Vasopressin: physiology, assessment and osmosensation.

Vasopressin (AVP) plays a major role in the regulation of water and sodium homeostasis by its antidiuretic action on the kidney, mediated by V2 receptors. AVP secretion is stimulated by a rise in plasma osmolality, a decline in blood volume or stress. V1a receptors are expressed in vascular smooth muscle cells, but the role of vasopressin in blood pressure regulation is still a matter of debate. AVP may also play a role in some metabolic pathways, including gluconeogenesis, through its action on V1a receptors expressed in the liver. It is now understood that thirst and arginine vasopressin (AVP) release are regulated not only by the classical homeostatic, intero-sensory plasma osmolality negative feedback, but also by novel, extero-sensory, anticipatory signals. AVP measurement is time-consuming, and AVP level in the blood in the physiological range is often below the detection limit of the assays. Recently, an immunoassay has been developed for the measurement of copeptin, a fragment of the pre-provasopressin molecule that is easier to measure. It has been shown to be a good surrogate marker of AVP.

Copeptin, a marker of vasopressin, in abdominal obesity, diabetes and microalbuminuria: the prospective Malmö Diet and Cancer Study cardiovascular cohort.

BACKGROUND: High plasma copeptin (copeptin), the C-terminal fragment of arginine vasopressin pro-hormone, has been associated with the metabolic syndrome (MetS), diabetes mellitus (DM) development and nephropathy. Here we tested whether elevated copeptin level is associated with later development of the MetS, its individual components and microalbuminuria. METHODS: We analysed copeptin at baseline (1991-1994) in the population-based Malmö Diet and Cancer Study cardiovasular cohort and re-examined 2064 subjects 15.8 years later (mean age 72.8 years, 59% women) with oral glucose tolerance test and measurement of MetS and its individual components. RESULTS: After age and sex adjustment, increasing quartiles of copeptin at baseline (the lowest quartile as reference) were associated with MetS (P for trend=0.008), incident abdominal obesity (P for trend=0.002), DM (P for trend=0.001) and microalbuminuria (P for trend=0.002). After additional adjustment for all the MetS components at baseline, increasing copeptin quartiles predicted incident abdominal obesity (odds ratios 1.55, 1.30 and 1.59; P for trend=0.04), DM (odds ratios 1.18, 1.32 and 1.46; P for trend=0.04) and microalbuminuria (odds ratios 1.05, 1.08 and 1.65; P for trend=0.02) but not MetS (P for trend=0.19) at the reexamination. Further, the relationship between copeptin and microalbuminuria was independent of baseline C-reactive protein, incident DM and incident hypertension. CONCLUSION: Copeptin independently predicts DM and abdominal obesity but not the cluster of MetS. Apart from predicting DM and abdominal obesity, elevated copeptin signals increased risk of microalbuminuria. Interestingly, the association between copeptin and later microalbuminuria was independent of both prevalent and incident DM and hypertension. Our findings suggest a relationship between a dysregulated vasopressin system and cardiometabolic risk, which could have implications for risk assessment and novel preventive treatments.

Long-term effects of vasopressin on the subcellular localization of ENaC in the renal collecting system.

Previous studies revealed that chronic (days) vasopressin treatment stimulates amiloride-sensitive sodium transport in isolated renal cortical collecting ducts and increases the abundance of beta- and gamma-subunits of the epithelial sodium channel (ENaC) in the kidney. The aim of the present work was to investigate in vivo the cellular basis of these effects. The long-term effect of V2 vasopressin agonist (1-deamino-8-D-arginine vasopressin (dDAVP)) on the abundance and subcellular localization of ENaC along the rat renal collecting system was determined by immunohistochemistry and laser confocal microscopy. Moreover, we studied by real-time reverse transcriptase-polymerase chain reaction the effect of vasopressin on proteins implicated in the regulation of ENaC (Nedd4-2, prostasin, Sgk1). After 5 days of administration, dDAVP markedly increased the intracellular pool of the beta- and gamma-ENaC subunits in the principal cells, with an increasing gradient from connecting tubule to the outer medullary collecting duct, but did not increase any subunit at the cell surface. The apical immunostaining of ENaC increased in response to sodium restriction, as expected, but dDAVP did not further enhance this apical labelling. dDAVP increased the gene expression of prostasin in the cortex but not that of Nedd4-2 and Sgk1. These findings suggest that the previously reported increase in sodium transport induced by sustained stimulation of vasopressin V2 receptor is probably mediated by other mechanism than an increase in the apical density of ENaC.

