Synchronous In Situ ATPase Activity, Mechanics, and Ca2+ Sensitivity of Human and Porcine Myocardium

Flash-frozen myocardium samples provide a valuable means of correlating clinical cardiomyopathies with abnormalities in sarcomeric contractile and biochemical parameters. We examined flash-frozen left-ventricle human cardiomyocyte bundles from healthy donors to determine control parameters for isometric tension (Po) development and Ca2+ sensitivity, while simultaneously measuring actomyosin ATPase activity in situ by a fluorimetric technique. Po was 17 kN m−2 and pCa50% was 5.99 (28°C, I = 130 mM). ATPase activity increased linearly with tension to 132 μM s−1. To determine the influence of flash-freezing, we compared the same parameters in both glycerinated and flash-frozen porcine left-ventricle trabeculae. Po in glycerinated porcine myocardium was 25 kN m−2, and maximum ATPase activity was 183 μM s−1. In flash-frozen porcine myocardium, Po was 16 kN m−2 and maximum ATPase activity was 207 μM s−1. pCa50% was 5.77 in the glycerinated and 5.83 in the flash-frozen sample. Both passive and active stiffness of flash-frozen porcine myocardium were lower than for glycerinated tissue and similar to the human samples. Although lower stiffness and isometric tension development may indicate flash-freezing impairment of axial force transmission, we cannot exclude variability between samples as the cause. ATPase activity and pCa50% were unaffected by flash-freezing. The lower ATPase activity measured in human tissue suggests a slower actomyosin turnover by the contractile proteins.