Determinants of Hypoxic and Posthypoxic Myocardial Contracture

The relationship between contracture (a rise in resting tension at constant length) and contractile activity was studied in isometric rat trabeculae carneae and cat right ventricular papillary muscles. Muscles at Lmax were paced at 12/min in solutions equilibrated at 29 degrees C with 95% O2-5% CO2. In rat myocardium, exposure to 60 min hypoxia (95% N2-5% CO2) at alkalotic pH produced contracture. In cat myocardium contracture was produced by the combination of exposure to digitalis and the recovery from hypoxia. Delayed relaxation was not responsible for the rise in resting tension in either preparation. In muscles not paced during hypoxia, contracture appeared despite total absence of contractile activity. Exposure to low calcium or acidosis reversed or prevented the rise in resting tension. Thus, contracture can be: 1) dynamic, reversible, and modified by factors which affect intracellular calcium availability; 2) independent of contractile activity and apparent activity of the relaxing system; and 3) distinct from delayed or incomplete relaxation. The extent of contracture during the recovery from hypoxia is not simply the function of the level of contractile activity during hypoxia but appears related to calcium availability at a critical time.