Donoso P, Hidalgo C
Instituto de Ciencias Biomédicas, Facultad de Medicina, Universidad de Chile, Casilla 70005, Santiago 7, Chile.
J Membr Biol. 2002 Feb 1;185(3):257-63. doi: 10.1007/s00232-001-0128-8. Epub 2002 Feb 13.
Triads and transverse tubules isolated from mammalian skeletal muscle actively accumulated Na+ in the presence of K+ and Mg-ATP. Active Na+ transport exhibited a fast single-exponential phase, lasting 2 min, followed by slower linear uptake that continued for 10 minutes. Valinomycin stimulated Na+ uptake, suggesting it decreased a pump-generated membrane potential gradient (Vm) that prevented further Na+ accumulation. At the end of the fast uptake phase transverse tubule vesicles incubated in 30 mM external [Na+] attained a ratio [Na+]in/[Na+]out=13.4. From this ratio and the transverse tubule volume of 0.35 microl/mg protein measured in this work, [Na+]in=400 mM was calculated. Determinations of active K+ transport in triads, using 86Rb+ as tracer, showed a 30% decrease in vesicular 86Rb+ content two minutes after initiating the reaction, followed by a slower uptake phase during which vesicles regained their initial 86Rb+ content after 10 minutes. Transverse tubule volume increase during active Na+ transport-as shown by light scattering changes of isolated vesicles--presumably accounted for the secondary Na+ and 86Rb+ uptake phases. These combined results indicate that isolated triads have highly sealed transverse tubules that can be polarized effectively by the Na+ pump through the generation of significant Na+ gradients.
从哺乳动物骨骼肌分离出的三联体和横小管在存在钾离子和镁 - 三磷酸腺苷（Mg - ATP）的情况下能主动积累钠离子。主动钠离子转运呈现出一个快速的单指数阶段，持续2分钟，随后是持续10分钟的较慢线性摄取阶段。缬氨霉素刺激钠离子摄取，这表明它降低了由泵产生的阻止进一步钠离子积累的膜电位梯度（Vm）。在快速摄取阶段结束时，在30 mM外部[钠离子]中孵育的横小管囊泡达到[钠离子]内/[钠离子]外 = 13.4的比率。根据这个比率以及在本研究中测得的0.35微升/毫克蛋白质的横小管体积，计算出[钠离子]内 = 400 mM。以86Rb +作为示踪剂对三联体中的主动钾离子转运进行测定，结果显示在反应开始两分钟后囊泡中86Rb +含量下降30%，随后是一个较慢的摄取阶段，在此期间囊泡在10分钟后恢复到其初始的86Rb +含量。如分离囊泡的光散射变化所示，主动钠离子转运过程中横小管体积增加，这可能解释了继发性钠离子和86Rb +摄取阶段。这些综合结果表明，分离出的三联体具有高度密封的横小管，钠离子泵可通过产生显著的钠离子梯度有效地使其极化。
