A comparison of the μ-conotoxins by [ 3H]saxitoxin binding assays in neuronal and skeletal muscle sodium channel

Abstract

Sodium channels from rat brain, rat skeletal muscle, chick brain, and eel electroplax were compared by using the μ-conotoxins GIIIA, PIIIA, and StIII and [ 3H]saxitoxin. Rat skeletal muscle and eel electroplax sodium channels are equally sensitive to GIIIA, PIIIA, and StIII, displacing >90% of the [ 3H]saxitoxin binding sites in rat skeletal muscle and eel electroplax membranes and exhibiting inhibitory concentrations at half-maximal percentage specific binding (IC50) of 0.97 nM for GIIIA, 1.3 nM for PIIIA in rat skeletal muscle, and concentrations of 3.5 nM for GIIIA and 2.8 nM for PIIIA in eel electroplax. PIIIA and GIIIA at all concentrations inhibit only up to 10% of [ 3H]saxitoxin binding sites in chick brain membranes. μ-Conotoxin StIII at all concentrations inhibits only up to 10% of [ 3H]saxitoxin binding sites in rat brain membranes and displays two-site binding inhibition of the [ 3H]saxitoxin binding sites in rat skeletal muscle. PIIIA displaces >60% of the [ 3H]saxitoxin binding sites (IC50 of 44 nM) while GIIIA blocks out only 30% of the binding sites in rat brain sodium channels (IC50 of 69 nM). Thus, sodium channel subtypes can be classified into two categories: μ-conotoxin sensitive (i.e., subtypes predominantly found in rat skeletal muscle and eel electroplax) and insensitive (i.e., subtypes predominantly found in rat and chick brain).