Length-dependence of Actin–Myosin Interaction in Skinned Cardiac Muscle Fibers in Rigor

It has been suggested that the length dependence of myofilament Ca2+sensitivity and of Ca2+binding to troponin C, observed over the ascending limb of the cardiac force–length curve, is based on variation in the number of interacting cross-bridges. This interaction would be reduced at short sarcomere length as a consequence of double overlap of oppositely polarized actin filaments and increased lateral separation of actin and myosin filaments. Based on current evidence, it is not clear to what extent the actin–myosin interaction is hindered at sarcomere lengths where Ca2+sensitivity is reduced. We have used two biochemical assays to assess cross-bridge attachment in rigor muscle at sarcomere lengths corresponding to the ascending limb of the cardiac force–length curve. These are based on (1) the inhibition of K+-activated myosin ATPase by the complexation of actin with myosin, and (2) the enhancement of Ca2+binding to troponin C by rigor bridge attachment to actin. Measurements were made with skinned fibers from bovine ventricle. As a check on our method, measurements were also made with skinned rabbit psoas muscle fibers. With both muscle types, a reduction in sarcomere length along the ascending limb of the force–length curve was associated with an increase in K+-activated ATPase activity and a reduction in Ca2+binding to the regulatory sites of troponin C. These results indicate that actin–myosin interaction is significantly reduced at short sarcomere length.