Direct sympathomimetic drugs act as agonists on adrenergic receptors. In contrast, indirectly acting sympathomimetic drugs act by increasing the concentration of noradrenaline. Indirectly acting sympathomimetics like tyramine, amphetamine or ephedrine are take up into sympathetic nerve terminals and are transported into the vesicles by the vesicular monoamine transporter in exchange for noradrenaline, which escapes into the cytosol. Most of the noradrenaline in the cytosol escapes via the transporter in the presynaptic membrane in exchange for the indirectly acting sympathomimetic monoamine. By this mechanism as well as by partly inhibiting the reuptake of noradrenaline and partly by inhibiting the degradation of noradrenaline by monoamine oxidase (MAO), they increase the concentration of noradrenaline in the synaptic cleft.

 α-adrenergic system

 β-adrenergic system