Zimmerberg J, Bezanilla F, Parsegian V A
Laboratory of Biochemistry and Metabolism, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
Biophys J. 1990 May;57(5):1049-64. doi: 10.1016/S0006-3495(90)82623-0.
We have applied solutions with varying osmotic pressures symmetrically to the inside and outside of perfused, TTX-treated, giant axons. The potassium conductance G decreased with increasing osmotic stress, but there was no effect on either the shape or the position of the voltage-current curve. One must distinguish three possible actions of the osmotic agent: osmotic stress, channel blocking, and lowered solution conductivity. To do so, we compared results obtained working with pairs of internal and external solutions of either (a) equal osmotic stress, (b) equal conductivity, or (c) the same blocking agent. There was the same change in G irrespective of the type of stressing species (sorbitol or sucrose); this provides some evidence against a blocking mechanism. The conductivity of the external solution had a small effect on K currents; internal solution conductivity had none. A change in series resistance of the Schwann cell layer could account for the small effect of external solution conductivity. The primary cause of G depression appears, then, to be the applied osmotic stress. Using this result, we have developed models in which the channel has a transition between closed states under voltage control but osmotically insensitive and a closed/open step that is voltage-independent but osmotically sensitive. We have assumed that the conductance of this open state does not change with osmotic stress. In this way, we estimate that an additional 1,350 +/- 200 A3 or 40-50 molecules of solute-inaccessible water appear to associate with the average delayed rectifier potassium channel of the squid axon when it opens.
我们已将具有不同渗透压的溶液对称地施加到经灌注、用河豚毒素处理的巨型轴突的内部和外部。钾电导G随着渗透压应激的增加而降低，但对电压-电流曲线的形状或位置均无影响。必须区分渗透剂的三种可能作用：渗透压应激、通道阻断和溶液电导率降低。为此，我们比较了使用以下两种情况的内部和外部溶液对所获得的结果：(a) 等渗透压应激，(b) 等电导率，或 (c) 相同的阻断剂。无论应激物质的类型（山梨醇或蔗糖）如何，G的变化都是相同的；这提供了一些反对阻断机制的证据。外部溶液的电导率对钾电流有较小影响；内部溶液电导率则无影响。施万细胞层串联电阻的变化可以解释外部溶液电导率的较小影响。那么，G降低的主要原因似乎是所施加的渗透压应激。利用这一结果，我们建立了模型，其中通道在电压控制下处于对渗透不敏感的关闭状态和与电压无关但对渗透敏感的关闭/开放步骤之间存在转变。我们假设这种开放状态的电导不会随渗透压应激而变化。通过这种方式，我们估计当鱿鱼轴突的平均延迟整流钾通道打开时，似乎有额外的1350±200 ų或40 - 50个溶质不可及水分子与该通道相关联。
