RESUMEN
We tested the prevailing paradigm that relaxation of corpus cavernosum smooth muscle (CCSM) and penile erection depends upon nitric oxide-induced elevation of myoplasmic cGMP and reduced Ca2+-dependent myosin regulatory light chain phosphorylation levels. This hypothesis invokes a reversal of normal activation pathways. Upon stimulation with 250 microM phenylephrine, phosphorylation of the 20 kD myosin regulatory light chains of rabbit or human CCSM increased approximately 4-fold coincident with contraction. Removal of the agonist was followed by a slow reduction in cross-bridge phosphorylation and force to basal levels. The NO donor, sodium nitroprusside elicited a dose-dependent increase in tissue [cGMP] associated with a rapid relaxation in the continued presence of phenylephrine, although cross-bridge phosphorylation remained significantly elevated. Thus the NO-cGMP inhibitory pathway in CCSM is not simply a reversal of excitatory signal transduction mechanisms. An unidentified mechanism contributes to relaxation by decreasing the rate of cross-bridge recruitment through phosphorylation.
Asunto(s)
GMP Cíclico/fisiología , Músculo Liso Vascular/fisiología , Pene/fisiología , Animales , Humanos , Técnicas In Vitro , Masculino , Relajación Muscular/fisiología , Músculo Liso Vascular/efectos de los fármacos , Pene/efectos de los fármacos , Fenilefrina/farmacología , Fosforilación , ConejosRESUMEN
PURPOSE: The primary mechanism for relaxation of corpus cavernosum smooth muscle (CCSM) and penile erection depends upon nitric oxide (NO)-induced elevation of myoplasmic cyclic guanosine monophosphate (cGMP). Agents that enhance the NO-cGMP signal transduction pathway may prove beneficial in treating erectile dysfunction. Sildenafil, a selective type-5 cGMP phosphodiesterase inhibitor, was investigated to determine the specific mechanism(s) involved in the therapeutic use of this compound to treat impotence. MATERIALS AND METHODS: Isolated strips of rabbit corpus cavernosum were stimulated isometrically with phenylephrine. Graded relaxations were induced using various concentrations of sodium nitroprusside (SNP) alone and in combination with sildenafil. At fixed times, the tissues were rapidly frozen and processed for myosin light chain (MLC) phosphorylation using isoelectric focusing with Western blot analysis, and cGMP content using radioimmunoassay techniques. RESULTS: Sildenafil alone reduced spontaneous tone in unstimulated CCSM, but had little effect on phenylephrine-induced isometric tension in the absence of a NO donor (SNP). Sildenafil sensitized the tissue to SNP for relaxation, but the relationship between relaxation and [cGMP] was unchanged by sildenafil. Relaxation from peak isometric force was correlated with [cGMP] but not MLC phosphorylation. CONCLUSIONS: Sildenafil relaxes CCSM by amplifying the effects of the normal, endogenous cGMP dependent relaxation mechanisms.
Asunto(s)
GMP Cíclico/fisiología , Inhibidores Enzimáticos/farmacología , Músculo Liso/efectos de los fármacos , Pene/efectos de los fármacos , Pene/fisiología , Piperazinas/farmacología , Animales , Técnicas In Vitro , Masculino , Relajación Muscular/efectos de los fármacos , Purinas , Conejos , Citrato de Sildenafil , SulfonasRESUMEN
Male infertility is a complicated and often poorly understood pathologic process. The cause of a man's infertility frequently remains unknown even after a thorough evaluation. Many of the currently available medical therapeutic options are empirical and lack documentation of efficacy. New diagnostic tests are emerging that may help define some previously obscure causes of infertility. Only when these abnormalities have been clearly identified will more specific and cost-effective treatment regimens be instituted.