[Influence of moderate body weight excess on the nycthemeral pattern of blood pressure, renal function and sodium and water excretion in patients with essential hypertension]. / Effet d'un excès de poids sur les variations nycthémérales de la pression artérielle, de la fonction rénale et de l'excrétion sodée chez des patients hypertendus.

Obesity is a risk factor for arterial hypertension. We studied the relationships between the body mass index (BMI) and the nycthemeral pattern of blood pressure (BP), renal function and sodium and water excretion (EX) in a group of 25 moderately hypertensive untreated men (41 +/- 2 y, 80 +/- 3 kg). Subjects were given a high sodium diet (6 g NaCl added to their usual diet, daily EX=200 mmol). On the 7th day, BP was monitored during 24 h and urine collected in 2 fractions (day=D, 8:00-22:00 and night=N, 22:00-8:00). Subjects were a posteriori divided into 2 groups according to the median BMI (26 kg/m2): Group 1, n=12, BMI 23.2 +/- 0.6 (mean +/- SEM) and Group 2, n=13, BMI 29.2 +/- 0.5 kg/m2. No difference was observed between the two groups for age, 24 h urine and sodium EX, or systolic and diastolic BP. However, heart rate was significantly higher during N in Group 2 (66 +/- 2 vs 57 +/- 2 b/min, p=0.012). Na and water EX were significantly higher during D than during N in Group 1, but lower during D than during N in Group 2. Creatinine clearance was higher in Group 2 than in Group 1 especially during N (D+29%, p=0.013; N+49%, p<0.001). In Group 2, subjects concentrated their urine more than in Group 1, as evaluated from the urine/plasma creatinine ratio (+49%, p=0.019). This ratio was positively correlated to BMI during D (r=0.561, p=0.004) but not during N. These results show that the glomerular hyperfiltration associated with overweight is more intense at night and that moderately overweight hypertensives have a reduced sodium and water EX during the day and a compensatory larger EX at night. The reduced diurnal EX goes along with an increased urine concentration. The nocturnal rise in EX is concomittant with a rise in heart rate. Given the growing health problems linked to obesity and hypertension, these results open a new field for the understanding of the difficulty to excrete sodium in this condition.

Influence of plasma amino acid level on vasopressin secretion.

OBJECTIVES: Vasopressin (VP) is known to be elevated in patients with diabetes mellitus (DM). While the influence of acute hyperglycemia has been ruled out, the mechanism or the osmotically active compound responsible for the increase in VP secretion is still not elucidated. Because the plasma level of several amino acids (AAs) is increased in DM, we evaluated whether AAs could represent an effective osmotic stimulus for VP secretion. RESEARCH DESIGN AND METHODS: In a cross-over study, eight healthy volunteers randomly received an infusion of isotonic saline (control) or mixed AA solution, i.v., at a low or a high rate (2 or 4.5 mg/min/kg BW, respectively). Plasma VP (P(VP)) was measured for two hours before and three hours during AA or control infusion. RESULTS: AA infusion induced a dose-dependent elevation in plasma AA concentration but did not alter P(VP). However, effective plasma osmolality (P(osm)) (osmolality minus urea concentration) remained unchanged because a concommittant fall in plasma sodium concentration (P(Na)), likely due to sodium-linked uptake of AA in peripheral cells, compensated for the rise in plasma AA. CONCLUSION: The stability of effective P(osm) may explain the lack of change observed in P(VP). Because sodium is a very efficient stimulus for VP secretion, it may be assumed that the fall in P(Na) occurring during AA infusion should have reduced VP secretion and thus P(VP). In this setting, the stability of P(VP) suggests that AAs induced an increase in VP secretion which counterbalanced the fall attributable to the decrease in P(Na). In conclusion, in acute experiments, AAs seem to represent an effective stimulus for VP secretion, almost equally potent as sodium. Further studies are needed to evaluate their contribution to the high P(VP) seen in the chronic setting of DM.

[Impaired urinary flow rate during the day: a new factor possibly involved in hypertension and in the lack of nocturnal dipping]. / Débit urinaire trop bas le jour mais élevé la nuit: un nouveau facteur pouvant contribuer à l'HTA et à l'absence de dipping nocturne.

