Differences in the properties of Na+ channels in muscle surface and T-tubular membranes revealed by tetrodotoxin derivatives.

The effect of ethylenediamine derivatives of tetrodotoxin (enTTXI and enTTXII) on frog skeletal muscle was studied both electrophysiologically and biochemically. Electrophysiological experiments with one of these molecules (enTTXI) showed that the concentrations needed to block the early phase of the inward sodium current (K0.5 = 7 nM) are much lower than those needed to block the late phase of inward current or muscle contraction (K0.5 = 40 nM). Conversely, tubular Na+ channels are more sensitive to enTTXII than are surface Na+ channels. Toxin binding to isolated muscle membranes was studied using 3H-enTTXI and 3H-enTTXII. The first derivative (3H-enTTXI) has a higher affinity Kd = 8 nM) for Na+ channels in the surface membrane than for Na+ channels in the T-tubular membrane (Kd greater than 20 nM). In contrast 3H-enTTXII has a higher affinity for the tubular Na+ channel (Kd = 0.2 nM) than for the receptor in surface membranes (Kd = 4nM). We conclude that Na+ channels in muscle surface and T-tubular membranes have different toxin-binding properties, which must reflect a difference in molecular structure.