Characterization of the effects of AHN 086, an irreversible ligand of "peripheral" benzodiazepine receptors, on contraction in guinea-pig atria and ileal longitudinal smooth muscle

The effects of AHN 086 and its reversibly acting structural analogue Ro 5-4864 were studied in the spontaneously beating guinea-pig atria and field-stimulated guinea-pig ileal longitudinal smooth muscle in the presence and absence of dihydropyridine calcium channel modulators. The treatment of guinea-pig atria with AHN 086 followed by extensive washing did not alter contraction. However, AHN 086 (0.5 μM) potentiated (88%) the positive inotropic responses by BAY K 8644, an effect that was not reversed by extensive washing of the tissue. Higher concentrations of AHN 086 (> 2 μM) irreversibly inhibited the intropic, but not the chronotropic responses to BAY K 8644, nifedipine, and isoproterenol. Ro 5-4864 (10 μM) produced a reversible enhancement of the inotropic responses and block of the chronotropic responses to BAY K 8644. In guinea-pig ileal longitudinal smooth muscle, both AHN 086 and Ro 5-4864 reversibly inhibited field-stimulated contractions. Neither Ro 5-4864 nor AHN 086 affected the ability of nifedipine to inhibit field-stimulated contractions of ileal longitudinal smooth muscle. Treatment of intact atria with 5 μM AHN 086 followed by extensive washing resulted in a significant inhibition (30–50%) of [3H]Ro 5-4864 binding to peripheral benzodiazepine receptors and of [3H]nitrendipine binding to voltage-operated calcium channels, but did not affect [3H]dihydroalprenolol binding to β-adrenergic receptors on atrial membranes. The same treatment applied to intact ileal longitudinal smooth muscle affected neither [3H] (−)-quinuclidinyl benzilate binding to muscarine receptors nor [3H]nitrendipine binding, but did result in a significant inhibition (30–50%) of [3H]Ro 5-4864 binding to ileal longitudinal smooth muscle membranes. The pharmacology of AHN 086 suggests that there is a different relationship between peripheral benzodiazepine receptors and voltage-operated calcium channels in guinea-pig atria and ileal longitudinal smooth muscle.Key words: calcium channels, peripheral benzodiazepine receptors, dihydropyridines, benzodiazepines, dihydropyridine binding sites.