De Weer P
J Gen Physiol. 1976 Aug;68(2):159-78. doi: 10.1085/jgp.68.2.159.
The free magnesium concentration in the axoplasm of the giant axon of the squid, Loligo pealei, was estimated by exploting the known sensitivity of the sodium pump to intracellular Mg2+ levels. The Mg-citrate buffer which, when injected into the axon, resulted in no change in sodium efflux was in equilibrium with a Mg2+ level of about 3--4 mM. Optimal [Mg2+] for the sodium pump is somewhat higher. Total magnesium content of axoplasm was 6.7 mmol/kg, and that of hemolymph was 44 mM. The rate coefficient for 28Mg efflux was about 2 X 10(-3) min-u for a 500-mum axon at 22-25degreesC, with a very high temperature coefficient (Q10=4-5). This efflux is inhibited 95% by injection of apyrase and 75% by removal of external sodium, and seems unaffected by membrane potential or potassium ions. Increased intracellular ADP levels do not affect Mg efflux nor its requirement for Na+/o, but extracellularl magnesium ions do. Activation of 28Mg efflux by Na+/o follows hyperbolic kinetics, with Mg2+/o reducing the affinity of the system for Na+/o. Lanthanum and D600 reversibly inhibit Mg efflux. In the absence of both Na+ and Mg2+, but not in their presence, removal of Ca2+ from the seawater vastly increased 28Mg efflux; this efflux was also strongly inhibited by lanthanum. A small (10(-14) mol cm-2) extra Mg efflux accompanies the conduction of an action potential.
利用已知的钠泵对细胞内镁离子水平的敏感性，对枪乌贼（Loligo pealei）巨大轴突轴浆中的游离镁离子浓度进行了估算。当注入轴突时，柠檬酸盐镁缓冲液不会导致钠外流发生变化，其与约3 - 4 mM的镁离子水平处于平衡状态。钠泵的最佳镁离子浓度略高。轴浆中的总镁含量为6.7 mmol/kg，而血淋巴中的为44 mM。对于一个500μm的轴突，在22 - 25℃时，28Mg外流的速率系数约为2×10⁻³ min⁻¹，具有非常高的温度系数（Q10 = 4 - 5）。这种外流被注射的腺苷三磷酸双磷酸酶抑制95%，被去除细胞外钠抑制75%，并且似乎不受膜电位或钾离子的影响。细胞内二磷酸腺苷水平的升高不会影响镁外流及其对细胞外钠的需求，但细胞外镁离子会有影响。细胞外钠激活28Mg外流遵循双曲线动力学，细胞外镁离子会降低该系统对细胞外钠的亲和力。镧和D600可逆地抑制镁外流。在没有细胞外钠和镁离子的情况下（但存在时不会），从海水中去除钙离子会极大地增加28Mg外流；这种外流也会被镧强烈抑制。一个小的（10⁻¹⁴ mol cm⁻²）额外镁外流伴随着动作电位的传导发生。
