REGULATION OF TONE IN PENILE CAVERNOUS SMOOTH-MUSCLE - ESTABLISHED CONCEPTS AND NEW FINDINGS

Since Benson, in 1983, reported on a potent nonadrenergic, noncholiner gic (NANC) transmitter postulated to relax penile vessels and the corp us cavernosum, much new information on the mechanisms of contraction a nd relaxation of corporeal smooth muscle and penile vasculature has be en obtained. The information currently available suggests that NANC tr ansmitters may be involved in both contractile and relaxant responses of penile erectile tissues. There is good experimental evidence to all ow the assumption that neurogenic nitric oxide (NO) is a mediator of p enile erection, but even if NO probably is the most important factor f or relaxation of penile vessels and the corpus cavernosum, this does n ot exclude the possibility that other agents released from nerves may have a modulatory function in this process. However, the roles of, for example, vasoactive intestinal polypeptide and related peptides as ne urotransmitters and/or neuromodulators in the nervous control of penil e erection have yet to be established. The restricted availability of human penile erectile tissues has led to the use of cavernous tissue a nd penile vessels from animals, both for screening and for detailed an alysis of mechanisms previously demonstrated to exist also in human ti ssues. When interpreting the results obtained, it is important to stre ss that there may be important differences between human and animal ti ssues, that each of the tissues only gives a piece of information an t he complex process of penile erection, and that the physiological and clinical importance of results from such experiments may be limited. T he differing responses in different parts of the vasculature within th e penis and the multiplicity of putative transmitters present in the c orpus cavernosum and in perivascular nerves make further investigation s necessary, as do the interactions between transmitters and neuromodu lators at the neuromuscular junction, and between the neural and endot helial control of vascular tone.