes to the sodium retention and renal hypoperfusion seen in cirrhosis. The hepatorenal syndrome is an extreme imbalance in these renal vasoconstrictor and vasodilator forces. In the therapy of ascites in Laennec's cirrhosis, abstention from alcohol, sodium restriction, and cautious diuresis are the principal therapeutic measures. A grave prognosis accompanies the diagnosis of the hepatorenal syndrome although recoveries have been reported.

Observations on a low molecular weight natriuretic and Na-K-ATPase inhibitory material in urine

Natriuretic and Na-K-ATPase inhibitory material prepared from urine by gel filtration on G25 Sephadex was previously found to be of low molecular weight, polar and non-peptide. Although activity appeared to depend on an amino group, tests and radioenzymatic assays for catecholamines suggested that these were not implicated in the natriuretic activity. Further purification of the material included solvent extraction, cation exchange and high performance liquid chromatography. At each stage, fractions were assayed for natriuretic activity, stimulation of G6PD and inhibition of Na-K-ATPase in cytochemical assays, and for digoxin-like activities i.e. inhibition of dog kidney Na-K-ATPase (Sigma), displacement of 3H ouabain bound to cell membranes and cross reaction with antidigoxin antibody. The crude material possessed all activities, but with successive purifications the activities separated from each other and were thus due to different substances. Analyses for catecholamines with HPLC and electrochemical detection revealed that the natriuretic activity was due to dopamine.

Natriuretic factor exerts a ouabain-like activity in the rat colon in vitro

A previously described natriuretic factor (NF) found in urine from man receiving a high salt diet has been postulated to be of hormonal nature. This factor inhibits Na-K ATPase and binds to ouabain receptors. In order to investigate if NF exerts its physiological activity through the Na-K ouabain sensitive pump, its capacity for inhibiting sodium transport has been tested in an in vitro rat colon preparation. Colonic mucosa from rats fed 0.55, 3.55, and 6.55 mmol Na a day were mounted in an Ussing chamber. Inhibition of short-circuit current (SCC) and PD was observed only when NF was added to the serosal side of the membrane and was similar to that observed with ouabain. In rats fed a low or normal salt diet, inhibition of SCC and PD starts after a lag period of 10 min and reaches its maximum inhibition (about 60%) after 90 min. By contrast colonic mucosa from rats receiving a high salt diet exhibits a higher SCC and PD (basal values) and inhibition of the sodium transport starts immediately after addition of NF to the bathing solution. The data demonstrate the similarity of physiological action of ouabain and natriuretic factor on the sodium transport by the colon in vitro. Experiments with rats receiving a high salt diet suggest that by contrast to the kidney, NaCl-dependent cotransport is strongly stimulated by the salt intake in the colon and could be directly inhibited by ouabain or a ouabain-like substance i.e. NF.

Micropuncture studies of the renal effects of atrial natriuretic substance

Micropuncture studies of the renal effects of atrial natriuretic substance. Injection of atrial extract produced by homogenization, boiling and centrifugation of atrial tissue from one heart caused a 10fold increase in urine flow rate and a 30-fold increase in Na excretion. Similarly prepared extracts of ventricle were without effect. To identify the site of action of atrial natriuretic substance, extract was infused intravenously at rates corresponding to 3 or 6 atria per hour. During infusion at a rate of 3 atria per hour mean urine flow increased from 9.5 +/- 2.8 to 17.2 +/- 1.2 microliter/min and Na excretion from 0.14 +/- 0.06 to 1.78 +/- 0.14 mumol/min. Glomerular filtration rate (GFR), single nephron filtration rate (SNGFR) and proximal and loop of Henle fluid absorption did not change significantly. During infusion of 6 atria per hour, paralleling a greater rise in urine flow rate (from 6.4 +/- 2.09 to 40.3 +/- 7.5 microliter/min) and in sodium excretion (from 0.18 +/- 0.0008 to 5.97 +/- 0.93 mumol/min), filtration rate, measured for either the single nephron or the whole kidney, rose. As a consequence of the rise in GFR, delivery of fluid and chloride into the distal tubule increased significantly. These data suggest that to a major extent the natriuresis is caused by transport inhibition along collecting tubules and collecting ducts. In addition, at high doses a rise in filtration rate contributes to the natriuretic effect of atrial extracts.

