Non-uniformity of myosin ATPase activity has been observed within single cells of rat ventricles during reperfusion following a brief period of ischemia and in old rats. In the reperfused hearts the non-uniformity is transitory. If the period of ischemia is sufficiently brief the contractility of the heart returns to pre-ischemic levels and the non-uniformity in myosin ATPase activity disappears as the contractility recovers.
The specific aim of this project is to determine the cause of this non-uniformity and the conditions that either predispose to it or make it less likely. The abnormality of the myofibrils may have important clinical implications because non-uniformity of myofibrillar function within a single cell may impair the ability of these cells to shorten uniformly and consequently may hamper mechanical performance of the heart. There are two possible explanations for the non-uniformity in myofibrils of old hearts: 1) non-uniform distribution of myosin isozymes; and 2) non-uniform specific activity of a single myosin isozyme. In the reperfused heart, the more likely cause is non-uniform specific activity because the rapidity of appearance is probably too great for change in myosin isozyme content of myofibrils. Ca-activated and actin-activated myosin ATPase will be determined by a microphotometrical technique that can localize and measure ATPase activity in a single A-band. The concentration of a specific isozyme of myosin will be measured in the adjacent serial sections by quantitative immunocytology. Since sections can be as thin as 3u, isozyme distribution in a cell and specific activity of myosin ATPase in a cell can be resolved within 2 sarcomere lengths. The relation of non-uniformity to activity of a cAMP dependent system that regulates myosin will be examined. The spatial relationship of the myofibrils with different myosin ATPase activity to other organelles and of cells with non-uniformity to tissue vasculature will be examined to determine if some causal relationship might exist. The effect of the duration of ischemia, the conditions of reperfusion and the degree of catecholamine stimulation on the appearance and disappearance of non-uniformity will be studied.
