New hormones are being discovered each year and the number of people afflicted with hormone-related diseases is enormous. One property of hormones is that they bind tightly to their receptor and act at very low concentrations. Nitric oxide, a gas, diffuses rapidly in three dimensions. It is rapidly broken down so it doesn't travel over a long distance. NO activates a number of enzymes which, in turn, regulate vascular tone and activate ion channels.
Growth factors are also paracrine signal molecules.
There are many growth factors and they are often named for their target organ. We discussed their signaling in the last class. Prostaglandins are modified fatty acids. Prostaglandins act in the reproductive system and in the immune system. One of the targets of PGs is smooth muscle.