Role of central and peripheral opioid receptors in the cardioprotection of intravenous morphine preconditioning.

BACKGROUND: Both central and peripheral opioid receptors activation produce cardioprotection. This study investigates the role of central and peripheral opioid receptors in intravenous morphine preconditioning (MPC) and ischemic preconditioning (IPC).
METHODS: Sixty-five anesthetized, open chests, male Sprague-Dawley rats were assigned to one of nine groups after intrathecal catheter placement. IPC was induced by three cycles of intermittent occlusion of left anterior descending artery (5 min occlusion interspersed with 5 min of reperfusion). MPC was induced by three consecutive intravenous infusions of 100 μg/kg morphine over five minutes. The opioid receptors antagonist naloxone methiodide (NM), was intravenously or intrathecally, at a dose of 20 mg/kg or 20 μg/kg, given 10 min before IPC or MPC (IVNM + IPC, ITNM + IPC, IVNM + MPC, ITNM +MPC). Control group (CON) and intravenously or intrathecally administered NM (IVNM, ITNM) were used as negative controls, respectively. All hearts were subjected to 30 min of ischemia follow by 2 h of reperfusion. Infarct size, as a percentage of the area at risk, was determined by 2, 3, 5-triphenyltetrazolium staining. Heart rate and mean arterial blood pressure were monitored.
RESULTS: The infarct size was significantly reduced in the IPC and MPC groups compared with control. The additional of intravenous or intrathecal NM both reversed the cardioprotective effects of MPC. In comparison only intravenous administration of NM before IPC could attenuate the cardioprotection.
CONCLUSIONS: MPC could mimic IPC, produce a similar cardioprotective effect. Both central and peripheral opioid receptors mediate in the cardioprotection of MPC, however, only peripheral opioid receptors in IPC.