Polyspecific organic cation transporters in epithelia play an important role in the elimination of many endogenous bioactive amines and therapeutically important drugs. Recently, the first human organic cation transporter (hOCT1) was cloned from liver. The purpose of the current study was to determine the effect of molecular size and hydrophobicity on the transport of organic cations by hOCT1. We studied the interaction of a series of n-tetraalkylammonium (n-TAA) compounds (alkyl chain length, N, ranging from 1 to 6 carbons) with hOCT1 in a transiently transfected human cell line, HeLa. [14C]tetraethylammonium (TEA) uptake was measured under different experimental conditions. Both cis-inhibition and trans-stimulation studies were carried out. With the exception of tetramethylammonium, all of the n-TAAs significantly inhibited [14C]TEA uptake. A reversed correlation of IC50 values (range, 3.0-260 microM) with alkyl chain lengths or partition coefficients (LogP) was observed. trans-Stimulation studies revealed that TEA, tetrapropylammonium, tetrabutylammonium, as well as tributylmethylammonium trans-stimulated TEA uptake mediated by hOCT1. In contrast, tetramethylammonium and tetrapentylammonium did not trans-stimulate [14C]TEA uptake, and tetrahexylammonium demonstrated an apparent "trans-inhibition" effect. These data indicate that with increasing alkyl chain lengths (N >/= 2), n-TAA compounds are more poorly translocated by hOCT1 although their potency of inhibition increases. Similar findings were obtained with nonaliphatic hydrocarbons. These data suggest that a balance between hydrophobic and hydrophilic properties is necessary for binding and subsequent translocation by hOCT1.