Alterations in the flow of blood to and from the penis are thought to be the most frequent causes of male erectile dysfunction and, therefore, the present review focuses on the penile vasculature. In the flaccid state, tonic noradrenaline release from the sympathetic nerves contracts penile arterial and corporal smooth muscle through activation of postjunctional α1-adrenoceptors, both by increasing intracellular calcium and by enhancing the sensitivity of the contractile apparatus for calcium. In addition, noradrenaline inhibits vasodilatatory neurotransmitter release by prejunctional α2-adrenoceptors. The exact role of the sympathetic neurotransmitters, neuropeptide Y and adenosine 5′-triphosphate, in erection is largely unknown. Penile vasodilatation during erection is mediated by nitric oxide (NO) through activation of guanylyl cyclase in the smooth muscle layer, followed by increases in cyclic guanosine monophosphate lowering of intracellular calcium and desensitisation of the contractile apparatus for calcium. Acetylcholine, vasoactive intestinal peptide as well as peptides in sensory nerves probably also play a role in penile vasodilation. Increased flow through the penile arteries stimulates the endothelium leading to release of NO, prostanoids and a non-NO non-prostanoid factor, and as such enhances the vasodilatation, while the role of endothelium-derived contractile factors in penile vasoconstriction is not clear. Erectile dysfunction shares arterial risk factors with ischaemic heart disease, and diabetes, age, and hypercholesterolaemia are associated with impairment of both neurogenic and endothelium-dependent vasodilator mechanisms in corpus cavernosum. Only few studies have investigated the impact of these risk factors on the penile vasculature, although recent evidence suggests that arterial insufficiency precedes changes in corpus cavernosum leading to erectile dysfunction.

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