The initial model of the electrical properties of the lens predicted the distribution of induced voltages when a current was injected into a central cell of a spherical syncytial tissue.
13 The model was based on the structure of the lens and was used to determine the membrane conductances of fiber and surface cells and to determine the effective resistances of intracellular and extracellular pathways.
4 Our impedance data suggest that most of the Na
+ leak conductance of the lens is associated with fiber cell membranes,
14 whereas others have localized Na,K-ATPase activity to the epithelium at the lens surface.
4 This spatial segregation of Na
+ influx (inner cells) from Na
+ efflux (surface cells) suggests that there is a circulation of Na
+. However, in our initial attempts to use the model to calculate the magnitude of this circulating Na
+ current, we naively neglected water flow. In the absence of water flow, the model predicted that voltage and ion gradients would develop in the intracellular and extracellular spaces that actually opposed the circulation. This result was contrary to experimental measurement of both surface current flows
12 and Na,K-ATPase activity, which suggests that a large Na
+ leak into the lens must exist to account for the large pump current. Even more bothersome was the prediction that large transmembrane osmotic gradients would develop, making it impossible to neglect water flow unless membrane water permeability was zero. Because it is clearly not zero,
15 it is now apparent that water flow cannot be neglected in modeling lens current flows.