Exchange diffusion of dopamine induced in planar lipid bilayer membranes by the ionophore X537A.

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Abstract
The ionophore X537A caused a large increase in the [14C]dopamine (a catecholamine) permeability of planar bilayer membranes. Dopamine transport increased linearly with the ionophore concentration. At relatively high concentrations in the presence of dopamine, the ionophore induced a conductance which was nearly ideally selective for the dopamine cation. The total dopamine flux as determined in tracer experiments was not affected by an electric field and was over 105 times larger than predicted from the estimated dopamine conductance. Increasing the dopamine concentration on the side containing radioactive dopamine (the cis side) saturated the dopamine transport. This saturation was relieved by trans addition of nonradioactive dopamine, tyramine, H+, or K+. With unequal concentrations of dopamine cis and trans (49 and 12.5 mM), the unidirectional dopamine fluxes were equal. Increasing H+ cis and trans decreased dopamine transport. At physiological pH, the X537A-induced transport of dopamine apparently occurs via an electrically silent exchange diffusion of dopamine cation with another cation (e.g., dopamine+, K+, or K+). X537A induces a Ca++-independent release of catecholamines from sympathetic nerves by interfering with intracellular storage within storage vesicles. X537A may cause an exchange of intravesicular catecholamine with a cytoplasmic cation (perhaps K+ or H+) across the storage vesicle membrane.