Correlations of purinergic, cholinergic and adrenergic functions in rabbit corporal cavernosal tissue

Erection involves cholinergic, adrenergic as well as non-cholinergic non-adrenergic nerves. Endothelial-derived relaxation factor plays an important role in mediating smooth muscle relaxation, which is crucial in initiating and maintaining erection. We previously showed that adenosine 5'-triphosphate (ATP) can induce significant relaxation in rabbit corporal cavernosal tissue. The present study presents effects of different neurotransmitters and the role of endothelium in controlling the contractile/relaxant status of rabbit cavernosal tissue. These studies utilized isolated tissue strips prepared from the corpus cavernosum of sexually mature male New Zealand white rabbits. The results can be summarized as follows: (1) field stimulation caused relaxations with rebound contractions in most strips; (2) bethanechol (250 μM), isoproterenol (20 μM) and ATP (1 mM) all induced relaxations, though the relaxation induced by bethanechol was poorly sustained; (3) removal of the endothelium by rubbing decreased the relaxation to field stimulation and virtually eliminated the relaxation induced by bethanechol, but had no effect on the relaxation responses to isoproterenol and ATP; (4) methoxamine (200 μM) stimulated a sustained contraction of corporal cavernosal tissue, an effect unaltered by rubbing the strips; (5) low dose epinephrine induced relaxation, whereas higher concentrations contracted the tissue, and (6) β-adrenergic inhibition with propranolol (20 μM) was significantly more effective than mascarinic blockade with atropine (20 μM) in eliminating relaxation caused by field stimulation. Three conclusions can be drawn from this study: (1) ATP (purinergic innervation) may play an important role in erectile physiology, and its actions are independent of the endothelium, as well as cholinergic and adrenergic neurotransmission; (2) muscarinically induced cavernosal relaxation is endothelium-dependent, but constitutes a relatively minor portion of the relaxant response to electric stimulation, and (3) β-adrenergic stimulation constitutes a relatively prominent component of field stimulation-induced relaxation, and cavernosal β-adrenoceptors are more sensitive to low levels of adrenergic stimulation than α-adrenoceptors.