BW373U86, a δ Opioid Agonist, Partially Mediates Delayed Cardioprotection via a Free Radical Mechanism that is Independent of Opioid Receptor Stimulation

Abstract

Opioids have been shown to produce both an early and delayed phase of cardioprotection; however, the signaling pathways involved, particularly in the delayed response, have not been well defined. Therefore, we investigated the potential of BW373U86 (BW), a potent δ opioid agonist, to produce delayed cardioprotection and characterized the role of opioid receptors and oxygen-derived free radicals (OFRs) in this delayed response. All rats underwent 30 min of ischemia followed by 2 h of reperfusion. The rats were divided into four groups. First, rats were pretreated with selective opioid receptor antagonists or the antioxidant, 2-mercaptopropionyl glycine (2-MPG), in the presence of BW and allowed to recover for 24 h before the ischemia-reperfusion protocol. Second, rats were pretreated with BW, allowed to recover for 24 h, and subsequently treated with either opioid antagonists or 2-MPG, 10 min prior to the ischemia-reperfusion protocol. Third, rats underwent ischemic preconditioning (IPC) (1×5 min occlusion) both with and without 2-MPG to determine the role of OFRs in acute cardioprotection. Fourth, rats were pretreated with TAN-67, an opioid agonist known to signal through theδ1 opioid receptor in the presence and absence of 2-MPG. Control rats were injected with saline and allowed to recover for 24 h. BW produced a bell-shaped dose-related reduction in infarct size with a maximal reduction observed at 0.1 mg/kg v control (16±3%v 60±3%, P<0.001). Surprisingly, the delayed protection induced by BW was only partially blocked by pretreatment with the δ1-selective antagonist, BNTX; however, it was completely blocked by pretreatment with 2-MPG (47±5%, P<0.001). Only naloxone given acutely inhibited the protective effects of BW; however, at the dose used, 2-MPG partially reduced the protective effect of acute IPC. TAN-67 (0.1 mg/kg) also produced a significant reduction in infarct size compared to control (18±4%v 60±3%, P<0.001). This protection was blocked by pretreatment with 2-MPG (42±4%, P<0.001). These data suggest that BW and TAN-67 mediate delayed cardioprotection via a free radical mechanism that appears to be only partially dependent on δ opioid receptor stimulation. Furthermore, it is the early burst in OFRs that is crucial to initiating the protective effect.