Chloramine-T alters the nerve membrane birefringence response

The change in birefringence during depolarizing voltage-clamp pulses of internally perfused squid giant axons are biphasic. There is a rapid decrease in birefringence with a 220-microsec half time at 8 degrees C followed by a slow decrease over the next several milliseconds. After the pulse there is a rapid recovery which is smaller than the initial rapid decrease followed by a slow recovery phase. The rate of change of the slow phase during the pulse is more rapid for larger depolarizations. After the pulse the rate of change is more rapid for more negative potentials. 3.6 mM chloramine-T, applied externally until the sodium currents were prolonged and inactivation was removed, removed the slow phase of the birefringence response both during and after the pulse and made the fast 'off' response as large as the fast 'on' response. Two anesthetics reduced the birefringence response by about 20%. A rocking helix model is presented which relates the birefringence findings and earlier gating current experiments.