INHIBITION OF [H-3] U69593 BINDING AND THE CARDIAC EFFECTS OF U50,488H BY CALCIUM-CHANNEL BLOCKERS IN THE RAT-HEART

1 The calcium channel blockers (CCBs), nifedipine, nicardipine, diltia zem and verapamil, were used to displace the binding of[H-3]-U69593 di nyl]-1-oxaspiro[4,5]dec-8-yl)-benzeneacetamide), a specific kappa-opio id agonist, in the rat cardiac sarcolemma. The CCBs competed with the binding of [H-3]-U69593 (4 nM) in a dose-dependent manner. The displac ing potency of verapamil was 55 times greater than that of nifedipine. 2 The effects of two CCBs, verapamil and nifedipine, on the arrhythmo genic action of kappa-receptor stimulation by a specific kappa-recepto r agonist, U50,488H s-(+/--3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl] cyclohexyl) benzeacetamide methanesulphonate), were also studied in t he rat isolated perfused heart. U50,488H 80-800 nmol dose-dependently induced arrhythmias, which were completely abolished by a selective ka ppa-receptor antagonist, nor-BNI (nor-binaltorphimine, '-6,6'-imino-7, 7'-binorphinan-3,4',14,14'-tetrol), at 100 nmol. The arrhythmogenic ef fect was also attenuated by both verapamil and nifedipine in a dose-de pendent 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 r elated to their well known potencies in inhibiting transmembrane Ca2influx in the cardiac muscle. 3 Measurement of [Ca2+](i) in the absenc e of free extracellular Ca2+ by a spectrofluorometric method, with fur a-2 as Ca2+ indicator, showed that U50,488H 5x10(-5) M slowly increase d [Ca2+](i) in single ventricular myocytes and this effect was abolish ed by pretreatment with nor-BNI (5 mu M), or ryanodine (5 mu M). Verap amil 1 and 10 mu 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 mu M had no effect at all. Neither verapamil nor nifedipine exerted any significant effect on the caffein e-induced Ca2+ transient. 4 The observations suggest that CCBs may inh ibit the actions of kappa-receptor stimulation at the level of the kap pa-receptor.