Comparison of tetrodotoxin and procaine in internally perfused squid giant axons

Squid giant axons were internally perfused with tetrodotoxin and procaine, and excitability and electrical properties were studied by means of current-clamp and sucrose-gap voltage-clamp methods. Internally perfused tetrodotoxin was virtually without effect on the resting potential, the action potential, the early transient membrane ionic current, and the late steady-state membrane ionic current even at very high concentrations (1,000–10,000 nM) for a long period of time (up to 36 min). Externally applied tetrodotoxin at a concentration of 100 nM blocked the action potential and the early transient current in 2–3 min. Internally perfused procaine at concentrations of 1–10 mM reversibly depressed or blocked the action potential with an accompanying hyperpolarization of 2–4 mv, and inhibited both the early transient and late steady-state currents to the same extent. The time to peak early transient current was increased. The present results and the insolubility of tetrodotoxin in lipids have led to the conclusion that the gate controlling the flow of sodium ions through channels is located on the outer surface of the nerve membrane.