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VOL. 73 (4), 963-985, 2007 PHISIOLOGY OF PENILE VASCULATURE...

Activation of sympathetic nerves still has antierectile effects under
conditions of a-adrenoceptor blockade suggesting that neuro-
transmitters other than noradrenaline may contribute to the
vasoconstrictor sympathetic activity in the penis (10). Neuro-
peptide Y (NPY), a 36-aminoacid peptide usually colocalized with
noradrenaline in sympathetic perivascular nerves, is widely
distributed in penile erectile tissues with a particularly high density
around helicine arteries (16, 17). Although NPY was initially
suggested to have a role in detumescence (10), earlier in vitro studies
on the contractile effects of the peptide on erectile tissue rendered
controversial results (16), and in vivo investigations showed that
intracavernous injection of NPY increased intracavernous pressure in
rabbits (18). We have recently provided an explanation for this
controversy between the rich presence of NPY-containing nerves in
the penis and the lack of clear antierectile/vasconstrictor effects of the
peptide (17). Thus, NPY has a dual facilitatory/inhibitory modulatory
role on the noradrenergic vasoconstriction of horse penile resistance
arteries, the ability to enhance and decrease noradrenaline-induced
contractions being achieved through a heterogenous population of
NPY receptors. Both Y1 and Y2 postsynaptic receptors are involved in
the NPY-induced enhancement of noradrenaline contractions, and
presynaptic inhibitory Y2 receptors limit noradrenaline release form
nerve terminals. The Y1 receptor is a G-protein-coupled receptor that
usually enhances other constrictors responses in small arteries by
depolarizing arterial smooth muscle and by inhibiting cAMP-
mediated relaxations (19, 20). Under conditions of Y1 and Y2 receptor
blockade, NPY can also induce NO-independent relaxations in penile
resistance arteries through atypical receptors located at the
endothelium, which could account for the in vivo proerectogenic
effects reported for the peptide (18).

Local factors regulating penile vasoconstriction

Several contractile prostanoids are locally synthesized and
metabolized in penile erectile tissues (21). Whereas in CC basal pro-
stanoid production is involved in the maintenance of the myogenic
spontanous tone consisting of phasic and tonic contractions (21), in
penile resistance arteries arterial spontaneous tone is modulated by

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