Interactions of D600 (methoxyverapamil) and local anesthetics with rat brain α-adrenergic and muscarinic receptors

D600 (methoxyverapamil) was found to inhibit the specific binding assayed in rat brain homogenates of the antagonist agents ³H]WB 4101 and ³H]QNB to the α-adrenergic and muscarinic receptors respectively. The IC₅₀ concentrations of D600 in standard binding experiments were 1.7 × 10_?6 M and 1.4 × 10 _?5M, with calculated k₁ values of 0.98 × 10_?6 M and 8.83 × 10_?6M.Scatchard analyses showed these inhibitions to be competitive. Lidocaine and tetracaine also inhibited radioligand binding to these receptors, with K₁ values of 5.25 × 10^?4M and 4.85 × 10^?5 M for the α-receptor and 8.2 × 10^t-5 M and 6.94 × 10^?6 M for the muscarinic receptor; these inhibitions also appeared to be competitive. Increasing the Ca²⁺ concentration in the assays to 10 mM did not influence the effects of D600 or the anesthetics. Analyses of inhibitions of muscarinic receptor binding produced by D600 and lidocaine over a range of pH indicated that the inhibitory species of D600 is the uncharged form, whereas the charged form of lidocaine is inhibitory. Interactions of D600 and lidocaine with the agonist site on the muscarinic receptor were studied by measuring the effects of these agents on the displacement of ³H]QNB by the muscarinic agonist carbachol. Comparison of these results with a theoretical model indicates that carbachol, ³H]QNB, and D600 or lidocaine competitively displace one another at the same agonist site. The binding of labeled naloxone to the opiate receptor was also inhibited by D600, the IC₅₀g being 4 × 10^t?6 M. These inhibitory effects of D600 and the local anesthetics on different receptors suggest that these agents may act by a common mechanism, namely by perturbing membrane structures. These results suggest caution in interpreting experiments in which D600 and verapamil are used analytically as Ca antagonists to assess the involvement of Ca in a biological system.