Evidence for facilitated diffusion of urea across the gill basolateral membrane of the rainbow trout (Oncorhynchus mykiss)

Biochim Biophys Acta. 2004 May 27;1663(1-2):89-96. doi: 10.1016/j.bbamem.2004.02.009.


Recent in vivo evidence suggests that the mechanism of branchial urea excretion in the ammoniotelic rainbow trout (Oncorhynchus mykiss) is carrier-mediated. Further characterization of this proposed mechanism was achieved by using an in vitro isolated basolateral membrane vesicle (BLMV) preparation in which isolated gill membranes were used to determine a variety of physiological properties of the transporter. BLMV demonstrated two components of urea uptake, a linear component at concentrations up to 17.5 mmol x l(-1) and a saturable component (K(0.5)=0.35+/-0.01 mmol x l(-1); V(max)=0.14+/-0.02 micromol mg protein(-1) h(-1)) with a Hill constant of 1.35+/-0.18 at low, physiologically relevant urea concentrations (<2 mmol x l(-1)). Saturable uptake of urea at 1 mmol x l(-1) by BLMV was reduced by 88.5% when incubated with 0.25 mmol x l(-1) phloretin, a potent blocker of UT-type facilitated diffusion urea transport mechanisms. BLMV also demonstrated differential handling of urea versus urea analogues at 1 mmol x l(-1) concentrations and total analogue/total urea uptake ratios were 32% for acetamide and 84% for thiourea. Saturable urea uptake at 1 mmol x l(-1) was significantly reduced by almost 100% in the presence of 5 mmol x l(-1) thiourea but was not affected by 5 mmol x l(-1) acetamide or 5 mmol x l(-1) N-methylurea. Lastly, total urea uptake at 1 mmol x l(-1) by BLMV was sensitive to temperatures above and below the temperature of acclimation with a Q(10)>2 suggesting a protein carrier-mediated process. Combined, this evidence indicates that a facilitated diffusion urea transport mechanism is likely present in the basolateral membrane of the rainbow trout gill.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acclimatization / physiology
  • Acetamides / pharmacokinetics
  • Animals
  • Biological Transport, Active / drug effects
  • Biological Transport, Active / physiology
  • Carbon Radioisotopes
  • Cell Membrane / metabolism*
  • Diffusion
  • Epithelial Cells / metabolism
  • Gills / metabolism*
  • Oncorhynchus mykiss / metabolism*
  • Oncorhynchus mykiss / physiology
  • Phloretin / pharmacology
  • Temperature
  • Thiourea / pharmacokinetics
  • Urea / analogs & derivatives
  • Urea / pharmacokinetics*


  • Acetamides
  • Carbon Radioisotopes
  • Urea
  • Thiourea
  • Phloretin