Prostacyclin and arterial wall biology.

Arachidonic acid, the precursor of all bisenoic prostaglandins, is the most common fatty acid present in cellular phospholipids and can be obtained directly from the diet or by desaturation and chain elongation from dietary linoleic acid (C18:2 w-6). Arachidonic acid is liberated from membrane phospholipids by the action of phospholipases, activated by changes in their chemical environment. Simple mechanical stimulation can result in generation of prostaglandins, as shown in many tissues (for review see reference 2). The enzymes that synthesize prostaglandins are present in most organs but some tissues, such as seminal vesicles, kidneys, and lungs, have a greater capacity for prostaglandin synthesis than others. The term "eicosanoids" is applied to all the 20carbon derivatives, whereas "prostanoids" refers only to those with a prostanoic acid skeleton. Once released from the membrane phospholipids, arachidonic acid is metabolized by two enzymes. The cyclooxygenase forms the prostaglandin endoperoxide PGG2. 4 This is converted to PGH, which then isomerizes enzymically or nonenzymically to the stable substances PGE2, PGF and PGD2 (figure 1). A 17carbon hydroxy acid called 12-hydroxy-5,8,10 heptadecatrienoic acid (HHT) is also formed, together with malondialdehyde (MDA). The prostaglandin endoperoxides are also transformed enzymically into