From the analysis of an extended set of selected galaxy rotation curves, it is found that for a given luminosity the rotation curves of spiral galaxies within the optical radius are a universal function. This result implies strong systematic variations of both the amplitude and the profil of the circular velocity with luminosity faint (bright) galaxies having low (high) velocities and steep (shallow) velocity gradients. Because luminous disk are self-similar, the observed progression of the shape of rotation curves with luminosity suggests that the dark-to-visible interplay varies with luminosity.