THE EFFECT OF CALCIUM-CHANNEL BLOCKERS ON THE H-+-ATPASE AND BIOENERGETICS OF CATECHOLAMINE STORAGE-VESICLES

A number of commonly used calcium channel blockers have been compared with respect to their effects on the bioenergetics of catecholamine storage vesicles. Chromaffin graunule ghosts with a well-preserved ability to actively transport and store catecholamines, were used as a model for adrenergic synaptic vesicles due to their functional similarity. Nicardipine, varapamil, terodiline and diltiazem were found to have effects comparable to that of prenylamine (Gronberg, M., O. Terland, E.S. Husebye and T. Flatmark, 1990, Biochem. Pharmacol. 40, 351) by inhibiting the generation of a transmembrane proton electrochemical gradient driven by the vesicular H+-ATPase, mainly by loose-coupling/uncoupling of this ATPase. Amlodipine inhibited the internal acidification of the vesicles in a tyramine-like manner and increased the steady-state membrane potential (positive inside) generated by the MgATP-dependent proton translocation. Nifedipine and felodipine also inhibited the efficiency of the proton pump, but their mechanisms of action require further investigation. The concentrations giving 50% inhibition of the H+-ATPase-dependent generation of a pH-gradient were found to be: 12-mu-M felodipine, 16-mu-M nicardipine, 25-mu-M terodiline, 50-mu-M nifedipine, 60-mu-M verapamil, 65-mu-M amlodipine and 150-mu-M diltiazem. The effects of the calcium channel blockers on the bioenergetics of chromaffin granules explain the release of catecholamines from sympathetic nerves and ganglia in vitro by the calcium channel blockers.