Almers W, McCleskey E W, Palade P T
J Physiol. 1984 Aug;353:565-83. doi: 10.1113/jphysiol.1984.sp015351.
Membrane currents were recorded from voltage-clamped, EGTA-loaded muscle fibres under conditions where currents through ordinary Na+, K+ and Cl- channels were prevented by drugs or by absence of permeant ions (K+, and Cl-). At 10 mM-external [Ca2+], substitution of Na+ for the large and presumably impermeant organic cations tetramethyl- (TMA+) or tetraethylammonium (TEA+) failed to increase peak inward current. Hence the Ca2+ channel was not significantly permeable to Na+ under these conditions. When external [Ca2+] was reduced to levels below 1 microM in the presence of external Na+, step depolarizations to negative potentials produced tetrodotoxin-resistant inward currents. At -20 mV, they rose to a peak of 30-200 microA/cm2 within 150 ms and declined thereafter. Ca2+ and several other divalent cations reversibly blocked this inward current. The sequence of blocking potencies was Ca2+ greater than Sr2+ greater than or equal to Co2+ greater than Mn2+ congruent to Cd2+ greater than Ni2+ congruent to Mg2+. Large inward currents may be carried by Li+, Na+, K+, Rb+ and Cs+ but not by TMA+ and TEA+. The effect of external Ca2+ ([Ca2+]o) was explored over a 10(8)-fold range in concentrations. Na+ was present at a fixed concentration. When [Ca2+]o was gradually increased from 10(-10) to 10(-2) M, inward current first diminished 10-fold, reached a minimum at [Ca2+]o = 60 microM and then increased again as [Ca2+]o was increased further and Ca2+ itself became a current carrier. Block of inward current at [Ca2+]o less than 10(-5) M could be described by binding of a single Ca2+ to a site, with a dissociation constant of the order of 0.7 microM at -20 mV.
在通过药物或去除通透离子（K⁺和Cl⁻）来阻止普通Na⁺、K⁺和Cl⁻通道电流的条件下，从电压钳制、加载EGTA的肌纤维中记录膜电流。在外部[Ca²⁺]为10 mM时，用大的且可能不可通透的有机阳离子四甲基铵（TMA⁺）或四乙铵（TEA⁺）替代Na⁺未能增加内向电流峰值。因此，在这些条件下Ca²⁺通道对Na⁺的通透性不显著。当在外部存在Na⁺的情况下将外部[Ca²⁺]降低到低于1 μM的水平时，向负电位的阶跃去极化产生了河豚毒素抗性内向电流。在-20 mV时，它们在150 ms内升至30 - 200 μA/cm²的峰值，随后下降。Ca²⁺和其他几种二价阳离子可逆地阻断这种内向电流。阻断效力顺序为Ca²⁺＞Sr²⁺≥Co²⁺＞Mn²⁺≈Cd²⁺＞Ni²⁺≈Mg²⁺。大的内向电流可能由Li⁺、Na⁺、K⁺、Rb⁺和Cs⁺携带，但不由TMA⁺和TEA⁺携带。在10⁸倍的浓度范围内研究了外部Ca²⁺（[Ca²⁺]o）的作用。Na⁺以固定浓度存在。当[Ca²⁺]o从10⁻¹⁰ M逐渐增加到10⁻² M时，内向电流首先减小10倍，在[Ca²⁺]o = 60 μM时达到最小值，然后随着[Ca²⁺]o进一步增加且Ca²⁺本身成为电流载体时再次增加。在[Ca²⁺]o＜10⁻⁵ M时内向电流的阻断可以用单个Ca²⁺与一个位点的结合来描述，在-20 mV时解离常数约为0.7 μM。
