The cation distribution set by surface charges explains a paradoxical membrane excitability behavior.

It is widely accepted that divalent cations can affect membrane excitability by interacting with negative charges on the membrane surface. This effect is generally supposed to arise from a modulation of the surface charge electrostatic field within the membrane. As an alternative mechanism, we propose that this effect may also be due to a generalized coupling between ion currents mediated by ionic composition changes at the membrane surface. To test this hypothesis we have computed the transmembrane potential using ionic current relations in which bulk external Ca2+ and K+ activities are substituted with superficial activities deduced from a Grahame-Langmuir isotherm. The model behavior agrees well with published results on Paramecium electrophysiology and furthermore explains 2 paradoxical observations on the cell membrane excitability upon changes of external Ca2+ and K+.