The Effect of High and Low Sodium Intake on Urinary Aquaporin-2 in Autosomal Dominant Polycystic Kidney Disease

Recruitment:

The patients with ADPKD are recruited from the Outpatient Nephrology Clinic of the Department of Medicine, Holstebro Hospital (Holstebro, Denmark). The control subjects are recruited by advertising in public institutions and private companies.

Number of Subjects:

A difference in u-AQP2CR of 40 ng/mmol is considered the minimal relevant difference. A sample size of 10 subjects, who can be evaluated, has 80% power to detect this difference assuming a level of significance of 5% and an SD of 30 ng/mmol. Because a few subjects are expected to drop out, 14-15 subjects will be included in each group.

Experimental Procedure Before the Study Day:

Five days before the study day, the subjects collect a standardized, HS (300 mmol sodium/day/17.5 g salt/day) or LS (30 mmol sodium/day/1.8 g salt/day), 4-day diet from the hospital kitchen. Depending on the individually estimated energy requirement, the participants are given either a diet of 8,000 or 11,000 kJ/day. The energy distribution is 55% carbohydrates, 15% proteins, and 30% lipids. The 4-day diet is started the following morning. The fluid intake is also standardized during the 4 days. The subjects are asked to drink exactly 250 ml/1,000 kJ/day and to abstain from coffee, tea, and alcoholic beverages. The subjects are instructed to keep their physical activity unchanged during the two experiments and to abstain from hard training. The subjects have to collect their urine for 24 h the day before the study day.

Experimental Procedure on the Study Day:

On the study day, the subjects are asked to drink 175 ml of water every 30 min from 7:00 AM. The subjects arrive at the department at 8:00 AM. Peripheral iv lines are inserted into the antecubital veins of both forearms, one for infusion of 51Cr-EDTA and hypertonic saline, and one for withdrawal of blood samples. The subjects will be kept in the supine position from 8:00 AM to 1:30 PM except during voiding, which will take place in the sitting or standing position. At 8:30 AM, a priming dose of 51Cr-EDTA is administered, followed by sustained infusion. After 60 min of equilibration, the study will continue with five clearance periods, the first two of 30-min duration (P1-P2), the last three of 60-min duration (P3-P5). The first two clearance periods are baseline periods. At 10:30 AM, 7 ml/kg of 3% saline is given over 30 min. Blood pressure and heart rate are measured every 30 min from 9:30 AM to 1:30 PM. Urine is collected in each clearance period and analyzed for sodium, osmolality, u-AQP2, u-ENaC-betaCR, u-cAMP, u-PGE2, and 51CrEDTA. Blood samples will be drawn every 30 min from 9:30 AM to 10:30 AM and every hour from 11:30 AM to 1:30 PM, and will be analyzed for sodium, osmolality, and 51Cr-EDTA. In addition, analysis of p-AVP, p-Renin, p-ANG II, p-Aldo, p-ANP, and p-BNP will be performed from blood samples drawn at 10:30 AM, 11:30 AM, 12:30 PM, and 1:30 PM.

Statistics:

Statistical analyses will be performed using SPSS version 15 (SPSS, Chicago, IL). Single baseline values are obtained by taking the weighted average of the measurements from the two baseline periods. The baseline values of the two groups will be compared by Student's t-test. The baseline values during HS and LS intake will be compared by paired samples t-tests. The investigators will use the "General Linear Model Repeated Measures" procedure in SPSS with time as the within-subject factor and group as the between-subject factor to compare the effect variables in patients and controls. The changes in response to the hypertonic saline infusion in each group will be analyzed with the General Linear Model Repeated Measures procedure with time as the within-subject factor and paired samples t-tests with Bonferroni correction as post hoc tests. P values Recruitment The patients with ADPKD will be recruited from the Outpatient Nephrology Clinic of the Department of Medicine, Holstebro Hospital (Holstebro, Denmark). The control subjects will be recruited by advertising in public institutions and private companies.

Number of Subjects A difference in u-AQP2CR of 40 ng/mmol is considered the minimal relevant difference. A sample size of 10 subjects, who could be evaluated, has 80% power to detect this difference assuming a level of significance of 5% and an SD of 30 ng/mmol. Because a few subjects are expected to drop out, 14-15 subjects will be included in each group.

Experimental Procedure Before the Study Day Five days before the study day, the subjects will collect a standardized, HS (300 mmol sodium/day/17.5 g salt/day) or LS (30 mmol sodium/day/1.8 g salt/day), 4-day diet from the hospital kitchen. Depending on the individually estimated energy requirement, the participants will be given either a diet of 8,000 or 11,000 kJ/day. The energy distribution will be 55% carbohydrates, 15% proteins, and 30% lipids. The 4-day diet will start the following morning. The fluid intake is also standardized during the 4 days. The subjects are asked to drink exactly 250 ml/1,000 kJ/day and to abstain from coffee, tea, and alcoholic beverages. The subjects will be instructed to keep their physical activity unchanged during the two experiments and to abstain from hard training. The subjects have to collect their urine for 24 h the day before the study day.

Experimental Procedure on the Study Day On the study day, the subjects are asked to drink 175 ml of water every 30 min from 7:00 AM. The subjects arrive at the department at 8:00 AM. Peripheral iv lines will be inserted into the antecubital veins of both forearms, one for infusion of 51Cr-EDTA and hypertonic saline, and one for withdrawal of blood samples. The subjects will be kept in the supine position from 8:00 AM to 1:30 PM except during voiding, which will take place in the sitting or standing position. At 8:30 AM, a priming dose of 51Cr-EDTA is administered, followed by sustained infusion. After 60 min of equilibration, the study will continue with five clearance periods, the first two of 30-min duration (P1-P2), the last three of 60-min duration (P3-P5). The first two clearance periods are baseline periods. At 10:30 AM, 7 ml/kg of 3% saline is given over 30 min. Blood pressure and heart rate will be measured every 30 min from 9:30 AM to 1:30 PM. Urine is collected in each clearance period and analyzed for sodium, osmolality, u-AQP2, u-ENaC-betaCR, u-cAMP, u-PGE2, and 51CrEDTA. Blood samples will be drawn every 30 min from 9:30 AM to 10:30 AM and every hour from 11:30 AM to 1:30 PM, and will be analyzed for sodium, osmolality, and 51Cr-EDTA. In addition, analysis of p-AVP, p-Renin, p-ANG II, p-Aldo, p-ANP, and p-BNP will be performed from blood samples drawn at 10:30 AM, 11:30 AM, 12:30 PM, and 1:30 PM.

Statistics Statistical analyses will be performed using SPSS version 15 (SPSS, Chicago, IL). Single baseline values are obtained by taking the weighted average of the measurements from the two baseline periods. The baseline values of the two groups will be compared by Student's t-test. The baseline values during HS and LS intake will be compared by paired samples t-tests. The investigators will use the "General Linear Model Repeated Measures" procedure in SPSS with time as the within-subject factor and group as the between-subject factor to compare the effect variables in patients and controls. The changes in response to the hypertonic saline infusion in each group will be analyzed with the General Linear Model Repeated Measures procedure with time as the within-subject factor and paired samples t-tests with Bonferroni correction as post hoc tests. P values ≤0.05 will be considered significant.