Ischemic heart disease remains the leading cause of death worldwide. Pharmacological agents that mimic the cardioprotective effects of ischemic preconditioning may have therapeutic potential as a secondary prevention strategy to resist infarction from subsequent cardiovascular events. Increased left ventricular end diastolic pressure (LVEDP) during ischemia, or ischemic peak pressure (IPP), is known to be correlated to infarct size. Recently our laboratory demonstrated that naltrindole (NTI), a selective delta opioid receptor antagonist, reduces IPP and infarct size when given prior to ischemia (preconditioning) in a Langendorff rat heart model. The purpose of this study was to examine the effects of NTI analogues naltriben (NTB, delta opioid receptor antagonist) and guanidinonaltrindole (GNTI, kappa opioid receptor antagonist) compared to NTI. Nor-binaltrophine (BNI, kappa opioid receptor antagonist) and naloxone (NX, broad-spectrum opioid receptor antagonist) were tested to evaluate cardioprotection by other opioid receptor antagonists.

Isolated hearts from male Sprague-Dawley rats (~300g) were subjected to 30-min global ischemia (I)/45-min reperfusion (R) with treatments infused for 5 min before I and during the first 5 min of R. LV cardiac function was measured using a pressure transducer. At the end of reperfusion, infarct size was assessed using 1% triphenyltetrazolium chloride staining and defined as infarcted tissue/total area at risk. Data were evaluated using ANOVA Student-Neuman-Keuls post-hoc analysis.

Control I/R hearts demonstrated an IPP of 39±3 mmHg compared to pre-ischemic LVEDP of 9±1 mmHg at baseline (n=12, p<0.01), resulting in substantial infarct at the end of 45 min R (32±4%). NTI (n=7) and NTB (n=6) elicited cardiodepressive effects during preconditioning by reducing the maximal rate in the rise of LV pressure (dP/dt max) to 1581±379 mmHg/s and 929±243 mmHg/s, respectively, compared to control (2471±72 mmHg/s, p<0.01). IPP was reduced by NTI (18±3 mmHg/s) and NTB (15±3 mmHg/s) compared to all groups (p<0.05). Post-reperfused dP/dt max and infarct size was most improved following NTI (1830±90 mmHg/s, 7±2%) and NTB (1846±140 mmHg/s, 7±2%) pretreatment, compared to control (777±142 mmHg/s, 32±4%, p<0.01). GNTI reduced infarct size (17 ± 4%, n=6, p<0.05), but did not exert a negative inotropic effect. Cardiac function and infarct size did not improve with BNI (n=7) or NX (n=6) pretreatment.

These results suggest that NTI and analogues, GNTI and NTB, are cardioprotective against myocardial I/R injury. The negative inotropic effects of NTI and NTB were associated with ~75% reduction in infarct size compared to control. GNTI decreased infarct size by ~50% and these results suggest that NTI, NTB, and GNTI exert tissue-salvaging effects independent of delta or kappa opioid receptor antagonism. In future studies, we will examine different ischemic time points to administer NTI and its analogues to determine optimal cardioprotection and investigate downstream effects on calcium handling.