Rodrigues-Simioni L, Prado-Franceschi J, Cintra A C, Giglio J R, Jiang M S, Fletcher J E
Department of Pharmacology, UNICAMP, Campinas, SP, Brazil.
Toxicon. 1995 Nov;33(11):1479-89. doi: 10.1016/0041-0101(95)00089-5.
The role of low levels of phospholipase A2 (PLA2) activity and intracellular Ca2+ stores in the pharmacological action of bothropstoxin (BthTX), a myotoxic Lys49 PLA2 homologue isolated from the venom of Bothrops jararacussu, was investigated. We examined the muscular effects of BthTX in the mouse diaphragm and its PLA2 activity in radiolabeled human and rat primary cultures of skeletal muscle. Although it is a Lys49 PLA2 homologue, BthTX had a low, but easily detectable, level of enzymatic activity relative to two Asp49 PLA2 enzymes from Naja naja kaouthia and Naja naja atra venoms, and this activity was reduced by about 85% in the presence of Sr2+ (4.0 mM). However, the replacement of 1.8 mM Ca2+ by 4 mM Sr2+ did not alter the BthTX-induced contracture and blockade of the muscle twitch tension. In addition, Sr2+ decreased by 50% the time required to cause 50% paralysis, and evoked approximately a four-fold increase in the number of spontaneous spikes. In isolated sarcoplasmic reticulum preparations, BthTX opened the intracellular Ca2+ release channel (ryanodine receptor) and lowered the threshold of Ca(2+)-induced Ca2+ release by a second, as yet unidentified, mechanism. However, in intact muscle, dantrolene, an antagonist of some forms of intracellular Ca2+ release, had no effect on the actions of BthTX. These findings do not support any role for the low levels of PLA2 activity, or dantrolene-sensitive intracellular Ca2+ stores, in the action of BthTX. The mechanism whereby Sr2+ stimulates the pharmacological activity of BthTX remains to be clarified.
研究了低水平磷脂酶A2（PLA2）活性和细胞内Ca2+储存库在矛头蝮蛇毒素（BthTX）药理作用中的作用。BthTX是从巴西矛头蝮蛇毒液中分离出的一种具有肌毒性的Lys49 PLA2同源物。我们检测了BthTX对小鼠膈肌的肌肉效应及其在放射性标记的人及大鼠骨骼肌原代培养物中的PLA2活性。尽管BthTX是一种Lys49 PLA2同源物，但其相对于眼镜王蛇和中华眼镜蛇毒液中的两种Asp49 PLA2酶，具有较低但易于检测到的酶活性水平，并且在存在Sr2+（4.0 mM）的情况下，该活性降低了约85%。然而，用4 mM Sr2+替代1.8 mM Ca2+并没有改变BthTX诱导的挛缩和肌肉抽搐张力的阻断。此外，Sr2+使导致50%麻痹所需的时间减少了50%，并使自发尖峰数量增加了约四倍。在分离的肌浆网制剂中，BthTX打开了细胞内Ca2+释放通道（兰尼碱受体），并通过另一种尚未确定的机制降低了Ca(2+)诱导的Ca2+释放阈值。然而，在完整肌肉中，丹曲林（一种细胞内Ca2+释放某些形式的拮抗剂）对BthTX的作用没有影响。这些发现不支持低水平PLA2活性或丹曲林敏感的细胞内Ca2+储存库在BthTX作用中的任何作用。Sr2+刺激BthTX药理活性的机制仍有待阐明。
