EFFECT OF AGING ON NITRIC OXIDE-MEDIATED PENILE ERECTION IN RATS

Aging is an important risk factor for impotence in men. Because nitric oxide (NO) appears to be the mediator of corpora cavernosal smooth mu scle relaxation, we have examined in 5-, 20-, and 30-mo-old rats, desi gnated ''adult,'' ''old,'' and ''senescent,'' respectively, whether ag ing causes a decrease of erectile response that may correlate with low er NO synthase (NOS) in the penis. Electric field stimulation (EFS) of the cavernosal nerve showed that the maximum intracavernosal pressure (MIP) declined in the old and senescent rats to 80 and 51% of the adu lt value, respectively. A low systemic dose of the NOS inhibitor, N-om ega-nitro-L-arginine methyl ester (L-NAME; 2 mg/kg), reduced the MIP b y only 38% in the adult rats but decreased it in the old and senescent rats by 72 and 80%, respectively. In the absence of EFS, intracaverno sal papaverine (phosphodiesterase inhibitor), or nitroglycerin (NO don or), caused a lower erectile response in the old and senescent rats co mpared with the adult animals (MIP: 41 and 14%, respectively; duration of the erection 46 and 21%, respectively). Tissue sections from old a nd senescent penises showed increasing degrees of sclerotic degenerati on. In comparison with the adult rats, the penile soluble NOS activity per gram of tissue that is sensitive to L-NAME decreased significantl y by 63% in the senescent rats but was elevated in the old rats. These results indicate that aging causes an erectile failure due to factors initially independent from an impairment of penile NO synthesis but w hich are compounded in the very old rats by the decrease of penile NOS activity.