Proximal Reabsorption Research Articles

Salt-Sensitivity of Proximal Reabsorption Alters Macula Densa Salt and Explains the Paradoxical Effect of Dietary Salt on Glomerular Filtration Rate in Diabetes Mellitus

GFR varies inversely with dietary NaCl in patients with early type I diabetes and in streptozotocin (STZ)-diabetic rats. To explain this paradox within the laws of physiology, it was hypothesized that it results from heightened sensitivity of the diabetic proximal tubule to dietary salt because changes in proximal reabsorption (Jprox) elicit reciprocal adjustments in GFR through the normal actions of tubuloglomerular feedback (TGF). Micropuncture was done in rats after 5 wk of moderately hyperglycemic STZ-diabetes and 1 wk of different NaCl diets. First, single-nephron GFR (SNGFR) and early distal tubular Na(+), Cl(-) and K(+) concentration (representing the TGF signal) were measured by collecting from early distal nephrons. In nondiabetics, dietary salt did not affect SNGFR or the TGF signal. In diabetics, the TGF signal varied directly with dietary salt while SNGFR varied inversely with dietary salt. Next, Jprox was measured by collecting from late proximal tubules. To control for different SNGFR, SNGFR was manipulated by perfusing Henle's loop to alter TGF activity. Controlling for SNGFR, dietary salt did not affect Jprox in nondiabetics but exerted a major inverse impact on Jprox in diabetics. In conclusion, normal rats acclimate to dietary NaCl by primarily adjusting transport downstream of the macula densa. In contrast, diabetes renders reabsorption in the proximal tubule sensitive to dietary NaCl with subsequent effects on the TGF signal. This explains the paradoxical effect of dietary NaCl on GFR in early diabetes.

Blood pressure in patients with primary aldosteronism is influenced by bradykinin B(2) receptor and alpha-adducin gene polymorphisms.

Primary aldosteronism (PA) is the most common cause of endocrine hypertension. PA is most frequently presented as moderate to severe hypertension, but the clinical and biochemical features vary widely. The aim of our study was to identify genetic variants that influence the phenotype of patients with PA. We hypothesized that genetic variants potentially affecting aldosterone production (aldosterone synthase, CYP11B2), renal proximal tubule reabsorption (alpha-adducin), or the mechanisms of counterbalance leading to vasodilatation and sodium excretion (bradykinin B(2)-receptor, B(2)R) could influence the clinical and biochemical characteristics of patients with PA. We studied three polymorphisms of these genes (C-344T of CYP11B2, G460W of alpha-adducin, and C-58T of B(2)R) in 167 primary aldosteronism patients (56 with aldosterone-producing adenoma and 111 with idiopathic hyperaldosteronism). B(2)R and alpha-adducin genotypes were strong independent predictors of both systolic and diastolic blood pressure levels; plasma renin activity and aldosterone also play a marginal role on BP levels. Body mass index, age, sex, and CYP11B2 genotype displayed no significant effect on the clinical parameters of our population. In particular, alpha-adducin and B(2)R polymorphisms accounted for 13.2% and 11.0% of the systolic and diastolic blood pressure variance, respectively. These data suggest that genetic variants of alpha-adducin and the bradykinin B(2)-R influence the blood pressure levels in patients with primary aldosteronism.

Indomethacin blocks enhanced paracellular backflux in proximal tubules.

Renal interstitial hydrostatic pressure (RIHP) is a link between increased arterial BP and natriuresis. The mechanism whereby increases in RIHP inhibits sodium and water transport across the mammalian proximal tubule epithelium may involve changes in flux across the tight junction of the proximal tubule. The purpose of this study was to determine the effects of increases in RIHP and inhibition of cyclooxygenase on paracellular backflux of an extracellular marker from the renal interstitium into the proximal tubule of the rat. During in vivo microperfusion of proximal tubules, the extracellular tracer of paracellular flux, lanthanum (La), was infused directly into the renal interstitium via a chronically implanted matrix. The net paracellular interstitium-to-lumen lanthanum backflux was measured before and after direct renal interstitial volume expansion (DRIVE) in the absence and presence of indomethacin. DRIVE significantly increased RIHP by 37% (Delta1.8 +/- 0.2 mmHg) and interstitium-to-lumen La backflux by 32% (Delta40.2 +/- 16.6 pg/min per mm), and it significantly decreased proximal reabsorption by 27% (Delta-7.7 +/- 3.8 nl/min; n = 6). In indomethacin-treated rats (n = 6), DRIVE again significantly increased RIHP by 40% (Delta1.9 +/- 0.2 mmHg), but it did not increase La backflux (Delta-39.0 +/- 24.4 pg/min per mm) or significantly decrease proximal reabsorption (Delta1.2 +/- 2.3 nl/min). These results demonstrate that increased RIHP increases paracellular backflux of lanthanum from the renal interstitium to the proximal tubule lumen in association with decreases in proximal reabsorption. Furthermore, indomethacin blocks the effects of increased RIHP on proximal reabsorption and paracellular backflux of lanthanum through the intercellular tight junctions of the proximal tubule epithelium.

