We have studied the effects of the anti-hypertensive agent hydralazine (1-100 nM) on the exocytotic process of single adrenal chromaffin cells. Hydralazine rapidly slows the rate of catecholamine release from individual exocytotic events and reduces the quantal size of catecholamine exocytosis but it does not change the frequency of exocytotic spikes. Hydralazine rapidly accumulates within secretory vesicles as shown by confocal and standard epifluorescence microscopy studies. Cell incubation with bafilomycin A₁, a compound that blocks the vesicular H⁺ pump, inhibits hydralazine uptake. Experiments with permeabilized cells show that hydralazine displaces catecholamines from secretory vesicles. The drug also displaces vesicular Ca²⁺, as shown by fura-2 microfluorimetry. These data suggest that hydralazine acts at least partially, by interfering with the storage of catecholamines. These effects of hydralazine occurred within seconds and at the tissue concentrations presumably reached in antihypertensive therapy; these concentrations are a thousand times lower than those described for relaxing vascular tissues ‘in vitro‘. We proposed that these novel effects could explain many of the therapeutic and side effects of this drug that are likely exerted in sympathetic nerve terminals.

We thank the personnel from ‘Matadero Insular de Tenerife’ for providing us with bovine adrenals. Supported by MCYT BFI2001-3135. M.M. is the recipient of a fellowship from Spanish Ministery of Science and Technology.