J. Starklint

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.

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

BackgroundTreatment 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.MethodsThe 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.ResultsIbuprofen 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.ConclusionDuring 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 systemTrial RegistrationClinical Trials Identifier: NCT00281762

Increased urinary aquaporin-2 excretion in response to furosemide in patients with chronic heart failure.

Objective. Patients with chronic heart failure (CHF) have decreased ability to excrete water and increased urinary excretion of aquaporin‐2 (U‐AQP2). The natriuretic and diuretic effects of furosemide are antagonized by an increased reabsorption of sodium and water in the collecting ducts. It is unknown whether aquaporin‐2 (AQP2) renal water channels are involved in this compensatory reabsorption. We tested the hypothesis that U‐AQP2 increases after a single intravenous dose of furosemide in CHF patients. Material and methods. In a randomized, single‐blind, placebo‐controlled, crossover study, we measured the effect of furosemide (80 mg) on U‐AQP2, urine volume, free water clearance (CH2O) and fractional excretion of sodium (FENa) in 12 CHF patients. Plasma concentrations of vasopressin (AVP), renin (PRC), angiotensin II (Ang II), aldosterone (Aldo), atrial (ANP) and brain natriuretic peptides (BNP) were measured during the study. U‐AQP2 and hormones were determined by radioimmunoassays. Results. Furosemide increased U‐AQP2 (140 %), urine volume (280 %), CH2O (95 %) and FENa by a factor of 15 (p<0.008 for all), and also AVP (51 %), PRC, Ang II (86 %) and Aldo (59 %) (p<0.021 for all). ANP and BNP did not change. Conclusions. In CHF, furosemide increased the vasopressin level, which stimulated water reabsorption via the APQ2 water channels. This is most likely a compensatory phenomenon in addition to the increase in the renin‐angiotensin system to prevent excess loss of sodium and water. However, both these effects were overridden by the effect of furosemide, as shown by increased free water clearance and sodium excretion.

Urinary excretion of aquaporin‐2 after furosemide and felodipine in healthy humans

Objective. Furosemide inhibits renal sodium and chloride reabsorption in the loop of Henle. A compensatory increased reabsorption of sodium and water takes place in the collecting duct. It is not known whether aquaporin‐2 (AQP2) renal water channels are involved in this compensatory reabsorption. In animals, dihydropyridine derivatives of calcium channel blockers down‐regulate AQP2 in the collecting duct, but the effect has not been studied in humans. We sought to test the hypotheses that urinary excretion of aquaporin‐2 (U‐AQP2) increases after a single intravenous dose of furosemide, and that U‐AQP2 decreases after a single oral dose of felodipine. Material and methods. In two randomized, single‐blind, placebo‐controlled, cross‐over studies, we measured the effect of furosemide and felodipine on U‐AQP2, urine volume, free water clearance (CH2O), and fractional excretion of sodium (FENa) in 13 healthy subjects in each study. Plasma concentrations of vasopressin (AVP), renin (PRC), angiotensin II (ang II), aldosterone (aldo), atrial (ANP), and brain natriuretic peptides (BNP) were measured during the study. Glomerular filtration rate (GFR) was measured by constant infusion technique. U‐AQP2 and hormones were determined by radioimmunoassay. Results. Furosemide treatment increased U‐AQP2 (202%), urine volume (214%), and FENa by a factor of 11, (p<0.001 for all), whereas CH2O and GFR were unchanged. After treatment with placebo, no differences were seen. Furosemide treatment increased AVP (18%), PRC (60%), ang II (100%), and aldo (98%) (p<0.032); ANP was decreased by 29% (p<0.001), whereas there was no change in BNP. The hormones were unchanged after placebo except for a minor decrease in ANP after placebo. Felodipine tended to increase U‐AQP2, to decrease CH2O and urine volume and GFR, and to increase FENa, but the effect was not significantly different from placebo. Felodipine increased PRC (82%) (p<0.003) and ang II, but decreased aldo, and increased AVP. After placebo, PRC was unchanged, whereas ang II, aldo and AVP were changed as after felodipine. Conclusions. Furosemide treatment increased U‐AQP2, AVP, and the activity of the renin‐angiotensin‐aldosterone system. These changes are most likely compensatory phenomena, which prevent an excess loss of sodium and water. Felodipine tended to increase U‐AQP2.

