Although opioid peptides (OP) are: coreleased with catecholamines from neuronal endings in the heart; are produced and secreted by cardiac myocytes; capable of activating OP receptors (OPR) which reside on cardiac cell membranes; the physiological role of OP's in cardiac function and pathophysiology,including the mechanism of signal transduction,has received meager investigation. Thus we have recently shown that OP's have a postsynaptic cardiac intracellular signalling role which involves a potent """"""""cross-talk"""""""" with the beta1-adrenergic receptor (beta1-AR) stimulation pathway. This work identified that the cross-talk between delta-OPR and beta1-AR occurs via a pertussis toxin sensitive (PTX) Gi-protein involved with adenylyl cyclase inhibition in intact isolated heart preparations and isolated cardiac myocytes. In non-cardiac cells, although delta-OPR exert direct effects which involve the activation of phospholipase C for phosphoinositide metabolism and IP3 formation, direct evidence that demonstrates delta-OPR coupling to either pertussis toxin-insensitive Gq-protein or PTX-sensitive Gi-protein is unavailable to date. This lab has previously shown that a dose-dependant negative inotropic effect, including inhibition of intracellular [Ca2+]and whole cell voltage, occurs in cardiac myocytes. We have also previously shown that OP's and their preproenkephalin mRNA are significantly increased with age. In light of these findings, a study of the direct effects of -OPR stimulation by leucine-enkephalin (LE) in 6-hydroxydopamine pretreated isolated hearts from young adult (6mo)and old rats (24mo)is in progress. LE reduced peak systolic pressure in a dose dependent manner in 6 and 24mo rats but to a greater extent in 24mo rats. The EC50 was approximately 10-8 M LE in 6 mo but 10-9M LE in 24mo hearts (p less than 0.05,n=4). A preliminary assessment of the role of PTX-sensitive Gi-protein in the direct effects of LE indicated that PTX pretreatment did not inhibit the LE-induced negative inotropy in either age group, as is LE's interactive effect with the beta1-AR signalling transduction pathway. Assays for the measurement of cAMP, IP3 and PKC in isolated hearts from all groups are in progress. The cross-talk between -OPR and beta1-AR signal transduction pathways is an important mechanism in the regulation of cardiac excitation-contraction coupling. Presently, our data indicates that two types of G-protein are coupled to the delta-OPR that are involved with the interactive and direct effects of opioid peptides. Complete elucidation of these intracellular signal transduction pathways and their mechanism of action, particularly during the process of aging or disease remains the focus of this project.
