U50,488H Inhibits Effects of Norepinephrine in Rat Cardiomyocytes—Cross-Talk between κ-opioid and β-adrenergic Receptors

In order to determine the effect ofκ-opioid receptor agonist on theβ1-adrenoceptor stimulation in the heart, the effects of norepinephrine (NE), aβ1-adrenoceptor agonist, on contraction and electrically induced intracellular calcium ([Ca2+]i) transient in the single rat ventricular myocyte pretreated with aκ-opioid receptor agonist, trans-(±)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl)-benzeneacetamide(U50,488H), at 0.01–1μ were studied with a video edge tracker method and a spectrofluorometric method using fura-2 as calcium indicator, respectively. NE at 0.01–10μ augmented both twitch amplitude and electrically induced [Ca2+]itransient dose-dependently, which were abolished by propranolol at 1μ , aβ-adrenoceptor antagonist. The effects of NE on both contraction and [Ca2+]itransient were attenuated in a dose-dependent manner by U50,488H at 0.01–1μ , which itself had no effect at all. The maximum response (Emax) was decreased, while the concentration that produces 50% of the maximum response (EC50) was enhanced, by U50,488H. The inhibitory effects of U50,488H onβ-adrenoceptor stimulation were completely blocked by pretreatment with norbinaltorphimine, a specificκ-opioid receptor antagonist at 1μ , or preincubation with pertussis toxin (PTX) at 200 ng/ml for 6 h. On the other hand, the inhibition on NE-induced augmentation in electrically induced [Ca2+]itransient by U50,488H was not affected by pretreatment with U73122, a specific inhibitor of phospholipase C (PLC), at 10μmfor 30 min. U50,488H attenuated the augmentation of the electrically stimulated [Ca2+]itransient induced by forskolin at 0.1 and 0.5μ . It did not, however, affect the augmentation of the electrically induced [Ca2+]itransient by N6,2′-O-dibutyryl adenosine cyclic monophosphate (DB-cAMP). The results suggest thatκ-opioid receptor stimulation by U50,488H at 10−6 or lower may inhibit the effects ofβ-adrenoceptor stimulation by acting at a PTX-sensitive G-protein and AC, but not via the phosphoinositol pathway.