A new sea urchin toxin and its effect on spontaneous transmitter release at frog neuromuscular junctions

The authors extracted a dialyzable substance from sea urchin (Diadematidae) collected in Fiji, and investigated its effect on the frequency of miniature endplate potentials (MEPPs) in the sartorius muscle of the frog. This substance was shown to be different from previously known biologically active substances in many respects. The increase of MEPP frequency by the application of toxin was significant and dose-dependent. The frequency began increasing immediately after application and returned to normal after washing. This increasing action of toxin on MEPP frequency was strengthened by increased calcium concentration, but even in Ca-depleted solution toxin action was significant. Toxin action in Ca-free Mg2+ solution was found to be just as remarkable as in Ca2+ solution, and was dependent on magnesium concentration. Toxin action was also found to depend significantly on sodium concentration. The toxin did not change membrane potential of the muscle appreciably. From these results, the depolarization of the nerve terminal was not considered to be the main effect of toxin on MEPP frequency. The increasing action of toxin on MEPP frequency was interpreted as being the result of increased permeability of the nerve terminal to both divalent cations and Na+.