硫酸鎂抗血小板凝集作用之機轉探討
Mechanisms involved in the Antiplatelet Activity of Magnesium Sulfate
中文摘要
Magnesium Sulfate (MgSO4)在臨床上注射用於抗痙攣(anticonvulsant)及 抗心律不整(antiarrhythemic) (Parra et al., 2001)。鎂離子(Mg2+)為必需 的微量元素，在人體神經肌肉功能及能量代謝上扮演一輔助因子 (cofactor)的 角色。正常濃度的鎂離子會減少血液凝集 (Gawaz et al.,1994)。對人體的病生 理上機轉並不清楚。本篇論文的研究目的是進一步探討硫酸鎂在人類血小板上的 作用。
在人類血小板懸浮液中，MgSO4 (0.6-3.0 mM)會隨著濃度的增加，而抑制 collagen (1 mg/l)， thrombin (0.02 IU/ml)，及 ADP (0.02 mM)所引起的 血小板的凝集及ATP 的釋放反應。所以，我們懷疑 MgSO4 是否是作用在血小 板凝集最後的共同路徑上(Santoro et al., 1989)；也就是纖維蛋白原
(fibrinogen)與血小板細胞膜上的 glycoprotein IIb/IIIa complex 的結合，進 而抑制血小板凝集反應。實驗發現MgSO4 不會影響 FITC 標定的 triflavin 結合 到細胞膜上glycoprotein IIb/IIIa complex；triflavin 是一種含 Arg-Gly-Asp (RGD)的蛇毒蛋白，已證實為一種 glycoprotein IIb/IIIa complex 的拮抗劑。
另外，MgSO4 在 3.0 mM 下能抑制 collagen (1 mg/l)所引起的
phosphoinositol (PI)的分解，細胞內鈣離子的增加，及抑制 thromboxane B2 的形成，但在 1.5 mM 時，並不會有意義的抑制 thromboxane B2 的形成。
在偵測細胞流動性的實驗中，MgSO4 (1.5 及 3.0 mM)會直接影響
diphylhexatriene (DPH)標定到細胞膜的程度，暗示著 MgSO4 會影響血小 板細胞膜的流動性。此外，MgSO4 會有意義的增加血小板內 cyclic AMP 的含 量，且會提高cyclic GMP 在血小板濃度。
綜合以上結果，MgSO4 對血小板的抑制作用機轉可能有以下二點：(一) MgSO4 直接改變血小板細胞膜的流動性，進而影響一些嵌在細胞膜上蛋白質 的作用，如phospholipase C (PLC)，使得 phosphoinositides (PI)分解後 產生inositol-1,4,5,-trisphosphate (IP3)與 1,2-diacylglycerol (DG)的含量 減少，IP3 的減少會降低細胞內鈣離子的濃度，進而抑制血小板的凝集及釋放 作用，而DG 減少則會進一步抑制 47 kDa protein phosphorylation。(二) MgSO4 使得血小板內 cyclic AMP 及 cyclic GMP 的含量增加，而抑制血小板 的凝集。
英文摘要
Magnesium Sulfate (MgSO4) used in human medicine as an injectable solution as
an an anticonvulsant and an antiarrhythemic(Parra et al., 2001). As essential element, Magnesium (Mg2+) plays a role of cofactor for many enzymes in human neuromuscular function and energy metabolism. Although concentrations of Mg2+ above the normal range have been shown to reduce platelet aggregation (Gawaz et al.,1994), the underlying pathophysiological mechanisms are still poorly understood. The aim of this thesis is to further investigate the mechanisms of MgSO4 in human platelets.
In human platelet suspension, we found that MgSO4 (0.6-3.0 mM) dise dependently potentiated platelet aggregation and ATP release by collagen (1 mg/l), thrombin (0.02 IU/ml), and ADP (0.02 mM). This may imply that whether MgSO4 inhibits platelet aggregation through directly interfering with fibrinogen binding to fibrinogen receptor associated with the glycoprotein IIb/IIIa
complex(Santoro et al., 1989). We found that MgSO4 did not significantly affect the FITC-triflavin binding to the glycoprotein IIb/IIIa complex；Triflavin, an Arg-Gly- Asp (RGD) containing antiplatelet peptide, was purified from Trimeresurus
flavoviridis snake venom.
In addition, MgSO4 (3.0 mM) inhibited the phosphoinositide breakdown and [Ca2+]i mobilization and thromboxane B2 formation in human platelet suspension stimulated by collagen (1 mg/l). At 1.5 mM, MgSO4 did not inhibit thromboxane B2 formation.
Measurememt of the platelet membrane fluidity, we found that MgSO4 (1.5-3.0 mM)capable of direct interaction with platelet membrane fluidity tragged with diphenylhexatriene (DPH). On the other hand, MgSO4 (1.5-3.0 mM) did
significantly affect cyclic AMP and cyclic GMP levels.
Therefore, based on the above observation, we suggested that there are two passibilities involving in the antiplatelet activity of MgSO4：(1) MgSO4 influenced the platelet membrane fluidity is the primary mechanism, followed by the
inhibition of phospholipase C activity, thereby leading to the inhibition of [Ca2+]i mobilization and platelet aggregation induced by agonist. (2) MgSO4 increased the cyclic AMP and cyclic GMP levels resulting in inhibitory platelet aggregation.
