Effects of U50488 and Bremazocine on [Ca2+]iand cAMP in Naive and Tolerant Rat Ventricular Myocytes: Evidence of Kappa Opioid Receptor Multiplicity in the Heart

To explore the existence of multiplicity ofκreceptor in the heart, two series of experiments were performed. In the first we studied the antagonistic actions of nor-BNI, a selectiveκ1antagonist, and quadazocine, a preferentialκ2antagonist, against the effects of U50488, a selectiveκ1agonist, and bremazocine, a universal agonist preferentially binding toκ2receptor, on the electrically stimulated [Ca2+]itransient and forskolin-stimulated cAMP accumulation in the rat ventricular myocyte. In the second series of experiments, we determined and compared the effects of above twoκreceptor agonists in the ventricular myocytes made insensitive toκ1andκ2agonists by prior exposure to the respective agonists. At the concentration range of 3×10−6–3×10−5 , both U50488 and bremazocine dose-dependently inhibited the [Ca2+]itransient induced by electrical stimulation. The inhibitory effects of U50488 and bremazocine were antagonized by nor-BNI and quadazocine. The antagonistic actions of nor-BNI were significantly greater against the effects of U50488, but smaller against the effects of bremazocine than those of quadazocine. At 1×10−6–5×10−5 , both U50488 and bremazocine dose-dependently and significantly inhibited the forskolin-induced cAMP accumulation. The inhibitory effect of 30μ U50488 on cAMP accumulation was significantly attenuated by 5μ nor-BNI, but not by quadazocine at the same concentration; whereas the effect of 30μ bremazocine was significantly blocked by 5μ quadazocine, but not by nor-BNI at the same concentration. The inhibitory effect of 30μ U50488 on electrically stimulated [Ca2+]iwas abolished by preincubation of myocytes with 10−6 U50488 for 24 h, but not with 10−6 bremazocine for 24 h; whereas the inhibitory effect of 30μ bremazocine on electrically stimulated [Ca2+]itransient was significantly attenuated after incubation of the myocyte with 10−6 bremazocine for 24 h, but not with 10−6 U50488 for 24 h. The observations indicate the existence ofκreceptor subtypes in the rat heart.