Two Small Molecule Inhibitors Of Myosin Decrease Force And Increase Rates Of Relaxation In Demembranated Rat Left Ventricular Tissue

Hypertrophic cardiomyopathy (HCM) is a genetic disease of the heart characterized by thickening of the left ventricle (LV), LV outflow obstruction, hypercontractility, and impaired relaxation. HCM is caused primarily by heritable mutations in sarcomeric proteins related to cardiac contraction, such as cardiac myosin binding protein C (cMyBP-C) and alpha or beta myosin heavy chain (α/βMHC). Current medications in clinical use for HCM do not directly target the underlying contractile changes in the sarcomere. Here, we investigate two small molecule compounds identified in a bovine cardiac myofibril high-throughput screen. These compounds are highly dependent on the presence of regulatory light chain to bind to α/βMHC and modulate its ATPase activity. In demembranated rat LV trabecula, both Compound A and Compound B decrease maximal Ca2+-activated force (pCa 4.0) and Ca2+ sensitivity of force, with Compound B showing larger decreases. Only Compound A showed a decrease in Hill coefficient, suggesting decreased cooperativity. In myofibrils isolated from rat LV, both maximal (pCa 4.0) and submaximal (pCa 5.4) Ca2+-activated force are reduced by nearly 50%. Additionally, the slow phase of relaxation is shorter with a faster rate than DMSO controls. Compound B shows a greater increase in rate than Compound A, especially at pCa 4.0. Both increased the rate of the fast phase of relaxation, but to a greater degree for Compound A at pCa 4.0. The rate of activation was slightly slower for both compounds. We will compare these results to a benchmark myosin S1 binding compound, Compound C. In conclusion, these two compounds inhibited the force and rate of cardiac contraction, while increasing the rate of relaxation, which could improve the underlying contractile changes associated with HCM.