Down-regulation of urinary AQP2 and unaffected response to hypertonic saline after 24 hours of fasting in humans

Differential roles of VPS and RAAS in water homeostasis and a risk for kidney dysfunction in rats undergoing rapid fasting/dehydration with regular exercise

Purpose

We examined the effects of rapid restriction of food and fluid intake on the pathways of water homeostasis, the vasopressinergic system (VPS), and the renin–angiotensin–aldosterone system (RAAS), in rats with or without regular exercise.

Methods

Sprague Dawley rats were divided into the following groups: no intervention, rapid restriction, regular exercise, and rapid restriction combined with regular exercise. Rats in the exercise group performed climbing exercise for 4 weeks. All rats consumed food ad libitum, and those in the rapid restriction group fasted for the last 3 days with no water on the last 1 day.

Results

Despite no significant differences in body weight among the groups, the kidney weight was decreased when rapid restriction and regular exercise were combined. Rapid restriction reduced the urine volume and increased the urine osmolality, whereas regular exercise did not. Rapid restriction but not regular exercise increased the levels of circulating aldosterone and the renal expression levels of the ion channel SGK‐1 compared to those without rapid restriction, indicating the stimulation of RAAS. Conversely, VPS showed no significant response to these interventions. Moreover, rapid restriction combined with regular exercise induced the renal expression levels of proinflammatory cytokines and increased the active forms of apoptotic effector caspase‐3 compared with the no intervention group.

Conclusions

Functional significance may differ between VPS and RAAS in water homeostasis in response to rapid restriction. Moreover, the combination of rapid restriction and regular exercise has potentially deleterious effects on the kidney.

Changes in urinary excretion of water and sodium transporters during amiloride and bendroflumethiazide treatment

AIM: To quantify changes in urinary excretion of aquaporin2 water channels (u-AQP2), the sodium-potassium-chloride co-transporter (u-NKCC2) and the epithelial sodium channels (u-ENaC) during treatment with bendroflumethiazide (BFTZ), amiloride and placebo.

METHODS: In a randomized, double-blinded, placebo-controlled, 3-way crossover study we examined 23 healthy subjects on a standardized diet and fluid intake. The subjects were treated with amiloride 5 mg, BFTZ 1.25 mg or placebo twice a day for 4.5 d before each examination day. On the examination day, glomerular filtration rate was measured by the constant infusion clearance technique with ⁵¹Cr-EDTA as reference substance. To estimate the changes in water transport via AQP2 and sodium transport via NKCC2 and ENaC, u-NKCC2, the gamma fraction of ENaC (u-ENaCγ), and u-AQP2 were measured at baseline and after infusion with 3% hypertonic saline. U-NKCC2, u-ENaCγ, u-AQP2 and plasma concentrations of vasopressin (p-AVP), renin (PRC), angiotensin II (p-ANG II) and aldosterone (p-Aldo) were measured, by radioimmunoassay. Central blood pressure was estimated by applanation tonometry and body fluid volumes were estimated by bio-impedance spectroscopy. General linear model with repeated measures or related samples Friedman’s two-way analysis was used to compare differences. Post hoc Bonferroni correction was used for multiple comparisons of post infusion periods to baseline within each treatment group.

RESULTS: At baseline there were no differences in u-NKCC2, u-ENaCγ and u-AQP2. PRC, p-Ang II and p-Aldo were increased during active treatments (P < 0.001). After hypertonic saline, u-NKCC2 increased during amiloride (6% ± 34%; P = 0.081) and increased significantly during placebo (17% ± 24%; P = 0.010). U-AQP2 increased significantly during amiloride (31% ± 22%; P < 0.001) and placebo (34% ± 27%; P < 0.001), while u-NKCC2 and u-AQP2 did not change significantly during BFTZ (-7% ± 28%; P = 0.257 and 5% ± 16%; P = 0.261). U- ENaCγ increased in all three groups (P < 0.050). PRC, AngII and p-Aldo decreased to the same extent, while AVP increased, but to a smaller degree during BFTZ (P = 0.048). cDBP decreased significantly during BFTZ (P < 0.001), but not during amiloride or placebo. There were no significant differences in body fluid volumes.

CONCLUSION: After hypertonic saline, u-NKCC2 and u-AQP2 increased during amiloride, but not during BFTZ. Lower p-AVP during BFTZ potentially caused less stimulation of NKCC2 and AQP2 and subsequent lower reabsorption of water and sodium.

