Effects of U50488 and bremazocine on [Ca2+](i) and cAMP in naive and tolerant rat ventricular myocytes: Evidence of kappa opioid receptor multiplicity in the heart

To explore the existence of multiplicity of kappa receptor in the heart, tw o series of experiments were performed. In the first we studied the antagon istic actions of nor-BNI, a selective kappa k(1) antagonist, and quadazocin e, a preferential kappa(2) antagonist, against the effects of U50488, a sel ective kappa(1) agonist, and bremazocine, a universal agonist preferentiall y binding to kappa(2) receptor, on the electrically stimulated [Ca2+](i) tr ansient and forskolin-stimulated cAMP accumulation in the rat ventricular m yocyte. In the second series of experiments, we determined and compared the effects of above two kappa receptor agonists in the ventricular myocytes m ade insensitive to kappa(1) and kappa(2) agonists by prior exposure to the respective agonists. At the concentration range of 3 x 10(-6)-3 x 10(-5) M, both U50488 and bremazocine dose-dependently inhibited the [Ca2+](i) trans ient induced by electrical stimulation. The inhibitory effects of U50488 an d bremazocine were antagonized by nor-BNI and quadazocine. The antagonistic actions of nor-BNI were significantly greater against the effects of U5048 8, but smaller against the effects of bremazocine than those of quadazocine . At 1 x 10(-6)-5 x 10(-5) M, both U50488 and bremazocine dose-dependently and significantly inhibited the forskolin-induced cAMP accumulation. The in hibitory effect of 30 mu M U50488 on cAMP accumulation was significantly at tenuated by 5 mu M nor-BNI, but not by quadazocine at the same concentratio n: whereas the effect of 30 mu M bremazocine was significantly blocked by 5 mu M quadazocine, but not by nor-BNI at the same concentration. The inhibi tory effect of 30 mu M U50488 on electrically stimulated [Ca2+](i) was abol ished by preincubation of myocytes with 10(-6) M U50488 for 24 h, but not w ith 10(-6) M bremazocine for 24 h; whereas the inhibitory effect of 30 mu M bremazocine on electrically stimulated [Ca2+](i) transient was significant ly attenuated after incubation of the myocyte with 10(-6) M bremazocine for 24 h, but not with 10(-6) M U50488 for 24 h, The observations indicate the existence of kappa receptor subtypes in the rat heart. (C) 1999 Academic P ress.