Allosteric interaction of trimebutine maleate with dihydropyridine binding sites

Abstract The effects of trimebutine maleate on [ 3 H]nitrendipine binding to guinea-pig ileal smooth muscle membranes and Ca 2+ -induced contraction of the taenia cecam were studied. Specific binding of [ 3 H]nitrendipine to smooth muscle membranes was saturable, with a K D value and maximum number of binding sites (B max ) of 0.16 nM and 1070 fmol/mg protein, respectively. Trimebutine inhibited [ 3 H]nitrendipine binding in a concentration-dependent manner with a K i value of 9.3 μM. In the presence of trimebutine (10 μM). Scatchard analysis indicated a competitive-like inhibition with a decrease in the binding affinity (0.31 nM) without a chane in B max (1059 fmol/mg protein). However, a dissociation experiment using trimebutine (10 or 100 μM) showed that the decreased affinity was due to an increase of the dissociation rate constant of [ 3 H]nitrendipine binding to the membrane. In mechanical experiments using the taenia cecum, trimebutine (3–30 μM) caused a parallel rightward shift of the dose-response curve for the contractile response to a higher concentration range of Ca 2+ under high-K + conditions in a noncompetitive manner. These results suggest that trimebutine has negative allosteric interactions with 1,4-dihydropyridine binding sites on voltage-dependent Ca 2+ channels and antagonizes Ca 2+ influx, consequently inhibiting contractions of intestinal smooth muscle.