P-glycoprotein-mediated drug secretion in mouse proximal tubule perfused in vitro

Abstract

To examine the functional significance of drug-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 [mdr 1a/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 (T1/2) as an index of the drug-transporting P-gp activity was regarded. In the wild-type mice, the T1/2 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 T1/2 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.