title: A decrease in aquaporin 2 excretion is associated with bed rest induced high calciuria.
abstract: BACKGROUND
  Exposure to microgravity or immobilization results in alterations of renal function, fluid redistribution and bone loss, which couples to a rise of urinary calcium excretion. We recently demonstrated that high calcium delivery to the collecting duct reduces local Aquaporin-2 (AQP2) mediated water reabsorption under vasopressin action, thus limiting the maximal urinary concentration and reducing calcium saturation. To investigate renal water balance adaptation during bed rest, a model to mimic the effects of microgravity on earth, the effect of changes in urinary calcium on urinary AQP2 excretion were assessed.
  
  
  METHODS
  Ten healthy men (aged 21-28 years) participated in the experiment. Study design included 7 days of adaptation and 35 days of continuous bed rest (days -6 to 0 and 1 to 35, respectively) under controlled diet. Food records and 24-hour urine samples were collected daily from day -3 to 35. Changes in blood hematocrit were used as an indirect index of plasma volume changes. AQP2 excretion was measured by ELISA.
  
  
  RESULTS
  Bed rest induced bone demineralization and a transient increase in urinary calcium followed by transient decrease in AQP2 excretion, which can reduce the urine concentrating ability causing plasma volume reduction. The return of calciuria to baseline was followed by a recovery of AQP2 excretion, which allows for a partial restoration of plasma volume.
  
  
  CONCLUSIONS
  These results further support the view that urinary calcium can modulate the vasopressin-dependent urine concentration through a down-regulation of AQP2 expression/trafficking. This mechanism could have a key role in the prevention of urine super-saturation due to hypercalciuria.
authors.forename: Grazia
authors.initials: G
authors.lastname: Tamma
authors.forename: Annarita
authors.initials: A
authors.lastname: Di Mise
authors.forename: Marianna
authors.initials: M
authors.lastname: Ranieri
authors.forename: Maria
authors.initials: M
authors.lastname: Svelto
authors.forename: Rado
authors.initials: R
authors.lastname: Pisot
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Bilancio
authors.forename: Pierpaolo
authors.initials: P
authors.lastname: Cavallo
authors.forename: Natale G
authors.initials: NG
authors.lastname: De Santo
authors.forename: Massimo
authors.initials: M
authors.lastname: Cirillo
authors.affiliation: Department of Biosciences, Biotechnologies and Biopharmaceutics, University of Bari Aldo Moro, Via Amendola 165/A, Bari 70126, Italy. giovanna.valenti@uniba.it.
authors.forename: Giovanna
authors.initials: G
authors.lastname: Valenti
mesh_terms: D000328:Adult; D051399:Aquaporin 2 / Q000652:urine*; D001510:Bed Rest; D002118:Calcium / Q000652:urine*; D004797:Enzyme-Linked Immunosorbent Assay; D006801:Humans; D012016:Reference Values
chemical_list: D051399:Aquaporin 2; D002118:Calcium