Apical uptake was higher than basolateral uptake at 37 degrees C and was decreased to 70% in the presence of NaN(3) or in the absence of extracellular Na(+). Besides alpha2-agonists, apical uptake was inhibited by verapamil, desipramine, and quinidine, but not by MPP(+) (1-methyl-4-phenylpyridinium), TEA (tetraethylammonium), decynium-22, carnitine, PHA (p-aminohippurate), alanine, or inosine.