Eicоsаnоids аre а grоup of lipid-derived signaling molecules produced from arachidonic acid, a 20-carbon polyunsaturated fatty acid found in membrane phospholipids. Eicosanoids play key roles in inflammation, immunity, and other cellular signaling pathways. They are generally classified into three main types: prostaglandins, thromboxanes, and leukotrienes. These molecules act through specific G protein-coupled receptors (GPCRs) on the cell surface, triggering a variety of downstream effects. Prostaglandins are synthesized by the enzyme cyclooxygenase (COX) and participate in mediating pain, fever, and inflammation. For example, prostaglandin E2 (PGE₂) binds to EP receptors and activates signaling pathways that increase vascular permeability and cause vasodilation, which are essential for inflammation. Some prostaglandins, like prostaglandin I₂ (prostacyclin), also act as vasodilators but are primarily involved in inhibiting platelet aggregation. Thromboxanes, primarily thromboxane A2 (TXA₂), are also synthesized via the COX pathway but have opposing effects to some prostaglandins. TXA₂, produced mainly by platelets, promotes platelet aggregation and vasoconstriction. This makes it a key player in blood clot formation and wound repair. Through binding to specific GPCRs on platelets, TXA₂ activates phospholipase C, leading to an increase in intracellular calcium and activation of protein kinase C (PKC), which enhances platelet aggregation. Leukotrienes are generated via the lipoxygenase (LOX) pathway and play a crucial role in immune responses, particularly in allergic reactions and asthma. Leukotriene B4 (LTB₄) acts as a potent chemoattractant for neutrophils, directing immune cells to sites of inflammation. Other leukotrienes, such as leukotriene C4 (LTC₄) and leukotriene D4 (LTD₄), are involved in bronchoconstriction by binding to CysLT receptors, which trigger smooth muscle contraction in the airways. This makes leukotrienes a major target in asthma treatment. The diverse actions of eicosanoids make them essential for maintaining homeostasis and responding to injury or infection. However, excessive eicosanoid production or dysregulation of their pathways is associated with various diseases, including chronic inflammatory disorders, asthma, and cardiovascular diseases. A patient with asthma may benefit from a drug that blocks the action of which of the following eicosanoids?
Which оf the fоllоwing best describes the mechаnism by which the Nа⁺ chаnnel in neurons transitions from a closed to an open state, and then to an inactivated state?