Internal cesium and the sodium inactivation gate in Myxicola giant axons

Internal cesium (CSi), relative to internal potassium (Ki), alters Na current (INa) time course in internally perfused Myxicola giant axons. CSi slows the time to peak INa, slows its decline from peak and increases the steady state to peak current ratio, INainfinity/INapeak. Neither activation nor deactivation kinetics are appreciably affected by CSi. Na current rising phases, times to half maximum and tail current time courses are similar in CSi and Ki. Inactivation time constants determined by both one (tau h) and two (tau c) pulses are also little changed by CSi. The CSi effects are due largely or entirely to an increased INainfinity/INapeak. CSi decreases the steady level of inactivation reached during a step in potential, preventing some fraction of inactivation gates from closing at all, the rest apparently closing normally. Inactivation block in CSi decreases with increasing inward current magnitude and in Ki inactivation block is appreciable only for outward Na channel current, suggesting the site of action is located somewhere in the current pathway. If this site mediates the normal operation of the inactivation gate, then a possible mechanism for gate closure could involve a positively charged structure moving to associate with a negative site near or into the inner channel mouth.