The permeability of collecting ducts to 22Na+ and 36Cl- in rat isolated papillae

1. The diffusional permeability of collecting ducts to 22Na+ and 36Cl- was measured in rat papillae in vitro. 2. The permeability of the collecting duct to 36Cl- was 0.72 (s.e.m. = 0.01; n = 356) microns/sec which was significantly higher than the value of 0.51 (s.e.m. = 0.01; n = 356) microns/sec measured for 22Na+. 3. Collecting ducts in papillae taken from rats on a high sodium intake had a 22Na+ permeability of 0.63 (s.e.m. = 0.04; n = 53) microns/sec which was significantly higher than the value on a normal salt intake (0.50, s.e.m. = 0.04; n = 46 microns/sec). 4. When papillae from normal rats were studied in plasma taken from salt loaded rats, the 22Na+ permeability of 0.59 (s.e.m. = 0.04; n = 18) microns/sec was significantly higher than when incubated in plasma from normal rats (0.44, s.e.m. = 0.05; n = 12) microns/sec. 5. An extract of urine with natriuretic activity had no effect on 22Na+ permeability when tested in this system. 6. Adrenalectomy, PGE2, indomethacin and antidiuretic hormone had no significant effect on 22Na+ and 36Cl- permeability. 7. A substance exists in plasma from salt loaded animals that increases the permeability of collecting ducts to sodium. This effect could explain the component of the natriuresis that follows saline infusion which is independent of changes in glomerular filtration rate, aldosterone, or proximal tubule reabsorption.

Progress in isolation and purification of an inhibitor of sodium transport obtained from dog urine

Among the potential modulators of transtubular sodium transport is the putative natriuretic hormone. Widespread efforts are underway to isolate this substance in pure form. The present studies describe a series of experiments directed to this goal. Urine samples in the amount of 150 liters were taken from normal, mineralocorticoid hormone "escape" dogs and were chromatographed through Sephadex G-25. The active fraction of the eluate (that is, the fraction containing the inhibitor of sodium transport) was then subjected to high pressure liquid chromatography (HPLC) in four consecutive steps using three different resins. Approximately 5% of the each column eluate was diverted by use of a stream-splitting apparatus, and the fluorescent pattern was measured and recorded, in most instances following the addition of fluorescamine. Based on the respective fluorescent patterns obtained from the eluates of the successive chromatographic steps, the residual portion (95%) of each eluate was divided into fractions, and each fraction was bioassayed. In each instance only the biologically active fraction was subjected to further purification. In the first step involving HPLC and a cation exchange resin, six fractions were obtained. Only one was active. When this fraction was subjected to reverse-phase chromatography, seven new fractions emerged. Again only one was active. When it was chromatographed using a second cation exchange resin, two fluorometrically detectable peaks, termed N ad H, were identifiable. N exhibited spontaneous fluorescence; H exhibited fluorescamine-dependent fluorescence. In the final step, N and H were separated and bio-assayed individually. N was inactive; H proved to be a potent inhibitor of sodium transport. Accumulation of H in sufficient quantity will determine whether it is a single compound and will permit analysis of its chemical nature.

Natriuretic factor in rats acutely expanded by Ringer's versus albumin solution

Previous studies have shown that it is possible to extract a natriuretic factor from the renal tissue of rats. This substance, which can be isolated by fractionation through a G-25 Sephadex column, shows a strong correlation with sodium elimination by the kidney. The present work was designed to compare the effects of expansion by a Ringer's solution versus an albumin solution on the presence of this factor and the natriuretic response in the rat. Albumin-expanded rats, like control nonexpanded rats, which have no natriuretic response, do not exhibit any active natriuretic factor in their kidneys. In contrast, significant activity of the natriuretic factor is found in the kidneys of Ringer-expanded rats, which also respond by a significant natriuresis (P less than 0.001). We conclude that the expansion of the extracellular fluid volume, as opposed to the expansion of the intravascular volume, only determines both the presence of the natriuretic factor and the physiologic natriuretic response. As the difference in the sodium transport by the renal tubule in these two modes of expansion takes place in the collecting duct, we suggest that the natriuretic factor acts at this level.

