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Abstract 905: Length Dependent Activation in Porcine Cardiac Myofilaments is Modulated by Mavacamten


 Length dependent activation (LDA) is a property of muscle where increased sarcomere length (SL) leads to increased force of contraction. Despite its key role in both normal and pathological states, the molecular mechanisms underlying LDA are not understood. Previous studies suggest that increased titin-based passive tension at longer SL triggers structural changes in the troponin complex leading to increased Ca
 2+
 sensitivity. Stretch also appears to release myosin heads from the OFF or super-relaxed state (SRX) increasing the number of available heads for cross-bridge formation, and, hence, maximum force. The small molecule inhibitor of myosin, mavacamten (mava) has been shown to stabilize the SRX state. Here, we used both stretch and mava on permeabilized porcine myocardium for force/pCa measurements coupled with x-ray diffraction to interrogate the mechanisms underlying LDA. Increasing SL from 2.0 to 2.3 um increases Ca
 2+
 sensitivity and elevates both diastolic and maximal tension (i.e. LDA). Mava abolished LDA while blunting the passive stiffness/tension relationship, suggesting increased compliance. The equatorial x-ray reflection intensity ratio, I
 1,1
 /I
 1,0
 , increases with stretch, indicating increased myosin head displacement towards actin. Stretch also decreases the intensities of both the m3 meridional and the MyBPC reflections, suggesting stretch-induced disordering of the thick-filament. Surprisingly, these findings in pig skinned muscle are opposite to those previously reported in intact rat muscle despite robust LDA in both preparations. Consistent with previous studies, stretch increases thick filament strain (indicated by changes in the m6 meridional spacing) and changes in the structure of the troponin complex (indicated by increased intensity of the third order troponin reflection), suggesting that the mechanism for increased Ca
 2+
 sensitivity are similar in both muscle systems. Interestingly, all diffraction pattern changes that occurred with stretch moved in the opposite direction in the presence of mava. Together, these data indicate that mava is not only capable of stabilizing the SRX state of myosin, but can modulate inter-filament interactions responsible for increasing Ca
 2+
 sensitivity and force.