Hyperuricemia, hypertension, and proteinuria associated with high-altitude polycythemia

Chronic exposure to high altitude is associated with the development of erythrocytosis, proteinuria, and, in some cases, hyperuricemia. We examined the relationship between high-altitude polycythemia and proteinuria and hyperuricemia in Cerro de Pasco, Peru (altitude, 4,300 m). We studied 25 adult men with hematocrits less than 65% and 27 subjects with excessive erythrocytosis (EE; hematocrit > 65%) living in Cerro de Pasco, Peru and compared them with 28 control subjects living in Lima, Peru (at sea level) and after 48 hours of exposure to high altitude. Serum urate levels were significantly elevated in patients with EE at altitude, and gout occurred in 4 of 27 of these subjects. Urate level strongly correlated with hematocrit (r = 0.71; P < 0.0001). Urate production (24-hour urine urate excretion and urine urate-creatinine ratio) was increased in this group compared with those at sea level. Fractional urate excretion was not increased, and fractional lithium excretion was reduced, in keeping with increased proximal reabsorption of filtrate. Significantly higher blood pressures and decreased renin levels in the EE group were in keeping with increased proximal sodium reabsorption. Serum urate levels correlated with mean blood pressure (r = 0.50; P < 0.0001). Significant proteinuria was more prevalent in the EE group despite normal renal function. Hyperuricemia is common in subjects living at high altitude and associated with EE, hypertension, and proteinuria. The increase in uric acid levels appears to be caused by increased urate generation secondary to systemic hypoxia, although a relative impairment in renal excretion also may contribute. © 2002 by the National Kidney Foundation, Inc.

Regulation of Na/Pi Transporter in the Proximal Tubule

The physiological tuning and pathophysiological alterations of renal proximal reabsorption of inorganic phosphate can be ascribed to the net amount of the Na/Pi-cotransporter NaPi-IIa localized in the brush border membrane. The net amount of NaPi-IIa appears to be the result of an endocytotic rate regulated by a complex network of different protein kinases. New approaches demonstrated that NaPi-IIa is part of heteromeric protein complexes, organized by PDZ (postsynaptic protein PSD95, Drosophila junction protein Disc-large, tight junction protein ZO-1) proteins. Such complexes are thought to play important roles in the apical positioning and regulated endocytosis of NaPi-IIa and therefore such interactions have to be considered when explaining proximal phosphate ion reabsorption.

The mechanism of improved sodium homeostasis of low-dose losartan in pre-ascitic cirrhosis

Renal sodium retention on standing is one aspect of the abnormal renal sodium handling in preascitic, well-compensated patients with cirrhosis. Recently, it has been shown that low doses (7.5 mg) of the angiotensin II (Ang II) receptor antagonist, losartan, can reverse renal sodium retention on high, 200-mmol sodium/d diet in these patients and restore them to sodium balance. Therefore, the effect of 7.5 mg of losartan on sodium excretion, when changing from supine to erect posture for 2 hours, was examined in 10 well-compensated patients with cirrhosis and 9 ageand sex-matched controls on the same sodium diet, under strictly controlled metabolic conditions. In contrast to control subjects, in whom sodium excretion was unaffected, single 7.5-mg doses of losartan again restored the preascitic patients with cirrhosis to sodium balance. In addition, it blunted the fall in erect posture– induced renal sodium excretion by a reduction in proximal and distal tubular reabsorption of sodium. These changes occurred without any significant changes in blood volumes, systemic and renal hemodynamics, or glomerular filtration rate (GFR) and filtered sodium load compared with controls, and despite activation of the systemic renin-angiotensinaldosterone system, which was still within normal levels. In conclusion, the beneficial natriuretic effects of low-dose losartan on erect posture – induced sodium retention in preascitic cirrhosis supports the suggestion that the pathophysiology of sodium retention in preascites is in part caused by an intrarenal tubular effect of Ang II in that posture. (HEPATOLOGY 2002; 35:1449-1458.)