Further observations on a low-molecular-weight natriuretic substance in the urine of normal man

Large quantities of the low-molecular-weight natriuretic material (F4), which appears after the salts when fractionated on G-25 Sephadex column, were obtained from the urine of normal man on a normal diet. The natriuretic substance in F4 was (1) untrafiltrable through a membrane with a claimed molecular-weight cut-off of 500 daltons (Amicon UMO5); (2) soluble in more polar organic solvents; (3) totally soluble in 95% acetone when specific activity was doubled; (4) relatively resistant to heating at 100 degrees C for 1 hour at a pH of 10, and to heating at 110 degrees C in 6 N hydrochloric acid for up to 90 hours under anaerobic conditions, and treatment with nitrous acid; it was less resistant to these procedures when extracted into 95% acetone; (5) not destroyed by trypsin, chymotrypsin, pronase, pepsin, leucine aminopeptidase, and subtilysin, nor was it destroyed by pepsin, leucine aminopeptidase, subtilysin, carboxypeptidase A and B, and aminopeptidase M, or by monoamine oxidase, aryl sulphatase, and beta-glucuronidase when extracted into 95% acetone. The natriuretic substance in the 95% acetone-soluble F4 was totally destroyed by incubation with prolidase. The least amount of 95% acetone-soluble F4 required to produce a significant natriuresis in the bioassay rat was that derived from a 7-min sample of urine. The maximal response was obtained from a 30-min sample of urine. Continuous i.v. infusion of the 95% acetone-soluble F4 for 40 min produced a sustained natriuresis, whereas a greater amount injected as a bolus produced an effect which was not sustained beyond 20 min.

Role of the basal sodium intake in the rats on their response to a natriuretic factor

A previously described natriuretic factor found in urine from man and dogs receiving a high salt diet has been postulated to be of hormonal nature. In order to test this hypothesis, natriuretic factor obtained from a pool of human urine of a standard activity has been injected into rats fed 0.55 (group A), 3.55 (group B), 6.55 (group C) mEq sodium a day as it has been shown that the endogenous activity of this factor depends on the sodium content of the diet. This material induces an increment of the UNaV and FENa in the rats averaging respectively 429.6 +/- 150.1% and 317.5 +/- 71.14% for the rats from group A, 103 +/- 29% and 99.7 +/- 26.25% for the rats from group B, 56.3 +/- 34.4% and 63.2 +/- 35.8% for the rats from group C reflecting the degree of statistical significance observed between the absolute values obtained during the control and experimental periods for each group. In addition, a dose-response curve has been established. Altogether the results suggest the hormonal nature of the natriuretic factor and design the kidney as the target organ. These data allow to define the best conditions under which the rats have to be in order to provide a valuable bioassay for this natriuretic material.

Uremic gastropathy in the dog

Stomachs of four dogs with uremia and four normal dogs were examined. Uremic stomachs represented four types of disease: atrophic, amyloidotic, ulcerative and necrotic gastropathy. Pathologic changes common to all uremic stomachs were expansion of the lamina propria, atrophy of gastric glands, and submucosal arteriopathy; lesions were limited to body and fundic zones. Lamina propria was markedly expanded by edema, mastocytosis, deposition of acidic mucosubstances, fibroplasia and mineralization. Capillaries in lamina propria had swollen endothelium and calcium salts were present extracellularly as amorphous granular laminae. Gastric glands were distorted and irregular and had fewer cells per unit of tissue. Parietal cells were swollen and had fragmentation of cytocavitary network and mitochondrial swelling with calcification. Chief cells were shrunken, agranular and atrophic with foci of glycogen and dilation of endoplasmic reticulum. Argentaffin cell content was diminished. Muscular arteries of submucosae had segmental degenerative lesions characterized by myocyte necrosis, calcification, and deposition of acidic mucosubstances and fibrin; thrombosis and obstructive arteriopathy were common. These studies suggest that uremic gastropathy is a disease of mucosal lamina propria and that lesions were due to anoxia caused by diffuse vascular injury and to altered parietal cell function.

