The Ca2+-antagonist and binding properties of the phenylalkylamine, anipamil

Abstract

1 Isolated, Langendorff-perfused rat hearts, isolated membranes, and pharmacological and receptor binding techniques were used to study the properties of the newly developed verapamil derivative, anipamil. 2 When added acutely to isolated, spontaneously beating or electrically paced hearts, anipamil (0.01-0.15 μm) exerted a dose-dependent negative inotropic effect which developed slowly and persisted after 60 min washout. 3 When added acutely (0.05-0.1 μm) to isolated hearts, or when given intravenously (2 mg kg⁻¹ body weight 1 h before the animals were killed), anipamil displaced the dose-response curves for the positive inotropic effect of (0.10-3.mm) Ca²⁺ and (10–50 nm) Bay K 8644 to the right. 4 When added to freshly isolated cardiac membranes, 0.1 μm anipamil increased the dissociation constant (K_D) of the phenylalkylamine (—)-[³H]-desmethoxyverapamil ((—)-[³H]-D888) from 1.22 ± 0.2 to 2.91 ± 0.46 nm, without any significant change in density (B_max; control: 163 ± 17; anipamil: 117 ± 20fmol mg⁻¹ protein). Bound (—)-[³H]-D888 was displaceable by (—)-D888 (K_i, 1.7 ± 0.4 nm) > (—)-D600 (K_i 12 ± 0.5 nm) > verapamil (K_i 55 ± 11 nm) > (+)-D600 (K_i 108 ± 12.2) > anipamil (K_i 471 ± 52 nm). 5 In cardiac membranes isolated from rats pretreated with anipamil (2 mg kg⁻¹ i.v.) 1 h before they were killed, the K_D of (—)-[³H]-D888 binding was increased (P < 0.05) from 1.59 ± 0.18 to 3.28 ± 0.65 nm with no significant change in density, compared to the placebo-treated (control) rats. 6 These results establish that anipamil interacts in a competitive manner with the phenylalkylamine binding sites in cardiac membranes, and that it resembles other Ca²⁺ antagonists in displacing the dose-reponse curve for the positive inotropic effect of Ca²⁺ to the right. The results also show that although anipamil binds tightly to the cardiac membranes, it binds to the (—)-[³H]-D888 recognition sites less potently than (—)-D888, (—)-D600 or verapamil.