Penile erectile functional measurements of ICP/MAP ratio, nNOS, Rho-kinase and nNOS/Rho-kinase ratio of Sprague-Dawley rats of different age. The protein expression of nNOS and Rho-Kinase was normalized to GAPDH. nNOS, neuronal nitric oxide synthase; ICP/MAP, intracavernosal pressure/mean arterial pressure; Rho-kinase, Rho-associated kinase.