Both cAMP and cGMP are key role players in this pathway. cAMP is the molecule that would lead to vasoconstriction through opening of calsium pores. cGMP is formed using NO and is a agonist of cAMP. In other words, it blocks the opening of the calsium pores by cAMP.

.Here is one recently published review on smooth muscle biology.

Here is the link for that review.

http://www.nature.com/nrgastro/journal/v9/n11/full/nrgastro.2012.168.html?WT.ec_id=NRGASTRO-201211