We evaluated the effects of prejunctional alpha-adrenoceptors on nitric oxide (NO)-mediated urethral relaxation in rabbits using a muscle bath technique and high-performance liquid chromatography coupled with a microdialysis procedure. The amount of NO(2)(-)/NO(3)(-) released during electrical field stimulation was measured by an NO(2)(-)/NO(3)(-) analyzer based on the Griess method. Pretreatment with phenylephrine (0.01 microM) and yohimbine (0.1-10 microM) significantly reduced the relaxation responses induced by electrical field stimulation. In contrast, pretreatment with clonidine (0.01 microM) and prazosin (0.01-1 microM) enhanced the relaxation responses. Cys-NO-induced relaxations of rabbit urethral smooth muscle were not affected by pretreatment with alpha-adrenoceptor agonists and antagonists. The amount of NO(2)(-)/NO(3)(-) released by electrical field stimulation increased after pretreatment with clonidine (0.01 microM) and prazosin (0.01-1 microM), but decreased after pretreatment with phenylephrine (0.01 microM) and yohimbine (0.1-10 microM). The results suggest that the release of NO from nitrergic nerves in the rabbit urethra is reduced and increased by stimulation of prejunctional alpha(1)- and alpha(2)-adrenoceptors, respectively.