Diphenhydramine transport by pH-dependent tertiary amine transport system in Caco-2 cells

Substrate specificity and pH dependence of the transport system for diphenh ydramine were investigated in Caco-2 cell monolayers. Diphenhydramine uptak e was not affected by any typical substrate for the renal organic cation tr ansport system except procainamide. Along with procainamide, tertiary amine compounds with N-dimethyl or N-diethyl moieties in their structures inhibi ted the diphenhydramine uptake. Moreover, accumulation of diphenhydramine w as stimulated by preloading the Caco-2 cells with these tertiary amines (tr ans-stimulation effect), indicating the existence of the specific transport system for tertiary amines with N-dimethyl or N-diethyl moieties. Efflux o f diphenhydramine from monolayers was enhanced by medium acidification. In addition, intracellular acidification resulted in marked stimulation of dip henhydramine accumulation. ATP depletion of the cells caused an enhancement of diphenhydramine accumulation, suggesting the involvement of an active s ecretory pathway. However, P-glycoprotein did not mediate the diphenhydrami ne transport. These findings indicate that a novel pH-dependent tertiary am ine transport system that recognizes N-dimethyl or N-diethyl moieties is in volved in diphenhydramine transport in Caco-2 cells.