This is an unusual one out of Turkey last year:
Kilic et al. The Effects of Oral 5-alpha Reductase Inhibitors on Penile Intracavernosal Pressures and Penile Morphology in Rat Model.
They treated rats with finasteride or dutasteride and then measured penile blood pressure and tissue composition.
They found more collagen density in the penis tissue of the treatment groups, as well as atrophy in the prostate tissue. Blood pressure differences were not statistically significant, but it was a low-power study with only 10 animals in each of the three groups.