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INTRODUCTION

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The eicosanoids are an important group of endogenous fatty acid autacoids that are synthesized from arachidonic acid, a 20-carbon fatty acid lipid in cell membranes. Major families of eicosanoids of clinical importance include the straight-chain derivatives (leukotrienes) and cyclic derivatives (prostacyclin, prostaglandins, and thromboxane).

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EICOSANOID AGONISTS

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A. Classification
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The principal eicosanoid subgroups are the leukotrienes and a group of cyclic molecules, including prostaglandins, prostacyclin, and thromboxane. The leukotrienes retain the straight-chain configuration of the parent arachidonic acid. Prostacyclin, thromboxane, and other members of the prostaglandin group are cyclized derivatives of arachidonic acid. There are several series for most of the principal subgroups, based on different substituents (indicated by letters A, B, etc) and different numbers of double bonds (indicated by a subscript number) in the molecule.

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B. Synthesis
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Active eicosanoids are synthesized in response to a wide variety of stimuli (eg, physical injury, immune reactions). These stimuli activate phospholipases in the cell membrane or cytoplasm, and arachidonic acid (a tetraenoic [4 double bonds] fatty acid) is released from membrane phospholipids (Figure 18–1). Arachidonic acid is then metabolized by several different enzymes. The 2 most important are lipoxygenase (LOX), which results in straight-chain leukotrienes, and cyclooxygenase (COX), which results in cyclization to prostacyclin, prostaglandins, or thromboxane. COX exists in at least 2 forms. COX-1 is found in many tissues; the prostaglandins produced by COX-1 appear to be important for a variety of normal physiologic processes (see later discussion). In contrast, COX-2 is found primarily in inflammatory cells; the products of its actions play a major role in tissue injury (eg, inflammation). In addition to these inflammatory functions, COX-2 is also responsible for synthesis of prostacyclin and of prostaglandins important in normal renal function. Thromboxane is preferentially synthesized in platelets, whereas prostacyclin is synthesized in the endothelial cells of vessels. Naturally occurring eicosanoids have very short half-lives (seconds to minutes) and are inactive when given orally.

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FIGURE 18–1

Synthesis of eicosanoid autacoids. Arachidonic acid is released from membrane lipids by phospholipase A2 and then converted into straight-chain derivatives by lipoxygenase or into cyclized derivatives by cyclooxygenase. Because many of the effects of these products are pathogenic, drugs that inhibit synthesis or prevent the actions of the products are clinically useful.

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High-Yield Terms to Learn
Abortifacient A drug used to cause an abortion. Example: prostaglandin F2α
Cyclooxygenase Enzyme that converts arachidonic acid to PGG and PGH, the precursors of the prostaglandins, including PGE, PGF, prostacyclin, and thromboxane
Dysmenorrhea Painful uterine cramping caused by prostaglandins released during menstruation
Great vessel transposition Congenital anomaly in which the pulmonary artery exits from the left ventricle and the aorta from the right ventricle. ...

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