Characterization of serotonin uptake in cultured pheochromocytoma cells. Comparison with norepinephrine uptake.

PC-12 cells, derived from a rat pheochromocytoma, were found to take up tritiated serotonin ([3H]5-HT) from the external medium by means of a saturable mechanism which follows Michaelis-Menten kinetics. The apparent Km of uptake was 0.39 microM and the Vmax was 0.40 pmole/min/10(6) cells. The uptake was temperature-dependent, partially sodium-dependent, and inhibited by selected metabolic inhibitors (sodium azide, 2,4-dinitrophenol, and iodoacetamide), PC-12 cells also accumulated tritiated norepinephrine ([3H]NE) by a saturable process, with an apparent Km of 1.13 microM and a Vmax of 1.72 pmole/min/10(6) cells. This NE uptake process was also temperature- and sodium-dependent and inhibited by metabolic inhibitors and ouabain. Desmethylimipramine (DMI, IC50 = 3.8 microM) was a better inhibitor of [3H]NE uptake than fluoxetine (IC50 = 24.6 microM). The NE uptake process was structurally specific, since unlabeled NE was a better inhibitor of [3H]NE uptake than 5-HT (IC50 = 19.6 and 171 microM, respectively). However, [3H]5-HT uptake in PC-12 cells appeared to be a less structurally specific process, as it was equally inhibited by unlabeled 5-HT and NE (IC50 4.9 microM and 4.3 microM, respectively). DMI was also a better inhibitor of [3H]5-HT uptake than fluoxetine (IC50 = 85 and 411 microM, respectively). The neurotoxins 6-hydroxydopamine and 5,6-dihydroxytryptamine were cytotoxic to PC-12 cells, causing a time- and concentration-dependent inhibition of [3H]thymidine incorporation into DNA. 5,7-Dihydroxytryptamine had little cytotoxic effect toward PC-12 cells in culture.