Specific blockage of squid axon resting potassium permeability by Haliclona viridis (Porifera: Haliclonidae) toxin (HvTX).

The action of partially purified HvTX, toxin of the marine sponge H. viridis, was explored on the giant axon of the tropical squids Doryteuthis plei and Sepioteuthis sepioidea. HvTX depolarizes the nerves dose dependently. The effect occurs after blocking sodium channels with tetrodoxin (1 microM), removing external Na+, blocking electrically excitable K+ channels with 3,4-diaminopyridine (10 mM) or internal and external application of tetraethylammonium (40 mM). Ouabain (up to 10 mM) does not modify HvTX effect. The action of HvTX occurs only when it is applied to the outer phase of the nerve membrane; microinjection of the toxin into the axons lacks depolarizing effects. HvTX reduces the dependence of membrane potential on external potassium concentration. The apparent 86Rb+ permeability (pi') was measured in axons of S. sepioidea. The value of pi' in normal artificial sea water was 80 (61,96) nm/sec (median and its 95% confidence interval, n = 8) and raised to 1030 (588, 2113) nm/sec (n = 7) when the axons were depolarized to 0 mV raising external K+ to 300 mM. In axons depolarized with HvTX (10 mM external K+) to 0 mV, pi' was 88 (55, 97) nm/sec (n = 8). HvTX could not prevent (P >> 0.05) the increase in pi' induced by 300 mM K+ when the ion concentration was raised before toxin application [pi' = 660 (354, 1876) nm/sec, n = 7]. Most of the 86Rb+ permeability increase in high K+ was prevented if HvTX was added before external K+ was raised [pi' = 298 (264, 337) nm/sec, n = 8]. All the measures of pi' were carried out in solutions containing 1 microM tetrodotoxin, 1 mM 3,4-diaminopyridine and 2 mM ouabain.