title: Free water transport, small pore transport and the osmotic pressure gradient.
abstract: BACKGROUND
  Water transport in peritoneal dialysis (PD) patients occurs through the small pores and water channels, the latter allowing free water transport (FWT). The osmotic gradient is known to be one of the major determinants of water transport. The objective of the study was to analyse the relation between each transport route and the osmotic gradient.
  
  
  METHODS
  The 4-h standard peritoneal permeability analyses of 80 stable PD patients were studied. Small pore transport (SPT) was calculated based on the transported amount of sodium. FWT was calculated by subtracting SPT from transcapillary ultrafiltration (TCUF). Water transport rates were determined. The osmotic gradient was calculated. The slope of the relation between FWT rate and osmotic gradient (slope(FWT)), and the elimination constant (K(e)) of the exponential relation between SPT rate and osmotic gradient (K(SPT)) were calculated for every patient.
  
  
  RESULTS
  The FWT rate was related to the osmotic gradient (P = 0.001). A similar correlation was also found between the SPT rate and osmotic gradient when fitted exponentially (P = 0.005). The rates of FWT decreased significantly between each time point during the whole dwell. The SPT rates decreased significantly within the first half of the dwell and levelled off thereafter. No correlations were found between the slope(FWT), K(SPT) and PD duration. The slope(FWT) of the relationship between the FWT and the osmotic gradient is an indirect measurement of the amount of functioning water channels. Similarly, the K(SPT) value represents the number of functioning small pores. The absence of a relationship of these parameters with the duration of PD suggests opposing mechanisms, for instance a lower number of functioning pores in combination with an increased vascular surface area. Conclusion. The curves of the relationship between FWR, SPT and OG support the assumption that FWR is much more dependent on the OG than SPT. Non-osmotic determinants are likely to be important in small pore fluid transport.
authors.affiliation: Division of Nephrology, Department of Medicine, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Alena
authors.initials: A
authors.lastname: Parikova
authors.forename: Watske
authors.initials: W
authors.lastname: Smit
authors.forename: Machteld M
authors.initials: MM
authors.lastname: Zweers
authors.forename: Dirk G
authors.initials: DG
authors.lastname: Struijk
authors.forename: Raymond T
authors.initials: RT
authors.lastname: Krediet
mesh_terms: D000328:Adult; D000368:Aged; D020346:Aquaporins / Q000502:physiology*; D001692:Biological Transport / Q000502:physiology; D002199:Capillary Permeability / Q000502:physiology*; D005260:Female; D006801:Humans; D007676:Kidney Failure, Chronic / Q000503:physiopathology* / Q000628:therapy*; D008297:Male; D008875:Middle Aged; D009997:Osmotic Pressure; D010529:Peritoneal Cavity / Q000503:physiopathology; D010530:Peritoneal Dialysis; D014462:Ultrafiltration; D014867:Water / Q000378:metabolism*
chemical_list: D020346:Aquaporins; D014867:Water