Uptake and accumulation of erythro- and threo-α-methyl-noradrenaline in adrenergic nerves

Abstract

α-Methylnoradrenaline (α-methyl-NA) has the molecular requirements for formation of a fluorescent product upon formaldehyde gas treatment according to the method of Falck and Hillarp. The stereoisomers, (±)-threo-, (±)-erythro-, (−)-erythro- and (+)-erythro-α-methyl-NA exhibited the same quantitative and qualitative fluorescence characteristics when enclosed in model protein layers and exposed to formaldehyde gas. The formed fluorophor is in all probability 3-methyl-4,6,7-trihydroxy-3,4-dihydroisoquinoline. Maximal excitation/emission at 410/480 mμ, which is identical to the spectral characteristics of the noradrenaline (NA) fluorophor (4,6,7-trihydroxy-3,4-dihydroisoquinoline).In vitro incubation of mouse atria and rat irides, depleted of their endogenous NA by reserpine, showed that (±)-erythro-α-methyl-NA is much more efficiently taken up and accumulated by the adrenergic nerves than is (±)-threo-α-methyl-NA, as observed histochemically. The (±)-erythro form was also considerably more effective in displaying 3H-NA previously taken up in adrenergic nerves of mouse atria, although the affinities of (±)-erythro- and (±)-threo-α-methyl-NA for the uptake sites located at the axonal membrane were similar; (±)-threo-α-methyl-NA for the uptake sites located at the axonal membrane were similar; (±)-threo-α-methyl-NA is not readily taken up by the adrenergic neuron. Fluorescence histochemistry also revealed a more efficient uptake and accumulation of (−)-erythro-α-methyl-NA than of (+)-erythro-α-methyl-NA. Furthermore, the (−)-erythro form was a more potent inhibitor of 3H-NA uptake in adrenergic nerves of mouse atria and more readily displaced 3H-NA previously taken up. The results indicate that the observed stereoselectivity must at least partly be related to the membrane pump mechanism located at the axonal membrane of the adrenergic nerves.