Structural Requirements for Cocaine-Sensitive and -Insensitive Uptake of Phenethylamines into the Adrenal Chromaffin Cell

Abstract: The adrenal medullary chromaffin cell is a commonly used model for the adrenergic neuron. Although much work has been done to study the transport system in the adrenal chromaffin vesicles, relatively little is known about cellular transport, especially with regard to structural features of phenethylamines required for intracellular accumulation. We have now investigated the structural requirements of phenethylamine-related compounds for their accumulation into cultured adrenal chromaffin cells. We find that two types of cellular uptake, previously described only for dopamine, norepinephrine, and epinephrine, are also present for ³H]tyramine. Although two types of accumulation occur, tyramine accumulation occurs mainly via a cocaine-insensitive process, whereas dopamine accumulation occurs predominantly via a cocaine-sensitive process. The accumulation of ¹⁴C]-phenethylamine and p-methoxyphenethylamine is not affected by cocaine, suggesting that a ring hydroxyl substituent is necessary for cocaine-sensitive accumulation. The compounds p-hydroxyphenylpropylamine and p-hydroxyphenyl-2-aminoethyl sulfide accumulate in the cell only via a cocaine-insensitive process, indicating that lengthening of the aminoalkyl side chain prevents cocaine-sensitive accumulation. We have performed conformational analyses of this series of compounds to determine whether the conformation of these compounds can be related to the kinetic data. For dopamine, tyramine, phenethylamine, and p-methoxyphenethylamine, two groups of energy-minimized conformers were found. We find that there is an approximately linear relationship between the K_m values for these phenethylamines and the differences in minimized energies between the low- and highest energy conformer groups of each compound. A similar correlation was found for p-hydroxyphenyl-2-aminoethyl sulfide. These results are consistent with the hypothesis that these compounds undergo a conformational change from the low-energy conformer to the highest energy conformer before their cocaine-insensitive accumulation.