Omecamtiv Mecarbil Abolishes Length-Mediated Increase in Guinea Pig Cardiac Myofiber Ca2+ Sensitivity

Abstract

Omecamtiv mecarbil (OM) is a pharmacological agent that augments cardiac contractile function by enhancing myofilament Ca sensitivity. Given that interventions that increase myofilament Ca sensitivity have the potential to alter length-dependent activation (LDA) of cardiac myofilaments, we tested the influence of OM on this fundamental property of the heart. This is significant not only because LDA is prominent in cardiac muscle but also because it contributes to the Frank-Starling law, a mechanism by which the heart increases stroke volume in response to an increase in venous return. We measured steady-state and dynamic contractile indices in detergent-skinned guinea pig (Cavia porcellus) cardiac muscle fibers in the absence and presence of 0.3 and 3.0 μM OM at two different sarcomere lengths (SLs), short SL (1.9 μm) and long SL (2.3 μm). Myofilament Ca sensitivity, as measured by pCa (-log of [Ca] concentration required for half-maximal activation), increased significantly at both short and long SLs in OM-treated fibers when compared to untreated fibers; however, the magnitude of increase in pCa was twofold greater at short SL than at long SL. A consequence of this greater increase in pCa at short SL was that pCa did not increase any further at long SL, suggesting that OM abolished the SL dependency of pCa. Furthermore, the SL dependency of rate constants of cross-bridge distortion dynamics (c) and force redevelopment (k) was abolished in 0.3-μM-OM-treated fibers. The negative impact of OM on the SL dependency of pCa, c, and k was also observed in 3.0-μM-OM-treated fibers, indicating that cooperative mechanisms linked to LDA were altered by the OM-mediated effects on cardiac myofilaments.

Omecamtiv mecarbil lowers the contractile deficit in a mouse model of nebulin-based nemaline myopathy

“…It can also explain the reduced OM effects at high OM levels reported by Nagy et al for both cardiac myocytes and diaphragm fibers[42], and seen here in soleus fiber bundles (S1A Fig): at high OM levels the OM-bound heads that do not generate force outnumber the additionally recruited OM-free force generating heads. The dose-response curve that we measured on cardiac muscle does not decline at high OM levels (S1B Fig) a finding similar to that of Gollapudi et al (their Fig 2)[41]. This can be explained by the short sarcomere length that was used in these studies (2.0 μm in our study and 1.9 μm in [41]), where baseline activation is low, and the long sarcomere length used by Nagy et al in their cardiac myocyte study (2.3 μm) where the baseline activation level is high.…”

“…From these studies, a model was proposed in which these OM-bound heads cooperatively activate the thin filament at submaximal activation levels, recruiting OM-free myosin heads and thereby increasing force [20]. This mechanism naturally explains the sarcomere length dependence of the magnitude of the OM-effect that has been found in sub-maximally activated cardiac muscle[41], with larger OM effects at short sarcomere length where the baseline activation level of the thin filament is lower and more OM-free heads can be recruited to the force generating pool than at longer sarcomere length. It can also explain the reduced OM effects at high OM levels reported by Nagy et al for both cardiac myocytes and diaphragm fibers[42], and seen here in soleus fiber bundles (S1A Fig): at high OM levels the OM-bound heads that do not generate force outnumber the additionally recruited OM-free force generating heads.…”

Cardiomyopathy mutation (F88L) in troponin T abolishes length dependency of myofilament Ca2+ sensitivity

“…We posit that increased XB-based cooperativity counteracts the negative impact of strained XBs on force-bearing XBs, leading to a decrease in γ at short SL. The augmenting effect of TnT F88L on XB-based cooperativity at short SL is also supported by the magnitude of ML-mediated XB recruitment, E R , which is sensitive to changes in XB-based cooperativity ( Campbell et al, 2004 ; Campbell and Chandra, 2006 ; Stelzer et al, 2006 ; Gollapudi et al, 2017 ). TnT F88L significantly increases E R at short SL (39%) but not at long SL, suggesting that TnT F88L imparts a greater effect on XB-based cooperativity at short SL.…”

Positive cardiac inotrope omecamtiv mecarbil activates muscle despite suppressing the myosin working stroke

“…Biochemical kinetic experiments have suggested that OM does not affect the actomyosin-detachment rate during cycling 1 , 6 , which is at odds with our findings. However, studies of tension development and relaxation rates of cardiac muscle preparations have indicated that detachment may be slowed by OM 11 , 12 , 34 . The previous biochemical conclusions 1 , 6 were based on the measurement of the rate of ADP release (the step that limits the rate of actin detachment at physiological ATP) from an AM·ADP complex formed by adding ADP to the rigor, AM complex in isolated actomyosin 1 , 6 , 9 Our results indicate that the step that limits actin detachment is not accessible by simply adding ADP, but occurs earlier in the cycle and may not be on the conventional ATPase pathway (Fig.…”