Titin provides which property in cardiac muscle?

• A. Desmosomal anchoring
• B. Active force generation
• C. Potent passive tension
• D. Calcium reuptake into SR

Answer: (C)

Titin acts as the molecular spring in cardiac muscle, providing potent passive tension as the sarcomere stretches. When the ventricle fills and sarcomere length increases, titin’s elastic regions extend and generate restoring force, contributing most of the passive tension in the myocardium. This passive tension helps set diastolic stiffness and supports the length-dependent activation that underlies the Frank-Starling mechanism.

Desmosomal anchoring involves cell–cell adhesion structures, not a sarcomeric elastic protein. Active force generation comes from actin–myosin cross-bridge cycling, which titin does not power. Calcium reuptake into the sarcoplasmic reticulum is mediated by SERCA pumps, not titin. Titin’s stiffness can vary with isoforms and posttranslational modifications, tuning how much passive tension is produced at a given stretch.