UNLABELLED: Previous studies suggest that a low urinary flow rate may reduce the capacity of the kidney to excrete sodium and could thus favour hypertension. In the present study, we evaluated the relationships between urinary flow rate (V) during day (D) and night (N), blood pressure (BP), and the day-night BP difference (delta in % of day BP) in 65 diabetic patients (glycosuria < 90 mmol/24 h) (35 F and 30 M, age 59 +/- 2 y) hospitalized for a 24 h urine collection and mean ambulatory blood pressure recording (AMBP). Urine was collected as two separate samples during D (8:00 am to 10:00 pm) and N (10:00 pm to 8:00 am). V, sodium excretion (NaEx) and mean systolic and diastolic BP (SBP and DBP, respectively) were calculated for the two periods. Patients were a posteriori classified according to the D/N ratio of V, and the mean values of 3 tertiles (T1, T2, T3, n = 22, 22, and 21 subjects) were calculated and compared by ANOVA (see table; *: p < 0.05; **: p < 0.01; ***: p < 0.001). [table: see text] Although total 24 h urine volume was similar in the three tertiles (1,670, 1,927 and 2,007 mL, NS), the fraction of urine excreted during D and N differed widely, with parallel differences in NaEx. With lower V during the day, BP tended to be higher and the fall in nocturnal BP to be reduced. CONCLUSIONS: This study shows the advantages of separate day and night urine collections in relation with AMBP. For a similar total diuresis, some subjects exhibit a too low urine flow rate and too low NaEx during the day, and a compensatory rise during the night. This low diurnal diuresis is associated with a higher blood pressure and a lower nocturnal fall (= nocturnal pressure diuresis). The factors responsible for a too low diuresis during the day need further investigation.

Chronic exposure to vasopressin upregulates ENaC and sodium transport in the rat renal collecting duct and lung.

Vasopressin is known to acutely stimulate sodium transport in the renal collecting duct. We investigated the long-term regulation by vasopressin of the epithelial sodium channel (ENaC) in the rat kidney. Five-day infusion of dDAVP (a V(2) receptor agonist) to Brattleboro rats lacking vasopressin induced a marked increase in beta- and gamma-subunit ENaC mRNA levels in the renal cortex (beta, 85%; gamma, 100%), with no change in alpha-ENaC mRNA. Expression of beta- and gamma-ENaC mRNAs was also enhanced in lung (beta, 49%; gamma, 33%) but not in distal colon (an organ devoid of V(2) receptors). Similar results were obtained in Sprague Dawley rats after either partial water restriction or dDAVP infusion for 5 days. Transepithelial voltage and transepithelial sodium and water net fluxes were measured in isolated perfused cortical collecting ducts of Brattleboro rats. Acute addition of 2x10(-10) mol/L dDAVP to the bath increased sodium and water fluxes in the same proportion, and to a far greater extent in dDAVP-infused than in control Brattleboro rats (change in Na(+) net flux, 337+/-30 versus 49+/-11 pmol. min(-1). mm(-1), respectively; P<0.001). These effects were abolished by amiloride. Extrarenal water losses, partly originating from the lung, were reduced by high plasma vasopressin level. This study shows that vasopressin increases sodium transport in the renal collecting duct and probably in the lung, through a differential transcriptional regulation of ENaC subunits. This effect is followed by isoosmotic water reabsorption and likely contributes to the process of water conservation. It could lead to less efficient sodium excretion, however, and thus participate in some forms of salt-sensitive hypertension.

Antidiuretic action of vasopressin: quantitative aspects and interaction between V1a and V2 receptor-mediated effects.

