Reexamination of the Applicability of the HIII Model of Muscle to Cat Myocardium

In cat papillary muscles, which were perfused with oxygenated physiologic saline at 26°C and contracting isometrically at 15 beats/min, shortening steps were imposed 330 msec after stimulation-near the peak of isometric twitch force-and 170 msec or 250 msec after stimulation-during force development. The force immediately following the shortening step was measured. From the results at 330 msec, we predicted the results at the earlier times assuming that the sudden drop in force was due to recoil of a series elastic component (SEC) alone; however, the actual results differed from the predicted results. The shortening step required to drop the force to zero at the earlier times was greater than was predicted; in other words, the force immediately following a shortening step predicted to drop the force to zero was finite. The results are not compatible with the hypothesis that the sudden drop in force accompanying the shortening step (the quick-release phenomenon) is due to the sudden recoil of the SEC, which is stretched to different lengths at the two times. They are compatible with the hypotheses that (1) the quick-release phenomenon is a property of the contractile component or (2) the SEC compliance is a function of the degree of activation. Therefore, we concluded that nonseries elasticity contributes to the quick-release phenomenon and should be considered when the Hill model is applied to heart muscle. In particular, isometric contraction cannot be modeled by a contractile component stretching an SEC of constant physical characteristics.