Sodium and water handling after gastric bypass surgery in a rat model

Background: It was the aim to investigate the influence of gastric bypass on renal sodium and water handling. The relationship between sodium and water absorption along the gastrointestinal tract and their renal excretion is poorly understood. Beneficial effects on blood pressure have been seen after bariatric surgery before significant weight loss has occurred. Methods: Male Wistar rats (348 +/- 19 g) underwent either gastric bypass (n = 14) or sham operation (n = 7) and were given a low-sodium diet with deionized water ad libitum. Before and after surgery, the rats received an oral sodium load (1.5 mmol/kg) as hyperosmolar saline (616 mM), and were then placed in individual metabolic cages so the urine volume, sodium content, and water intake for 8 hours could be recorded. The urine sodium concentration was also measured. Results: The rats that had undergone gastric bypass had a significantly lower body weight than the sham-operated controls throughout the follow-up period (346 +/- 21 g versus 501.3 +/- 8.0 g at day 60; P = .0004). An oral sodium load after gastric bypass led to an increase in water intake (.07 +/- .01 mL/g versus .03 +/- .01 mL/g; P = .023), urine output (.03 +/- .01 mL/g versus .02 +/- .002 mL/g; P = .027), and sodium excretion (65.99 +/- 10.7 mol versus 31.71 +/- 8.7 mol; P = .020). No change was seen in water intake, urine output, or sodium excretion after sham surgery. Conclusion: Urine output, water intake, and sodium excretion are all increased after gastric bypass surgery in rats given an oral sodium load compared with sham-operated controls. More rapid excretion, and less retention, of a dietary sodium load could be a part of the mechanism underlying the beneficial effect of bariatric surgery on blood pressure. (Surg Obes Relat Dis 2011;7:68-73.) (C) 2011 American Society for Metabolic and Bariatric Surgery. All rights reserved.