Abstract
Transmural nerve stimulation of isolated atria, obtained from reserpine-pretreated guinea-pigs, in the presence of atropine and the β 1-adrenoceptor-blocking drug CGP 20712A, induced a positive inotropic effect. [D-Ala 2, N-Me-Phe 4, Gly 5-ol] enkephalin (DAGO), [D-Ala 2, D-Leu 5]enkephalin (DADLE), morphine and dynorphin dose dependently reduced the cardiac response to transmural nerve stimulation. The δ receptor selective agonist [D-Pen 2, D-Pen 5]enkephalin (DPDPE), and the κ receptor agonist, U50488, were unable to modify the response. The inhibitory effect of all the active opioid agonists was antagonized by naloxone but not by the selective δ and κ opioid receptor antagonists, ICI 174.864 and MR 2266. These results suggest the presence on sensory nerve terminals of inhibitory opioid receptors belonging to the μ, but not to the δ and κ subtypes.
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