The effects of pH on the activity of coryneine and related phenolic quaternary ammonium salts on the frog rectus preparation.

The activity of m-hydroxybenzyltrimethylammonium, coryneine (3:4-dihydroxyphenethyltrimethylammonium, 'quaternary dopamine'), and m-hydroxyphenylpropyltrimethylammonium relative to tetramethylammonium has been measured on the frog rectus preparation (Rana pipiens) at pH 7 and pH 9. 2 The compounds are more active in the more acid environment indicating that ionization of the phenolic group reduces activity to between one-half and one-tenth of that of the form with the intact hydroxyl group. 3 In contrast with the situation at aminoacid receptors, there is no reason to believe that at other receptors zwitterions are likely to be more active than the uncharged forms with which they are in equilibrium.