The Role of Titin's N2B Element in Limiting Energy Loss of Mouse Myocardium During Repeated Loading Cycles

Abstract

Utilizing a N2B knockout (KO) mouse model in which the exon that encodes the cardiac-specific N2B element (exon 49) has been deleted, we investigated the mechanical role of the N2B element, one of the three extensible regions of titin. It has been proposed that the extensibility provided by the N2B element limits unfolding of titin's Ig domains that reside in series with the N2B element, thereby reducing energy loss during stretch and shortening (i.e., during diastole and systole) cycles of the beating heart. We were able to show significant increases in hysteresis, a measure of energy loss determined from the area between the stretch and release force-SL curves, through triangular stretch/release protocols using a range of velocities (10, 100 and 1000 % /s) and three amplitudes (0.2, 0.3, and 0.4 µm/sarcomere). In order to more closely simulate physiological conditions, triangular stretch/release traces were concatenated in excess of 1000 cycles to properly precondition the tissue. Results showed that hysteresis decayed over preconditioning cycles (30% of initial hysteresis at 200 preconditioning cycles) but held the significance increase in KO compared to wildtype (WT) mice. (Hysteresis was 650±90pJ/mm2/sarcomere vs. 300±50pJ/mm2/sarcomere, p<0.005, at 200 preconditioning cycles) Using skinned muscle fibers, it is known that they exhibit expanded sarcomere lattice spacing, in order to eliminate this difference we ran this preconditioning protocol in 3% dextran at 37.5oC. Returning the lattice spacing to physiological conditions increased the amount of hysteresis in both WT and KO mice, this increase was concentration dependent though. At 3% dextran the KO held a significant increase in hysteresis over WT tissue (1229±174pJ/mm2/sarcomere vs. 572±65pJ/mm2/sarcomere, p<0.05, at 200 preconditioning cycles).