  In this work, we examine effects of large permanent charges on ionic flow
through ion channels based on a quasi-one dimensional Poisson-Nernst-Planck
model. It turns out large positive permanent charges inhibit the flux of cation
as expected, but strikingly, as the transmembrane electrochemical potential for
anion increases in a particular way, the flux of anion decreases. The latter
phenomenon was observed experimentally but the cause seemed to be unclear. The
mechanisms for these phenomena are examined with the help of the profiles of
the ionic concentrations, electric fields and electrochemical potentials. The
underlying reasons for the near zero flux of cation and for the decreasing flux
of anion are shown to be different over different regions of the permanent
charge. Our model is oversimplified. More structural detail and more
correlations between ions can and should be included. But the basic finding
seems striking and important and deserving of further investigation.