Extracellular cAMP inhibits proximal reabsorption: are plasma membrane cAMP receptors involved?

Glucagon binding to hepatocytes has been known for a long time to not only stimulate intracellular cAMP accumulation but also, intriguingly, induce a significant release of liver-borne cAMP in the blood. Recent experiments have shown that the well-documented but ill-understood natriuretic and phosphaturic actions of glucagon are actually mediated by this extracellular cAMP, which inhibits the reabsorption of sodium and phosphate in the renal proximal tubule. The existence of this "pancreato-hepatorenal cascade" indicates that proximal tubular reabsorption is permanently influenced by extracellular cAMP, the concentration of which is most probably largely dependent on the insulin-to-glucagon ratio. The possibility that renal cAMP receptors may be involved in this process is supported by the fact that cAMP has been shown to bind to brush-border membrane vesicles. In other cell types (i.e., adipocytes, erythrocytes, glial cells, cardiomyocytes), cAMP eggress and/or cAMP binding have also been shown to occur, suggesting additional paracrine effects of this nucleotide. Although not yet identified in mammals, cAMP receptors (cARs) are already well characterized in lower eukaryotes. The amoeba Dictyostelium discoideum expresses four different cARs during its development into a multicellular organism. cARs belong to the superfamily of seven transmembrane domain G protein-coupled receptors and exhibit a modest homology with the secretin receptor family (which includes PTH receptors). However, the existence of specific cAMP receptors in mammals remains to be demonstrated. Disturbances in the pancreato-hepatorenal cascade provide an adequate pathophysiological understanding of several unexplained observations, including the association of hyperinsulinemia and hypertension, the hepatorenal syndrome, and the hyperfiltration of diabetes mellitus. The observations reviewed in this paper show that cAMP should no longer be regarded only as an intracellular second messenger but also as a first messenger responsible for coordinated hepatorenal functions, and possibly for paracrine regulations in several other tissues.

Site of action of moxonidine in the rat nephron.

Moxonidine is a centrally acting antihypertensive agent which has been found to exert its blood pressure lowering effect by interaction with (alpha2-adrenoceptors and imidazoline receptors of the I(1)-type. These receptors have also been demonstrated to be present in the rat kidney. In the present study, clearance and micropuncture techniques were applied to anaesthetized rats to localize the site of action of moxonidine within the nephron. The clearance data show that moxonidine (0.25 mg/kg i.v., followed by a continuous i.v. infusion of 0.25 mg/h) induced a marked increase in urine flow and urinary excretion of sodium, chloride and potassium. The changes in urine flow and urinary solute excretion were accompanied by an enhanced glomerular filtration rate. The micropuncture experiments revealed that moxonidine significantly increased glomerular filtration rate of superficial nephrons, and significantly inhibited fractional reabsorption of fluid, sodium, potassium and chloride by similar amounts (by 9.0%-9.8%) in superficial proximal tubules. Regarding fluid and sodium reabsorption, the proximal effect of moxonidine was continuously weakened by a compensatory increase of reabsorption in the loop of Henle and the subsequent distal nephron segments. The inhibitory effect of moxonidine on fractional proximal potassium reabsorption was completely compensated in the loop of Henle, but the drug induced a net secretion of potassium into the segments lying beyond the early distal tubule, probably as a consequence of the increased tubule fluid and sodium load delivered to them. The experiments have identified the proximal tubule as the principal nephron site where the diuretic action of moxonidine arises. The proximal effect may be related to the increased glomerular filtration rate and to a direct inhibitory interaction of moxonidine with the proximal Na+/H+ exchanger.

Parathyroid hormone-mediated regulation of renal phosphate co-transporter

The key player in proximal tubular Pi reabsorption, the type IIa Na(+)/Pi co-transporter, is internalized upon PTH treatment and degraded in the lysosomes. PTH action is initiated by apically and basolaterally localized PTH receptors, whereby apical effects are most probably via cAMP-independent (via PKC) and basolateral effects via cAMP-dependent (via PKA) regulatory mechanisms. The molecular determinants at the level of the transporter protein or at the level of interacting (regulatory) proteins are not yet known.

