Effect of calcium on in vitro para-aminohippurate accumulation by rat renal cortical slices

In previous studies para-aminohippurate (PAH) accumulation by renal cortical slices was reduced in calcium-free medium when slices are pretreated with calcium (Ca) chelators. Addition of Ca to the incubation medium of these slices reversed the effects of Ca chelation. Based on these studies, it has been proposed that Ca is required for maximum PAH transport by renal cortical epithelium. However, in the studies presented here, we found that PAH accumulation by rat renal cortical slices not exposed to a chelator varied inversely with incubation medium Ca. Differences in accumulation of PAH was not demonstrable until 90 minutes incubation. Also unaffected was the apparent Vmax and Km of initial accumulation and efflux constant. Addition of EDTA to Ca++-free medium reduced PAH accumulation suggesting chelating agents act by a different mechanism than removal of extracellular calcium.