Glomerular filtration rate and blood pressure are unchanged by increased sodium intake in atorvastatin‐treated healthy men

Objective. Improved cardiovascular survival during statin treatment might be due to effects in addition to cholesterol lowering. We hypothesize that sodium intake affects renal function and vasoactive hormones in atorvastatin‐treated healthy subjects. Methods. In a randomized, placebo‐controlled, double‐blind, crossover study we measured the effect of a moderate change in sodium intake on glomerular filtration rate (GFR), blood pressure (BP), renal tubular function, plasma concentrations of vasoactive hormones and urinary excretion of aquaporin‐2 (u‐AQP2) in 22 healthy subjects. The subjects were randomized to standardized fluid intake and diet corresponding to the need for calories in the 4 days before each of the 2 examination days. In one of the periods they were randomized to receive sodium chloride tablets (2 g) thrice daily for 4 days. Two doses of atorvastatin (80 mg) were given; one at 2200 h the evening before the study day, the other at 0830 h in the morning. Results. 24‐h urinary sodium excretion increased by 23 %. GFR and BP were unchanged. Sodium clearance, fractional excretion of sodium and u‐AQP2 increased, whereas free water clearance decreased during high sodium intake. PRC and aldosterone were suppressed during the high sodium diet. Conclusions. A change in dietary sodium intake of approximately 100 mmol daily does not change GFR and BP in atorvastatin‐treated healthy men. The lack of change in BP might reflect that the subjects studied were not sodium sensitive, or that atorvastatin treatment modified sodium sensitivity. Trial registration: ClinicalTrials.gov identifier: NCT00678184.

Acute Effects of Atorvastatin on Glomerular Filtration Rate, Tubular Function, Blood Pressure, and Vasoactive Hormones in Patients With Type 2 Diabetes

Statins improve cardiovascular survival in both nondiabetic and diabetic patients, but diabetic patients benefit more, in both primary and secondary prevention. Statins seem to have multiple effects beyond cholesterol lowering, that is, pleiotropic effects that may include changes in renal function. This study tests the hypothesis that acute treatment with atorvastatin may change glomerular filtration rate, tubular function, vasoactive hormones, blood pressure, and pulse rate in patients with type 2 diabetes. In an acute, randomized, placebo‐controlled, double‐blinded, crossover trial, the effects of atorvastatin on renal function, vasoactive hormones, blood pressure, and pulse rate are measured in 21 patients with type 2 diabetes. Patients are randomized to either 2 doses of atorvastatin 80 mg or placebo before 2 different study days. Treatment with atorvastatin induces a significant reduction in fractional sodium excretion compared with placebo, and sodium clearance tends to be reduced. No significant differences in glomerular filtration rate, albumin/creatinine ratio, vasoactive hormones, and blood pressure by acute treatment with atorvastatin are found in diabetic patients. Acute treatment with atorvastatin reduces renal fractional sodium excretion in patients with type 2 diabetes. No changes are measured in glomerular filtration rate, albumin/creatinine ratio, vasoactive hormones, and blood pressure.

Hypochromic reticulocytes, hypochromic erythrocytes and p-transferrin receptors in diagnosing iron-deficient erythropoiesis

Background: The purpose of this study was to test the hypothesis that hypochromic reticulocytes, hypochromic erythrocytes and p-transferrin receptors are sensitive variables in detecting iron-deficient erythropoiesis in healthy subjects and hemodialysis patients. Methods: Study 1: Twenty-one blood donors donated 450 mL blood. During the following 2 weeks blood samples were analyzed for the variables mentioned above. Study 2: Twenty-eight blood donors received 10 000 U recombinant human erythropoietin (rHuEPO) twice in the first week or placebo, after they had donated 450 mL blood. During the following 3 weeks the blood samples were analyzed for the variables mentioned in Study 1. Study 3: Eighteen hemodialysis patients receiving rHuEPO and iron treatment had either iron treatment discontinued for 4 weeks, after which iron was resumed, or received unchanged treatment. During 8 weeks blood samples were analyzed for the variables mentioned in Study 1. Results: Study 1: Blood donation induced an increase in hypochromic reticulocytes of 178%, in hypochromic erythrocytes the increase was 267%, and in p-transferrin receptors 32%. Study 2: Treatment with rHuEPO induced a more pronounced increase than placebo in hypochromic reticulocytes (232% vs. 158%) and hypochromic erythrocytes (1240% vs. 300%), but not in p-transferrin receptors. Study 3: Discontinuation of iron treatment did not cause any significant differences in the variables mentioned above between the two groups, but caused a 25% decrease in p-ferritin. When iron treatment was resumed, p-ferritin increased by 19%. We found no significant changes in the control group. Conclusions: Hypochromic reticulocytes, hypochromic erythrocytes and p-transferrin receptors are sensitive variables in the early detection of iron-deficient erythropoiesis in healthy subjects, but in this study the iron withdrawal period was too short to show the value of these variables in the detection of iron-deficient erythropoiesis in hemodialysis patients.