Purpose: Nitric oxide (NO), the neurotransmitter responsible for mediating penile erection in the rat, is synthesized from L arginine by nitric oxide synthase (NOS) in a reaction blocked by L-NAME (N-omega-nitro-L-arginine methyl ester). To determine whether dietary supplementation of L-arginine can stimulate penile erection and whether ancillary pathways for penile erection may exist, a series of experiments were conducted in the Fischer 344 rat.
Materials and methods: Adult male (5 month old) and aged (20 month old) rats were fed L-arginine (2.25%) and L-NAME (0.7%) dissolved in tap water for 8 weeks. Animals (n = 6) underwent electrical field stimulation (EFS) of the cavernosal nerve to induce erection and both maximal intracavernosal pressure (MIP) and mean arterial pressure (MAP, mm. Hg +/- SEM) were measured. Tissue and serum levels of L-arginine were measured by an automated amino acid analyzer. Penile eNOS (endothelial) and nNOS (neuronal) content were measured by western blot densitometry. Total penile NOS enzyme activity was measured by the L-arginine to L-citrulline conversion assay.
Results: The L-arginine fed animals demonstrated a significant increase in EFS-induced MIP when compared to the controls in both the adult (104 +/- 4 vs. 86 +/- 6, p = 0.04) and aged (87 +/- 5 vs. 66 +/- 4, p = 0.02) animals, without changes in MAP. L-NAME virtually abolished the MIP in adult rats (8 +/- 3, p < 0.0001), while increasing the MAP (186 +/- 8, p < 0.0001). Serum and penile tissue levels of L-arginine were increased by 64-148% in all groups compared to control animals. Penile eNOS and nNOS content remained unchanged in control and treated animals. Penile NOS activity was increased nearly 100% in the L-arginine treated groups vs. controls.
Conclusions: Long-term oral administration of supra-physiologic doses of L-arginine improves the erectile response in the aging rat. We postulate that L-arginine in the penis may be a substrate-limiting factor for NOS activity and that L-arginine may up-regulate penile NOS activity but not its expression. The blockade of penile erection by EFS with L NAME suggests that if ancillary corporeal vasodilator mechanisms develop, a basal level of NO synthesis is still required for activation and relaxation of the corporeal smooth muscle. These data support the possible use of dietary supplements for treatment of erectile dysfunction.