True statements regarding eicosanoids include which of the following?

A. Prostaglandins and thromboxanes are synthesized via the cyclo-oxygenase pathway.

B. The vasoconstricting, platelet-aggregating, and bronchoconstricting effects of thromboxane A 2 are balanced by the actions of prostacyclin, which produces the opposite effects.

D. The principal prostaglandins have a short circulation half-life and exert most of their effects locally. 

DISCUSSION: The eicosanoids are a group of compounds arising from the metabolism of arachidonic acid. The prostaglandins and thromboxanes are synthesized via the cyclo-oxygenase pathway; thus, their synthesis is blocked by NSAIDs. Leukotrienes, on the other hand, are synthesized via the lipoxygenase pathway. Prostacyclin, produced largely by vascular endothelium, inhibits platelet aggregation and causes vasodilatation as well as bronchodilatation. Its effects are balanced by those of thromboxane A 2, which is produced by platelets and also local actions, including platelet aggregation, vasoconstriction, and bronchoconstriction. The leukotrienes also have pulmonary and hemodynamic effects and may be involved in the physiologic responses associated with anaphylactic and septic shock.