Sjodin R A, Ortiz O E, Montes J G
Department of Biophysics, University of Maryland School of Medicine, Baltimore 21201.
J Membr Biol. 1989 Apr;108(1):61-71. doi: 10.1007/BF01870426.
Giant axons from the marine annelid, Myxicola infundibulum, were internally dialyzed with ATP-free media and with media with lower than normal ATP levels in an attempt to determine quantitatively the ATP requirement of the Na pump in these cells. This was accomplished by using 22Na ions to measure Na efflux. When [ATP]i in dialysis fluid fell to values within the range of 20-40 microM, a marked stimulation of Na efflux was observed even though an essentially normal ouabain sensitivity of Na efflux persisted; when axons were dialyzed with ATP-free solutions with ouabain present in the external medium throughout the dialysis period, the stimulation of Na efflux still occurred. The stimulation of Na efflux produced by low [ATP]i levels could be reversed by reintroducing normal ATP levels into the dialysis medium. Reversibility was complete provided axons were not depleted of ATP for periods longer than about 1 hr. Longer periods of ATP depletion led to larger and ultimately irreversible increases in Na efflux. The increases in Na efflux occasioned by ATP depletion either prevented or obscured the decrease in Na efflux expected to occur from unfueling the Na pump. Since [ATP]i levels required to significantly unfuel the Na pump lie below the levels at which the Na efflux stimulation occurred, it is problematic to quantitatively assess the influence of [ATP]i levels on Na pump rate by measurements of Na efflux in this preparation. Substitutes for ATP failed to prevent increases in Na efflux. The large increases in Na efflux observed at low [ATP]i occurred with no important changes in the resting membrane potential, and also occurred in Na-free and Ca-free external media. At least part of the increased Na efflux under these conditions may be due to a Na/Na exchange component, as a significant dependence of Na efflux on [Na]o appropriate for this kind of exchange was observed in the ATP-depleted axons. Whether the highly reproducible anomalous effect on Na efflux in Myxicola axons has some fundamental significance in its own right is a matter for future investigation. A few possible explanations of the anomalous effect of reduced ATP levels are discussed.
对海生环节动物——漏斗海豆芽的巨轴突进行内部透析，分别使用无ATP的介质以及ATP水平低于正常水平的介质，试图定量确定这些细胞中钠泵对ATP的需求量。这是通过使用²²Na离子来测量钠外流实现的。当透析液中的[ATP]i降至20 - 40微摩尔范围内的值时，即使钠外流对哇巴因的敏感性基本正常，仍观察到钠外流有显著刺激；当轴突在整个透析期间用外部介质中存在哇巴因的无ATP溶液进行透析时，钠外流的刺激仍然发生。低[ATP]i水平产生的钠外流刺激可通过将正常ATP水平重新引入透析介质来逆转。只要轴突ATP耗尽时间不超过约1小时，逆转就是完全的。ATP耗尽时间更长会导致钠外流更大且最终不可逆转的增加。ATP耗尽引起的钠外流增加要么阻止了要么掩盖了因钠泵供能不足预期会发生的钠外流减少。由于显著耗尽钠泵所需的[ATP]i水平低于钠外流刺激发生的水平，通过测量该制剂中的钠外流来定量评估[ATP]i水平对钠泵速率的影响存在问题。ATP的替代物未能阻止钠外流增加。在低[ATP]i时观察到的钠外流大幅增加，静息膜电位没有重要变化，并且在无钠和无钙的外部介质中也会发生。在这些条件下，增加的钠外流至少部分可能归因于钠/钠交换成分，因为在ATP耗尽的轴突中观察到钠外流对适合这种交换的[Na]o有显著依赖性。漏斗海豆芽轴突中对钠外流高度可重复的异常效应本身是否具有某种根本意义，有待未来研究。文中讨论了ATP水平降低的异常效应的一些可能解释。
