Delta Opioid Receptors: The Link between Exercise and Cardioprotection

<div><p>This study investigated the role of opioid receptor (OR) subtypes as a mechanism by which endurance exercise promotes cardioprotection against myocardial ischemia-reperfusion (IR) injury. Wistar rats were randomly divided into one of seven experimental groups: 1) control; 2) exercise-trained; 3) exercise-trained plus a non-selective OR antagonist; 4) control sham; 5) exercise-trained plus a kappa OR antagonist; 6) exercise-trained plus a delta OR antagonist; and 7) exercise-trained plus a mu OR antagonist. The exercised animals underwent 4 consecutive days of treadmill training (60 min/day at ∼70% of maximal oxygen consumption). All groups except the sham group were exposed to an in vivo myocardial IR insult, and the myocardial infarct size (IS) was determined histologically. Myocardial capillary density, OR subtype expression, heat shock protein 72 (HSP72) expression, and antioxidant enzyme activity were measured in the hearts of both the exercised and control groups. Exercise training significantly reduced the myocardial IS by approximately 34%. Pharmacological blockade of the kappa or mu OR subtypes did not blunt exercise-induced cardioprotection against IR-mediated infarction, whereas treatment of animals with a non-selective OR antagonist or a delta OR antagonist abolished exercise-induced cardioprotection. Exercise training enhanced the activities of myocardial superoxide dismutase (SOD) and catalase but did not increase the left ventricular capillary density or the mRNA levels of HSP72, SOD, and catalase. In addition, exercise significantly reduced the protein expression of kappa and delta ORs in the heart by 44% and 37%, respectively. Together, these results indicate that ORs contribute to the cardioprotection conferred by endurance exercise, with the delta OR subtype playing a key role in this response.</p></div

Hemodynamic parameters in response to IR injury.

<p>The values represent the means ± SD, n = 10 for all groups. CT, control group; Exe, exercised group; Exe+Nal, exercised + naloxone group; SAP, systolic arterial pressure (mmHg); MAP, mean arterial pressure (mmHg); DAP, diastolic arterial pressure (mmHg); HR, heart rate (bpm).</p><p>*P<0.05 <i>vs.</i> all groups.</p><p>Hemodynamic parameters in response to IR injury.</p

Left ventricle structural capillary density.

<p>The bars represent capillary density in the control and exercised groups (A). The representative images used to determine the structural capillary density with capillaries stained with FITC-conjugated <i>G</i>. <i>simplicifolia</i> lectin. Magnification, ×200; scale bar, 100 µm in the CT (B) and Exe groups (C). Exe, exercised group (n = 5); CT, control group (n = 5).</p

Total superoxide dismutase (A) and catalase (B) activities in the control and exercised groups.

<p>Exe, exercised group (n = 5); CT, control group (n = 5). * P<0.05 <i>vs.</i> CT.</p

NADPH Oxidases mRNA levels in liver (A) and heart (B) of rats treated with Deca Durabolin (50 mg.mL⁻¹ Organon, 1 mg/100 g bw, im), once a week, for 8 weeks.

<p>mRNA levels were determined by real-time PCR and was expressed relative to the control group. Data were obtained with 10 animals from at least two independent experiments and are shown as mean ± SEM. * p<0.05.</p

Protein carbonyl content of liver (A), kidney (B), and heart (C) of rats treated with Deca Durabolin (50 mg.mL⁻¹ Organon, 1 mg/100 g bw, im), once a week, for 8 weeks.

<p>Protein carbonyl content was evaluated based on a reaction with dinitrophenylhidrazine (DNPH), evaluated in a spectrophotometer at 370 nm. Data were obtained with 6 animals from at least two independent experiments and are shown as mean ± SEM. * p<0.05; **p<0.001.</p

NADPH Oxidase activity in liver (A), kidney (B), and heart (C) of rats treated with Deca Durabolin (50 mg.mL⁻¹ Organon, 1 mg/100 g bw, im), once a week, for 8 weeks.

<p>H₂O₂ production was determined in the microssomal fraction by the Amplex Red/Horseradish Peroxidase assay. Data were obtained with 10 animals from at least two independent experiments and are shown as mean ± SEM. * p<0.05; **p<0.001.</p

Reduced thiol content of liver (A), kidney (B), and heart (C) of rats treated with Deca Durabolin (50 mg.mL⁻¹ Organon, 1 mg/100 g bw, im), once a week, for 8 weeks.

<p>Total sulfhydryl groups were measured by the reaction of thiols with DTNB, evaluated in a spectrophotometer at 412 nm. Data were obtained with 6 animals from at least two independent experiments and are shown as mean ± SEM. * p<0.05; **p<0.001.</p

Functional parameters at the end of the experiment.

<p>The results are presented as the means ± SEMs. BWt, body weight; RWt, renal weight; SBP, systolic blood pressure; SCr, serum creatinine; eGFR, estimated glomerular filtration rate; DM, diabetic group; FCS, diabetic group treated with fucosylated chondroitin sulfate; ENX, diabetic group treated with enoxaparin;</p><p>*all of the diabetic groups vs. control;</p>#<p>DM vs. FCS and ENX groups;</p><p>**<i>p<</i>0.01 DM vs. control.</p><p>Functional parameters at the end of the experiment.</p

The GAGs effectively reduced heparanase-1 expression in the glomeruli of treated DM rats.

<p>From A–D, immunofluorescence photomicrographs of heparanase-1–stained renal glomerular sections with an evident increase in expression in the DM (B) animals that was prevented by FCS (C) and ENX (D) administration, as confirmed by a semiquantitative scoring system (E). The data represent the means ± SEMs. *<i>p<</i>0.0001 vs. groups; #<i>p<</i>0.01 vs. ENX. Bar = 25 µm.</p