oxidation of pyruvate through the citric acid cycle. APE-LM-preserved hearts consumed a significantly higher amount of oxygen than hearts preserved with other media. The higher oxygen and pyruvate consumption in these hearts indicated that the hearts had a greater metabolic oxidative activity during preservation than the other hearts. The higher oxidative activity may have been reflective of greater tissue perfusion, especially in the coronary beds, and thereby perfusion of oxygen to a greater percentage of myocardial cells. Another factor contributing to the effectiveness of APE-LM as a transplantation media is its biologically compatible lipid emulsifier, which consists primarily of phospholipids and cholesterol. The lipid provides a favorable environment for myocardial membranes and may prevent perfusion-related depletion of lipids from cardiac membranes. The cholesterol contains a bulky steroid nucleus with a hydroxyl group at one end and a flexible hydrocarbon tail at the other end. The hydrocarbon tail of the cholesterol is located in the non polar core of the membrane bilayer. The hydroxyl group of cholesterol hydrogen-bonds to a carbonyl oxygen atom of a phospholipid head group. Through this structure, cholesterol prevents the crystallization of fatty acyl chains by fitting between them. Thus, cholesterol moderates the fluidity of membranes.8The reason there are currently such strict limits on the amount of time a heart can remain viable out of the body is because there must be a source of energy for the heart tissue if it is to stay alive. Once the supply of energy runs out, the tissue suffers irreversible damage and dies. Therefore, this tissue cannot be used for transplantation. If hypothermic hearts are not given exogenous substrates that they can transport and consume, like pyruvate, then they must rely on glycogen or lipid stores for energy metabolism. The length of time that the heart can be preserved in vitro is thus rel...
