Vol. 282, Issue 1, 440-444, 1997

p-Glycoprotein-Mediated Transport of a Fluorescent Rapamycin Derivative in Renal Proximal Tubule¹

David S. Miller , Gert Fricker and Juergen Drewe

Laboratory of Pharmacology and Chemistry, National Institute of Environmental Health Sciences, National Institutes of Health (D.S.M.), Research Triangle Park, North Carolina, Institut fur Pharmazeutische Technologie und Biopharmazie (G.F.), INF 366, D-69120 Heidelberg, Germany, University of Basel/Kantonsspital (J.D.), CH-4031 Basel, Switzerland, and Mount Desert Island Biological Laboratory (D.S.M., G.F., J.D.), Salsbury Cove, Maine

The transport of a fluorescent rapamycin derivative was measured in killifish (Fundulus heteroclitus) renal proximal tubules by means of confocal microscopy and image analysis. Renal cells and tubular lumens rapidly accumulated the rapamycin analog from the medium and attained steady state within 60 min. At steady state, luminal fluorescence intensity was two to four times higher than cellular fluorescence. Cellular fluorescence intensity was a linear function of medium substrate concentration and was not affected by any treatment used. In contrast, luminal fluorescence exhibited a saturable component as the medium concentration of the rapamycin derivative was increased. Secretion into the lumen was blocked by KCN, rapamycin, cyclosporin A and substrates for p-glycoprotein (verapamil, PSC-833 and FK506), but not by substrates for the renal organic anion or organic cation transport systems, such as p-aminohippurate, leukotriene C₄ or tetraethylammonium. Finally, rapamycin blocked p-glycoprotein-mediated secretion of a fluorescent cyclosporin A derivative. The data are consistent with the fluorescent rapamycin analog entering proximal tubule cells by simple diffusion and then being pumped into the tubular lumen by p-glycoprotein. They suggest that the parent compound, rapamycin, would be handled similarly.