Eicosanoids mediate inflammation in a variety of ways. Of the following statements, which are true with regard to this?

b. Eicosanoids include prostaglandins, thromboxanes, leukotrienes and lipoxins

d. Physiologic responses to eicosanoids include vasodilatation, vasoconstriction, increased vascular permeability and both chemotaxis and chemoattractant inhibition

The eicosanoids are derived from arachidonic acid (eicosatetraenoic acid) and consist of prostaglandins, thromboxanes, leukotrienes and lipoxins. The eicosanoids are not stored in cells but are rapidly synthesized by cells in response to a variety of stimuli. They have potent effects on vascular and bronchial smooth muscle including vasodilatation, vasoconstriction, bronchodilation and bronchoconstriction. In addition, they directly regulate vascular permeability. LTB4 is a potent, neutrophil chemoattractant whereas lipoxin A4 inhibits other chemoattractants. It appears that eicosanoids are important regulators of the endogenous inflammatory response. The rapid destruction of eicosanoids in the circulation limits their role primarily to that of mediators of local inflammatory changes. The local effects can be substantial. In general, the eicosanoids are rapidly metabolized or are so chemically unstable that they primarily exert their effects near the site of synthesis. Arachidonic acid does not exist in cells but is esterified to membrane phospholipids. Thus, the first step in the production of eicosanoids is phospholipase action, which liberates arachidonic acid. (Figure 6-6)