Effect of potassium supplementation on renal tubular function, ambulatory blood pressure and pulse wave velocity in healthy humans

BACKGROUND

Potassium is the main intracellular cation, which contributes to keeping the intracellular membrane potential slightly negative and elicits contraction of smooth, skeletal and cardiac muscle. A change in potassium intake modifies both cardiovascular and renal tubular function. The purpose of the trial was to investigate the effect of dietary potassium supplementation, 100 mmol daily in a randomized, placebo-controlled, crossover trial of healthy participants during two periods of 28 days duration. The participants (N = 21) received a diet that was standardized regarding energy requirement, and sodium and water intake.

METHODS

24-hour ambulatory blood pressure (ABP) and applanation tonometry were used to assess blood pressure, pulse wave velocity (PWV), augmentation index (AIx) and central blood pressure (CBP). Immunoassays were used for measurements of plasma concentrations of vasoactive hormones: renin (PRC), angiotensin II (Ang II), aldosterone (Aldo), atrial natriuretic peptide (ANP), vasopressin (AVP), pro-brain natriuretic peptide (pro-BNP),endothelin (Endo), urinary excretions of aquaporin 2 (AQP2), cyclic AMP (cAMP), and the β-fraction of the epithelial sodium channel (ENaC(ß)).

RESULTS

AQP2 excretion increased during potassium supplementation, and free water clearance fell. The changes in urinary potassium excretion and urinary AQP2 excretion were significantly and positively correlated. Aldo increased. GFR, u-ENaC- β, PRC, Ang II, ANP, BNP, Endo, blood pressure and AI were not significantly changed by potassium supplementation, whereas PWV increased slightly.

CONCLUSIONS

Potassium supplementation changed renal tubular function and increased water absorption in the distal part of the nephron. In spite of an increase in aldosterone in plasma, blood pressure remained unchanged after potassium supplementation.

Effect of high and low sodium intake on urinary aquaporin-2 excretion in healthy humans

The degree of water transport via aquaporin-2 (AQP2) water channels in renal collecting duct principal cells is reflected by the level of the urinary excretion of AQP2 (u-AQP2). In rats, the AQP2 expression varies with sodium intake. In humans, the effect of sodium intake on u-AQP2 and the underlying mechanisms have not previously been studied. We measured the effect of 4 days of high sodium (HS) intake (300 mmol sodium/day; 17.5 g salt/day) and 4 days of low sodium (LS) intake (30 mmol sodium/day; 1.8 g salt/day) on u-AQP2, fractional sodium excretion (FE(Na)), free water clearance (C(H2O)), urinary excretion of PGE(2) (u-PGE(2)) and cAMP (u-cAMP), and plasma concentrations of vasopressin (AVP), renin (PRC), ANG II, aldosterone (Aldo), atrial natriuretic peptide (ANP), and brain natriuretic peptide (BNP) in a randomized, crossover study of 21 healthy subjects, during 24-h urine collection and after hypertonic saline infusion. The 24-h urinary sodium excretion was significantly higher during HS intake (213 vs. 41 mmol/24 h). ANP and BNP were significantly lower and PRC, ANG II, and Aldo were significantly higher during LS intake. AVP, u-cAMP, and u-PGE(2) were similar during HS and LS intake, but u-AQP2 was significantly higher during HS intake. The increases in AVP and u-AQP2 in response to hypertonic saline infusion were similar during HS and LS intake. In conclusion, u-AQP2 was increased during HS intake, indicating that water transport via AQP2 was increased. The effect was mediated by an unknown AVP-independent mechanism.

Increased renal sodium absorption by inhibition of prostaglandin synthesis during fasting in healthy man. A possible role of the epithelial sodium channels

BACKGROUND

Treatment with prostaglandin inhibitors can reduce renal function and impair renal water and sodium excretion. We tested the hypotheses that a reduction in prostaglandin synthesis by ibuprofen treatment during fasting decreased renal water and sodium excretion by increased absorption of water and sodium via the aquaporin2 water channels and the epithelial sodium channels.

