THE CATION DISTRIBUTION SET BY SURFACE-CH ARGES EXPLAINS A PARADOXICAL MEMBRANE EXCITABILITY BEHAVIOR

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