(1) Vasopressin (VP), or antidiuretic hormone, is secreted in response to either increases in plasma osmolality (very sensitive stimulus) or to decreases in plasma volume (less sensitive stimulus). Its normal plasma level is very low (about 1 pg/ml, i.e. 10(-12) M), close to the detection limit of present immunoassays, and distinct antidiuretic effects are observed after infusion of small undetectable amounts of VP. (2) This antidiuretic action results from three main effects of VP on principal cells of the collecting duct (CD) mediated by occupancy of peritubular V2 receptors. (i) Increase in water permeability along the entire CD (via AQP2). (ii) Increase in urea permeability in only the terminal inner medullary CD (via UT-A1). (iii) Stimulation of sodium reabsorption, mainly in the cortical and outer medullary CD (via ENaC). VP also acts on medullary vasculature (V1a receptors) to reduce blood flow to inner medulla without affecting blood flow to outer medulla. Besides these actions, all concurring to increase urine osmolality in different and additive ways, other VP effects, probably exerted through V1a receptors located on luminal membrane, tend to limit the antidiuretic effects of the hormone. They induce the formation of prostaglandins which reduce V2-dependent cAMP accumulation in these cells and thus partially inhibit all three V2 effects. (3) Because urine is first diluted along the nephron before being concentrated in the medulla, VP is required, not only for urine concentration, but first for re-equilibration of tubular fluid osmolality with plasma osmolality, a step taking place in the renal cortex, and achieved through the reabsorption of large quantities of water (more than what is subsequently reabsorbed in the medulla to concentrate urine). Accordingly, VP effects on urine flow-rate are not linear. Small changes in plasma VP in the low range of urine osmolality will induce wide changes in urinary flow-rate, whereas in the upper range of urine osmolality larger changes in plasma VP induce much more limited further reduction in urine flow-rate. (4) Most likely, the different effects of VP require different levels of VP concentration to occur and are thus recruited successively with progressive rise in VP secretion.

Aquaporin-2 and urea transporter-A1 are up-regulated in rats with type I diabetes mellitus.

AIMS/HYPOTHESIS: Although the urine flow rate is considerably higher in diabetes mellitus, water reabsorption is greatly increased to concentrate an increased amount of solutes. Our study evaluated the expression of aquaporins and urea transporters, which are essential to the urinary concentration process. METHODS: Northern blot and immunoblot were used to quantify mRNA and proteins for aquaporin-2 (AQP2) as well as urea transporters UT-A1, UT-A2 and UT-B1, in subzones of the renal medulla of rats with streptozotocin-induced diabetes. RESULTS: In these rats, glycaemia, urine flow rate and water reabsorption were respectively fourfold, nine-fold and fourfold those of control rats. The AQP2 protein isoforms were significantly up-regulated in outer and inner medulla. In the base and tip of inner medulla, UT-A1 mRNA was significantly up-regulated (three- and 1.3-fold, respectively) as well as the 117 kD protein (ten- and threefold, respectively) whereas the 97 kD protein was not changed or decreased twofold, respectively. This suggests that, in diabetes, the inner medullary collecting duct is endowed with more UT-A1, especially in its initial part. In the case of mRNA and proteins of UT-A2, located in thin descending limbs in the inner stripe of outer medulla, they were respectively not changed and down-regulated in diabetic rats. CONCLUSION/INTERPRETATION: This study shows that in diabetes, the increased expression of AQP2 and UT-A1 in medullary collecting duct is consistent with an improved concentrating activity. In addition, the underexpression of UT-A2 and the overexpression of UT-A1 in the initial medullary collecting duct are reminiscent of the changes seen after experimental reduction of urine concentration or low protein feeding.

Selective blockade of vasopressin V2 receptors reveals significant V2-mediated water reabsorption in Brattleboro rats with diabetes insipidus.

BACKGROUND: In a previous study we observed that acute administration of the selective antagonist of vasopressin (AVP) V2 receptors, SR 121463A (SR), aggravated the symptoms of diabetes insipidus (DI) in homozygous Brattleboro rats (an AVP-deficient strain). The present study investigates in more details the acute and chronic effects of SR in DI rats. METHODS AND RESULTS: In experiment A, different groups of rats received acute i.p. injections of SR (0.001-10 mg/kg) or vehicle alone, and urine was collected for the next 24 h. SR dose-dependently increased urine flow rate and decreased urine osmolality with no significant change in solute excretion, thus confirming a pure 'aquaretic' effect. In experiments B and C, the chronic effects of orally administered SR were evaluated over 8 days in Brattleboro DI rats (experiment B, 1 mg/kg/day) and in adult Sprague-Dawley rats with normal AVP secretion (experiment C, 3 mg/kg/day). In DI rats, the aquaretic effects of SR persisted with the same intensity over the 8 days. In Sprague-Dawley rats, SR induced a sustained, stable aquaretic effect and also increased non-renal water losses, suggesting an effect of AVP on water conservation in extrarenal sites. Because oxytocin (OT) synthesis is elevated in DI rats and OT is known to bind to V2 receptors, we evaluated the antidiuretic effects of OT in DI rats in experiment D. Chronic infusion of OT (3 microg/kg/h, i.p.) induced a marked antidiuresis, and acute SR (1 mg/kg) in OT-treated DI rats completely abolished this antidiuretic effect, thus indicating that it was due to binding of OT to V2 receptors. CONCLUSION: (i) SR is a potent orally active aquaretic and induces stable effects during 1 week in rats with or without endogenous AVP secretion. (ii) Significant V2 receptor-mediated water reabsorption occurs in collecting ducts of Brattleboro DI rats because their usual urine osmolality is about twofold higher than the minimum observed during SR-induced maximum diuresis. (iii) This V2 agonism could be mediated in part by OT binding to V2 receptors. Small amounts of endogenous AVP, known to be produced by adrenal and testis in DI rats, could also contribute to this V2 agonism, as well as a possible constitutive activation of the V2 receptors. (iv) In normal rats, AVP probably reduces water losses through extrarenal sites, probably the lungs.

