Diazoxide amelioration of myocardial injury and mitochondrial damage during cardiac surgery

Abstract

Background: Recently, we have shown that the selective opening of mitochondrial ATP-sensitive potassium channels with diazoxide significantly decreases myocardial injury. The purpose of this study was to determine the effects of diazoxide on apoptosis and the mechanisms modulating apoptosis and myocardial injury in a blood-perfused model of acute myocardial infarction.

Methods: Pigs (32 to 42 kg) undergoing total cardiopulmonary bypass underwent left anterior descending coronary artery occlusion for 30 minutes. The aorta was cross-clamped and magnesium-supplemented potassium cold-blood cardioplegia (DSA; n = 6) or magnesium-supplemented potassium cardioplegia containing 50 micromol/L diazoxide (DZX; n = 6) was administered, followed by 30 minutes of global ischemia and 120 minutes of reperfusion. Left ventricular tissue samples from DSA and DZX hearts were obtained after reperfusion. Apoptosis was determined by TUNEL, caspase-3 and PARP cleavage, and caspase-3 activity. Bax and bcl-2 levels were determined and tissue morphology was examined by light and transmission electron microscopy.

Results: Apoptosis, as estimated by TUNEL-positive nuclei/3,000 myocardial cells, was 120.3 +/- 48.8 in DSA hearts and was significantly decreased to 21.4 +/- 5.3 in DZX hearts (p < 0.05 vs control). Caspase-3 and poly-ADP-ribose polymerase cleavage and pro-apoptotic bax protein levels were significantly decreased with diazoxide (p < 0.05 vs DSA). Light and transmission electron microscopy indicated severe disruption of tissue with capillary dilatation, mitochondrial cristae damage, and evidence of increased presence of mitochondrial granules in DSA as compared with DZX hearts.

Conclusions: The addition of diazoxide (50 micromol/L) to cardioplegia significantly decreases regional myocardial apoptosis and mitochondrial damage, and provides an additional modality for achieving myocardial protection.

Fig 1

TUNEL-positive nuclei/3,000 myocytes in the regional and global ischemic zones. Representative photomicrographs of TUNEL photomicrographs of regional ischemia (Regional) and global ischemia (Global) sections (6 μm) from hearts receiving Deaconess Surgical Associates (DSA) cardioplegia or DSA cardioplegia containing 50 μmol diazoxide (DZX) after 120 minutes of reperfusion. Control hearts received cardiopulmonary bypass and sham reperfusion only and are shown for comparative purposes. Myocyte nuclei were visualized by indirect digoxigenin fluorescence labeling of DNA with propidium iodide (PI), shown as red in top row. TUNEL-positive cells were visualized by FITC labeling and are shown as green in bottom row. Results indicate a significant increase in TUNEL-positive nuclei/3,000 myocytes in DSA regional as compared with DZX regional and DSA and DZX global.

Fig 2

The effects of Deacon Surgical Associates (DSA) and diazoxide (DZX) cardioplegia on poly ADP-ribose polymerase (PARP) and caspase-3 cleavage and antiapoptotic bcl-2 and pro-apoptotic bax levels determined by Western blot in the regional and global ischemic zones. Protein equivalency and transfer efficiency were confirmed by Ponceau red staining and actin immunoblotting. A representative 10% polyacrylamide gel stained with Coomassie brilliant blue R-250 is shown on the left. Molecular mass (kDa) is shown. Representative immunoblots for PARP, actin, caspase-3, bax, and bcl-2 are shown on the right. Latent zymogens for PARP and caspase-3 are shown at 116 and 32 kDa, respectively. Catalytically active proteases for PARP and caspase-3 are shown at 85 kDa for PARP and 20 and 11 kDa for caspase-3. Protein equivalency and transfer efficiency were confirmed by Ponceau red staining and actin immunoblotting. Results indicate a significant increase in caspase-3 cleavage in DSA regional as compared with DZX regional and DSA and DZX global.

Fig 3

The effects of Deaconess Surgical Associates (DSA) and diazoxide (DZX) cardioplegia on bcl-2/bax ratio (arbitrary units) in the regional and global ischemic zones. All results are shown as mean ± SEM for n = 6 for each sample. Significant differences at p less than 0.05 are shown as *versus control and as **versus DZX. Results indicate a significant decrease in anti- to pro-apoptotic protein levels in DSA regional as compared with DZX regional and DSA and DZX global ischemic zones of hearts.

Fig 4

The effects of Deaconess Surgical Associates (DSA) and diazoxide (DZX) cardioplegia on catalytically active caspase3–like activity (pmol DVED-pNA/μg protein/min) in the regional and global ischemic zones. Catalytically active caspase-3–like activity (pmol DVED-pNA/μg protein/min) was significantly increased (p < 0.05) approximately threefold in DSA as compared with DZX hearts in the regional ischemic zone. Standard curve using known concentrations of DVED-pNA is shown in inset. All results are shown as mean ± SEM for n = 6 for each sample. Significant differences at p less than 0.05 are shown as *versus control and as **versus DZX. Results indicate a significant increase in caspase-3–like activity in DSA regional as compared with DZX re gional and DSA and DZX global.

Fig 5

Representative hematoxylin and eosin (H&E, ×40) and Masson trichrome (×40) stained myocardial sections (6 μm) from regional and global ischemic zones of Deaconess Surgical Associates (DSA) and diazoxide (DZX) hearts. Results indicate severe tissue disruption with capillary dilatation in DSA hearts that was not evident in DZX hearts.

Fig 6

(A) Representative transmission electron micrographs (×10,500) from regional ischemic zones from control, Deaconess Surgical Associates (DSA), and diazoxide (DZX) hearts. Results show myocytes in DSA hearts are separated by numerous densely packed, dilated mitochondria with extensive swelling of the matrix and disruption of the cristae and numerous large mitochondrial granules. Control and DZX hearts show preserved myofilament structure with electron-dense intracristae matrix and few small mitochondrial granules. (Ca = calcium granules; M = mitochondrion; N = nucleus; S = sarcomere.) (B) Representative transmission electron micrographs (×62,500) of mitochondria from regional ischemic zones from DSA and DZX hearts. Results show disrupted mitochondrial structure and large mitochondrial granules in DSA hearts. DZX hearts show preserved mitochondrial structure with smaller significantly less mitochondrial granules as compared with DSA hearts. Mitochondrial granules are indicated by arrows.