This study was conducted on human K562 lymphocytes to investigate the mechanisms implicated in the regulation of the serotonin transport process. The uptake of serotonin in these cells was saturable (Km, 3.37 microM; Vmax, 2.03 nmol/10(6) cells) and Na+ dependent; isoosmotic replacement of Na+ with choline chloride in the assay medium resulted in the decreased uptake process. Augmentation of intracellular free calcium, [Ca2+]i, by thapsigargin decreased the uptake of serotonin in these cells. Similarly, addition of calcium ionophores (A23187) and ionomycin also inhibited serotonin transport. In Fura-2-loaded cells, these agents increased the [Ca2+]i contents. These results suggest that an increase in [Ca2+]i is implicated with a decrease in serotonin transport. Since an increase in [Ca2+]i is known to activate calmodulin (CaM), we employed CaM antagonists. Calmodulin antagonists W-7 (N-[6-aminohexyl]-5-chloro-1-naphthalene-sulfonamide) and mellitin inhibited serotonin uptake in K562 cells, suggesting that CaM is involved in serotonin transport regulation. Furthermore, acute exposure of K562 cells to known protein kinase C (PKC) activators, phorbol-12-myristate-13-acetate (PMA) and sn-1,2-dioctanoylglycerol (DiC8), curtailed serotonin uptake by these cells. However, staurosporine (a PKC inhibitor) failed to abolish the inhibitory effects of PMA on serotonin transport in these cells, indicating that the target of PMA is not PKC. Nonetheless, the PMA-induced inhibitory effects are specific as 4 alpha-phorbol-12,13,didecanoate (a phorbol ester known not to activate PKC) failed to mimic PMA-like actions on serotonin transport in K562 cells. DiC8 not only exerted higher inhibitory effects than PMA but also had additive effects in the presence of the latter on serotonin transport. These results suggest that in addition to PKC, there are other cellular targets (of PMA) implicated in serotonin transport regulation.