Vasopressin and diabetes mellitus.

In diabetes mellitus (DM), the urine flow rate is increased, and the fluid turnover in the body is accelerated because of the glucose-induced osmotic diuresis. On the other hand, plasma vasopressin (VP) is elevated in both type 1 and type 2 DM. This elevation seems to be due to a resetting of the osmostat. A high VP level is beneficial in the short term because it limits to some extent the amount of water required for the excretion of a markedly enhanced load of osmoles (mainly glucose). However, in the long run, it may have adverse effects by favoring the development of diabetic nephropathy. VP has been shown in normal rats to induce kidney hypertrophy, glomerular hyperfiltration, and an increase in urinary albumin excretion (features also occurring in association in the period preceding diabetic nephropathy). Moreover, VP has been shown to participate in the progression of renal failure in rats with five-sixths reduction in renal mass. In recent studies, we have shown (1) that creatinine clearance, albuminuria and renal mass increased much less during experimental DM in Brattleboro rats unable to secrete VP than in their VP-replete Long-Evans controls, and (2) that albuminuria was prevented during experimental DM in Wistar rats when a VP nonpeptidic, highly selective V2 receptor antagonist was administered chronically for 9 weeks. Taken together, these results strongly suggest that VP plays a crucial role in the onset and aggravation of the renal complications of DM. The mechanisms by which VP exerts these adverse V2-dependent effects are not yet elucidated. They are most likely indirect and may involve several intermediate steps comprising VP-induced changes in the composition of the tubular fluid in the loop of Henle (due to solute recycling in the renal medulla associated with improved concentrating activity of the kidney), inhibition of the tubuloglomerular feedback control of glomerular function, and alterations in glomerular hemodynamics by the intrarenal renin-angiotensin system.

Massive reduction of urea transporters in remnant kidney and brain of uremic rats.

BACKGROUND: The facilitated urea transporters (UT), UT-A1, UT-A2, and UT-B1, are involved in intrarenal recycling of urea, an essential feature of the urinary concentrating mechanism, which is impaired in chronic renal failure (CRF). In this study, the expression of these UTs was examined in experimentally induced CRF. METHODS: The abundance of mRNA was measured by Northern analysis and that of corresponding proteins by Western blotting in rats one and five weeks after 5/6 nephrectomy (Nx). RESULTS: At five weeks, urine output was enhanced threefold with a concomitant decrease in urine osmolality. The marked rise in plasma urea concentration and fall in urinary urea concentration resulted in a 30-fold decrease in the urine/plasma (U/P) urea concentration ratio, while the U/P osmoles ratio fell only fourfold. A dramatic decrease in mRNA abundance for the three UTs was observed, bringing their level at five weeks to 1/10th or less of control values. Immunoblotting showed complete disappearance of the 97 and 117 kD bands of UT-A1, and considerable reduction of UT-A2 and UT-B1 in the renal medulla. Similar, but less intense, changes were observed at one-week post-Nx. In addition to the kidney, UT-B1 is also normally expressed in brain and testis. In the brain, its mRNA expression remained normal one-week post-Nx, but decreased to about 30% of normal at five-weeks post-Nx, whereas no change was seen in testis. CONCLUSIONS: (1) The decline in urinary concentrating ability seen in CRF is largely due to a major reduction of UTs involved in the process of urea concentration in the urine, while factors enabling the concentration of other solutes are less intensely affected. (2) The marked reduction of brain UT expression in CRF may be responsible for brain edema of dialysis disequilibrium syndrome observed in some patients after fast dialysis.

Regulation by sodium intake of type 1 angiotensin II receptor mRNAs in the kidney of Sabra rats.

