Chen L Y, Mehta J L
Department of Medicine, University of Florida College of Medicine, Gainesville, USA.
J Investig Med. 1997 Feb;45(2):69-74.
Besides endothelial cells, platelets possess active nitric oxide synthase (NOS) enzyme, which converts L-arginine to NO and L-citrulline. Nitroglycerin (NTG) inhibits platelet aggregation by increasing intracellular cGMP levels, an effect similar to that of NO. In this study we examined the regulation of platelet NOS activity by NTG and authentic NO.
Nitric oxide synthase activity was measured as formation of L-citrulline from L-arginine. As expected, incubation of platelet-rich plasma with NTG resulted in a concentration-dependent (40-400 mumol/L) increase in nitrite levels and an increase in cGMP accumulation. Concurrently, NTG was found to exert an inhibitory effect on platelet NOS activity. Exposure of washed platelets to authentic NO also resulted in approximately 50% decrease in NOS activity. Western analysis showed that NTG and authentic NO had no effect on NOS protein expression in human platelets.
This study indicates that NO, either authentic or derived from NTG, decreases NOS activity without affecting NOS protein expression in human platelets. The downregulation of NOS activity may have a bearing on the phenomenon of nitrate tolerance.
除内皮细胞外，血小板也拥有活性一氧化氮合酶（NOS），该酶可将L-精氨酸转化为NO和L-瓜氨酸。硝酸甘油（NTG）通过提高细胞内cGMP水平来抑制血小板聚集，这一作用与NO类似。在本研究中，我们检测了NTG和天然NO对血小板NOS活性的调节作用。
通过检测从L-精氨酸生成L-瓜氨酸的量来测定一氧化氮合酶活性。正如预期的那样，富含血小板的血浆与NTG孵育后，亚硝酸盐水平呈浓度依赖性（40 - 400 μmol/L）升高，cGMP积累增加。同时，发现NTG对血小板NOS活性有抑制作用。将洗涤后的血小板暴露于天然NO也导致NOS活性降低约50%。蛋白质免疫印迹分析表明，NTG和天然NO对人血小板中NOS蛋白表达没有影响。
本研究表明，无论是天然的还是由NTG衍生的NO，均可降低人血小板中NOS活性，而不影响NOS蛋白表达。NOS活性的下调可能与硝酸盐耐受性现象有关。
