title: The frequently used intraperitoneal hyponatraemia model induces hypovolaemic hyponatraemia with possible model-dependent brain sodium loss.
abstract: What is the central question of this study? The brain response to acute hyponatraemia is usually studied in rodents by intraperitoneal instillation of hypotonic fluids (i.p. model). The i.p. model is described as 'dilutional' and 'syndrome of inappropriate ADH (SIADH)', but the mechanism has not been explored systematically and might affect the brain response. Therefore, in vivo brain and muscle response were studied in pigs. What is the main finding and its importance? The i.p. model induces hypovolaemic hyponatraemia attributable to sodium redistribution, not dilution. A large reduction in brain sodium is observed, probably because of the specific mechanism causing the hyponatraemia. This is not accounted for in current understanding of the brain response to acute hyponatraemia. Hyponatraemia is common clinically, and if it develops rapidly, brain oedema evolves, and severe morbidity and even death may occur. Experimentally, acute hyponatraemia is most frequently studied in small animal models, in which the hyponatraemia is produced by intraperitoneal instillation of hypotonic fluids (i.p. model). This hyponatraemia model is described as 'dilutional' or 'syndrome of inappropriate ADH (SIADH)', but seminal studies contradict this interpretation. To confront this issue, we developed an i.p. model in a large animal (the pig) and studied water and electrolyte responses in brain, muscle, plasma and urine. We hypothesized that hyponatraemia was induced by simple water dilution, with no change in organ sodium content. Moderate hypotonic hyponatraemia was induced by a single i.v. dose of desmopressin and intraperitoneal instillation of 2.5% glucose. All animals were anaesthetized and intensively monitored. In vivo brain and muscle water was determined by magnetic resonance imaging and related to the plasma sodium concentration. Muscle water content increased less than expected as a result of pure dilution, and muscle sodium content decreased significantly (by 28%). Sodium was redistributed to the peritoneal fluid, resulting in a significantly reduced plasma volume. This shows that the i.p. model induces hypovolaemic hyponatraemia and not dilutional/SIADH hyponatraemia. Brain oedema evolved, but brain sodium content decreased significantly (by 21%). To conclude, the i.p. model induces hypovolaemic hyponatraemia attributable to sodium redistribution and not water dilution. The large reduction in brain sodium is probably attributable to the specific mechanism that causes the hyponatraemia. This is not accounted for in the current understanding of the brain response to acute hyponatraemia.
authors.affiliation: Department of Anaesthesiology, Aarhus University Hospital NBG, Aarhus, Denmark.|Institute of Clinical Medicine, Aarhus University, Aarhus University Hospital, Skejby, Denmark.
authors.forename: Christian
authors.initials: C
authors.lastname: Overgaard-Steensen
authors.affiliation: The MR Research Centre, Aarhus University Hospital, Skejby, Denmark.
authors.forename: Hans
authors.initials: H
authors.lastname: Stødkilde-Jørgensen
authors.affiliation: Hedenstierna Laboratory, Department of Surgical Sciences, Uppsala University, Uppsala, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Larsson
authors.affiliation: Department of Anaesthesiology, Aarhus University Hospital NBG, Aarhus, Denmark.|Institute of Clinical Medicine, Aarhus University, Aarhus University Hospital, Skejby, Denmark.
authors.forename: Else
authors.initials: E
authors.lastname: Tønnesen
authors.affiliation: Institute of Clinical Medicine, Aarhus University, Aarhus University Hospital, Skejby, Denmark.|The Water and Salt Research Centre, Aarhus University, Aarhus, Denmark.
authors.forename: Jørgen
authors.initials: J
authors.lastname: Frøkiaer
authors.affiliation: Department of Nephrology, Aalborg University Hospital, Aalborg, Denmark.
authors.forename: Troels
authors.identifier: 0000-0002-0193-6316
authors.initials: T
authors.lastname: Ring
mesh_terms: D000818:Animals; D001202:Ascitic Fluid / Q000378:metabolism*; D001921:Brain / Q000378:metabolism* / Q000503:physiopathology; D004195:Disease Models, Animal; D004573:Electrolytes; D005260:Female; D007010:Hyponatremia / Q000378:metabolism* / Q000503:physiopathology*; D020896:Hypovolemia / Q000378:metabolism* / Q000503:physiopathology*; D007177:Inappropriate ADH Syndrome / Q000378:metabolism / Q000503:physiopathology; D008279:Magnetic Resonance Imaging / Q000379:methods; D009132:Muscles / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D013552:Swine; D014867:Water / Q000378:metabolism
chemical_list: D004573:Electrolytes; D014867:Water; D012964:Sodium