The family of substances known as prostaglandins resemble other known hormones in many of their effects; chemically, however, they are a quite different class of compounds. Prostaglandins are fatty acids, variants of a 20-carbon carboxylic acid incorporating a five-member cyclopentane ring. They are synthesized in many tissue of the body from polyunsaturated fatty acids. The key to understanding prostaglandins and linking information about them to the rest of molecular biology is the fact that the main source of the precursors to prostaglandins is the phospholipids of the cell membrane. It seems that the cell membrane is a prime site of prostaglandin synthesis. These "hormones", if they can be so called, are present in very small quantities and are rapidly broken down. They are rarely concentrated, except in semen where there may be 100 micrograms per gram of wet weight; 100 times the usual amount found in other tissues.
The scarcity of prostaglandins resulted in a search for means of synthesizing them using precursors from plants or animals. Prostaglandins are metabolized very rapidly after intravenous injection, disappearing within a few minutes. They have a wide range of effect. On type (PG2) lowers blood pressure, another (PGF2-alpha) raises it. But both PG2 and PG2-alpha stimulate uterine contractions and are used to facilitate childbearing labor. Others /E1 or E2) inhibit gastric secretion in dogs and may prove useful in preventing peptic ulcers.