Lysosomal Hypothesis in Evolution of Myocardial Infarction

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Abstract
Twenty-two cat hearts were perfused according to the Langendorff technique and myocardial regional ischemia was induced by occlusion of the left anterior coronary artery. Separation of particulate (bound) from soluble (free) fraction and subsequent fractionation into plasma membranes, lysosomes, sarcoplasmic reticulum and mitochondria were performed by sucrose density gradient ultracentrifugation. With ischemia for 60 min, particle-bound activity of cathepsin D decreased from 4.2 .+-. 0.24 U[unit]/mg protein of 3.2 .+-. 0.31 U/mg protein (P < 0.01). The particle-bound activity of .beta.-glucuronidase decreased from 11.9 .+-. 0.92 U/mg protein to 6.2 .+-. 1.28 U/mg protein (P < 0.01). Free/bound activity ratios of cathepsin D increased from 0.8 to 1.9 and .beta.-glucuronidase from 0.9 to 2.8, respectively. A conspicuous fall from 12.8 .+-. 0.6 U/mg protein to 8.0 .+-. 0.97 U/mg protein (P < 0.01) in absolute specific activity of cathepsin D bound to the lysosomal fraction presents definitive evidence of lysosomal release of the acid hydrolases during the early phase of myocardial ischemia. EM observation of the ischemic myocytes revealed ultrastructural alterations of the lysosomes suggestive of autophagic degradation of various subcellular organelles.

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