Tetrodotoxin sensitivity of muscle action potentials in pufferfishes and related fishes

Abstract

The effect of tetrodotoxin (TTX) was examined on action potentials in various fishes obtained in the waters of the Great Barrier Reef. The action potentials in muscles of all pufferfishes examined (seven species) were found to be insensitive to 3 × 10−6 M TTX. On the other hand, TTX blocked the action potentials in common hardy-head (Pranesus ogilbyi), slender remora (Echeneis naucrates) and parrotfish (Callyodon pyrrhostethus) at a concentration of 3 × 10−7 M. The dose-response curve fits well the curve predicted by the Langmuir adsorption isotherm. The dissociation constants thus determined in the above mentioned fishes were 3 × 10−8 M, 6 × 10−8 M and 5 × 10−8 M respectively. In contrast to the small dissociation constants found in ordinary fishes, some fishes closely related to pufferfishes have large dissociation constants. They are 3 × 10−7 M in porcupinefish (Diodon holacanthus), 1 × 10−7 M in boxfish (Rhynchostracion nasus) and 5 × 10−7 M in triggerfish (Rhinecanthus aculeatus). We speculate that the resistivity of pufferfish action potentials to TTX is a result of evolutionary change and that the selection may have been done by TTX itself. The effect of saxitoxin (STX) on pufferfish action potentials (Arothron hispidus) was examined and it was found that the action potentials are also resistant to 3 × 10−7 M STX whereas those of common hardy-head (Pranesus ogilbyi) muscle were blocked by 3 × 10−8M. Possible molecular mechanisms of the TTX resistivity are discussed.