Wolfe, et. al., "In vivo muscle amino acid transport involves two distinct processes." Am J Physiol Endocrinol Metab. 2004 Jul;287(1):E136-41.
Most studies and real world experience show that NO boosters are useless for muscle gain. The pumps they can give certainly have a nice psychological benefit. The study above shows the possibility of increased blood flow leading to more amino delivery to the muscles. If the limiting factor is how much can get past the cell membrane, then increased blood flow probably wouldn't matter. However, if the limiting factor in the delivery of aminos into the cells is the raw amount making it to the cell, then increased blood flow through the muscle could possibly make a difference in hypertrophy. The billion other factors that play into it aside, of course.
