Effects of imipramine and some tryptamine derivatives on the efflux of 3H-5-hydroxytryptamine from rabbit platelets.

The efflux of 3H-5-hydroxytryptamine (3H-5-HT) from rabbit platelets (monoamine oxidase inhibited; pretreatment with reserpine) was measured in the absence and presence of various concentrations of imipramine or a number of tryptamine derivatives. The maximum efflux-accelerating effect (Emax) of 5-HT and some other tryptamines (e.g., N-methyl-5-HT, 5-methoxytryptamine) far exceeded that of imipramine, whereas the Emax for 2-methyl-5-HT did not. It is concluded that tryptamines that are more effective in releasing 3H-5-HT than imipramine have the property of being substrates of the 5-HT transporter.