OBJECTIVE: To study the relationship between the sensitivity to sodium content of the diet in terms of development of hypertension and the regulation of the expression of type 1 angiotensin II receptor subtypes by such a diet. METHODS: The expression of angiotensin II receptor subtype (AT1A and AT1B) mRNAs was studied by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) in the four zones of the kidneys of Sabra rats, sensitive or resistant to DOCA salt-induced hypertension (SBH/y and SBN/y, respectively). Rats were fed a high (8%) or normal (0.4%) NaCl diet. As vasopressin is known to be elevated in SBH/y rats and to be involved in DOCA-salt hypertension, we studied an additional group of SBH/y rats, fed a high sodium diet, enriched in water. RESULTS: With the absence of DOCA, SBH/y rats did not develop hypertension. The high sodium diet induced a greater fall in the plasma renin activity in the SBH/y (-95%) than in the SBN/y (-63%). In the cortex (C) and inner stripe (IS), the high sodium diet decreased AT1A and AT1B mRNAs in SBH/y and SBN/y, with a higher magnitude for SBH/y, than for SBN/y (C, -28 versus -20%; IS, -42 versus -20%). The addition of water to the high sodium diet lessened the effect of sodium in the C and IS, although the plasma renin activity (PRA) was not altered. CONCLUSION: A high sodium diet significantly decreases both AT1A and AT1B gene expression in two specific zones of the rat kidney containing the target cells of angiotensin II (C and IS). This down-regulation is organ-specific since it was observed in the kidney and adrenals, but not in the liver. Finally, SBH/y and SBN/y rats differ in the basal level of AT1 mRNA expression in the IS, and in the ability to modulate AT1 mRNA level under sodium intake.

Renal urea transporters. Direct and indirect regulation by vasopressin.

Urea is the most abundant urinary solute and is excreted in urine at a much higher concentration than in other body fluids. Urea concentration is achieved in the kidney through complex urea movements between blood vessels and renal tubules, which involve facilitated urea transport. Three major urea transporters expressed in the kidney have been cloned, UT-A1, UT-A2 and UT-B1, the first two derived from the same gene by differential transcription. These membrane proteins enable facilitated diffusion of urea through specific parts of the nephron (UT-A) and through renal vasculature (UT-B) in the medulla. UT-A1 is localised in the terminal part of the inner medullary collecting ducts and accounts for the vasopressin-dependent increase in urea permeability of this segment. UT-A2 is found in the descending thin limbs of Henle's loops. UT-B1 is expressed in the endothelium of the descending vasa recta supplying blood to the renal medulla, and in red cells. All three urea transporters are primarily involved in the process of intrarenal urea recycling, which enables the establishment, and prevents the dissipation, of a high concentration of urea in the inner medulla. This is an essential feature for producing a concentrated urine and thus for water economy in mammals. Vasopressin, upon binding to V2 receptors in the inner medullary collecting ducts, increases urea permeability through activation of UT-A1 molecules, thus enabling urea to diffuse into the inner medullary interstitium. Urea then taken up in ascending vasa recta is returned to the inner medulla via UT-A2 and UT-B1 by countercurrent exchange. These latter two urea transporters are not influenced acutely by vasopressin, but UT-A2 expression is markedly increased in the descending thin limbs of the loops of Henle after sustained exposure to vasopressin or its V2 agonist dDAVP. This effect is indirect because vasopressin receptors are lacking in the descending limbs. The acute direct and delayed indirect actions of vasopressin on renal urea transporters will increase medullary urea accumulation and thus the ability of the kidney to conserve water. Atrial natriuretic peptide inhibits the vasopressin-dependent increase in urea permeability in the inner medullary collecting ducts. The interruption of urea recycling probably contributes to the natriuresis. Impairing in this way the capacity of the kidney to concentrate urea enhances its capacity to concentrate sodium in the urine.

Vasopressin contributes to hyperfiltration, albuminuria, and renal hypertrophy in diabetes mellitus: study in vasopressin-deficient Brattleboro rats.