Thyroid hormone-aldosterone interaction on Na+ transport in toad bladder

Repeated administration of thyroxine (T4) in vivo (2 microgram/100 g body wt for 6 days) lowered by 2.3 times (P less than 0.025, df = 18) the basal rate of Na+ transport measured by the short-circuit current (SCC) in vitro in the urinary bladder of the toad (Bufo marinus). This difference was not accounted for by a change in the plasma aldosterone concentration. Moreover the response of the SCC to aldosterone in vitro was markedly inhibited in bladders from T4-treated animals (P less than 0.001, df = 18). These findings raised the possibility of a direct interaction between thyroid hormone and aldosterone in the target cell. The effects of L-triiodothyronine (T3) and aldosterone were examined in vitro. T3 alone (60 nM) had no significant effect on the base-line SCC (deltamuA = -14 +/- 11 (SE) muA per hemibladder; P greater than 0.3, n = 10). By contrast, T3 (60 nM) inhibited the response of the SCC to aldosterone from 6 to 8 h after its addition (deltamuA = -98 +/- 19 muA per hemibladder; P less than 0.001, n = 10). The inhibition by T3 was present at 6 nM (P less than 0.01, n = 10) and became not significant at 0.6 nM. T3 had no significant effect on base-line or aldosterone-stimulated H+ transport. Thyroid hormone might therefore regulate the late response of the SCC to aldosterone at the level of its target cell.

Presence of sodium transport inhibiting factor in dog plasma during volume expansion

Plasma dialysates from volume-expanded dogs (E) were compared directly to dialysates from the same dogs when hydropenic. In a double-blind study, E caused relative inhibition of short-circuit current in toad urinary bladder. We therefore confirm the presence of a sodium transport inhibiting factor in plasma of volume-expanded dogs.

Urine-reinfusion natriuresis: evidence for potent natriuretic factors in rat urine

In awake rats the entire urine output was continuously reinfused i.v. Urine-reinfusion (UR) consistently led to the appearance, within one to two hours, of massive, sustained natriuresis and diuresis, suggesting the existence of potent natriuretic factors in the urine. At the time of maximal natriuresis, mean sodium excretion rate and urine flow rate were 25 and 15 times their respective values in control rats. Ths "urine-reinfusion natriuresis" could be demonstrated despite treatment with desoxycorticosterone acetate, blockage of prostaglandin synthesis by indomethacin or meclofenamate, reduction of plasma urea by pretreatment with a protein-free diet, or heating the urine to 100 degrees C. The natriuresis was not prevented by the absence of vasopressin (in Brattleboro rats) and was augmented by vasopressin infusion. In the Brattleboro rats, a marked increase in (CH2O + CNa)/GFR with only a slight rise in CH2O/GFR during UR suggests inhibition of both proximal and distal tubular reabsorption. Renal blood flow and plasma flow increased markedly during UR with a lesser rise in GFR, consistent with post-glomerular vasodilatation. Thus, the phenomenon of urine-reinfusion natriuresis suggests the presence in rat urine of potent, heat stable natriuretic factors, whose action is largely independent of changes in mineralocorticoids, prostaglandins, urea, or vasopressin. Renal vasodilatation with decreased sodium reabsorption at both proximal and distal nephron sites, appears to play an important role in the natriuresis.

