Cota G, Stefani E
Can J Physiol Pharmacol. 1982 Apr;60(4):513-23. doi: 10.1139/y82-071.
Effects of external Ca2+ concentration reduction on the amplitude and time course of K+ contractures were studied in single muscle fibres. The resting potential, effective resistance, threshold for the Na current, action potential and K+-induced depolarizations did not change when 1.8 mM Ca2+ was replaced by 3 mM Mg2+ (3--6 microM Ca2+). Identical results were obtained after the addition of 5 mM EGTA (less than or equal to 10(-9) M Ca2+; Ca-free saline). The rate of tension development during the initial phase of K+ contractures was independent of external Ca2+ while the amplitude, the duration, and the time constant of spontaneous relaxation decreased progressively as Ca2+ concentration was diminished. The activation curve shifted by 3--5 mV towards more positive potentials while the inactivation curve shifted by 16--18 mV in the opposite direction and both curves became steeper in Ca-free saline. External Ca2+ may play a role in excitation--contraction coupling during K contractures either via the inward Ca current or via specific interactions between external Ca2+ ions and the coupling mechanism or both.
在单根肌纤维中研究了细胞外钙离子浓度降低对钾离子挛缩幅度和时程的影响。当用3 mM镁离子（3 - 6 microM钙离子）替代1.8 mM钙离子时，静息电位、有效电阻、钠离子电流阈值、动作电位和钾离子诱导的去极化均未改变。加入5 mM乙二醇双四乙酸（小于或等于10(-9) M钙离子；无钙生理盐水）后也得到了相同的结果。钾离子挛缩初始阶段的张力发展速率与细胞外钙离子无关，而随着钙离子浓度降低，自发松弛的幅度、持续时间和时间常数逐渐减小。激活曲线向更正电位方向移动3 - 5 mV，而失活曲线向相反方向移动16 - 18 mV，并且在无钙生理盐水中两条曲线都变得更陡。细胞外钙离子可能在钾离子挛缩期间的兴奋 - 收缩偶联中发挥作用，要么通过内向钙电流，要么通过细胞外钙离子与偶联机制之间的特异性相互作用，或者两者皆有。
