ENDOTHELIA and SMOOTH MUSCLES
Smooth muscle in say blood vessles - notably arterioles - work in close association with the overlying epithelial cells. Action of hormones, neurotransmitters (ACh) or deformation of the epithelial cell by flow of blood can trigger reactions which can stimulate or inhibit associated smooth muscle beds.
These effects may operate via second messenger systems such as phospholipase A2 (PLA2)which may in turn activate cyclo-oxygenase (COX) / prostacyclin synthase (PCS) enzyme systems to produce prostaglandins (PGI2) which diffuse readily through the tissue fluids to act on the smooth muscle cells.
Alternatively activation of nitric oxide synthase (L-arginase) (NOS) may result in production of the highly diffusible gaseous "neurotransmitter" NO. These paracrine agents act on the smooth muscle cells either through G-protein systems or directly on ion channels.