22.10
一酸化窒素シグナル伝達経路
無機ガスである一酸化窒素 (NO) は、ほとんどの動植物の組織において強力なセカンドメッセンジャーとして機能します。 NO は、それを生成する細胞から拡散し、隣接する細胞に入り、下流の応答を生成します。 NO シンターゼ (NOS) は、アミノ酸アルギニンの脱アミノ化による NO 生成を触媒します。…
一酸化窒素(NO)は、ガス状のシグナル伝達分子であり、平滑筋の弛緩、神経伝達、視覚などのプロセスを媒介する独自のセカンドメッセンジャーです。
アセチルコリン結合GPCRは、PLC-βを刺激してIP3産生を誘導し、カルシウム放出を誘発します。
カルシウムイオンはカルモジュリンと結合して、NOシンターゼを活性化する複合体を形成します。この酵素は内皮細胞でNOを産生し、内皮細胞は隣接する平滑筋細胞に拡散して作用します。
NOはグアニルシクラーゼに結合して活性化し、プロテインキナーゼGまたはPKGを活性化する環状GMPを産生します。
PKGはホスファターゼを活性化し、ホスファターゼがミオシン軽鎖を脱リン酸化します。これにより、ミオシンヘッドの折り畳みが誘導され、アクチンフィラメントから解離します。
また、PKGはIP3依存性カルシウムチャネルを阻害し、細胞質カルシウム濃度を低下させます。カルシウムレベルが低いと、ミオシン軽鎖キナーゼが不活性化され、アクチン-ミオシンの再構築と筋肉の収縮が妨げられます。
平滑筋が弛緩し、血管が拡張して血流が増加します。
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Q1: How does nitric oxide function as a second messenger in cells?
Nitric oxide is a gaseous signaling molecule that diffuses out of producing cells and enters neighboring cells to generate downstream responses. NO binds and activates guanylyl cyclase, which converts GTP to cyclic GMP. This rapid increase in cGMP levels triggers protein kinase G activation, initiating the cellular response within seconds. NO's short half-life of 2-30 seconds allows it to act locally on adjacent cells.
Q2: What role does calmodulin play in nitric oxide production?
Calcium ions released from intracellular stores bind calmodulin to form a complex that activates NO synthase. This enzyme catalyzes NO production through deamination of the amino acid arginine. The calmodulin-dependent signaling pathway is essential for initiating nitric oxide synthesis in response to acetylcholine-bound GPCRs and subsequent IP3-mediated calcium release.
Q3: How does protein kinase G cause smooth muscle relaxation?
Protein kinase G activates a phosphatase that dephosphorylates myosin light chains, causing myosin heads to fold and dissociate from actin filaments. PKG also inhibits IP3-gated calcium channels, reducing cytosolic calcium concentration. Low calcium inactivates myosin light chain kinases, preventing actin-myosin reassembly and muscle contraction, resulting in smooth muscle relaxation.
Q4: Why does nitric oxide cause blood vessels to dilate?
NO diffuses from endothelial cells into neighboring smooth muscle cells, where it activates guanylyl cyclase to produce cyclic GMP. This triggers protein kinase G activation, which disassembles the actin-myosin contractile machinery. Smooth muscle relaxation allows blood vessels to dilate, increasing blood flow to tissues. This mechanism is crucial for regulating vascular tone and blood pressure.
Q5: What are the three isoforms of NO synthase and where are they found?
Endothelial NOS (eNOS) is found in endothelial cells lining blood vessels. Neuronal NOS (nNOS) is present in nerve and muscle cells. Inducible NOS (iNOS) is produced by macrophages upon exposure to various pathogens. Each isoform catalyzes NO production through arginine deamination in different tissue contexts and physiological conditions.
Q6: How do phosphodiesterase inhibitors like Viagra affect cGMP levels?
cGMP phosphodiesterase normally degrades cyclic GMP, balancing its cytosolic levels. Drugs like Viagra inhibit this enzyme, causing cGMP to remain elevated for prolonged periods. Sustained cGMP levels maintain smooth muscle relaxation in penile blood vessels, increasing blood flow and enabling erection. This mechanism demonstrates how feedback regulation of signaling molecules controls physiological responses.
Q7: How is nitroglycerine used to treat angina?
Nitroglycerine has been used since the 1860s to treat angina, a severe chest pain caused by improper blood flow to the heart muscle. The drug metabolizes into nitric oxide once inside the body. NO then dilates blood vessels, increasing blood flow to the heart and relieving chest pain. This therapeutic application demonstrates NO's critical role in regulating vascular function and tissue perfusion.