EVIDENCE FOR UPTAKE(2)-MEDIATED O-METHYLATION OF NORADRENALINE IN THEHUMAN AMNION FL CELL-LINE

The uptake and metabolism of H-3-noradrenaline has been examined in th e FL cell-line derived originally from human amnion. Cell cultures met abolised H-3-noradrenaline (1.0 mumol/1) to H-3-normetanephrine and, t o a lesser extent, to metabolites (not distinguished) of the 0-methyla ted deaminated fraction; primary deaminated metabolites were not detec ted. H-3-normetanephrine formation a) was not saturable in the noradre naline concentration range 0.2- 150 mumol/l, b) was decreased to 20%-3 0% of control levels by uptake2 inhibitors (O-methylisoprenaline, 20 a nd 100 mumol/l; cimetidine, 10 mumol/l; hydrocortisone, 200 mumol/1) a nd c), was almost insensitive to uptake1 inhibitors (cocaine, 30 mumol /l; desipramine, 3 mumol/1). Uptake of noradrenaline was manifested af ter 30 minutes as a 6-fold increase in the cell content of the amine f ollowing inhibition of catechol-0-methyl transferase, either alone or in conjunction with inhibition of monoamine oxidase. Uptake was decrea sed maximally to 40% of control levels by 0-methylisoprenaline. IC50 v alues for inhibition of the 0-methylisoprenaline-sensitive component o f uptake were (in mumol/1): corticosterone (0.3), papaverine (1.1), 0- methylisoprenaline (3.0), cimetidine (6.0), (-)noradrenaline (460), an d tetraethylammonium (2230). Except for the last agent, for which a co mparative value is not available, the IC50's are in good agreement wit h those for inhibition of uptake2 in the Caki-I cell-line reported by other investigators. The component of uptake resistant to 0-methylisop renaline was depressed by papaverine (a 50% decrease at 50 mumol/1), b ut was not affected by the other uptake2 inhibitors or by cocaine (30 mumol/l). It is concluded that the FL cell possesses an extraneuronal metabolising system very similar to the system in tissues such as hear t and smooth muscle where transport of noradrenaline into the cell by uptake2 is followed by rapid 0-methylation via catechol-O-methyl trans ferase. The only difference appears to be the absence of saturation of H-3-normetanephrine formation in the FL cell at low micromolar concen trations of H-3-noradrenaline. The presence of a second uptake process is suggested by the inhibitory effect of papaverine on uptake resista nt to O-methylisoprenaline; lack of effect of cocaine implies that thi s second process is not uptake1.