High-affinity binding sites for [3H]saxitoxin are associated with voltage-dependent sodium channels in portal vein smooth muscle.

Saturable, high-affinity binding sites for [3H]saxitoxin were identified in equine portal vein smooth muscle membranes. These sites had a dissociation constant of 0.29 nM and a maximal binding capacity of 115 fmol.mg-1 of protein. A similar dissociation constant was obtained with cells prepared from rat portal vein. Specific binding of [3H]saxitoxin was completely displaced by unlabelled saxitoxin and tetrodotoxin, with inhibition constants of 0.42 and 2.10 nM, respectively. Tetrodotoxin blocked the fast Na+ current in single cells of rat portal vein in a concentration-dependent manner, with an IC50 of 3.15 nM. These results suggest that the high-affinity binding sites for tetrodotoxin and saxitoxin may be associated with voltage-dependent Na+ channels in vascular myocytes.