METHODS

The effect of ibuprofen, 600 mg thrice daily, was measured during fasting in a randomized, placebo-controlled, double-blinded crossover study of 17 healthy humans. The subjects received a standardized diet on day 1, fasted at day 2, and received an IV infusion of 3% NaCl on day 3. The effect variables were urinary excretions of aquaporin2 (u-AQP2), the beta-fraction of the epithelial sodium channel (u-ENaCbeta), cyclic-AMP (u-cAMP), prostaglandin E2 (u-PGE2). Free water clearance (CH2O), fractional excretion of sodium (FENa), and plasma concentrations of vasopressin, angiotensin II, aldosterone, atrial-, and brain natriuretic peptide.

RESULTS

Ibuprofen decreased u-AQP2, u-PGE2, and FENa at all parts of the study. During the same time, ibuprofen significantly increased u-ENaCbeta. Ibuprofen did not change the response in p-AVP, u-c-AMP, urinary output, and free water clearance during any of these periods. Atrial-and brain natriuretic peptide were higher.

CONCLUSION

During inhibition of prostaglandin synthesis, urinary sodium excretion decreased in parallel with an increase in sodium absorption and increase in u-ENaCbeta. U-AQP2 decreased indicating that water transport via AQP2 fell. The vasopressin-c-AMP-axis did not mediate this effect, but it may be a consequence of the changes in the natriuretic peptide system and/or the angiotensin-aldosterone system

TRIAL REGISTRATION

Clinical Trials Identifier: NCT00281762.

Abnormal function of the vasopressin-cyclic-AMP-aquaporin2 axis during urine concentrating and diluting in patients with reduced renal function. A case control study

Background

The kidneys ability to concentrate and dilute urine is deteriorated during progressive renal insufficiency. We wanted to test the hypothesis that these phenomena could be attributed to an abnormal function of the principal cells in the distal part of the nephron.

Methods

Healthy control subjects and patients with chronic kidney diseases were studied. Group 1 comprised healthy subjects, n = 10. Groups 2-4 comprised patients with chronic kidney disease (Group 2, n = 14, e-GFR ? 90 m1/min; Group 3, n = 11, 60 m1/min ? e-GFR < 90 ml/min; and Group 4, n = 16, 15 ml/min ? e-GFR < 60 ml/min). The subjects collected urine during 24 hours. A urine concentrating test was done by thirsting during the following 12 hours. Thereafter, a urine diluting test was performed with a water load of 20 ml/kg body weight. The effect variables were urinary excretions of aquaporin2 (u-AQP2), cyclic-AMP (u-c-AMP), urine volume (UV), free water clearance (C_H2O), urine osmolarity (u-Osm), and plasma arginine vasopressin (p-AVP).

Results

After fluid deprivation, u-Osm increased. In all groups, UV and C_H2O decreased and u-AQP2 and u-c-AMP increased in Groups 1 and 2, but were unchanged in Group 3 and 4. P-AVP was significantly higher in Group 4 than in the other groups. During urine diluting, UV and C_H2O reached significantly higher levels in Groups 1-3 than Group 4. Both before and after water loading, u-AQP2 and p-AVP were significantly higher and u-c-AMP was significantly lower in Group 4 than the other groups. Estimated-GFR was correlated negatively to p-AVP and positively to u-c-AMP.

Conclusions

Patients with moderately severe chronic kidney disease have a reduced renal concentrating and diluting capacity compared to both patients with milder chronic kidney disease and healthy control subjects. These phenomena can be attributed, at least partly, to an abnormally decreased response in the AVP-c-AMP-AQP2 axis.

ClinicalTrials.Gov Identifier: NCT00313430

Effect of nutrition on plasma C-type natriuretic peptide forms in adult sheep: evidence for enhanced C-type natriuretic peptide degradation during caloric restriction

Previous studies in lambs and children show that the plasma concentration of amino terminal pro-C-type natriuretic peptide (NTproCNP), a stable product of proCNP, is strongly correlated with skeletal growth and markers of bone formation. Consistent with these findings, CNP expression is sensitive to nutritional status and is reduced by caloric restriction (CR) in both the fetus and the postnatal lamb. However, the effect of nutritional status on CNP in the adult, once linear growth is complete, is unknown. Hypothesizing that reduced CNP synthesis during CR is contingent on the presence of active growth plates, we studied the effect of CR ( 25% of maintenance) or loading (CL, 200% of maintenance) on CNP forms and alkaline phosphatase (ALP) in adult ewes and compared the findings to responses in a control group (C) fed a maintenance diet of 10.6 MJ of metabolizable energy. Live body weight was reduced (17%) in the CR group and increased (10%) in the CL group after 16 days of intervention. Plasma CNP concentration and ALP both fell in CR sheep and were significantly lower than C (P < .05 for both), returning toward basal levels 1 week after refeeding. In contrast, plasma NTproCNP did not differ (CR vs C). There were no significant changes in CNP forms and ALP in CL sheep compared with C. Fall in plasma CNP but not in NTproCNP in CR adult sheep suggests that CNP degradation (not synthesis) is altered, and contrasts with previous findings in growing lambs where CR reduces both CNP forms.

