Sodium flux ratio in voltage-clamped squid giant axons.

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Abstract
The Na flux ratio across the axolemma of internally perfused, voltage-clamped giant axons of L. pealei was measured at various membrane potentials. The flux ratio exponent obtained from these measurements was about unity and independent of membrane voltage over the 50 mV range from about -20 to +30 mV. These results, combined with previous measurements of ion permeation through Na channels, show that the Na channel behaves like a multi-ion pore with 2 ion binding sites that are rarely simultaneously occupied by Na.