Micturition in conscious rats with and without bladder outlet obstruction: role of spinal alpha 1-adrenoceptors.

1. In normal rats and rats with bladder hypertrophy secondary to outflow obstruction, undergoing continuous cytometry, we examined the responses to the selective alpha 1-adrenoceptor antagonist doxazosin given intrathecally (i.t.) and intra-arterially (i.a.). In addition, we investigated the effects of the drug on L-dopa-induced bladder hyperactivity in normal, unobstructed rats. 2. Doxazosin 50 nmol (approximately 60 micrograms kg-1), given i.t., decreased micturition pressure in normal rats and in animals with post-obstruction bladder hypertrophy. The effect was much more pronounced in the animals with hypertrophied/overactive bladders. Doxazosin did not markedly affect the frequency or amplitude of the unstable contractions observed in obstructed rats. In contrast, however, doxazosin reduced L-dopa-induced bladder overactivity. When tested, the enantiomers of doxazosin produced qualitatively similar effects to doxazosin, but there was no evidence of stereoselectivity. 3. The results suggest that in addition to the well documented action on prostatic and lower urinary tract smooth muscle, and an effect on the sympathetic outflow to the bladder, bladder neck, prostate, and external urethral sphincter, doxazosin may have an action at the level of the spinal cord and ganglia, thereby reducing activity in the parasympathetic nerves to the bladder. This effect is more pronounced in rats with bladder hypertrophy than in normal rats.