The flux ratio of the Na-Cl cotransport mechanism in the frog corneal epithelium.

The stoichiometry of the Na-Cl cotransport mechanism at the basolateral membrane of the frog corneal epithelium was studied with the aid of intracellular microelectrodes by analyzing the effects of ouabain on intracellular recordings (PDSC) and transepithelial short-circuit current (ISC). Under short-circuit conditions, the total current across the basolateral membrane was equated to the Cl- transport across the apical membrane. Ouabain produces a quick and steep decrease of PDSC (7.1 mV) and ISC (3.0 microA/cm2) that was ascribed to a sudden inhibition of the electrogenic component of the Na-K pump. From these effects, and from results in previous studies, the maximum value for the Na+ flux via the cotransport mechanism (IcNa) was estimated and compared to the apical Cl- transport (a minimum estimate of the Cl- flux cotransported with Na+c). In all experiments, ICl was larger than IcNa. Considering that Na+ also recirculates across the basolateral membrane via a passive pathway, the Cl:Na flux ratio of the cotransport system may be as high as 4.