Two natriuretic substances in extracts of urine from normal man when salt-depleted and salt-loaded

When the extra-cellular fluid volume is expanded, the subsequent rise in urinary sodium excretion that occurs is due in part to a change in the concentration of some circulating natriuretic substance. Two natriuretic substances with different characteristics on Sephadex chromatography have been previously been identified separately by different workers. Extracts prepared from the urine of 31 normal subjects were tested for these two natriuretic materials in the normal conscious water-loaded rat. Two natriuretic fractions were found. The larger of the two was prepared on G50 Sephadex, and the smaller on G25 Sephadex. The natriuresis produced by the larger material was slow to develop and persisted for two hours. The natriuresis produced by the smaller material was maximal in the first 20 min, declined rapidly within the next 40 min, and tended to rise again during the subsequent 60 min. The amount of natriuretic activity that could be extracted from the freeze-dried urine was diminished by high concentrations of sodium chloride. The natriuretic activity of both materials was greater in the urine of the subjects when they were salt-loaded than when they were salt-depleted. The urine of salt-depleted subjects contained significant amounts of natriuretic material.

Enhanced end-organ responsiveness of the uremic kidney to the natriuretic factor

In chronic renal disease, the addition of a fixed quantity of Na to the extracellular fluid (ECF) will evoke a natriuretic response per nephron which is inversely proportional to the glomerular filtration rate (GFR). One factor that could contribute to this "magnification" phenomenon is an increased sensitivity of residual nephrons to physiologic natriuretic forces. The present studies were designed to examine this possibility. Natriuretic urine fractions from uremic patients, infused into one renal artery of normal rats, produced a small but significant unilateral natriuresis. Infusion of the same fractions in identical amount into remnant kidneys of stage II nonuremic rats (i.e., rats with a contralateral normal kidney in situ) produced a natriuresis in the remnant kidney only which was equivalent to that observed in the normal kidneys. The i.v. infusion of natriuretic fractions into stage II rats produced comparable increments in the fractional excretion of sodium (FENa) bilaterally. However, when the natriuretic fractions were infused into remnant kidneys of stage III rats (no contralateral kidney), deltaFENa was significantly greater than in the foregoing groups. Because stage III rats have increased control values for FENa, baseline FENa was increased to an equivalent level in normal rats by unilateral renal denervation. Natriuretic factor was administered into the ipsilateral renal artery. Although the natriuretic response was increased, it was significantly less than in the stage III remnant kidneys. The data support the view that the uremic state per se is associated with an enhanced responsiveness of the residual nephrons to the natriuretic factor found in the urine (and blood) of uremic patients.

On the influence of the natriuretic factor from patients with chronic uremia on the bioelectric properties and sodium transport of the isolated mammalian collecting tubule

A gel filtration fraction of urine from patients with chronic renal disease (natriuretic factor) has been shown previously to cause natriuresis in rats and to inhibit sodium transport in the isolated toad bladder. The effect of this fraction on transtubular potential difference and sodium transport was examined on the isolated perfused cortical collecting tubule of the rabbit. A rapid inhibition of potential difference from -22.5 mV to -12 mV (P less than 0.001) was observed when the fraction was applied to the peritubular surface. This effect was accompanied by a decrease in net sodium flux from 6.29 to 3.21 pmol/cm per s (P less than 0.001). Unidirectional fluxes using isotopic sodium revealed that the inhibition of net sodium transport was due to a decrease in flux from the lumen to the peritubular surface, i.e., an inhibition of active sodium transport. There was no change in sodium flux in the reverse direction. These changes were all rapidly reversed by removal of the fraction from the peritubular surface. The addition of the fraction to the lumen had no effect on potential difference or net sodium flux. Control studies using the same fraction from the urine of normal subjects had no effect on any of the parameters studies. Where both a uremic and a normal fraction were sequentially applied to the peritubular surface of the same tubule, inhibition of potential difference was obtained only with the former. In the light of evidence implicating the collecting duct fraction from normal animals, the data are consistent with the view that the natriuretic factor may be biologically important in the regulation of sodium balance via it's regulatory role in active sodium transport in the collecting tubule.