Effect of Ca++ on Vmax measured in absence of external or internal load

Myofibrils 1 sarcomere wide (single myofibrils) were prepared from rat heart. Mean initial sarcomere length was 2.68 mu. Contraction was initiated with 20, 25 or 100 muM MgATP and terminated with EGTA and EDTA. Single fibrils shortened at the theoretical Vmax for the in vitro conditions until sarcomere length reached 2.05 mu. At this length they encountered a significant internal afterload. Reaction time, substrate, and temperature were adjusted so that final sarcomere length was greater than or equal to 2.05 mu in most experiments. Ca++ increased Vmax of single fibrils 2- 4-fold. Results were similar at 20 and 100 muM MgATP, and in reaction mixture containing 100 or 140 mM KCI. We conclude that Ca++ controls not only the number of cross-bridges, but also the rate at which the cross-bridges turn over.