Host: R. Perez-Jimenez

The cellular basis of the Frank-Starling mechanism is sarcomere length (SL) modulation of myofilament Ca²⁺ sensitivity (LDA). The molecular mechanism(s) that underlie LDA are unknown, but recent evidence has implicated the giant protein titin as possible sarcomeric strain sensor responsible for LDA by an, as of yet unidentified, signal transduction pathway. Accordingly, the aim of the present study was to elucidate the impact of SL (from slack=2.0 µm to stretch=2.4 µm) on LDA and sarcomere structure in isolated rat myocardium from either wild-type (WT) or mutant (HM) rats expressing a giant splice isoform of titin. At stretch, WT muscles showed reduced increase in passive tension (-20%) and twitch force (-58%); also, LDA was significantly blunted at the myofilament level. Time-resolved small angle x-ray diffraction of intact twitching muscles during diastole revealed at stretch a significant increase in intensity and spacing of: myosin M2 (+121% & +0.4%) and troponin T3 (+174% & +1.0%); Myosin binding protein C (MyoBPC) also trended to increase (C1%C2). These SL dependent changes in sarcomere structure were absent in HM muscles. Cross-bridge radial spacing (layer line analysis) was significantly reduced at stretch in WT (-8.0%); in HM muscles radial spacing was further, but similarly reduced at both slack and stretch (~-20%). Equatorial spacings and intensity ratios were similar between WT and HM at both slack and stretch. Electron density reconstruction revealed, only in WT, increased mass in both thick and thin filament, and the appearance of an as of yet unidentified moiety spanning the space between the thick and thin filaments at stretch.  These results were independently confirmed in skinned myocyte fragments in which endogenous TnC was replaced with fluorescently labeled TnC employing confocal microscopy. Conclusion: Stretch induces structural changes in both thick and thin filaments mediated by titin strain. Moreover, MyoBPC may interact with actin to mediate LDA.