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(Circulation Research. 2002;91:830.)
© 2002 American Heart Association, Inc.

Cellular Biology

Hydralazine Reduces the Quantal Size of Secretory Events by Displacement of Catecholamines From Adrenomedullary Chromaffin Secretory Vesicles

José D. Machado, José F. Gómez, Gema Betancor, Marcial Camacho, Miguel A. Brioso, Ricardo Borges

From the Unidad de Farmacología, Facultad de Medicina, Universidad de La Laguna, Tenerife, Spain.

Correspondence to Dr Ricardo Borges, Unidad de Farmacología, Facultad de Medicina, Universidad de La Laguna, 38071 La Laguna, Tenerife, Spain. E-mail rborges{at}ull.es

The effects of the antihypertensive agent hydralazine (1 to 100 nmol/L) on the exocytotic process of single adrenal chromaffin cells have been studied using amperometry. Hydralazine does not reduce the frequency of exocytotic spikes but rapidly slows the rate of catecholamine release from individual exocytotic events by reducing the quantal size of catecholamine exocytosis. Confocal and standard epifluorescence microscopy studies show that hydralazine rapidly accumulates within secretory vesicles. The blockade of the vesicular H⁺ pump with bafilomycin A₁ 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.

Key Words: amperometry • bafilomycin A₁ • catecholamines • chromaffin granules • intragranular pH

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