Cardioprotective effects by a novel opioid peptide in myocardial ischemia/reperfusion injury

Preliminary studies in mice have shown that a novel tri-peptide (Phe-D-Arg-Phe-Amide, MW=468 g/mol) attenuates ventilation induced diaphragm dysfunction when given as a pretreatment. Tri-peptide is structurally similar to the SS-20 peptide (Phe-D-Arg-Phe-Lys-Amide), which has been shown to be cardioprotective when given before ischemia, but thought not to have opioid properties (Cho et al., 2007). Our study aims to determine whether pre- or posttreatment with tri-peptide is more efficacious in attenuating the deleterious effects of myocardial ischemia/reperfusion (I/R) injury and whether this cardioprotection is mediated by opioid receptor activation. This was tested by observing whether tri-peptide-induced cardioprotection was abolished when administered in conjunction with naloxone, a nonselective opioid receptor antagonist. Tri-peptide (50μM) was administered prior to I (pretreatment, n=8) or during R (posttreatment, n=8) in isolated perfused rat hearts subjected to I(30 min)/R(45 min) and compared to untreated control hearts (n=7) and hearts pre-treated with tri-peptide (50 μM) + naloxone (Nlx, 10μM, n=8). Tri-peptide pretreatment hearts significantly recovered post-reperfused left ventricular developed pressure (LVDP) to 59±6% of initial baseline values compared to untreated control, posttreatment tri-peptide, and pretreatment tri-peptide + Nlx hearts in which LVDP recovered to only 33±7%, 34±8%, and 26±4% of initial baseline values respectively (p<0.01). Furthermore, tri-peptide pretreatment hearts exhibited significantly reduced infarct sizes of 26±1% compared to untreated control and posttreatment tri-peptide hearts, in which infarct sizes were 38±4% and 37±2% respectively (p<0.05) assessed using 1% triphenyltetrazolium chloride staining. Pretreated tripeptide + Nlx hearts showed infarct sizes of 32±3% which was not significantly different from untreated controls. These data suggest that pretreatment and not posttreatment with this novel tri-peptide was cardioprotective and that this protection was mediated via opioid receptor activation. Future studies will attempt to determine the specific opioid receptor subtype involved in tri-peptide cardioprotection by testing whether delta or kappa opioid receptor antagonists block the cardioprotective effects of tri-peptide pretreatment.