Oxygen-dependent quenching of phosphorescence used to characterize improved myocardial oxygenation resulting from vasculogenic cytokine therapy

Abstract

This study evaluates a therapy for infarct modulation and acute myocardial rescue and utilizes a novel technique to measure local myocardial oxygenation in vivo. Bone marrow-derived endothelial progenitor cells (EPCs) were targeted to the heart with peri-infarct intramyocardial injection of the potent EPC chemokine stromal cell-derived factor 1α (SDF). Myocardial oxygen pressure was assessed using a noninvasive, real-time optical technique for measuring oxygen pressures within microvasculature based on the oxygen-dependent quenching of the phosphorescence of Oxyphor G3. Myocardial infarction was induced in male Wistar rats (n = 15) through left anterior descending coronary artery ligation. At the time of infarction, animals were randomized into two groups: saline control (n = 8) and treatment with SDF (n = 7). After 48 h, the animals underwent repeat thoracotomy and 20 μl of the phosphor Oxyphor G3 was injected into three areas (peri-infarct myocardium, myocardial scar, and remote left hindlimb muscle). Measurements of the oxygen distribution within the tissue were then made in vivo by applying the end of a light guide to the beating heart. Compared with controls, animals in the SDF group exhibited a significantly decreased percentage of hypoxic (defined as oxygen pressure ≤ 15.0 Torr) peri-infarct myocardium (9.7 ± 6.7% vs. 21.8 ± 11.9%, P = 0.017). The peak oxygen pressures in the peri-infarct region of the animals in the SDF group were significantly higher than the saline controls (39.5 ± 36.7 vs. 9.2 ± 8.6 Torr, P = 0.02). This strategy for targeting EPCs to vulnerable peri-infarct myocardium via the potent chemokine SDF-1α significantly decreased the degree of hypoxia in peri-infarct myocardium as measured in vivo by phosphorescence quenching. This effect could potentially mitigate the vicious cycle of myocyte death, myocardial fibrosis, progressive ventricular dilatation, and eventual heart failure seen after acute myocardial infarction.

Fig. 1.

Schematic of in vivo myocardial oxygen pressure measurements. Dashed white ellipse denotes infarcted LV. Asterisk indicates coronary ligation point. Green ellipses indicate typical sites of intramyocardial Oxyphor G3 injection and tissue oxygen pressure measurement. Remote left hindlimb muscle not pictured. BZ, border zone; SC, scar.

Fig. 2.

Measurement of oxygen histograms in vivo on the beating heart. The tips of the light guides were brought into contact with the myocardium, but care was taken not to apply pressure that might restrict flow in the surface blood vessels. The obtained signal was used to calculate the distribution of phosphorescence lifetimes which was subsequently converted into the distribution of Po₂ in the sample.

Fig. 3.

Compared with the saline control group, animals in the stromal cell-derived factor 1α (SDF) group exhibited a significantly decreased percentage of hypoxic peri-infarct myocardium (9.7 ± 6.7% vs. 21.8 ± 11.9% of measured peri-infarct myocardium with oxygen pressure ≤ 15.0 Torr, P = 0.017).

Fig. 4.

The peak oxygen pressures in the peri-infart region of the animals in the SDF/sargramostim [granulocyte-macrophage colony stimulating factor (GM-CSF)] group were significantly higher than the saline controls (39.5 ± 36.7 Torr vs. 9.2 ± 8.6 Torr, P = 0.02).

Fig. 5.

Representative oxygen pressure histogram of control animal peri-infarct myocardium.

Fig. 6.

Representative oxygen pressure histogram of SDF-treated animal peri-infarct myocardium.