Binding of [3H]saxitoxin to the voltage-dependent Na channels and inhibition of 22Na influx in bovine adrenal medullary cells.

The binding characteristics of [3H]saxitoxin and its binding site were examined in bovine adrenal medullary cells. These cells showed a specific binding of [3H]saxitoxin which was saturable and reversible. Scatchard analysis showed a single class of high-affinity binding sites with an equilibrium dissociation constant of 5.8 nM and a maximum binding capacity of 427.2 fmoles/10(7) cells (124.2 fmoles/mg of cell protein). A Hill plot revealed that there were no co-operative interactions among the binding sites. Unlabeled saxitoxin inhibited the specific binding of [3H]saxitoxin as well as veratridine-induced 22Na influx with a similar potency as did tetrodotoxin. However, veratridine, aconitine and scorpion venom, at concentrations that increased 22Na influx, did not inhibit [3H]saxitoxin binding. These results indicate that saxitoxin binds to a specific site on voltage-dependent Na channels and inhibits the influx of 22Na. [3H]Saxitoxin would be useful for the detailed analysis of voltage-dependent Na channels in adrenal medullary cells.