Allosteric Stimulation of Voltage-Dependent Sodium Channels by Brevetoxin in Cultured Bovine Adrenal Medullary Cells

The effects of Ptychodiscus brevis toxin (PbTx-3) on 22Na influx, 45Ca influx and catecholamine secretion were examined in cultured bovine adrenal medullary cells and compared with the effects of classical neurotoxins. PbTx-3 alone had no effects, but greatly enhanced veratridine (30 microM)-induced Na influx, Ca influx and secretion, with a EC50 of 30, 25 and 23 nM, respectively. PbTx-3 (1 microM) reduced EC50 values of veratridine approximately 3-fold and increased the maximal responses caused by saturating concentration (300 microM) of veratridine approximately 1.3 fold. alpha- and beta-Scorpion venom shifted the concentration-response curves of veratridine to the left without altering maximal responses. PbTx-3 in combination with either alpha- or beta-scorpion venom showed only additive effects on Na influx, but augmented veratridine (30 microM)-induced Na influx to a greater extent than PbTx-3, alpha- or beta-scorpion venom alone. Na influx due to these toxins was abolished by 1 microM saxitoxin. Our results suggest that Na channels in adrenal medullary cells have neurotoxin receptors for brevetoxin that allosterically stimulate Na influx initiated by veratridine, leading to increased Ca influx and catecholamine secretion. Allosteric interactions do not exist between brevetoxin and alpha-scorpion venom, or between brevetoxin and beta-scorpion venom, but once Na channels are gated by veratridine, these toxins cooperatively augment Na influx.