Inhibition of [ 3 H]‐U69593 binding and the cardiac effects of U50,488H by calcium channel blockers in the rat heart

The calcium channel blockers (CCBs), nifedipine, nicardipine, diltiazem and verapamil, were used to displace the binding of [ 3 H]‐U69593 ((5a,7a,8b)‐(+)‐N‐methyl‐N‐(7‐[1‐pyrrolidinyl]‐1‐oxaspiro[4,5]dec‐8‐yl)‐benzeneacetamide), a specific κ‐opioid agonist, in the rat cardiac sarcolemma. The CCBs competed with the binding of [ 3 H]‐U69593 (4 n m ) in a dose‐dependent manner. The displacing potency of verapamil was 55 times greater than that of nifedipine. The effects of two CCBs, verapamil and nifedipine, on the arrhythmogenic action of κ‐receptor stimulation by a specific κ‐receptor agonist, U50,488H ( trans ‐(±‐3,4‐dichloro‐N‐methyl‐N‐(2‐[1‐pyrrolidinyl] cyclohexyl) benzeacetamide methanesulphonate), were also studied in the rat isolated perfused heart. U50,488H 80–800 nmol dose‐dependently induced arrhythmias, which were completely abolished by a selective κ‐receptor antagonist, nor‐BNI (nor‐binaltorphimine,17,17′‐(dicyclopropylmethyl)‐6,6′,7,7′‐6,6′‐imino‐7,7′‐binorphinan‐3,4′,14, 14′‐tetrol), at 100 nmol. The arrhythmogenic effect was also attenuated by both verapamil and nifedipine in a dose‐dependent manner. The ED 50 values for verapamil and nifedipine were 2.75 and 63.7 nmol, respectively. The antiarrhythmic potencies of these two CCBs were correlated to their displacing potencies and inversely related to their well known potencies in inhibiting transmembrane Ca 2+ influx in the cardiac muscle. Measurement of [Ca 2+ ] i in the absence of free extracellular Ca 2+ by a spectrofluorometric method, with fura‐2 as Ca 2+ indicator, showed that U50,488H 5×10 −5 m slowly increased [Ca 2+ ] i in single ventricular myocytes and this effect was abolished by pretreatment with nor‐BNI (5 μ m ), or ryanodine (5 μ m ). Verapamil 1 and 10 μ m abolished the effect of U50,488H in 37.5% (3 out of 8) and 100% (12 out of 12) of the cells studied, respectively. On the other hand, nifedipine 10 and 100 μ m had no effect at all. Neither verapamil nor nifedipine exerted any significant effect on the caffeine‐induced Ca 2+ transient. The observations suggest that CCBs may inhibit the actions of κ‐receptor stimulation at the level of the κ‐receptor.British Journal of Pharmacology (1997) 120 , 827–832; doi: 10.1038/sj.bjp.0700985