We studied effects of Ca2+ in the incubation medium on [3H]dopamine ([3H]DA) uptake by rat striatal synaptosomes. Both the duration of the preincubation period with Ca2+ (0-30 min) and Ca2+ concentration (0-10 mM) in Krebs-Ringer medium affected [3H]DA uptake by the synaptosomes. The increase was maximal at a concentration of 1 mM Ca2+ after a 10-min preincubation (2.4 times larger than the uptake measured without preincubation), which reflected an increase in Vmax of the [3H]DA uptake process. On the other hand, [3H]DA uptake decreased rapidly after addition of ionomycin in the presence of 1 mM Ca2+. The Ca(2+)-dependent enhancement of the uptake was still maintained after washing synaptosomes with Ca(2+)-free medium following preincubation with 1 mM Ca2+. Protein kinase C inhibitors did not affect apparently Ca(2+)-dependent enhancement of the uptake, whereas 1(-)[N,O-bis(1,5-isoquinolinesulfonyl)-N-methyl-L- tyrosyl]-4-phenylpiperazine (KN-62; a Ca2+/calmodulin-dependent kinase II inhibitor) and wortmannin (a myosin light chain kinase inhibitor) significantly reduced it. Inhibitory effects of KN-62 and wortmannin appeared to be additive. N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7; a calmodulin antagonist) also remarkably inhibited the enhancement. These results suggest that Ca(2+)-dependent enhancement of [3H]DA uptake is mediated by activation of calmodulin-dependent protein kinases.