Abstract P315: Shortening The Thick Filament By Partial Deletion Of Titin’S C-zone Alters Cardiac Function By Reducing The Operating Sarcomere Length Range Of The Heart.

Titin’s C-zone is the inextensible part of titin that binds along the thick filament at its cMyBP-C -containing region. Previously it was shown that deletion of titin’s super-repeats C1 and C2 ( Ttn ΔC1-2 mouse model) results in shorter thick filaments and contractile dysfunction, but LV chamber stiffness is normal. Here we studied the contraction-relaxation kinetics from the time-varying elastance of the left ventricle (LV) and from cellular work loops of intact loaded cardiac myocytes. Ca 2+ transients were also measured as well as crossbridge cycling kinetics and Ca 2+ sensitivity of force. It was found that intact cardiomyocytes of Ttn ΔC1-2 mice exhibit systolic dysfunction and impaired relaxation. The time-varying elastance of the LV chamber showed that the kinetics of LV activation are normal but that relaxation is slower in Ttn ΔC1-2 mice. The slowed relaxation was, in part, attributable to an increased myofilament Ca 2+ sensitivity and slower early Ca 2+ reuptake. Dynamic stiffness at the myofilament level showed that cross-bridge kinetics are normal, but that the number of force-generating cross-bridges is reduced. In vivo sarcomere length (SL) measurements in the mid-wall region of the LV revealed that the operating SL range is shifted in Ttn ΔC1-2 mice towards shorter lengths. This normalizes the apparent cell and LV chamber stiffness but reduces the number of force generating cross-bridges due to suboptimal thin and thick filament overlap. Thus the contractile dysfunction in Ttn ΔC1-2 mice is not only due to shorter thick filaments but also to a reduced operating sarcomere length range. Overall these results reveal that for normal cardiac function, thick filament length regulation by titin’s C-zone is critical.