Abercrombie R F, Masukawa L M, Sjodin R A, Livengood D
J Gen Physiol. 1981 Oct;78(4):413-29. doi: 10.1085/jgp.78.4.413.
The binding and release of 45Ca by axoplasm isolated from Myxicola giant axons were examined. Two distinct components of binding were observed, one requiring ATP and one not requiring ATP. The ATP-dependent binding was largely prevented by the addition of mitochondrial inhibitors, whereas the ATP-independent component was unaffected by these inhibitors. The ATP-independent binding accounted for roughly two-thirds of the total 45Ca uptake in solutions containing an ionized [Ca2+] = 0.54 microM and was the major focus of this investigation. This fraction of bound 45Ca was released from the axoplasm at a rate that increased with increasing concentrations of Ca2+ in the incubation fluid. The ions Cd2+ and Mn2+ were also able to increase 45Ca efflux from the sample, but Co2+, Ni2+, Mg2+, and Ba2+ had no effect. The concentration-response curves relating the 45Ca efflux rate coefficients to the concentration of Ca2+, Cd2+, and Mn2+ in the bathing solution were S-shaped. The maximum rate of efflux elicited by one of these divalent ions could not be exceeded by adding a saturating concentration of a second ion. Increasing EGTA concentration in the bath medium from 100 to 200 microM did not increase 45Ca efflux; yet increasing the concentration of the EGTA buffer in the uptake medium from 100 to 200 microM and keeping ionized Ca2+ constant caused more 45Ca to be bound by the axoplasm. These results suggest the existence of high-affinity, ATP-independent binding sites for 45Ca in Myxicola axoplasm that compete favorably with 100 microM EGTA. The 45Ca efflux results are interpreted in terms of endogenous sites that interact with Ca2+, Cd2+, or Mn2+.
对从巨型黏液虫轴突分离出的轴浆中45Ca的结合与释放进行了检测。观察到两种不同的结合成分，一种需要ATP，另一种不需要ATP。添加线粒体抑制剂可大大抑制依赖ATP的结合，而不依赖ATP的成分不受这些抑制剂的影响。在含有离子化[Ca2+]=0.54微摩尔的溶液中，不依赖ATP的结合约占45Ca总摄取量的三分之二，是本研究的主要关注点。这部分结合的45Ca从轴浆中释放的速率随孵育液中Ca2+浓度的增加而增加。离子Cd2+和Mn2+也能够增加样品中45Ca的流出，但Co2+、Ni2+、Mg2+和Ba2+没有影响。将45Ca流出速率系数与浴液中Ca2+、Cd2+和Mn2+浓度相关的浓度-反应曲线呈S形。添加第二种离子的饱和浓度不能超过这些二价离子之一引发的最大流出速率。将浴液介质中EGTA的浓度从100微摩尔增加到200微摩尔不会增加45Ca的流出；然而，将摄取介质中EGTA缓冲液的浓度从100微摩尔增加到200微摩尔并保持离子化Ca2+不变会导致轴浆结合更多的45Ca。这些结果表明，在巨型黏液虫轴浆中存在与Ca2+、Cd2+或Mn2+相互作用的内源性位点。这些结果表明，巨型黏液虫轴浆中存在对45Ca具有高亲和力、不依赖ATP的结合位点，这些位点能与100微摩尔EGTA有效竞争。45Ca流出结果是根据与Ca2+、Cd2+或Mn2+相互作用的内源性位点来解释的。
