On the Persistent Sodium Current in Squid Giant Axons

R. F. Rakowski, D. C. Gadsby, and P. DeWeer have reported a persistent, tetrodotoxin-sensitive sodium ion current (I NaP) in squid giant axons having a low threshold (-90 mV) and a maximal inward amplitude of −4 μA/cm2 at −50 mV. This report makes the case that most of I NaP is attributable to an ion channel mechanism distinct from the classical rapidly activating and inactivating sodium ion current,I Na, which is also tetrodotoxin sensitive. The analysis of the contribution ofI Na toI NaP is critically dependent on slow inactivation of I Na. The results of this gating process reported here demonstrate that inactivation ofI Na is complete in the steady-state for V > −40 mV, thereby making it unlikely thatI NaP in this potential range is attributable to I Na. Moreover, −90 mV is well below I Na threshold, as demonstrated by the C. A. Vandenberg and F. Bezanilla model ofI Na gating in squid giant axons. Their model predicts a persistent current having a threshold of −60 mV and a peak amplitude of −25 μA/cm2...