Characterization of two Bunodosoma granulifera toxins active on cardiac sodium channels

1 Two sodium channel toxins, BgII and BgIII, have been isolated and purifie d from the sea anemone Bunodosoma granulifera. Combining different techniqu es, we have investigated the electrophysiological properties of these toxin s. 2 We examined the effect of BgII and BgIII oil rat ventricular strips. Thes e toxins prolong action potentials with EC50 values of 60 and 660 nm and mo dify the resting potentials. 3 The effect on Na+ currents in rat cardiomyocytes was studied using the pa tch-clamp technique. BgII and BgIII slow the rapid inactivation process and increase the current density with EC50 values of 58 and 78 nm, respectivel y. 4 On the cloned hH1 cardiac Na+ channel expressed in Xenopus laevis oocytes , BgII and BgIII slow the inactivation process of Na+ currents (respective EC50 values of 0.38 and 7.8 mum), shift the steady-state activation and ina ctivation parameters to more positive potentials and the reversal potential to more negative potentials. 5 The amino acid sequences of these toxins are almost identical except for an asparagine at position 16 in BgII which is replaced by an aspartic acid in BgIII. In all experiments, BgII was more potent than BgIII suggesting th at this conservative residue is important for the toxicity of sea anemone t oxins. 6 We conclude that BgII and BgIII, generally known as neurotoxins, are also cardiotoxic and combine the classical effects of sea anemone Na+ channels toxins (slowing of inactivation kinetics, shift of steady-state activation and inactivation parameters) with a striking decrease on the ionic selectiv ity of Na+ channels.