Mack_1987_Biochem.Pharmacol_36_991

The neurotoxin veratridine is well known for its ability to open sodium channels in neuronal and muscle tissues in micromolar concentrations. It has also been shown that veratridine is an inhibitor of the potent muscarinic receptor antagonist L-quinuclidinyl benzilate (QNB) at these concentrations. These findings prompted us to examine the relationships between action potential sodium channels and muscarinic receptors in a glass-fiber filtration assay for [3H]QNB binding to mouse hindbrain membranes using agents known to affect interconversion of the affinity states in some muscarinic receptor populations, i.e. guanosine triphosphate (GTP) and magnesium (Mg2+). The actions of the sodium channel antagonist tetrodotoxin (TTX) were also examined. Veratridine inhibited [3H]QNB binding with a Ki value of approximately 2.5 microM. This inhibition exhibited a competitive mechanism at higher concentrations (5-10 microM), while showing an apparent non-competitive action at low concentrations (1 microM). Magnesium caused a parallel shift to the right in the inhibition curve with a 32% increase in the veratridine Ki. GTP caused a non-parallel shift to the left with the greatest displacement occurring at lower veratridine concentrations (2-5 microM). The addition of magnesium to GTP did not alter the action of GTP significantly. TTX (5 microM) caused a parallel shift of the veratridine inhibition curve to the right. In addition, TTX alone inhibited the binding of [3H]QNB. Therefore, it appears that there may be more than one binding site for veratridine which may be linked to the muscarinic system and that these may be action potential sodium channels.