When blood flow forces are low, the cells lining blood vessels initially make more of a gas called nitric oxide (which helps blood vessels relax), but then they stop producing as much of it over time.

The study applied the same low shear stress (2 dyn/cm2) to the same human umbilical vein cells as described in the claim, and found exactly what the claim describes: nitric oxide production went up briefly at 5 minutes, then dropped and stayed low for the rest of the experiment.