Cardioprotection by kappa-opioid receptor agonist U50488H is mediated by opioidergic regulation but not by calcium current modulation

Abstract

Background: Because the kappa-opioid receptor (OR) agonist U50488H stimulates opioidergic regulation and inhibits L-type Ca(2+) channels, this study was aimed at assessing the roles of OR and L-type Ca(2+) channels on U50488H-induced cardioprotection.

Methods: Langendorff-perfused rat hearts were subjected to 30 min of regional ischemia and 2 h of reperfusion. Isolated hearts were treated with U50488H with or without the kappa-OR antagonist nor-binaltorphimine (nor-BNI) or the Ca(2+) channels activator BAY K 8644. Infarct size was measured with 2,3,5-triphenyltetrazolium chloride staining.

Results: U50488H treatment at reperfusion: (1) significantly reduced infarct size (11.3 +/- 1.3%) compared to control hearts (27.7 +/- 1.1%, P < 0.001), an effect that was completely blocked by nor-BNI (24.0 +/- 0.9%, P < 0.001 vs. U50488H) but not by BAY K 8644 (7.1 +/- 1.7%, P > 0.05 vs. U50488H); (2) significantly increased left ventricular developed pressure (65.3 +/- 4.8%) after 2 h of reperfusion compared to control hearts (44.8 +/- 3.6%, P < 0.05), an effect that was abrogated by nor-BNI (40.5 +/- 4.5%, P > 0.05 vs. control) but not by BAY K 8644 (64.3 +/- 5.6%, P < 0.01 vs. control); and (3) significantly decreased heart rate (P < 0.01 vs. control), an effect that was completely abrogated by both nor-BNI and BAY K 8644.

Conclusions: U50488H significantly limits myocardial infarction and stunning in isolated rat hearts after ischemia-reperfusion induction. The infarct size limitation and contractility improvement observed with U50488H treatment during reperfusion are entirely mediated by OR stimulation and not by Ca(2+) channel modulation.

Keywords: Coronary occlusion; Myocardial infarction; Opioid receptors; Reperfusion.

Fig. 1

Area of necrosis (AN) as percentage of area at risk (AR) as evaluated by triphenyltetrazolium chloride staining following 30 min regional ischemia and 2 h reperfusion in an isolated rat heart model. U50488H treatment during the early reperfusion period greatly reduces infarct size. Infarct size limitation by U50488H treatment is completely abolished by the selective κ-OR antagonist nor-binaltorphimine but not by the L-type Ca²⁺ channel activator BAY K 8644. CON: control, U50: U50488H, BNI: nor-binaltorphimine, BAY: BAY K 8644. Each circle represents 1 heart. Horizontal bars depict mean of the group. Values are means ± SEM. ^*P < 0.05 vs. CON.

Fig. 2

Percentage changes in heart rate (HR) after 30 min regional ischemia and 2 h reperfusion. HR is significantly decreased in U50488H-treated hearts. Both nor-binaltorphimine and BAY K 8644 completely blocks the decrease in HR in U50488H-treated hearts. CON: control, U50: U50488H, BNI: nor-binaltorphimine, BAY: BAY K 8644. Values are means ± SEM. ^*P < 0.05 vs. CON, ^†P < 0.05 vs. U50.

Fig. 3

Percentage changes in left ventricular developed pressure (LVDP) and rate-pressure product (RPP) after 2 h of reperfusion compared to baseline levels in isolated rat hearts. U50488H significantly increases LVDP after reperfusion. The increase in LVDP by U50488H is blocked by nor-binaltorphimine but not by BAY K 8644. There is no significant change in RPP between U50488H-treated hearts and untreated control hearts. CON: control, U50: U50488H, BNI: nor-binaltorphimine, BAY: BAY K 8644. Values are means ± SEM. ^*P < 0.05 vs. CON, ^†P < 0.05 vs. U50 + BNI.

Fig. 4

Percentage changes in velocity of contraction (+dP/dt_max) and velocity of relaxation (-dP/dt_min) after 2 h of reperfusion compared to baseline in isolated rat hearts. There are no statistical significances among the groups. CON: control, U50: U50488H, BNI: nor-binaltorphimine, BAY: BAY K 8644. Values are means ± SEM.