Force generation in experimental tetanus, KCl contracture, and oxygen and glucose deficiency contracture in mammalian myocardium.

We studied the amount and time-course of tension recovery after quick releases (0.25 mm) during experimental tetanus, potassium chloride contracture (KCl), hypoxic contracture (H) as well as joint glucose and oxygen deficiency contracture (HG) in left ventricular papillary muscles of rat and right ventricular papillary muscles of cat myocardium. Both in experimental tetanus and KCl contracture, the tension recovery was finished within 200 ms after the release, and was 77% and 60% of initial tension fall, respectively. The Q10 value for the time constant of half of recovery tension was 2.5, and the time constants differed by the same factor between rat and cat myocardium. In H or HG contracture, we never could find any significant tension recovery process during the first 200 ms after a release, neither in late nor in very early stages of contracture tension generation. However, when longer observation periods (90 s) after a release were monitored, a slow tension recovery was observed which was at least 18% of initial tension fall. This data indicates fast, calcium-mediated cross-bridge cycling in experimental tetanus and KCl contracture. In contrast, the very slow tension recovery in H and HG contracture, which is consistent with recent myothermal data, can be interpreted as rigorlike cross-bridges with a very slow cycling rate, a long time of attachment in force-generating position and low energy turnover.