Topiramate‐induced metabolic acidosis: report of two cases

Two children who presented with symptomatic metabolic acidosis after being put on topiramate (TPM) are reported. The first patient was an 11‐year‐old male with refractory complex partial epilepsy who was put on TPM for 13 months. He developed hyperventilation 1 week after increasing the dose to 300mg/day. Arterial blood gas revealed hyperchloraemic metabolic acidosis with partial respiratory compensation: pH 7.36, PCO2 27.2 mmHg, bicarbonate 14.9 mEq/L, base excess ‐8.9 mmol/L. Hyperventilation and acidosis resolved after administration of sodium bicarbonate and reduction of the dose of TPM. The second patient was a female who developed increasing irritability at age 16 months and 21 months, each time associated with introduction of TPM and resolved promptly upon withdrawal of the drug. Venous blood gas taken during the second episode revealed pH 7.34, PCO2 37.4 mmHg, bicarbonate 20.4 mEq/L, base excess ‐4.2 mmol/L. The predominant mechanism of TPM‐induced hyperventilation involves inhibition of carbonic anhydrase at the proximal renal tubule, resulting in impaired proximal bicarbonate reabsorption. The occurrence of hyperpnoea or mental status change in any patient who is on TPM should prompt an urgent blood gas sampling, with correction of the acid‐base disturbances accordingly.

Regulation of the renal proximal tubule second sodium pump by angiotensins.

For several years it was believed that angiotensin II (Ang II) alone mediated the effects of the renin-angiotensin system. However, it has been observed that other peptides of this system, such as angiotensin-(1-7) (Ang-(1-7)), present biological activity. The effect of Ang II and Ang-(1-7) on renal sodium excretion has been associated, at least in part, with modulation of proximal tubule sodium reabsorption. In the present review, we discuss the evidence for the involvement of Na+-ATPase, called the second sodium pump, as a target for the actions of these compounds in the regulation of proximal tubule sodium reabsorption.

Chronic heart failure: pathophysiology and therapeutic approaches — why is the kidney so important?

The leading symptoms of severe chronic heart failure (CHF), low arterial blood pressure, dyspnoea and oedema, reflect the pathophysiological key features of CHF, i.e. arterial underfilling with subsequent sodium and water retention. Arterial underfilling is sensed by extrarenal as well as renal baroreceptors, which activate hormonal effectors such as catecholamines, the renin-angiotensin aldosterone systems, endothelin and vasopressin. Subsequently, sodium and water retention is induced by the following renal/adrenal mechanisms: (1) reduced renal plasma flow; (2) constriction of the efferent glomerular arteriole; (3) enhanced proximal tubular sodium reabsorption; (4) enhanced distal tubular sodium reabsorption; (5) increased renal water reabsorption; (6) increased renin release; and (7) increased release of aldosterone. In early heart failure, vasoconstriction as well as renal sodium and water retention are partly counter-regulated by activation of vasodilatory and natriuretic substances, such as natriuretic peptides, prostaglandins, nitric oxide and adrenomedullin. In advanced heart failure, however, vasoconstriction with subsequent avid sodium and water retention predominates. Pharmacotherapy in CHF should aim at antagonizing the detrimental effects of neurohumoral activation. The sequence of pathophysiological alterations predicts that a combined treatment with ACE inhibitors (or angiotensin II antagonists), diuretics and beta-blockers would best antagonize the activated renal/ adrenal mechanisms with avid salt and water retention.

Hypertrophy and hyperfunction of the diabetic kidney.

Hypertrophy and hyperfunction of the diabetic kidney.

Short-term regulation of the proximal tubule Na+,K+-ATPase: increased/decreased Na+,K+-ATPase activity mediated by protein kinase C isoforms.

In different species and tissues, a great variety of hormones modulate Na+,K+-ATPase activity in a short-term fashion. Such regulation involves the activation of distinct intracellular signaling networks that are often hormone- and tissue-specific. This minireview focuses on our own experimental observations obtained by studying the regulation of the rodent proximal tubule Na+,K+-ATPase. We discuss evidence that hormones responsible for regulating kidney proximal tubule sodium reabsorption may not affect the intrinsic catalytic activity of the Na+,K+-ATPase, but rather the number of active units within the plasma membrane due to shuttling Na+,K+-ATPase molecules between intracellular compartments and the plasma membrane. These processes are mediated by different isoforms of protein kinase C and depend largely on variations in intracellular sodium concentrations.

Renal enlargement precedes renal hyperfiltration in early experimental diabetes in rats.