Diabetic nephropathy represents a major complication of diabetes mellitus (DM), and the origin of this complication is poorly understood. Vasopressin (VP), which is elevated in type I and type II DM, has been shown to increase glomerular filtration rate in normal rats and to contribute to progression of chronic renal failure in 5/6 nephrectomized rats. The present study was thus designed to evaluate whether VP contributes to the renal disorders of DM. Renal function was compared in Brattleboro rats with diabetes insipidus (DI) lacking VP and in normal Long-Evans (LE) rats, with or without streptozotocin-induced DM. Blood and urine were collected after 2 and 4 weeks of DM, and creatinine clearance, urinary glucose and albumin excretion, and kidney weight were measured. Plasma glucose increased 3-fold in DM rats of both strains, but glucose excretion was approximately 40% lower in DI-DM than in LE-DM, suggesting less intense metabolic disorders. Creatinine clearance increased significantly in LE-DM (P < 0.01) but failed to increase in DI-DM. Urinary albumin excretion more than doubled in LE-DM but rose by only 34% in DI-DM rats (P < 0.05). Kidney hypertrophy was also less intense in DI-DM than in LE-DM (P < 0.001). These results suggest that VP plays a critical role in diabetic hyperfiltration and albuminuria induced by DM. This hormone thus seems to be an additional risk factor for diabetic nephropathy and, thus, a potential target for prevention and/or therapeutic intervention.

Vasopressin and urinary concentrating activity in diabetes mellitus.

In diabetes mellitus (DM), the high urine flow rate suggests that urinary concentrating capacity is impaired. However, several studies have shown that vasopressin is elevated in DM and the consequences of this elevation have not yet been characterized. This study reevaluated renal function and water handling in male Wistar rats with Streptozotocin-induced DM, and in control rats. During five weeks after induction of DM, urine was collected in metabolic cages and a blood sample was drawn during the third week. Control rats (CONT) were studied in parallel. On week 3, urine flow rate was tenfold higher in DM than in CONT rats and urinary osmolality was reduced by half along with a markedly higher osmolar excretion (DM/CONT = 5.87), due for a large part to glucose but also to urea (DM/CONT = 2.49). Glucose represented 52% of total osmoles (90.3 +/- 6.5 mmol/d out of 172 +/- 14 mosm/d). Free water reabsorption was markedly higher in DM rats compared to CONT (326 +/- 24 vs 81 +/- 5 ml/d). In other rats treated in the same way, urinary excretion of vasopressin was found to be markedly elevated (15.1 +/- 4.1 vs 1.44 +/- 0.23 ng/d). In DM rats, glucose concentration in urine was 17 fold higher than in plasma, and urea concentration 14 fold higher. Both urine flow rate and free water reabsorption were positively correlated with the sum of glucose and urea excretions (r = 0.967 and 0.653, respectively) thus demonstrating that the urinary concentrating activity of the kidney increased in proportion to the increased load of these two organic solutes. These results suggest that vasopressin elevation in DM contributes to increase urinary concentrating activity and thus to limit water requirements induced by the metabolic derangements of DM. The possible deleterious consequences of sustained high level of vasopressin in DM are discussed.

Contribution of vasopressin to progression of chronic renal failure: study in Brattleboro rats.

We have previously shown that a chronic reduction in plasma vasopressin level slowed the progression of chronic renal failure (CRF) in Sprague Dawley rats. The aim of the present study was to determine the respective contribution of pressor (V1) and antidiuretic (V2) effects of vasopressin on progression. Male homozygous Brattleboro rats with hereditary central diabetes insipidus were submitted to 5/6 nephrectomy. They were divided into three groups, two of which received chronic i.p. infusion of AVP (V1 + V2 effects) or dDAVP (V2 effects). The third group served as control (CONT). The doses of AVP and dDAVP were chosen so as to produce urine osmolality similar to that observed in 5/6 Nx Sprague Dawley rats. All rats ate the same amount of food and drank water ad libitum. Renal function was studied for 13 weeks. All three groups showed a marked hypertension. Rats infused with dDAVP, but not those infused with AVP, had a higher creatininemia, anemia and urinary protein excretion than CONT rats. In the dDAVP but not the AVP group, fractional excretion of urea was markedly decreased and plasma urea concentration rose much more than that of creatinine. These results show that V2 but not V1 effects play a major role in the deleterious influence of vasopressin on progression, at least in Brattleboro rats. The more severe progression seen in dDAVP rats could indirectly result from the V2-mediated effects on the collecting duct resulting in a decreased efficiency of urea excretion, an increased intrarenal urea recycling, and a rise in plasma urea concentration. Both the toxic effects of urea and the recently demonstrated V2-mediated increase in glomerular hemodynamics might be involved in the deleterious influence of V2 agonism.

