Identification and kinetic properties of the current through a single Na+ channel

The kinetic properties of a single Na+ channel in the tunicate egg cell membrane were studied by the patch-recording technique. A conventional micropipette filled with a solution of 600 mM NaCl and 1.5 mM MnCl2 was used as the patch electrode. The seal resistance between patch electrode and egg surface was more than 1000 M omega. In the patch recording, the current fluctuations at a given membrane potential consisted of pulse-like events of a uniform amplitude. The amplitude was 1 pA at -56 mV and decreased as the membrane potential was made more positive. It is suggested that the fluctuation in the patch current is the current through a single Na+ channel for the following reasons. First, the reversal potential of the pulse-like fluctuation in the patch current was approximately equal to that of the total membrane Na+ current in a solution equivalent to that in the patch electrode. Second, the charges transferred by the patch current and by the total membrane Na+ current during a 170-msec command pulse showed parallel dependence on membrane potential. Third, the kinetic properties of the pulse-like fluctuations in the patch current were analyzed according to the Hodgkin-Huxley model, and it was shown that the time sequences of the pulse-like events were compatible with those for a single Na+ channel.