NO donors—Nitric oxide (NO) is the most
important mediator of smooth-muscle relaxation.
During pregnancy, contractions of the myometrium
are inhibited by an NO-mediated rise in intracellular
cGMP synthesis and a resulting efflux of calcium
from the myocytes (Figure 4). In eleven randomized
trials, the transdermal application of an NO donor
was found to be at least as effective as betamimetics
for tocolysis lasting 48 hours or seven days, with a
significantly better maternal side-effect profile (24).
A placebo-controlled trial demonstrated a significant