Energy-metabolic aspects of ischemia and pre-treatment: studies in porcine myocardium

Abstract: The focus of this thesis is to clarify mechanisms involved in protective pre-treatment of ischemia by carbon monoxide (CO) and ischemic preconditioning (IP), so that new pro-tective therapies can be designed. This is studied in heart muscle, where the clinical gain would ultimately be to be able to prolong the period during a threatened myocardial infarction where permanent injury can be prevented. The aim is to elucidate energy meta-bolic relations as a basis for temporary metabolic adaptation to threatened injury in heart muscle, with focus on the biological relevance of this mechanism. All studies were conducted in an open-chest, anesthetized pig model using microdialysis sampling. Methods: In anesthetized 40 kg pigs, regional myocardial ischemia was produced by transient snare-ligation of a branch of the left anterior descending coronary artery. Micro-dialysis catheters were used for local sampling of interstitial fluid in the ischemic area. In Studies 1 and 2, CO was administered before prolonged ischemia in a clinically relevant dose (5% increase in carboxyhemoglobin). In Study 2, 45Ca2+ was administered locally by microperfusate to ischemic myocardium with 45Ca2+ recovery used as a marker for intra-cellular calcium overload during ischemia. Myocardial injury markers glycerol and glut-amate (and taurine in Studies 3 and 4) were measured in microdialysate. In Studies 3 and 4, IP was performed by 4 brief transient cycles of coronary occlusion and reperfusion before a prolonged index ischemic episode was performed. In Study 3, 14C-marked adenosine was administered locally via microdialysis catheters in the heart muscle wall, and when this was metabolized during ischemia as an energy source, it was detected as 14C-marked lactate. In Study 4, a water-soluble purine nucleoside phosphorylase inhibitor was administered to heart muscle via microdialysis which was treated by IP before an index ischemia. Markers of glycolysis were measured serially before and during ischemia for Studies 1-4. Radio-labelled markers were analyzed using liquid chromatography and scintigraphy. Results: Study 1 results showed clear signs of metabolic advantage as far as glycolytic markers related to CO during myocardial ischemia. Study 2 results demonstrated no apparent energy metabolic advantage including for 45Ca2+ recovery and no diminishment of injury markers related to the single tested carbon monoxide dose during ischemia. Study 3 showed that IP led to enhanced radio-marked adenosine consumption as an energy-metabolic substrate, and that glycolytic flow (as less glucose consumption and lactate formation) was slower in IP-treated heart muscle. Study 4 showed that local purine nucleoside phosphorylase blockade inhibits adenosine utilization as an energy-metabolic substrate during ischemia, but this did not have an effect on glycolysis or injury markers during prolonged ischemia after IP. Conclusions: From Studies 1 and 2, we concluded that CO in this dose could show effects on glycolysis during ischemia but does not seem to confer cell protection during ischemia or early reperfusion, though CO protective effects in other doses or time frames cannot be ruled out. From Study 3 we concluded that there may be an immediate energy-metabolic explanation for why more IP-treated cells survive during prolonged ischemia. From Study 4 we concluded that experimental purine nucleoside phosphorylase blockade appears to allow interruption of IP-related adenosine utilization as an energy-metabolic substrate during prolonged ischemia without obvious effects on glycolysis, and that this requires further study to test if adenosine as an energy resource during ischemia is associated with protection during infarction. Key words: myocardial ischemia, preconditioning, carbon monoxide, adenosine