蛙肌膜中的一种非选择性阳离子电导被微摩尔浓度的细胞外钙离子所阻断。
A non-selective cation conductance in frog muscle membrane blocked by micromolar external calcium ions.
作者信息
Almers W, McCleskey E W, Palade P T
出版信息
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。
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