Low-threshold, slow-inactivating Na⁺ potentials in the cockroach giant axon

Electrophysiological properties of the ventral giant axon in the abdominal nerve cord of the cockroach were studied by recording intracellular potentials following partial or complete block of the K⁺ conductance. When the K⁺ conductance was completely blocked by tetraethylammonium (TEA) and 3,4-diaminopyridine, the intensity of depolarizing currents necessary for eliciting the action potential was markedly decreased, and the action potential was followed by a prolonged plateau potential. During the plateau potential following the spike, the input resistance was significantly reduced. The plateau potential was not affected by changing the external Ca²⁺ concentration but depended on the external Na⁺ concentration in a manner expected from the Nernst equation and was blocked by tetrodotoxin (TTX). During the plateau potential, the Na⁺ conductance responsible for the spike was inactivated, whereas immediately after the plateau potential a newly evoked spike was not followed by a plateau potential, suggesting different inactivation kinetics between the spike and plateau Na⁺ conductances. When the K⁺ conductance was partially blocked by TEA alone, slow depolarizing responses were evoked at voltage levels a few millivolts more positive than the resting potential. The 'threshold' for the slow potential was much lower than that for the spike potential. The slow potential produced after partial block of the K⁺ conductance was not affected by alterations of the external Ca²⁺ concentration but was blocked by TTX or in a Na⁺-free solution. Even in normal medium, a small TTX-sensitive depolarizing response was discernible. This response was similar in its time course and threshold to the slow potential observed after partial block of the K⁺ conductance. It is concluded that the cockroach giant axon has two populations of Na⁺ channels, which can be distinguished by differences in time course and voltage levels for activation and that the slow, low-threshold Na⁺-dependent potential is largely masked by delayed increases in the K⁺ conductance under normal conditions. It remains uncertain whether the low-threshold slow potential and the plateau potential originate from the same or different populations of Na⁺ channels.