Piperine disrupts the OFF state of the resting thick filament of rat skeletal muscle, enhancing dynamic contractility in a fibre-type-dependent manner.

The myosin-containing thick filament has been shown to alter its resting structure in response to multiple diseases and therapeutics. Changes in thick filament resting structure are caused by myosin heads transitioning between ordered and disordered OFF conformational states. Functionally, this modulation of thick filament structure is a key regulatory step in muscle contraction and a promising therapeutic target. The availability of disordered myosin heads governs dynamic contractility, which is critical to physical function and well-being. At present, there is a lack of compounds favouring this disordered state in resting skeletal muscle. Piperine is a molecule known to bind to myosin and increase submaximal isometric contractility in fast and slow skeletal muscle. However, the effects on dynamic contractility and the underlying mechanism responsible for the observed effects in skeletal muscles remain unclear. Here, we used fibre small-angle X-ray diffraction and intact-muscle ex vivo contractility experiments to determine the effects of piperine on resting myosin structure and dynamic contractility in fast and slow rat muscles. X-ray diffraction data suggest that piperine promotes a priming of myosin in resting skeletal muscle from an ordered OFF state to a disordered OFF state, increasing the availability of myosin heads for force generation. Functionally, piperine substantially enhanced submaximal dynamic contractility in both muscle types, while only leading to improvements in slow muscle during maximal electrical stimulation. These findings establish piperine as a tool for priming the thick filament in skeletal muscle, highlighting muscle-type-specific effects of thick-filament modulation on the recruitment of the contractile reserve capacity. KEY POINTS: Piperine is a compound known to bind to skeletal muscle myosin and enhance isometric contractility in fast and slow muscles, but its effect on dynamic muscle function and the underlying molecular mechanism remain unknown. We show that piperine disrupts the ordered OFF state of the myosin-containing thick filament in resting fast and slow skeletal muscle, which likely explains the effect of piperine on contractile function. Piperine substantially increases contractile power of both fast and slow skeletal muscles at low-frequency stimulation; however, it only enhances power in slow skeletal muscle at high-frequency stimulation. Our data reveal potentiation of dynamic contractility with fibre-type-dependent magnitudes in response to piperine-induced priming of the resting thick filament, which is a phenomenon requiring further investigation. Dynamic contractility drives locomotion in vivo; therefore, thick-filament priming may ultimately be exploited in the treatment of diseases characterised by skeletal muscle weakness.