Myocardial heterogeneity and regional variations in response to injury

Different regional variations in the heart with respect to structure, function, and metabolism are known. However, regional heterogeneity with respect to histochemical localization of enzymes under normal as well as different pathologic conditions remain to be described.

Histochemical localization of glycolytic, oxidative, and hydrolytic enzymes was done in right and left ventricles, atria, nodal tissue, and Purkinje system in isolated perfused rat hearts under control as well as different experimental conditions including calcium paradox, ischemia-reperfusion and oxygen-free radicals.

Histochemical reaction for glycolytic enzyme glycogen phosphorylase as well as for acetylcholine esterase was more intense in the nodal tissue and Purkinje fibers, whereas oxidative enzymes showed intense reaction in the contractile cells of the ventricles. Calcium paradox caused more histochemical enzyme changes in the subepicardial layer. Ischemia reperfusion had more damaging effect in the subendocardial layer. Oxygen-free radicals caused transmurally focal injuries. This pattern of histochemical changes also correlated with ultrastructural injury in these regions. Ventricular muscle suffered marked decreases, whereas atrial muscle showed a relatively less decrease in different histochemical enzyme activities. Purkinje fibers were the least affected tissue in this respect. The differential changes were most sensitively reflected by glycogen phosphorylase followed by lactate dehydrogenase, alkaline phosphatase, isocitrate dehydrogenase and beta-hydroxybutyric dehydrogenase.

Regional heterogeneity with respect to histochemical distribution of enzymes in a normal heart may have contributed to unique regional responses to different injuries, whereas transmural inhomogeneity in the ventricular response may have to do with the functional and/or metabolic transmural differences.