Direct effect of methylprednisolone on renal sodium and water transport via the principal cells in the kidney

BACKGROUND

Glucocorticoids influence renal concentrating and diluting ability. We tested the hypothesis that methylprednisolone treatment increased renal water and sodium absorption by increased absorption via the aquaporin-2 (AQP2) water channels and the epithelial sodium channels (ENaCs) respectively.

METHODS

The effect of methylprednisolone was measured during fasting in a randomized, placebo-controlled, single-blinded cross-over study of 15 healthy humans. The subjects received a standardized diet on day 1, fasted on day 2, and received 500 mg methylprednisolone intravenously on day 3. The effect variables were urinary excretions of AQP2 (u-AQP2), urinary excretion of the beta-fraction of the ENaC (u-ENaC(beta)), cAMP (u-cAMP), prostaglandin E(2) (u-PGE(2)), free water clearance (C(H2O)), and fractional excretion of sodium (FE(Na)), and plasma vasopressin (p-AVP), angiotensin II (p-Ang II), aldosterone (p-Aldo), atrial natriuretic peptide (p-ANP), and brain natriuretic peptide (p-BNP).

RESULTS

Methylprednisolone treatment increased u-AQP2, u-ENaC(beta), and p-AVP significantly, but did not change u-cAMP, c(H2O), and FE(Na). P-ANP increased during methylprednisolone treatment, but after the increase in u-AQP2 and u-ENaC(beta). U-PGE(2), p-Ang II, and p-BNP were unchanged. Heart rate increased and diastolic blood pressure fell.

CONCLUSIONS

Methylprednisolone increased u-AQP2 and u-ENaC. Neither the AVP-cAMP axis nor changes in the renin-angiotensin-Aldo system, or the natriuretic peptide system seems to bear a causal relationship with the increase in either u-AQP2 or u-ENaC. Most probably, the effect is mediated via a direct effect of methylprednisolone on the principal cells. The lack of decrease in urinary output and sodium reabsorption most likely can be attributed to the diuretic and natriuretic properties of the increased secretion of ANP.

Protein-enriched diet increases water absorption via the aquaporin-2 water channels in healthy humans

BACKGROUND

According to animal experiments, a protein-enriched diet increased renal absorption of sodium and water. We wanted to test the hypothesis that a protein-enriched diet would increase the expression of the aquaporin-2 water channels and the epithelial sodium channels in the distal part of the nephron using biomarkers for the activity of the two channels.

METHODS

We performed a randomized, placebo controlled crossover study in 13 healthy humans to examine the effect of a protein-enriched diet on renal handling of water and sodium during baseline condition and during hypertonic saline infusion. We measured the effect of the protein-enriched diet on urinary excretions of aquaporin-2 (u-AQP2), the beta-fraction of the epithelial sodium channels (u-ENaC(beta)), free water clearance (C(H2O)), fractional excretion of sodium and vasoactive hormones.

RESULTS

During baseline conditions, u-AQP2 increased, and C(H2O) decreased during the protein-enriched diet, whereas u-ENaC(beta) was unchanged, although the urinary sodium excretion increased. During hypertonic saline infusion, the response in the effect variables did not deviate between protein-enriched and normal diet. Plasma concentrations of angiotensin II and aldosterone increased as well as pulse rate. Vasopressin in plasma was unchanged, and prostaglandin E(2) fell during the protein-enriched diet.

CONCLUSIONS

The protein-enriched diet increased water absorption via an increased transport via the aquaporin-2 water channels. The increased u-AQP2 might be due to a reduced prostaglandin level. The increase in renal sodium excretion seems to be mediated in another part of the nephron than the epithelial sodium channels.