Oxygen and oxygenation in stem-cell therapy for myocardial infarction

Abstract

Myocardial infarction (MI) is caused by deprivation of oxygen and nutrients to the cardiac tissue due to blockade of coronary artery. It is a major contributor to chronic heart disease, a leading cause of mortality in the modern world. Oxygen is required to meet the constant energy demands for heart contractility, and also plays an important role in the regulation of heart function. However, reoxygenation of the ischemic myocardium upon restoration of blood flow may lead to further injury. Controlled oxygen delivery during reperfusion has been advocated to prevent this consequence. Monitoring the myocardial oxygen concentration would play a vital role in understanding the pathological changes in the ischemic heart following myocardial infarction. During the last two decades, several new techniques have become available to monitor myocardial oxygen concentration in vivo. Electron paramagnetic resonance (EPR) oximetry would appear to be the most promising and reliable of these techniques. EPR utilizes crystalline probes which yield a single sharp line, the width of which is highly sensitive to oxygen tension. Decreased oxygen tension results in a sharpening of the EPR spectrum, while an increase results in widening. In our recent studies, we have used EPR oximetry as a valuable tool to monitor myocardial oxygenation for several applications like ischemia-reperfusion injury, stem-cell therapy and hyperbaric oxygen therapy. The results obtained from these studies have demonstrated the importance of tissue oxygen in the application of stem-cell therapy to treat ischemic heart tissues. These results have been summarized in this review article.

Figure 1

Representative EPR spectra obtained from OxySpin in the heart. EPR spectra of a suspension of OxySpin in PBS equilibrated with room air (A) and 100% nitrogen (B). EPR spectra obtained from an infarcted mouse heart, in vivo, 1 day (C) or 2 weeks (D) after implantation of OxySpin-labeled skeletal myoblast (SM) cells. The peak-peak width (w) of the spectrum is a measure of oxygen concentration. The measured pO₂ values are noted on the respective spectrum.

Figure 2

Long-term monitoring of myocardial pO₂ values in beating hearts. The measurements were performed repeatedly for 16 weeks using in vivo EPR oximetry in murine hearts implanted with microcrystals of LiNc-BuO (OxySpin) probe in the mid-ventricular region without LAD coronary artery ligation. Data represent mean±SD;(n=6). The results show the feasibility of pO₂ measurements for more than 3 months in the beating hearts of mice.

Figure 3

Myocardial pO₂ in the infarcted heart at the site of cell transplantation. Myocardial pO₂ values were measured repeatedly for 4 weeks using in vivo EPR oximetry in murine hearts transplanted with OxySpin-labeled SM cells. (A) Tissue pO₂ at 4 weeks after treatment with SM cells (MI+SM) was significantly higher when compared to untreated (MI) hearts. (B) The time-course values of myocardial pO₂ measured from infarcted hearts (MI), and infarcted hearts treated with SM cells (MI+SM) are shown. Values are expressed as mean±SD; (n=7). *p<0.05 versus MI group.

Figure 4

Values of pO₂ in rat hearts under conditions of ischemia-reperfusion and stem-cell therapy for myocardial infarction. (A) Effect of pretreatment of rats with verapamil (VER), HO-4038, SPZ, and TMZ on myocardial pO₂ during regional ischemia followed by reperfusion. Myocardial tissue pO₂ values were measured by EPR oximetry at 30 min of ischemia, followed by at 1 h of reperfusion. *p<0.05 versus Saline group; n=6. (B) Measurement of myocardial pO₂ in rat hearts transplanted with mesenchymal stem cells (MSC) at 2 weeks. The results show an increase in myocardial oxygenation levels in hearts treated with MSC and further enhancement in pO₂ was observed in hearts treated in conjunction with HBO and MSCs The “baseline” data were obtained from hearts before induction of ischemia. Data represent mean±SD; n=6. *p<0.05 versus Saline group. **p<0.05 versus MSC group.