P-Glycoprotein-Mediated Drug Secretion in Mouse Proximal Tubule Perfused In Vitro

. To examine the functional significance of durg-transporting P-glycoprotein (P-gp), studies were conducted in the isolated and perfused proximal tubule S2 segment from mice disrupting both mdr1a and mdr1b genes [mdr1a/1b(—)(—)] and their wild-type mice. Efflux of the intracellular fluorescence of rhodamine 123, a fluorescence substrate of P-gp, into the lumen was measured, and the decay half-time of the intracellular fluorescence (T 1/2 ) as an index of the drug-transporting P-gp activity was regarded. In the wild-type mice, the T 1/2 was 34 ± 4 s ( n = 36) at the basal period and was increased to 434 ± 41 s by the addition of luminal verapamil, a P-gp inhibitor. In the mdr1a/1b(—)(—) mice, the T 1/2 was 407 ± 16 s ( n = 10) at the basal period and was no longer affected by the luminal addition of verapamil. The digoxin content in the kidney after a repeated administration of the drug was markedly elevated in the mdr1a/1b(—)(—) mice. In conclusion, P-gp—mediated drug efflux capacity indeed exists in the apical membrane of proximal tubule cells from the wild-type mice, whereas it is absent in that of the mdr1a/1b(—)(—) mice.