Veratridine Modification of the Sodium Channel a-Polypeptide from Eel Electroplax Purified

In the interest of continuing structure-function studies, highly puri-fied sodium channel preparations from the eel electroplax were incorporated into planar lipid bilayers in the presence of veratridine. This lipoglycoprotein originates from muscle-derived tissue and consists of a single polypeptide. In this study it is shown to have properties analogous to sodium channels from another muscle tissue (Garber, S. S., and C. Miller. 1987. J0urna / of General Physiology. 89:459-480), which have an additional protein subunit. However, significant qualitative and quantitative differences were noted. Comparison of veratridine-modified with batrachotoxin-modified eel sodium channels revealed common properties. Tetrodotoxin blocked the channels in a voltage-dependent manner indistinguishable from that found for batrachotoxin-modified channels. Veratridine-modified channels exhibited a range of single-channel conductance and subconductance states. The selectivity of the veratridine-modified sodium channels for sodium vs. potassium ranged from 6-8 in reversal potential measurements, while conductance ratios ranged from 12-15. This is similar to BTX-modified eel channels, though the latter show a predominant single-channel conductance twice as large. In contrast to batrachotoxin-modified channels, the fractional open times of these channels had a shallow voltage dependence which, however, was similar to that of the slow interaction between veratridine and sodium channels in voltage-clamped biological membranes. Implications for sodium channel structure are discussed.