EFFECTS OF TETRODOTOXIN AND TETRAETHYLAMMONIUM CHLORIDE ON INSIDE OF NODAL MEMBRANE OF XENOPUS-LAEVIS

Voltage clamp experiments on single myelinated nerve fibres of Xenopus were done in which tetrodotoxin (TTX) and tetraethylammonium chloride (TEA) were applied to the excitable membrane from outside (by superfusion) or inside (by diffusion through the axoplasm from the cut end of the fibre). The following results were obtained: 1. TTX (10-7 g/ml) in the external fluid selectively blocks the sodium current. 2. Even at a higher concentration TTX appears to have no effect from inside the membrane. 3. 0.3 mM TEA in the external fluid slows down the increase of the potassium current during a depolarizing pulse and reduces its steady state value. 4. TEA applied from the cut end of the fibre to the inside of the nodal membrane also reduces the steady state potassium current but hastens its increase. 5. On the assumption that either side of the membrane is equally sensitive to TEA the diffusion coefficient for TEA ions in the axoplasm is obtained as 0.48×10-5 cm2sec-1 at 18° C. Control experiments gave a diffusion coefficient of 0.65×10-5 cm2sec-1 for sodium ions in the axoplasm. The following conclusions were drawn: The properties of the sodium channel are different on the inner and outer surface of the nodal membrane. This agrees with similar findings on the squid axon membrane. The properties of the potassium channel are similar on either side. In this respect the nodal membrane differs from the squid axon membrane. There is no evidence that TTX or TEA can penetrate the nodal membrane. © 1969 Springer-Verlag.