Immunohistochemical and functional evidence for a noradrenergic regulation in the horse penile deep dorsal vein

Our aim was to study the presence of noradrenergic nerves and to characterize the α -adrenergic receptors involved in the contractions to electrical field stimulation and to α -adrenergic agonists of the horse penile deep dorsal vein. Noradrenergic fibres were visualized by immunohistochemistry using an antibody against dopamine-beta-hydroxylase (DBH). For functional studies, the responses of the venous rings to electrical field stimulation and to α -adrenergic agonists (noradrenaline, phenylephrine and BHT 920) were studied in the absence and the presence of noradrenergic transmission- and neuronal sodium channel-blockers (guanethidine and tetrodotoxin, respectively) and of α 1 - and α 2 -adrenergic antagonists (prazosin and rauwolscine, respectively). DBH-immunoreactive fibres were present in the adventitia and in the media layer of the venous rings. Electrical field stimulation (0.5–32 Hz) caused frequency-dependent contractions that were abolished by guanethidine (10 −6 M) and tetrodotoxin (10 −6 M) and reduced by prazosin (10 −9 –10 −7 M) and rauwolscine (3 × 10 −8 –3 × 10 −7 M). Noradrenaline, phenylephrine and BHT 920 induced equipotent contractions of the rings. Prazosin and rauwolscine competitively antagonized the contractions to phenylephrine and BHT 920, respectively. In conclusion, DBH-immunoreactive nerve fibres are present in the horse penile dorsal vein. Both transmural nerve stimulation and α -adrenergic agonists induce contraction of the venous rings through a heterogeneous population of α 1 - and α 2 -adrenoceptors.