Choi O H, Padgett W L, Nishizawa Y, Gusovsky F, Yasumoto T, Daly J W
Laboratory of Bioorganic Chemistry, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
Mol Pharmacol. 1990 Feb;37(2):222-30.
Maitotoxin (MTX) increases formation of [3H]inositol phosphates from phosphoinositides and release of [3H]arachidonic acid from phospholipids in pheochromocytoma PC12 cells. Formation of [3H]inositol phosphates is detected within 1 min of incubation even with concentrations as low as 0.3 ng/ml (90 pm) MTX, whereas release of [3H]arachidonic acid is not detected until 20 min even with concentrations as high as 1 ng/ml (300 pm) MTX. Stimulation of arachidonic acid release can be detected at 0.03 ng/ml (9 pm) MTX, whereas 0.1 ng/ml (30 pm) MTX is the threshold for detection of phosphoinositide breakdown. Organic and inorganic calcium channel blockers, except Cd2+ and a high concentration of Mn2+, have no effect on MTX-elicited phosphoinositide breakdown, whereas inorganic blockers (e.g., Co2+, Mn2+, Cd2+), but not organic blockers (nifedipine, verapamil, diltiazem), inhibit MTX-stimulated arachidonic acid release. All calcium channel blockers, however, inhibited MTX-elicited influx of 45Ca2+ and the MTX-elicited increase in internal Ca2+ measured with fura-2 was markedly reduced by nifedipine. MTX-elicited phosphoinositide breakdown and arachidonic acid release are abolished or reduced, respectively, in the absence of extracellular calcium plus chelating agent. The calcium ionophore A23187 has little or no effect alone but, in combination with MTX, A23187 inhibits MTX-elicited phosphoinositide breakdown and enhances arachidonic acid release, the latter even in the absence of extracellular calcium. The results suggest that different sites and/or mechanisms are involved in stimulation of calcium influx, breakdown of phosphoinositides, and release of arachidonic acid by MTX.
刺尾鱼毒素（MTX）可增加嗜铬细胞瘤PC12细胞中磷酸肌醇生成[³H]肌醇磷酸酯以及磷脂释放[³H]花生四烯酸的量。即使在MTX浓度低至0.3 ng/ml（90皮摩尔）的情况下孵育1分钟后，也能检测到[³H]肌醇磷酸酯的生成；而即使在MTX浓度高达1 ng/ml（300皮摩尔）的情况下，直到20分钟后才能检测到[³H]花生四烯酸的释放。在MTX浓度为0.03 ng/ml（9皮摩尔）时可检测到对花生四烯酸释放的刺激作用，而MTX引起磷酸肌醇分解的检测阈值为0.1 ng/ml（30皮摩尔）。除了Cd²⁺和高浓度的Mn²⁺外，有机和无机钙通道阻滞剂对MTX引发的磷酸肌醇分解均无影响，而无机阻滞剂（如Co²⁺、Mn²⁺、Cd²⁺）可抑制MTX刺激的花生四烯酸释放，有机阻滞剂（硝苯地平、维拉帕米、地尔硫䓬）则无此作用。然而，所有钙通道阻滞剂均抑制MTX引发的⁴⁵Ca²⁺内流，并且用fura - 2测定的MTX引发的细胞内Ca²⁺增加量被硝苯地平显著降低。在无细胞外钙并加入螯合剂的情况下，MTX引发的磷酸肌醇分解和花生四烯酸释放分别被消除或减少。钙离子载体A23187单独作用时几乎没有影响，但与MTX联合使用时，A23187可抑制MTX引发的磷酸肌醇分解并增强花生四烯酸释放，即使在无细胞外钙的情况下也是如此。结果表明，MTX刺激钙内流、磷酸肌醇分解和花生四烯酸释放涉及不同的位点和/或机制。
