title: Unstirred Water Layers and the Kinetics of Organic Cation Transport.
abstract: OBJECTIVE
  Unstirred water layers (UWLs) present an unavoidable complication to the measurement of transport kinetics in cultured cells, and the high rates of transport achieved by overexpressing heterologous transporters exacerbate the UWL effect. This study examined the correlation between measured Jmax and Kt values and the effect of manipulating UWL thickness or transport Jmax on the accuracy of experimentally determined kinetics of the multidrug transporters, OCT2 and MATE1.
  
  
  METHODS
  Transport of TEA and MPP was measured in CHO cells that stably expressed human OCT2 or MATE1. UWL thickness was manipulated by vigorous reciprocal shaking. Several methods were used to manipulate maximal transport rates.
  
  
  RESULTS
  Vigorous stirring stimulated uptake of OCT2-mediated transport by decreasing apparent Kt (Ktapp) values. Systematic reduction in transport rates was correlated with reduction in Ktapp values. The slope of these relationships indicated a 1500 μm UWL in multiwell plates. Reducing the influence of UWLs (by decreasing either their thickness or the Jmax of substrate transport) reduced Ktapp by 2-fold to >10-fold.
  
  
  CONCLUSIONS
  Failure to take into account the presence of UWLs in experiments using cultured cells to measure transport kinetics can result in significant underestimates of the apparent affinity of multidrug transporters for substrates.
authors: Department of Physiology, College of Medicine, University of Arizona, Tucson, Arizona, 85724, USA.
authors: Takahiro
authors: T
authors: Shibayama
authors: Mark
authors: M
authors: Morales
authors: Xiaohong
authors: X
authors: Zhang
authors: Lucy J
authors: LJ
authors: Martínez-Guerrero
authors: Alfred
authors: A
authors: Berteloot
authors: Timothy W
authors: TW
authors: Secomb
authors: Stephen H
authors: SH
authors: Wright
mesh_terms: D015655:1-Methyl-4-phenylpyridinium / Q000378:metabolism; D000818:Animals; D001692:Biological Transport / Q000502:physiology; D016466:CHO Cells; D002460:Cell Line; D003412:Cricetulus; D007700:Kinetics; D027701:Organic Cation Transport Proteins / Q000378:metabolism*; D019789:Tetraethylammonium / Q000378:metabolism; D014867:Water / Q000378:metabolism*
chemical_list: D027701:Organic Cation Transport Proteins; D014867:Water; D019789:Tetraethylammonium; D015655:1-Methyl-4-phenylpyridinium