Sodium current inhibition by internal calcium: a combination of open-channel block and surface charge screening?

Internal application of millimolar concentrations of calcium to batrachotoxin (BTX)-activated rat skeletal muscle sodium channels, bathed symmetrically in 200 mM NaCl, causes a reduction in apparent single-channel amplitude without visibly increasing noise at a bandwidth of 50 Hz. A greater calcium-induced reduction occurred upon removal of external sodium ions. Internal calcium acted similarly in high ionic strength solutions (3M NaCl), where surface charges are effectively screened, suggesting that calcium acts, in part, by binding within the pore and occluding the conducting pathway. In low ionic strength solutions (20 mM NaCl), internal addition of N-Methyl-Glucamine (NMG) ions decreased the single channel amplitude consistent with screening of negative surface charges. An accurate description of the dose dependence of calcium inhibition, using either a simple blocking model, or rate theory calculations of ion permeation and block, also required surface charge screening. Hence, our data support the view that sodium current inhibition by internal calcium arises from a combination of both open-channel block and surface charge effects.