Low-dose vasopressin restores diuresis both in patients with hepatorenal syndrome and in anuric patients with end-stage heart failure.

OBJECTIVES: The purpose of this study was to confirm earlier reports that low-dose vasopressin (LDVP) analogues promote urine output in patients with hepatorenal syndrome (HRS) and to check whether this mode of therapy could also be effective in renal shutdown due to nonhepatic conditions. DESIGN: A prospective, open, interventional study. SETTING: An intermediate-level (step-down) medical intensive care unit within a general medical ward of a large university-affiliated hospital. SUBJECTS: Eighteen successive hospitalized patients with HRS (mean age 65 +/- 13 years) and 11 patients with end-stage congestive heart failure (CHF) (mean age 81 +/- 5 years) who failed to restore urine output with conventional treatment (fluids, dopamine, and diuretics) given for at least 24 h. INTERVENTIONS: The patients received LDVP (1 IU h-1) continuously in addition to the conventional treatment. MAIN OUTCOME MEASURES: Urine output and creatinine clearance every 24 h. RESULTS: In the HRS group, before treatment the urine output was 155 +/- 9 mL 24 -1h (mean +/- SD). After treatment with LDVP for 24, 48, and 72 h, urine output improved to 1067 +/- 87, 1020 +/- 501, and 1311 +/- 988 mL 24 -1h, respectively (P < 0.0001 for all measures; two-tailed paired t-test). In the CHF group, before treatment the urine output was 99 +/- 99 mL 24 -1h. After treatment with LDVP for 24, 48, and 72 h, this improved to 1125 +/- 994 mL 24 -1h (P = 0.0028), 1821 +/- 1300 mL 24 -1h (P = 0.004), and 2920 +/- 2423 mL 24 -1h (P = 0.0012), respectively. The improvement in urine output was not accompanied by a parallel improvement in creatinine clearance. The overall outcome did not change, and all patients except two in each group succumbed to their end-stage disease, due to nonrenal causes. CONCLUSIONS: LDVP is effective in restoring urine output both in HRS and in CHF. This suggests that LDVP affects mechanisms not specifically related to liver disease. LDVP may be useful in critical patients with renal shutdown whilst awaiting liver or heart transplantation.

Glucagon receptor gene mutation (Gly40Ser) in human essential hypertension: the PEGASE study.

A missense mutation (Gly40Ser) in exon 2 of the glucagon receptor gene (GCG-R) was shown to reduce ligand affinity and impair cAMP response. We conducted a case-control study with a sample of 741 French hypertensive patients with moderate to severe hypertension and 412 normotensive control subjects, who were genotyped for this biallelic variant by use of hybridization with allele-specific oligonucleotides. The Gly40Ser polymorphism was not significantly associated with hypertension in the whole study population, although the frequency of 40Ser carriers in hypertensive subjects was double that in normotensive subjects (3.1% in hypertensives versus 1.5%; P=0.087). However, the separate analysis of both genders revealed that 40Ser allele carriers were significantly more frequent (P=0. 035) among male patients (17/429; 4.0%) than among normotensive male controls (2/242; 0.8%), whereas no significant difference was observed in female subjects (6/312 in hypertensives and 4/170 in normotensives). Further studies are required to interpret the significance of this association.

mRNA expression of renal urea transporters in normal and Brattleboro rats: effect of dietary protein intake.

Differences in dietary protein level induce differences in fractional excretion of urea, in arginine vasopressin (AVP) plasma level, and in urine concentrating activity (in which intervene the renal urea transporters (UT)). The abundance of mRNA for UT-A1 (of the inner medullary collecting duct, IMCD) UT-A2 (of the descending thin limb) and UT-B1 (of descending vasa recta) was determined by Northern analysis of total RNA extracted from medullary subregions of Sprague-Dawley rats fed for 1 week, a low, normal, or high protein diet. The implication of AVP was then examined by studying AVP-deprived (Brattleboro) rats. Our results show that none of these transporters is affected by the level of protein intake, except UT-A1 that is reduced in terminal IMCD by low protein diet in the absence of AVP (Brattleboro rats). These data suggest that (1) the previously reported effect of kidney medulla hypertonicity on UT-A2 and UT-B1 mRNA expression is somehow obliterated by protein intake deficiency or excess, and (2) AVP influences the mRNA abundance of the UT-A1 of the terminal IMCD during protein deficiency.