The effect of chlorpromazine on bioenergetic processes in chromaffin granule membranes

Chromaffin granules, the storage vesicles of the adrenal medulla, take up catecholamines via a ‘chemiosmotic’ transport system which is inhibited by chlorpromazine. Because chlorpromazine inhibits a number of other ‘chemiosmotic’ energy-coupling systems, the effect of the drug on some bioenergetic processes in chromaffin-granule membranes was explored. The chromaffin granule membranes possess an electrogenic proton-translocating adenosine 5'-triphosphatase which generates an adenosine 5'-triphosphate-dependent membrane potential. This potential is abolished by relatively high concentrations of chlorpromazine. At these concentrations, adrenaline uptake is also inhibited whereas adenosine 5'-triphosphatase activity is not affected. An ATP-dependent enhancement of the fluorescence of 1-anilinonaphthalene-8-sulphonate is inhibited by very low concentrations of chlorpromazine. The fluorescence enhancement is caused by increased binding of the fluorophor to the membrane; chlorpromazine reduces this binding and does not affect the quantum yield of the membrane-bound fluorophore. At these lower drug concentrations, neither adrenaline uptake nor the adenosine 5'-triphosphate-dependent membrane potential are affected. These results suggest that chlorpromazine inhibits catecholamine transport by dissipating the membrane potential (uncoupling) and not by competitively inhibiting the catecholamine translocator. Because the enhancement of the fluorescence of 1-anilinonaphthalene-8-sulphate is sensitive to low chlorpromazine concentrations, it may be useful in studying subtle effects of the drug on membrane structure.