Deletion of the titin N2B region accelerates myofibrillar force development but does not alter relaxation kinetics

Cardiac titin is the main determinant of sarcomere stiffness during diastolic relaxation. To explore whether titin stiffness affects the kinetics of cardiac myofibrillar contraction and relaxation, we used subcellular myofibrils from the left ventricles of homozygous and heterozygous N2B-knockout mice which express truncated cardiac titins lacking the unique elastic N2B region. Compared with myofibrils from wild-type mice, myofibrils from knockout and heterozygous mice exhibit increased passive myofibrillar stiffness. To determine the kinetics of Ca²⁺+-induced force development (rate constant k_ACT), myofibrils from knockout, heterozygous and wildtype mice were stretched to the same sarcomere length (2.3 μm) and rapidly activated with Ca²⁺+. Additionally, mechanically induced force-redevelopment kinetics (rate constant k_TR) were determined by slackening and re-stretching myofibrils during Ca²⁺+-mediated activation. Myofibrils from knockout mice exhibited significantly higher k_ACT, k_TR and maximum Ca²⁺+-activated tension than myofibrils from wild-type mice. By contrast, the kinetic parameters of biphasic force relaxation induced by rapidly reducing [Ca²⁺+] were not significantly different among the three genotypes. These results indicate that increased titin stiffness promotes myocardial contraction by accelerating the formation of force-generating crossbridges without decelerating relaxation.