Do local anaesthetics interact with dihydropyridine binding sites on neuronal L-type Ca2+ channels?

We have examined the interaction of procaine, prilocaine, lignocaine,bupivacaine, amylocaine and R(+) and S(-) ropivacaine with L-typevoltage-sensitive Ca2+ channels in rat cerebrocortical membranes. Membraneswere prepared in Tris HCl 50 mmol litre-1, pH 7.4, by homogenization andcentrifugation. Binding assays were performed in 1- ml volumes of Tris HCl50 mmol litre-1, pH 7.4, for 90 min at room temperature using approximately200 micrograms of protein. Non-specific binding was defined in the presenceof nifedipine 10(-5) mol litre-1, and bound and free radioactivity wereseparated by vacuum filtration. The effects of local anaesthetics weredetermined by displacement of [3H]PN200-110 (approximately 0.2 nmollitre-1), a radiolabelled 1,4- dihydropyridine (DHP) L-channel antagonist.The concentration of displacer producing 50% displacement was corrected forthe competing mass of [3H]PN200-110 to yield the affinity constant, K50.All local anaesthetics displaced [3H]PN200-110 in a dose-dependent mannerwith a rank order potency of (K50, mmol litre-1) bupivacaine (0.48),amylocaine (0.74), lignocaine (1.09), prilocaine (2.06) and procaine(2.09). Ropivacaine enantiomers did not show stereo-selective displacement,with K50 values of 0.99 and 0.92 mmol litre-1 for R(+) and S(-)ropivacaine, respectively. There was a significant correlation between pK50and p (octanol:buffer partition coefficient) (r2 = 0.872, P = 0.020), pK50and p (local anaesthetic potency) (r2 = 0.816, P = 0.036), pK50 and p(relative conduction blocking potency) (r2 = 0.843, P = 0.028) and betweenpK50 and p (IC50 for inhibition of cardiac output) (r2 = 0.897, P = 0.015).These data suggest that DHP binding sites may be involved in both themechanism of local anaesthesia and the cardiotoxicity of these agents.