Metabolic component in the epithelial intracellular potential of rabbit cornea

Electrical signaling in control of ocular cell behaviors

Epithelia of the cornea, lens and retina contain a vast array of ion channels and pumps. Together they produce a polarized flow of ions in and out of cells, as well as across the epithelia. These naturally occurring ion fluxes are essential to the hydration and metabolism of the ocular tissues, especially for the avascular cornea and lens. The directional transport of ions generates electric fields and currents in those tissues. Applied electric fields affect migration, division and proliferation of ocular cells which are important in homeostasis and healing of the ocular tissues. Abnormalities in any of those aspects may underlie many ocular diseases, for example chronic corneal ulcers, posterior capsule opacity after cataract surgery, and retinopathies. Electric field-inducing cellular responses, termed electrical signaling here, therefore may be an unexpected yet powerful mechanism in regulating ocular cell behavior. Both endogenous electric fields and applied electric fields could be exploited to regulate ocular cells. We aim to briefly describe the physiology of the naturally occurring electrical activities in the corneal, lens, and retinal epithelia, to provide experimental evidence of the effects of electric fields on ocular cell behaviors, and to suggest possible clinical implications.

Origins of the transient anterior-posterior asymmetry in the frog lens fiber potential

The origin of the transient asymmetry of intracellular resting potentials between the anterior and posterior lens fibers was investigated in the isolated American bullfrog lens by a conventional microelectrode technique. In high K+, Rb+, Cs+, or NH+4 test solution applied only to the lens anterior or posterior side, anterior fibers depolarized at a slower rate than posterior ones. After a long exposure, however, the transient potential difference disappeared. The magnitude of the depolarizations of the lens fibers was in the order of K+ greater than Rb+ greater than Cs+ greater than NH+4. The resting potentials plotted as a function of external K+ concentrations ([K]0) were in agreement with Nernst equation predictions with a slope of 58 mV/decade ion concentration change. A small Na+ permeability is unmasked at a [K]0 less than 10 mM. It was concluded that the transient difference measured in potentials of anterior and posterior lens fibers on increasing external K+, Rb+, Cs+ or NH+4 depends on the anterior epithelial cell layer, which is a diffusional barrier for ions penetrating into the lens interior.

Rheogenic sodium transport in a tight epithelium, the amphibian skin

1. Intracellular potentials from frog and toad skins were measured to identify rheogenic components of active Na transport across the basolateral membrane. Transcellular current flow and associated R . I-drops were blocked with amiloride or Na-free mucosal solution. 2. The potential difference across the basolateral membrane was found to be hyperpolarized by 18 . 5 +/- 1 . 6 mV above the steady-state value immediately after blockage of apical membrane Na conductance. The hyperpolarization disappeared within 15--25 min. 3. The final steady-state value of 93 . 1 +/- 2 . 5 mV was slightly less than reasonable estimates of the K equilibrium potential. 4. The hyperpolarization could not be observed 3--5 min after addition of ouabain (10(-4) M). 5. Both the magnitude and duration of the hyperpolarization correlate directly with the amount of Na accumulated in the intracellular space. 6. A fraction of the intracellular potential was missing when Na transport was re-established after long term blockage of apical membrane Na entry. It reappeared within 10--20 min. 7. It is suggested that the hyperpolarization is due to rheogenic Na transport across the basolateral membranes. This transport mechanism may contribute some 30--50% of the electrical gradient for passive Na entry across the mucosal membrane. 8. A coupling ratio between pumped fluxes of Na and K of about 2:1 is calculated from the data.