Energy-dependent accumulation of calcium antagonists in catecholamine storage vesicles

Abstract

The calcium antagonists verapamil, nitrendipine, mibefradil, and amlodipine accumulate in chromaffin granule ghosts with apparent equilibrium partition coefficients [(mol/mg membrane lipid)/(mol/mg solvent water)] of 246 ± 105 (N = 8), 2700 ± 600 (N = 4), 7400 ± 2200 (N = 4), and 8100 ± 1100 (N = 5), respectively. In the presence of 1.2 mM MgATP, the partition coefficients were 854 ± 206 (N = 10), 2300 ± 600 (N = 4), 32,700 ± 8,900 (N = 7), and 20,300 ± 5,000 (N = 11) for verapamil, nitrendipine, mibefradil, and amlodipine, respectively. Except for nitrendipine, the apparent partition coefficients in the presence of MgATP were significantly different from the control ( P < 0.001). For amlodipine and verapamil, the vacuolar H +-ATPase inhibitors bafilomycin A 1 (30 nM) and N-ethylmaleimide (2 mM) and the protonophore (uncoupler) carbonyl cyanide m-chlorophenylhydrazone (CCCP, 10 μM) completely blocked the increase in partition coefficients in response to MgATP. The extra amlodipine, mibefradil, and verapamil that accumulated in response to MgATP were released into the medium by CCCP (10 μM) by 18% (N = 5), 30% (N = 5), and 88% (N = 5) for amlodipine, mibefradil, and verapamil, respectively. Thus, amlodipine, mibefradil, and verapamil, but not nitrendipine, accumulate in catecholamine storage vesicles in response to Δμ H+ generated by the endogenous V-type H +-ATPase, and are partially released by de-energetisation. Hence, these calcium antagonists can reach unexpectedly high concentrations in certain target cells, and give pharmacodynamic properties not shared by nitrendipine.