Temperature dependence of tetrodotoxin effect in squid giant axons.

1. The effects of temperature changes from 30 to 5 degrees C on some properties of giant axons of the squid Doryteuthis plei were studied under voltage-clamp conditions. 2. The maximum inward sodium current of giant axons held at their normal resting potential decreases from 8 to 2 mA/cm2 when the temperature is lowered from 21 to 5 degrees C. 3. The effect of tetrodotoxin on those axons and its steady-state kinetics are markedly temperature dependent. It appears that there are two types of receptors in squid axon membranes, which differ in their temperature dependence. 4. One group of receptors (type 1) increases its affinity for the toxin when the temperature is increased (maximum affinity at 30 degrees C, K1 = 0.03 +/- 0.01 nM). The other group of receptors (type 2) shows an increase in its affinity when the temperature is lowered (maximum affinity at 5 degrees C, K2 = 0.07 +/- 0.02 nM). At 10 degrees C the two populations have the same affinity (K1 = K2 = 0.67 +/- 0.06 nM).