Sarcomere Length Dependent Effects on Ca2+-Induced Troponin Regulation within Chemically Skinned Cardiac Muscle Fibers

Abstract

Sarcomere length dependent activation (LDA) of myocardial force development is the cellular basis underlying the Frank-Starling law of the heart, but it is still elusive how sarcomere detects the SL changes and converts it into altered activation of thin filament. Among the myofilament proteins, troponin and myosin are two key components that are likely involved in the LDA. The two components are functionally linked through Ca2+ activation and cross-bridge feedback. Although the active cross-bridge feedback has been strongly implicated in LDA, there was no evidence linking sarcomere length changes and troponin regulation until our recent study showing that Ca2+-cTnC interaction in cardiac muscle can be modulated by sarcomere length through cross-bridge feedback [Biophysical Journal, 107(3), 682-93 (2014)]. In this study, we continue our efforts to understand the role of the C-domain of cTnI in LDA. Specifically, we used in situ time-resolved FRET measurements to determine how the switch region of cTnI is affected by Ca2+, sarcomere length, and cross-bridge in skinned cardiac muscle fibers. To monitor the Ca2+-induced structural transition of the switch region, skinned myocardial fibers were reconstituted with troponin complex containing FRET donor (AEDANS) modified cTnI(167C)AEDANS and acceptor (DDPM) modified cTnC(89C)DDPM. The measured FRET distance changes show that Ca2+, strong cross-bridges and sarcomere length all influence the structural transition of the switching region of cTnI within myocardial fibers. The results provide a mechanism by which sarcomere length can modulate Ca2+-troponin regulation via strong cross-bridge binding and suggest that the sarcomere length dependent cross-bridge effect plays an important role in the Frank-Starling law of the heart.