A NON-SELECTIVE CATION CONDUCTANCE IN FROG-MUSCLE MEMBRANE BLOCKED BY MICROMOLAR EXTERNAL CALCIUM-IONS
A NON-SELECTIVE CATION CONDUCTANCE IN FROG-MUSCLE MEMBRANE BLOCKED BY MICROMOLAR EXTERNAL CALCIUM-IONS
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DOI：
10.1113/jphysiol.1984.sp015351
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发表时间：
1984-01-01
影响因子：
5.5
通讯作者：
PALADE, PT
中科院分区：
文献类型：
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作者：
ALMERS, W;MCCLESKEY, EW;PALADE, PT
在电压钳制的乙二醇双(β-氨基乙醚)N，N，N‘’N‘’N‘-四乙酸[EGTA]负载的肌纤维上记录到膜电流，条件是通过普通Na+，K+和Cl-通道的电流被药物阻止或不存在外在离子(K+和Cl-)。在10 mM外[Ca~(2+)]时，用Na~+取代大的有机阳离子四甲基(TMA+)或四乙基铵(TEA+)不能增加峰内向电流。因此，在这些条件下，钙离子通道对Na+没有明显的通透性。当在外加Na+的情况下，外加[Ca~(2+)]还原到1µm以下时，阶跃去极化为负电位产生河豚毒素抗性内向电流。在-20 mV时，它们在150ms内上升到30-200µA/cm2的峰值，然后下降。Ca2+和其他几个二价阳离子可逆地阻断了这一内向电流。阻断能力的大小顺序为：Ca~(2+)、~(2+)、~(2+)。Co2+>；Mn2+>；Mn2+.simeq.Cd2+>；Ni2+.simeq.镁离子。大的内向电流可由Li+、Na+、K+、Rb+和Cs+携带，但不能由TMA+和TEA+携带。在108倍的浓度范围内探索了外部钙离子([Ca~(2+)]o)的影响。Na+以固定浓度存在。当[Ca~(2+)]_o从10~(-10)M逐渐增加到10~(-2)M时，内向电流先减小10倍，在[Ca~(2+)]_o=60微米时达到极小值，然后随着[Ca~(2+)]_o的进一步升高而增加，钙离子本身成为电流载体。在[Ca~(2+)]10~(-5)M处的内向电流阻断可用单个Ca~(2+)与一个部位的结合来描述，在-20 mV处的Kd约为0.7微米。
Membrane currents were recorded from voltage-clamped, ethylene glycol-bis(.beta.-aminoethyl ether) N,N,N''N''-tetraacetic acid [EGTA]-loaded muscle fibers 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 .mu.M 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 .mu.A/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+ > Sr2+ .gtoreq. Co2+ > Mn2+ > Mn2+ .simeq. Cd2+ > Ni2+ .simeq. 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 108-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 mininum at [Ca2+]o = 60 .mu.M and then increased again as [Ca2+]o was increased further and Ca2+ itself became a current carrier. Block of inward current at [Ca2+]o < 10-5 M could be described by binding of a single Ca2+ to a site, with a Kd of the order of 0.7 .mu.M at -20 mV.