Abstract

The biphasic nature of the efflux of amines from isolated adrenal storage vesicles suggests that there are two storage pools. The uptake, storage, and distribution of 11 amines were determined to elucidate the structural specificity of the uptake mechanism and storage system in each pool. The half-times for efflux of newly incorporated amines from the two pools were 5 min ("fast pool") and 0.5-3 hr ("slow pool"). Both pools were located in the heavy vesicle fraction, as demonstrated by density gradient centrifugation of vesicles labeled with ¹⁴C-epinephrine (a slow pool seeker) and ³H-metaraminol (a fast pool seeker). The following moieties increased fast pool uptake when substituted on the basic β-phenylethylamine structure: p-hydroxyl, m-methoxy, and α-methyl. Slow pool uptake was increased by p- and m-hydroxyl substitution and by replacement of the phenyl ring with 5-hydroxyindole. β-Hydroxyl and catechol groups contributed to the stability of storage in the slow pool, but not in the fast pool. Amines which enter the slow pool were more dependent on ATP-Mg²⁺-stimulated uptake (and therefore more sensitive to blockade of uptake by reserpine) than were amines which primarily enter the fast pool. Other amines inhibited the uptake of epinephrine in both pools, and the slow pool uptake mechanism was saturated at lower substrate concentrations. β-Phenylethylamine was not taken up into either pool, but inhibited epinephrine incorporation, suggesting that the uptake process is independent of the storage process. Binding of amines to chromogranins cannot account for the storage of amines in either pool, but may contribute to the specificity of binding in the slow pool.