Korenbrot J I, Brown D T, Cone R A
J Cell Biol. 1973 Feb;56(2):389-98. doi: 10.1083/jcb.56.2.389.
Freshly isolated frog rod outer segments are sensitive osmometers which retain their photosensitivity; their osmotic behavior reveals essentially the same light-sensitive Na(+) influx observed electrophysiologically in the intact receptor cell. Using appropriate osmotic conditions we have examined freeze-etch replicas of freshly isolated outer segments to identify the membrane which regulates the flow of water and ions. Under isosmotic conditions we find that the disc to disc repeat distance is almost exactly twice the thickness of a disc. This ratio appears to be the same in a variety of vertebrate rod outer segments and can be reliably measured in freeze-etch images. Under all our osmotic conditions the discs appear nearly collapsed. However, when the length of the outer segment is reduced by hyperosmotic shocks the discs move closer together. This markedly reduces the ratio of repeat distance to disc thickness since disc thickness remains essentially constant. Thus, the length reduction of isolated outer segments after hyperosmotic shocks primarily results from reduction of the extradisc volume. Since the discs are free floating and since they undergo negligibly small changes in volume, the plasma membrane alone must be primarily responsible for regulating the water flux and the light-sensitive Na(+) influx in freshly isolated outer segments. On this basis we calculate, from the osmotic behavior, that the plasma membrane of frog rod outer segment has a Na(+) permeability constant of about 2.8 x 10(-6) cm/s and an osmotic permeability coefficient of greater than 2 x 10(-3) cm/s.
刚分离出的青蛙视杆细胞外段是敏感的渗透计，能保持其光敏性；其渗透行为揭示了在完整受体细胞中通过电生理学观察到的基本相同的光敏性钠离子内流。利用适当的渗透条件，我们检查了刚分离出的外段的冷冻蚀刻复制品，以确定调节水和离子流动的膜。在等渗条件下，我们发现盘与盘之间的重复距离几乎正好是一个盘厚度的两倍。在各种脊椎动物视杆细胞外段中，这个比例似乎是相同的，并且可以在冷冻蚀刻图像中可靠地测量。在我们所有的渗透条件下，这些盘看起来几乎是塌陷的。然而，当外段的长度因高渗冲击而减小时，这些盘会靠得更近。由于盘的厚度基本保持不变，这显著降低了重复距离与盘厚度的比例。因此，高渗冲击后分离出的外段长度减小主要是由于盘外体积的减小。由于这些盘是自由漂浮的，并且它们的体积变化可以忽略不计，所以仅质膜必须主要负责调节刚分离出的外段中的水通量和光敏性钠离子内流。基于此，我们从渗透行为计算得出，青蛙视杆细胞外段的质膜的钠离子渗透常数约为2.8×10⁻⁶厘米/秒，渗透渗透系数大于2×10⁻³厘米/秒。
