Published online before print October 3, 2002, doi: 10.1161/01.RES.0000039530.30495.6F
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Submitted on January 14, 2002
Revised on September 13, 2002
Accepted on September 19, 2002
Hydralazine Reduces the Quantal Size of Secretory Events by Displacement of Catecholamines From Adrenomedullary Chromaffin Secretory Vesicles
José D. Machado ;
From the Unidad de Farmacología, Facultad de Medicina, Universidad de La Laguna, Tenerife, Spain.
* To whom correspondence should be addressed. 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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