Metz S A
Biochem Pharmacol. 1986 Oct 1;35(19):3371-81. doi: 10.1016/0006-2952(86)90438-7.
The effects of the basic polypeptide melittin on islet phospholipid degradation and insulin release were studied in static incubations of intact rat islets as a possible model of endogenous phospholipase A2 (PLA2) activation. Melittin (2 micrograms/ml) increased [3H]-arachidonic acid [( 3H]-AA) release from prelabeled islets (at 1.7 mM glucose) to 371% of basal values. Concomitantly, melittin induced degradation of islet phospholipids labeled with [3H]-AA or [14C]-stearic acid, and led to the accumulation of stearate-labeled (but not AA-labeled) lysophosphatidylcholine (LPC, 605% of control). These findings suggested, for the first time in intact rat islets, the presence and activation of a PLA2. Under identical conditions, melittin initiated insulin secretion (at 1.7 mM glucose) in a manner that represented stimulation of physiologic exocytosis--that is, it was dose-dependent, reversible (albeit slowly), unassociated with impairment of other physiologic islet processes (i.e. the response to 16.7 mM glucose after removal of the drug) and inhibitable by reduced ambient temperature. The effect of melittin seemed to be independent of extracellular Ca2+ influx or mobilization of intracellular Ca2+ stores but was blocked by nickel or lanthanum, indirectly suggesting that the effects of this cationic amphiphile might involve a superficial pool of Ca2+. Unexpectedly, melittin-induced insulin release (at least at low glucose concentrations) was not greatly or consistently altered by a battery of inhibitors of endogenous PLA2 or of enzymes affecting AA oxygenation. Furthermore, significant contamination by bee venom PLA2 of the commercially-available melittin preparation was found, and insulin release could be induced by pure bee venom PLA2, probably through the generation of lysophospholipids. Although estimates of the amount of PLA2 in the melittin preparation suggested that such contamination was insufficient to explain at least some of the islet phospholipid hydrolysis and insulin release caused by melittin, we conclude that this agent does not serve as a specific probe of the role of endogenous PLA2 or of AA lipoxygenation in hormone release.
在完整大鼠胰岛的静态孵育实验中，研究了碱性多肽蜂毒肽对胰岛磷脂降解和胰岛素释放的影响，以此作为内源性磷脂酶A2（PLA2）激活的一种可能模型。蜂毒肽（2微克/毫升）使预先标记的胰岛（在1.7毫摩尔葡萄糖浓度下）中[3H] - 花生四烯酸[（3H] - AA）的释放量增加至基础值的371%。同时，蜂毒肽诱导了用[3H] - AA或[14C] - 硬脂酸标记的胰岛磷脂降解，并导致硬脂酸标记（而非AA标记）的溶血磷脂酰胆碱（LPC）积累（为对照的605%）。这些发现首次在完整大鼠胰岛中提示了PLA2的存在及激活。在相同条件下，蜂毒肽引发胰岛素分泌（在1.7毫摩尔葡萄糖浓度下），其方式表现为对生理性胞吐作用的刺激——即它具有剂量依赖性、可逆性（尽管缓慢），与其他胰岛生理过程的损害无关（即去除药物后对16.7毫摩尔葡萄糖的反应），且可被降低的环境温度抑制。蜂毒肽的作用似乎独立于细胞外Ca2+内流或细胞内Ca2+储存的动员，但被镍或镧阻断，这间接表明这种阳离子两亲物的作用可能涉及一个表面的Ca2+池。出乎意料的是，一系列内源性PLA2抑制剂或影响AA氧化的酶并未显著且持续地改变蜂毒肽诱导的胰岛素释放（至少在低葡萄糖浓度下）。此外，发现市售蜂毒肽制剂被蜂毒PLA2严重污染，纯蜂毒PLA2可能通过溶血磷脂的生成诱导胰岛素释放。尽管对蜂毒肽制剂中PLA2含量的估计表明这种污染不足以解释至少部分由蜂毒肽引起的胰岛磷脂水解和胰岛素释放，但我们得出结论，该试剂并非内源性PLA2或AA脂氧合作用在激素释放中作用的特异性探针。
