Digital fluorescence imaging of organic cation transport in freshly isolated rat proximal tubules

The secretion of cationic drugs and endogenous metabolites is a major function of the kidney. This is accomplished by organic cation transport systems, mainly located in the proximal tubules. Here, we describe a model for continuous measurement of organic cation (OC) transport. In this model, organic cation transport in individual freshly isolated rat proximal tubules is investigated by use of digital fluorescence imaging. To directly measure organic cation transport across the basolateral membrane, the fluorescent organic cation 4-(4-dimethylaminostyryl)-N-methylpyridinium (ASP(+)) is used with a customized perfusion chamber. ASP(+) uptake in this model displayed the characteristics of organic cation transport. Over the tested range of 1 to 50 mu M, it showed a concentrationdependent uptake across the basolateral membrane. In the presence of competitive inhibitors of OC transport such as N-1-methylnicotinamide(+), tetraethylammonium(+), and choline(+), a concentration-dependent and reversible inhibition of ASP(+) uptake could be documented. In conclusion, continuous measurement of organic cation transport in freshly isolated rat proximal tubules by digital fluorescence imaging using ASP(+) is a useful tool for investigation of drug transport and interactions and, furthermore, may be helpful for investigation of organic cation transport under pathophysiological conditions.