The order of appearance between renal hypertrophy and hyperfunction in early experimental diabetes is still disputed. The reason for previous discrepant results is believed to be methodologic problems, as most previous studies of renal function have been performed in anesthetized animals. In the present study in nondiabetic and streptozotocin-diabetic animals, renal volume was measured by a noninvasive magnetic resonance imaging technique, while renal function parameters were measured in conscious, chronically catheterized animals. To avoid artifacts caused by the procedures associated with induction of streptozotocin-diabetes (fasting, brief anesthesia, and transient hypoglycemia) on renal growth and function, diabetic animals were injected with insulin to obtain euglycemia for 4 d before study start. At day 0, insulin administration was withdrawn and all animals developed hyperglycemia within 12 h. Renal volume and kidney function were measured on days 0, 1, 2, 3, 5, and 7. Renal enlargement was detectable at day 1 (20%) and reached an increase of 40% at day 7. No changes were seen in effective renal plasma flow or effective renal vascular resistance within the first 7 d after development of hyperglycemia. GFR tended to rise on day 5 and was increased by 16% at day 7. The absolute proximal reabsorption showed a pronounced rise (30%) at day 7, whereas no change was seen in the proximal tubular fluid output. It is concluded that renal enlargement precedes renal hyperfunction in the early phase after onset of experimental diabetes.

Diuretics and kidney diseases

The main indications for diuretic treatment of renal patients are edema and hypertension. Pharmacokinetics and pharmacodynamics of diuretics are altered in patients with proteinuria and/or impaired renal function. These patients exhibit avid sodium retention. Diuretics are partially inactivated by binding to proteins in tubular fluid. The natriuretic response to diuretics is limited by counter-regulation, specifically increased proximal tubular reabsorption in response to hypovolemia and increased distal tubular sodium reabsorption in response to increased sodium load. At higher serum creatinine values, thiazides are no longer sufficiently effective in monotherapy. The diuretics of first choice are then loop diuretics, potentially in combination with thiazide diuretics. Potassium-sparing diuretics are contraindicated. The most important side effects of diuretics are hypovolemia with orthostatic hypotension, hypokalemia, metabolic alkalosis, increase of creatinine concentration and (rarely) hyponatremia. Diuretic treatment should be accompanied by reduction of dietary sodium intake. Important points are selection of an adequate dose (in problematic cases dose-finding via urine sodium measurements) and selection of proper dosing intervals. If cases do not respond to loop diuretic monotherapy, combination with thiazide diuretics or intravenous administration of loop diuretics should be considered.

Dual effect of secretin on nephron filtration and proximal reabsorption depending on the route of administration

Secretin is a vasoactive peptide capable of acting on transmembrane volume fluxes. We measured nephron filtration (SNGFR) and resorption during secretin microinjection (MIJ) into the tubular lumen or microperfusion (MP) into peritubular capillaries. In 24 rat nephrons, SNGFR, measured by collections from the distal tubule, rose from 25 ± 4 to 61 ± 8 nl/min during MIJ of saline containing secretin 10 − 9 M into the last convolution of the proximal tubule (LP). Percent and absolute resorptions rose from 70 to 90% and from 20 ± 4 to 56 ± 8 nl/min, respectively. During MIJ of secretin, 3 × 10 − 9 M into the first convolution of the proximal tubule, SNGFR, measured at LP, rose from 32 ± 4 to 61 ± 8 nl/min, percent and absolute reabsorptions from 52 ± 4 to 78 ± 3% and from 16 ± 2 to 50 ± 7 nl/min, respectively ( n = 30). During MP of secretin, 1.5 × 10 − 9 M, SNGFR fell from 39 ± 6 to 15 ± 4, resorption from 19 ± 4 to 9 ± 2 nl/min, while percent resorption rose from 43 ± 6 to 59 ± 5% ( n = 15). While all MIJ and MP changes were significant ( P < 0.001), paired pre- versus post-MIJ and MP values were not. Secretin is a powerful vasoconstrictor when perfused into peritubular capillary blood, unlike systemic and intra-arterial injections. When injected into the tubular lumen, it up-regulates SNGFR and increases reabsorption directly.

Citrate and succinate transport in proximal tubule cells.

Urinary citrate, which inhibits calcium nephrolithiasis, is determined by proximal reabsorption via an apical dicarboxylate transporter. Citrate is predominantly trivalent at physiological pH, but citrate(-2) is transported at the apical membrane. We now demonstrate that low-Ca solutions induce transport of citrate(-2) and succinate in opossum kidney cells. With 1.2 mM extracellular Ca, citrate uptake was pH insensitive and not competed by succinate(-2). In contrast, with low extracellular Ca, citrate uptake increased twofold, was inhibited by succinate (and other dicarboxylates), was stimulated by lowering extracellular pH (consistent with citrate(-2) transport), and increased further by lowering extracellular Mg. The effect of Ca was incrementally concentration dependent, between 0 and 1.2 mM. The effect of Ca was not simply complexation with citrate because succinate (which is complexed significantly less) was affected by Ca similarly. Incubation of cells for 48 h in a low-pH media increased citrate transport (studied at control pH) more than twofold, suggesting induction of transporters.

