Figure 1
From: Cyclic AMP signalling pathways in the regulation of uterine relaxation
Cyclic AMP pathways in myometrial tissue. Activation of membrane receptors (GPCR) coupled to Gs activates adenylyl cyclase (ADCY) which converts ATP to cAMP. The levels of cAMP are tightly regulated by phosphodiesterases (PDE), especially PDE4 isoforms. It is thought that cAMP induces uterine relaxation via activation of a specific protein kinase (PRKA) which phosphorylates and inhibits myosin light chain kinase (MYLK). PRKA may also oppose the effect of stimulatory receptors which operate through the phospholipase C (PLC)/calcium pathway. However the precise targets for PRKA phosphorylation in human myometrium are under investigation (
stimulation, 
inhibition)