Temma K, Akera T
J Pharmacol Exp Ther. 1982 Nov;223(2):490-6.
The rate of development of the positive inotropic action of ouabain is enhanced when the heart is stimulated at higher frequencies. A hypothesis that this enhancement is due to a stimulation of the glycoside binding to sarcolemmal Na+,K+-adenosine triphosphatase (ATPase) caused by an increase in intracellular Na+ available to the sodium pump was tested in isolated left atrial muscle preparations of guinea-pig heart, incubated at 30 degrees C and electrically stimulated at 0.5, 1 or 2 Hz. The rate of development of the positive inotropic action of ouabain was dependent on the frequency of stimulation. Each preparation was homogenized at a predetermined time and the fractional occupancy of Na+,K+-ATPase by ouabain was estimated from the decrease in the initial velocity of ATP-dependent [3H]ouabain binding reaction. A parallel relationship was observed between effects of stimulation frequency of the positive inotropic action and those on the occupancy of Na+,K+-ATPase by ouabain. In quiescent preparations, a sodium ionophore, monensin, enhanced the development of contracture caused by a toxic concentration of ouabain and also the glycoside binding to Na+,K+-ATPase. Similar effects on the ouabain-induced contracture and on the glycoside binding were observed with either grayanotoxin I or batrachotoxin, agents known to increase sodium influx, when muscle preparations were exposed to these agents under 1.5 Hz stimulation and were subsequently tested for the actions of ouabain in quiescence. When the exposure to ouabain and either grayanotoxin I or batrachotoxin was restricted to quiescent period, the development of ouabain-induced contracture and glycoside binding to Na+,K+-ATPase were not significantly altered. Monensin, grayanotoxin I or batrachotoxin failed to significantly affect [3H]ouabain binding to muscle homogenates when added to the medium for the labeled glycoside binding assay. These results indicate that intracellular sodium ions promote the ouabain binding to Na+,K+-ATPase and thereby enhance the development of glycoside actions in the isolated atrial muscle of guinea-pig heart. The "beat-dependent" onset of the glycoside action is at least partially explained from the effect of membrane depolarization to increase Na+ available to the sodium pump and to enhance the glycoside binding.
当以更高频率刺激心脏时，哇巴因正性肌力作用的发展速率会加快。在30℃孵育并以0.5、1或2Hz进行电刺激的豚鼠心脏离体左心房肌制备物中，对这样一种假说进行了检验，即这种增强是由于钠泵可利用的细胞内Na⁺增加，刺激了糖苷与肌膜Na⁺,K⁺-腺苷三磷酸酶（ATP酶）的结合。哇巴因正性肌力作用的发展速率取决于刺激频率。在预定时间将每个制备物匀浆，并根据ATP依赖性[³H]哇巴因结合反应初始速度的降低来估计哇巴因对Na⁺,K⁺-ATP酶的占据分数。在正性肌力作用的刺激频率效应与对哇巴因占据Na⁺,K⁺-ATP酶的效应之间观察到平行关系。在静止的制备物中，一种钠离子载体莫能菌素增强了由毒性浓度的哇巴因引起的挛缩的发展，也增强了糖苷与Na⁺,K⁺-ATP酶的结合。当肌肉制备物在1.5Hz刺激下暴露于已知会增加钠内流的灰藓毒素I或箭毒蛙毒素，并随后在静止状态下测试哇巴因的作用时，观察到对哇巴因诱导的挛缩和糖苷结合有类似影响。当将哇巴因与灰藓毒素I或箭毒蛙毒素的暴露限制在静止期时，哇巴因诱导的挛缩的发展以及糖苷与Na⁺,K⁺-ATP酶的结合没有明显改变。当将莫能菌素、灰藓毒素I或箭毒蛙毒素添加到用于标记糖苷结合测定的培养基中时，它们未能显著影响[³H]哇巴因与肌肉匀浆的结合。这些结果表明，细胞内钠离子促进哇巴因与Na⁺,K⁺-ATP酶的结合，从而增强了哇巴因在豚鼠心脏离体心房肌中的作用发展。糖苷作用的“搏动依赖性”起始至少部分可由膜去极化增加钠泵可利用的Na⁺并增强糖苷结合的效应来解释。
