Structural Changes in Both the Troponin Complex and the Thick Filament May Underlie Myofilament Length Dependent Activation

Abstract

The main cellular mechanism that underlies the so-called “Frank-Starling Law of the Heart” is an increase in the responsiveness of cardiac myofilaments to activating Ca2+ at longer sarcomere lengths (SL). The structural basis of this “Length Dependent Activation” (LDA) is not known. 2D X-ray diffraction patterns were obtained using the BioCAT beamline 18ID at the Advanced Photon Source from electrically stimulated (0.2 Hz) intact, twitching papillary muscle isolated from rat hearts during a 10 ms time window in diastole just prior to electrical stimulation. Diffraction patterns were compared from muscles that were stretched to Lmax (SL= ∼2.3 μm) to those taken following a quick release to slack length (SL=∼1.9 μm). We previously reported that myosin heads moved radially inwards at longer SL suggesting that an increased radial extent of crossbridges at longer length cannot explain increased calcium sensitivity so other explanations must be sought. It is known that changes in isoform composition of the troponin complex can markedly affect calcium sensitivity but the role of troponin in the length sensing mechanism underlying LDA is not clear. Here we analyzed the meridional patterns which showed that the 3rd order troponin repeat distance, the 3rd - order troponin reflection intensity and the 2nd order myosin (“forbidden”) meridional reflection all increased significantly (P < 0.01) at Lmax as compared to slack length. Thus, stretching intact heart muscle in diastole induces changes in the structure of both the thick filaments and the thin filaments. It appears, then, that the length sensing mechanism underlying LDA must involve connections of some kind that transmit strain between the thick and thin filament that alter the structure of the troponin complex and, presumably, myofilament contractile properties. Supported by NIH HL075494 and RR08630.