Allen T J, Baker P F
J Physiol. 1986 Sep;378:53-76. doi: 10.1113/jphysiol.1986.sp016207.
Experiments are described in which the [Ca]o-dependent component of 22Na efflux is monitored under conditions of membrane potential control by voltage clamp. The apparent affinity of the efflux system for external Ca is very low in choline sea water (apparent KD approximately 50 mM); but increases dramatically when choline is replaced isosmotically by Li or K (apparent KD approximately 1-2 mM). Ca influx changes in a parallel fashion. Tris behaves much like choline and guanidinium is about two-thirds as effective as Li. Replacement of Li by K has little effect on the apparent affinity for external Ca but brings about a small (30-40%) increase in the maximal flux. The increase in maximum flux can be removed by electrical hyperpolarization to the potential before application of K and, in the absence of K, can be mimicked by electrical depolarization. These experiments suggest that the stimulatory effect of K on the Ca-dependent Na efflux into Li sea water is electrical in origin. Partial replacement of choline by K stimulates the Ca-dependent Na efflux; but only part of this stimulation can be removed by electrical hyperpolarization and, in the absence of K, electrical depolarization only brings about a relatively small stimulation. This is because only part of the stimulation that follows addition of K to choline sea waters is electrical in origin: the rest reflects an increase in the apparent affinity for external Ca that is brought about by K acting chemically. The maximum efflux into K is about 40% higher than that into choline. That this may reflect an electrical effect is supported by the observation that electrical depolarization increases the flux into choline sea water containing 110 mM-Ca where the Ca-binding site is close to saturation. The voltage clamp was used to determine the voltage dependence of the Ca-dependent Na efflux into Li sea water, choline sea water and choline sea water containing 100 mM-Na. In all three cases the flux increased with depolarization and was still rising at +70 mV. The dependence on potential was not very steep, an e-fold increase occurred over approximately 50 mV.
本文描述了一些实验，即在电压钳控制膜电位的条件下监测22Na外流的[Ca]o依赖性成分。在胆碱海水中，外流系统对外部Ca的表观亲和力非常低（表观KD约为50 mM）；但当胆碱被等渗的Li或K取代时，表观亲和力会显著增加（表观KD约为1 - 2 mM）。Ca内流以平行方式变化。Tris的行为与胆碱非常相似，胍的效果约为Li的三分之二。用K取代Li对外部Ca的表观亲和力影响不大，但会使最大通量小幅增加（30 - 40%）。最大通量的增加可以通过在施加K之前将电位电超极化到之前的电位来消除，并且在没有K的情况下，可以通过电去极化来模拟。这些实验表明，K对Ca依赖性Na外流进入Li海水中的刺激作用源于电效应。用K部分取代胆碱会刺激Ca依赖性Na外流；但这种刺激只有部分可以通过电超极化消除，并且在没有K的情况下，电去极化只会带来相对较小的刺激。这是因为在胆碱海水中加入K后，只有部分刺激源于电效应：其余部分反映了K的化学作用导致的对外部Ca表观亲和力的增加。进入K的最大外流比进入胆碱的约高40%。这可能反映电效应的证据是，电去极化会增加进入含有110 mM - Ca的胆碱海水中的通量，此时Ca结合位点接近饱和。使用电压钳来确定Ca依赖性Na外流进入Li海水、胆碱海水和含有100 mM - Na的胆碱海水中的电压依赖性。在所有三种情况下，通量都随着去极化而增加，并且在+70 mV时仍在上升。对电位的依赖性不是很陡峭，在大约50 mV内发生了e倍的增加。
