It has been found that cardiac myocytes produce and secrete opioids and hav opioid peptide receptors (OPR). We have previously shown that the k OPR agonist, leucine-enkephalin (LE), specifically reduces twitch, cytosolic Ca2+ transient (Cai) and L-type Ca2+ channel current amplitude in adult rat ventricular myocytes. As it has been shown that opioid peptides are coreleased with catecholamines from autonomic nerve endings within the heart, we propose that OPR stimulation interacts with beta-adrenergic receptor (beta-AR) stimulation in a negative feedback role. Although it has been reported that OPR stimulation effects involve increased phospholipase C activity and thus IP3 formation, OPR stimulation also may decrease cardia cAMP levels. In contrast, beta-AR stimulation by norepinephrine (NE) elevates basal cAMP levels. In an isolated heart preparation peak developed pressure was increased to 217% of control by NE (10-7M), addition of LE (10 8M) resulted in a marked reduction in developed pressure to 66% of control within 15-25min. Further addition of the OPR antagonist naloxone (10-8M) to the LE+NE buffer rapidly reversed the LE effect (<1-2min) to 188% of contro systolic pressure. LE alone at 10-8M had no significant effect on developed pressure but was highly potent following NE. A non-hydrolyzable analog of cAMP, CPTcAMP, at 3x10-5M increased peak developed pressure to 176% of control but LE (10-8M) +CPTcAMP had no significant effect on peak pressure. Additional experiments were performed in single rat ventricular myocytes where cytosolic Ca2+ transient and contraction amplitude were measured during an identical series of experiments. Results from these experiments support those obtained with the isolated heart preparation. These results indicate that there is a distinct interaction of OPR and beta-AR stimulatio at the postsynaptic level which may alter receptor coupling to adenylate cyclase or inactivate adenylate cyclase itself. Further elucidation of this opioid signal transduction mechanism may shed light on an important negativ feedback control beta-adrenergic cardiac effects.
