Ion concentrations, fluxes and electrical properties of the embryonic chicken lens

The membrane properties of embryonic chicken lenses were characterized using isotopic and electrical techniques. The lenses had a relatively high water content (80%) and large extracellular space (12·5%). Isotopic uptake measurements indicated that the lens cytoplasm contained 118 mm K⁺ and 26 mm Cl^-. A value for intracellular Na⁺ of 14 mm was obtained using Na⁺-sensitive microelectrodes. A doubleexponential model was used to fit the efflux of ⁸⁶Rb⁺, ²²Na⁺, ³⁶Cl^- and [³H]mannitol (an extracellular space marker) from the lens. When perfused with artificial aqueous humor (AAH) solution, embryonic lenses exhibited membrane potentials of between -20 and -40 mV. The more negative values were generally observed in lenses from older embryos. A ouabain-sensitive component, contributing -7 mV to the membrane potential, was also identified. The relatively depolarized membrane potentials suggested that the lens membranes were only weakly selective for K⁺ over Na⁺. To test this further, lenses were perfused with AAH containing varying concentrations of K⁺. The resulting changes in potential were interpreted in terms of the Goldman model. The best fit of the Goldman potential equation indicated that, in the presence of ouabain, the chicken lens membranes had a relative permeability to K⁺, Na⁺ and Cl^- of 1·0, 0·36, 0·51 respectively. Replacing most or all of the Na⁺ in the AAH caused only a small change in the membrane potential rather than the large hyperpolarization towards the K⁺ equilibrium potential predicted by the Goldman model. Including the K⁺ ionophore valinomycin in the low Na⁺-AAH solutions caused a large increase in ⁸⁶Rb⁺ efflux but did not result in additional hyperpolarization. This suggested that the insensitivity of the membrane potential to reduced extracellular Na⁺ was not due to voltage or pH inactivation of lens K⁺ channels.