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