Kinetics of tyramine transport and permeation across chromaffin-vesicle membranes

Tyramine permeates chromaffin-granule membranes via a reserpine-insensitive mechanism. The rate is unsaturable and increases with pH, indicating permeation of the unprotonated form of the amine. Reserpine-insensitive dopamine uptake is at least 10 times slower, consistent with dopamine's lesser lipophilicity. Dopamine is transported into chromaffin-granule membrane vesicles via a saturable, reserpine-sensitive, proton-linked mechanism. Tyramine inhibits dopamine transport with a Ki of 5-10 microM. Tyramine is not accumulated nearly as well as dopamine because inward transport is opposed by outward permeation. Nevertheless, the velocity of reserpine-sensitive tyramine transport can be deduced from the steady-state level of tyramine accumulation and the rate of permeation. Vmax for tyramine transport is about one-third of the value for dopamine transport. Therefore, two aromatic hydroxyls are not needed for monoamine transport but are required for efficient accumulation and storage.