Significance: This paper represents a major advance in understanding intrinsic heart beat regulation, because it provides an integrated view of the Frank–Starling law that combines mechanical, structural, and energetic aspects of heart performance. We show that mechanosensing in the myosin filament adjusts the number of myosin motors recruited from the off, ATP hydrolysis-unavailable state to the systolic force. In this way, the energetic cost of the heart beat is tuned to the end systolic pressure–volume relation. These results elucidate a plausible mechanistic link between the mutations that affect the structure of the myosin filament and cause hypercontractility and/or loss of contractile efficiency and dysregulation of the control by the stress-sensing mechanism on the state of the myosin motors.

Key points: Force and shortening in muscle are caused by ATP‐driven working strokes of myosin II motors, during their cyclic interactions with the actin filament in each half‐sarcomere. Crystallographic studies indicate that the working stroke consists in an interdomain movement of the myosin motor associated with the release of inorganic phosphate (P_i). Here the coupling of the working stroke with the release of P_i is studied in situ using fast half‐sarcomere mechanics on skinned fibres from rabbit psoas. The isotonic velocity transient following stepwise force reductions superimposed on isometric contraction measures the mechanical manifestation of the working stroke and its rate of regeneration. The results indicate that the release of P_i from the catalytic site of an actin‐attached myosin motor can occur at any stage of the working stroke, and a myosin motor uses two consecutive actin monomers to maximize the power during shortening. Abstract  Skeletal muscle shortens faster against a lower load. This force–velocity relationship is the fundamental determinant of muscle performance in vivo and is due to ATP‐driven working strokes of myosin II motors, during their cyclic interactions with the actin filament in each half‐sarcomere. Crystallographic studies suggest that the working stroke is associated with the release of phosphate (P_i) and consists of 70 deg tilting of a light‐chain domain that connects the catalytic domain of the myosin motor to the myosin tail and filament. However, the coupling of the working stroke with P_i release is still an unsolved question. Using nanometre–microsecond mechanics on skinned muscle fibres, we impose stepwise drops in force on an otherwise isometric contraction and record the isotonic velocity transient, to measure the mechanical manifestation of the working stroke of myosin motors and the rate of its regeneration in relation to the half‐sarcomere load and [P_i]. We show that the rate constant of the working stroke is unaffected by [P_i], while the subsequent transition to steady velocity shortening is accelerated. We propose a new chemo‐mechanical model that reproduces the transient and steady state responses by assuming that: (i) the release of P_i from the catalytic site of a myosin motor can occur at any stage of the working stroke, and (ii) a myosin motor, in an intermediate state of the working stroke, can slip to the next actin monomer during filament sliding. This model explains the efficient action of muscle molecular motors working as an ensemble in the half‐sarcomere.

A conventional five-step chemo-mechanical cycle of the myosin–actin ATPase reaction, which implies myosin detachment from actin upon release of hydrolysis products (ADP and phosphate, Pi) and binding of a new ATP molecule, is able to fit the [Pi] dependence of the force and number of myosin motors during isometric contraction of skeletal muscle. However, this scheme is not able to explain why the isometric ATPase rate of fast skeletal muscle is decreased by an increase in [Pi] much less than the number of motors. The question can be solved assuming the presence of a branch in the cycle: in isometric contraction, when the force generation process by the myosin motor is biased at the start of the working stroke, the motor can detach at an early stage of the ATPase cycle, with Pi still bound to its catalytic site, and then rapidly release the hydrolysis products and bind another ATP. In this way, the model predicts that in fast skeletal muscle the energetic cost of isometric contraction increases with [Pi]. The large dissociation constant of the product release in the branched pathway allows the isometric myosin–actin reaction to fit the equilibrium constant of the ATPase.