Sweatt J D, Johnson S L, Cragoe E J, Limbird L E
J Biol Chem. 1985 Oct 25;260(24):12910-9.
We have observed previously that removal of extraplatelet Na+ blocks platelet secretion of dense granule contents in response to epinephrine, ADP, and 0.004 unit/ml thrombin, all agents which must mobilize arachidonic acid for its subsequent conversion to cyclooxygenase products in order to elicit platelet secretion. The present studies demonstrate that removal of extraplatelet Na+ blocks arachidonic acid mobilization in response to epinephrine, ADP, and 0.004 unit/ml thrombin without altering arachidonic acid conversion to thromboxane A2. The data also provide several lines of evidence which suggest that the blockade of arachidonic acid release due to removal of extraplatelet Na+ is a manifestation of blockade of Na+/H+ exchange system. 1) There is a concentration-dependent effect of extraplatelet Na+ (EC50 congruent to 55 mM) on [3H]arachidonic acid release such that mobilization is observed when [Na+]o greater than [Na+]i. 2) Increasing extraplatelet [H+] (i.e. decreasing extraplatelet pH from pH 7.35 to 6.8) causes a concentration-dependent decline in stimulus-provoked [3H]arachidonic acid release. 3) Ethylisopropylamiloride and other potent 5-amino analogs of amiloride block [3H]arachidonic acid release with a potency that parallels their effects on Na+/H+ exchange in other cellular systems. None of the above manipulations alter primary aggregation induced by epinephrine, ADP, or 0.004 unit/ml thrombin, indicating that stimulus-receptor binding, subsequent exposure of fibrinogen receptors, and fibrinogen-mediated platelet-platelet cross-linking are not significantly inhibited by [3H]arachidonic acid release in response to greater than 0.1 unit/ml thrombin, a stimulus that can elicit platelet secretion in the absence of products of the cyclooxygenase pathway. Therefore, Na+/H+ exchange may selectively modulate arachidonic acid mobilization in response to the so-called "weak agonists," agonists that require this mobilization to effect vigorous platelet aggregation and dense granule secretion.
我们之前观察到，去除血小板外的钠离子会阻断血小板致密颗粒内容物的分泌，这种分泌是对肾上腺素、二磷酸腺苷（ADP）和0.004单位/毫升凝血酶的反应，所有这些试剂都必须动员花生四烯酸以便其随后转化为环氧化酶产物，从而引发血小板分泌。目前的研究表明，去除血小板外的钠离子会阻断对肾上腺素、ADP和0.004单位/毫升凝血酶的花生四烯酸动员，而不会改变花生四烯酸向血栓素A2的转化。这些数据还提供了几条证据，表明由于去除血小板外的钠离子而导致的花生四烯酸释放受阻是钠离子/氢离子交换系统受阻的表现。1）血小板外的钠离子对[3H]花生四烯酸释放有浓度依赖性影响（半数有效浓度约为55毫摩尔），使得当细胞外钠离子浓度大于细胞内钠离子浓度时可观察到动员。2）增加细胞外氢离子浓度（即细胞外pH从7.35降至6.8）会导致刺激引发的[3H]花生四烯酸释放呈浓度依赖性下降。3）乙基异丙基氨氯吡咪和其他有效的氨氯吡咪5-氨基类似物阻断[3H]花生四烯酸释放的效力与其在其他细胞系统中对钠离子/氢离子交换的作用相当。上述操作均未改变由肾上腺素、ADP或0.004单位/毫升凝血酶诱导的初始聚集，这表明刺激-受体结合、随后纤维蛋白原受体的暴露以及纤维蛋白原介导的血小板-血小板交联不会因对大于0.1单位/毫升凝血酶的反应中[3H]花生四烯酸释放而受到显著抑制，大于0.1单位/毫升凝血酶这种刺激在没有环氧化酶途径产物的情况下也能引发血小板分泌。因此，钠离子/氢离子交换可能选择性地调节对所谓“弱激动剂”的花生四烯酸动员，这些激动剂需要这种动员来实现有力的血小板聚集和致密颗粒分泌。
