AJP - Regulatory, Integrative and Comparative Physiology, Vol 269, Issue 2 370-R379, Copyright © 1995 by American Physiological Society

ARTICLES

Daunomycin secretion by killfish renal proximal tubules

D. S. Miller
Intracellular Regulation Section, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina 27709, USA.

Epifluorescence microscopy and video-image analysis were used to measure the uptake of the fluorescent anthracycline daunomycin by intact killifish renal proximal tubules. When tubules were incubated in medium containing 2-5 microM daunomycin, the drug accumulated in the cells and the tubular lumen. At steady state, luminal fluorescence was two to three times greater than cellular fluorescence. Luminal accumulation of daunomycin was reduced when tubules were exposed to the multidrug-resistance (MDR) transporter modifiers verapamil and cyclosporin A (CSA), but not tetraethylammonium (TEA), a model substrate for the renal organic cation transport system. NaCN and vanadate reduced luminal drug accumulation. In contrast, cellular daunomycin accumulation was not affected by verapamil, CSA, TEA, or vanadate and was only slightly reduced by NaCN. When the pH of the buffer solution was decreased from 8.25 to 7.25, luminal, but not cellular, accumulation of daunomycin was again reduced by CSA; however, TEA now reduced cellular and luminal accumulation. These findings are consistent with daunomycin being actively secreted in killifish proximal tubule by two mechanisms. At pH 8.25, daunomycin crossed the basolateral membrane by simple diffusion and was secreted into the tubular lumen by the MDR transporter. At pH 7.25, daunomycin was transported across the basolateral membrane by simple diffusion and carrier-mediated uptake on the organic cation transporter and was secreted into the lumen by the MDR transporter and the organic cation/H+ exchanger.

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