Inhibitors of mdr1-dependent transport activity delay accumulation of the mdr1 substrate rhodamine 123 in primary rat hepatocyte cultures

Toxicology. 2001 Oct 5;167(1):47-57. doi: 10.1016/s0300-483x(01)00457-7.


P-glycoproteins (P-gps) encoded by mdr1 (multidrug resistance) genes mediate extrusion of numerous lipophilic xeno- and endobiotics through the plasma membrane. Rhodamine 123 (Rh123), a fluorescent dye which is accumulated by mitochondria, is a mdr1 substrate and a well-established tool to study mdr1 transport activity. Inhibitors of mdr1-dependent transport such as verapamil or cyclosporin A have been found to decrease Rh123 efflux from mdr1-expressing cells. Mdr1b gene expression increases with time in primary rat hepatocyte culture. In hepatocytes cultured for 4 days and expressing high levels of P-gp, intracellular Rh123 accumulation was enhanced in the presence of mdr1 inhibitors (cyclosporin A, 8 and 80 microM, verapamil, 8 and 80 microM, or triton X-100, 8 microM). Surprisingly, in hepatocytes expressing low levels of P-gp (after 1 day of culture), time-dependent Rh123 accumulation was not enhanced, but delayed by cyclosporin A, verapamil or triton X-100. In these cells orthovanadate (50 microM), an inhibitor of P-glycoprotein ATPase activity, suppressed Rh123 accumulation, while tetraethylammonium (200 microM), an organic cation transporter (OCT) substrate, had no effect. The paradoxical delay in Rh123 accumulation by verapamil and cyclosporin A occurred eventhough these compounds decreased dye extrusion from Rh123 pre-loaded cells. These observations suggest that a hitherto unknown mechanism which is sensitive to modulators of mdr1-activity contributes to Rh123 uptake or accumulation in primary rat hepatocytes.

MeSH terms

  • ATP Binding Cassette Transporter, Subfamily B, Member 1 / antagonists & inhibitors*
  • ATP Binding Cassette Transporter, Subfamily B, Member 1 / biosynthesis
  • ATP Binding Cassette Transporter, Subfamily B, Member 1 / metabolism
  • Animals
  • Biological Transport / drug effects
  • Calcium Channel Blockers / pharmacology
  • Cyclosporine / pharmacology*
  • Enzyme Inhibitors / pharmacology
  • Excipients / pharmacology
  • Fluorescent Dyes / metabolism
  • Fluorescent Dyes / pharmacokinetics*
  • Hepatocytes / drug effects
  • Hepatocytes / metabolism*
  • Male
  • Octoxynol / pharmacology*
  • Rats
  • Rats, Wistar
  • Rhodamine 123 / metabolism
  • Rhodamine 123 / pharmacokinetics*
  • Vanadates / pharmacology
  • Verapamil / pharmacology*


  • ATP Binding Cassette Transporter, Subfamily B, Member 1
  • Calcium Channel Blockers
  • Enzyme Inhibitors
  • Excipients
  • Fluorescent Dyes
  • Rhodamine 123
  • Vanadates
  • Cyclosporine
  • Octoxynol
  • Verapamil