Renal acid-base and sodium handling in hypoxia and subsequent mild metabolic acidosis in foetal sheep.

1. To measure the renal contribution to acid-base homeostasis during hypoxia (not associated with hypercapnia) and in response to the subsequent mild metabolic acidosis and to determine the effects of this hypoxia on the renal handling of sodium, studies were performed in six chronically catheterized foetal sheep (129-138 days gestation) before, during and for 1 h after a 2 h period of hypoxia. 2. Hypoxia was induced in the conscious ewe by infusing nitrogen into the trachea. Foetal arterial oxygen tension fell to 12.0 +/- 0.6 mmHg (P < 0.001). Carbon dioxide tension fell during hypoxia (P < 0.001) and was still somewhat reduced in the recovery period (P < 0.005). Arterial pH fell progressively to 7.19 +/- 0.08 in the recovery period (P < 0.05). Plasma bicarbonate concentrations fell (P < 0.001) and lactate rose (P < 0.001). 3. Urinary pH and the excretion rates of bicarbonate, titratable acid, ammonium and net acid did not change during hypoxia. Ammonium excretion and, hence, generation of new bicarbonate increased in the recovery period (P < 0.05). 4. Renal sodium excretion progressively increased and was greatest after normoxia was restored (P < 0.05). This natriuresis was due to a fall in the reabsorption of sodium by the proximal tubule (P < 0.05). Proximal reabsorption of sodium was directly related to foetal pH (P < 0.0001) and bicarbonate reabsorption (P < 0.001). 5. It was concluded that: (i) the foetal kidneys began to contribute to the maintenance of acid-base balance within the first hour of recovery from a 2 h episode of hypocapnic hypoxia, even though the acidosis was relatively mild; and (ii) a reduction in bicarbonate reabsorption was probably the most important factor that limited sodium reabsorption by the renal tubule during this experiment.

Normal Responses of Atrial Natriuretic Factor and Renal Tubular Function to Sodium Loading in Hypertension-Prone Humans

Background: In order to explore the hypothesis of an atrial natriuretic factor (ANF) deficiency in prehypertension, we compared the response to sodium loading on ANF and renal function in subjects with positive and negative histories of hypertension. Methods: Twenty-two offspring of hypertensive parents (OH) and 20 offspring of normotensive parents (ON) were studied after 4 days of low (50 mmol/day) or high (300 mmol/day) dietary sodium intake. The diets were allocated randomly. Blood pressure (BP), renal function, plasma concentration of ANF, cyclic guanosine monophosphate (cGMP), renin, angiotensin I and II, aldosterone, endothelin and catecholamines were determined during a clearance period of 90 min on both diets. Neurohormones were measured by radioimmunoassays. Renal function was determined by simultaneous measurements of 51Cr-ethylenediaminetetraacetate (a marker of glomerular filtration rate), lithium and sodium clearances. Results: Supine systolic and diastolic BPs were significantly elevated in OH, with both low and high dietary sodium intake. There was no difference in ANF and cGMP concentrations on the low sodium diet. Increasing sodium intake caused a similar increase in ANF in OH and ON but cGMP did not change significantly. As expected the activity of the renin-angiotensin-aldosterone system was decreased by enhancing sodium intake but with both low and high sodium intake plasma renin concentration was significantly higher in OH than in ON. Activation of the sympathetic nervous system with low sodium intake was indicated by a moderate increase in plasma concentrations of epinephrine and norepinephrine in both groups. The renal effects were characterized by significant increases in GFR, lithium and sodium clearances with increasing sodium intake. There were no differences between OH and ON. Estimated values of fractional proximal and distal tubular sodium reabsorption decreased significantly and in a similar way in both OH and ON. Conclusion: These results indicate that the renal and neuroendocrine responses to dietary sodium loading are similar in both OH and ON. The only difference was a higher BP and an elevated plasma renin concentration on both dietary regimens in OH compared with ON. In particular, in OH and ON an identical increase in plasma ANF concentration in response to sodium loading was found. Thus, this study cannot support the hypothesis of a dysregulation of ANF in hypertension-prone humans.