Measuring the mechanical efficiency of a working cardiac muscle sample at body temperature using a flow-through calorimeter

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Activation heat arises from two sources during the contraction of striated muscle. It reflects the metabolic expenditure associated with Ca pumping by the sarcoplasmic reticular Ca -ATPase and Ca translocation by the Na /Ca exchanger coupled to the Na ,K -ATPase. In cardiac preparations, investigators are constrained in estimating its magnitude by reducing muscle length to the point where macroscopic twitch force vanishes. But this experimental protocol has been criticised since, at zero force, the observed heat may be contaminated by residual crossbridge cycling activity. To eliminate this concern, the putative thermal contribution from crossbridge cycling activity must be abolished, at least at minimal muscle length. We achieved this using blebbistatin, a selective inhibitor of myosin II ATPase. Using a microcalorimeter, we measured the force production and heat output, as functions of muscle length, of isolated rat trabeculae from both ventricles contracting isometrically at 5 Hz and at 37°C. In the presence of blebbistatin (15 μmol l ), active force was zero but heat output remained constant, at all muscle lengths. Activation heat measured in the presence of blebbistatin was not different from that estimated from the intercept of the heat-stress relation in its absence. We thus reached two conclusions. First, activation heat is independent of muscle length. Second, residual crossbridge heat is negligible at zero active force; hence, the intercept of the cardiac heat-force relation provides an estimate of activation heat uncontaminated by crossbridge cycling. Both results resolve long-standing disputes in the literature.

Section: Limitations Of the Studymentioning

confidence: 75%

Does the intercept of the heat–stress relation provide an accurate estimate of cardiac activation heat?

Pham

Tran

Mellor

et al. 2017

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“…B is insufficient to generate detectably greater shortening heat, despite the 10 nW resolution of our microcalorimeter (Taberner et al . ). Hence we see no energetic disadvantage conferred on the right ventricle, in terms of heat of shortening, as a consequence of its greater velocity of shortening.…”

Section: Discussionmentioning

confidence: 97%

“…) with a 10‐fold increase in thermal resolution (10 nW) (Taberner et al . ). In the measurement chamber, the trabecula was held between two platinum hooks connected to a custom‐built force transducer at one end and a linear motor for length control at the other.…”

Section: Methodsmentioning

confidence: 97%

Do right‐ventricular trabeculae gain energetic advantage from having a greater velocity of shortening?

Pham

Han

Taberner

et al. 2017

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Our study aimed to ascertain whether the interventricular difference of shortening velocity, reported for isolated cardiac tissues in vitro, affects interventricular mechano-energetic performance when tested under physiological conditions using a shortening protocol designed to mimic those in vivo. We isolated trabeculae from both ventricles of the rat, mounted them in a calorimeter, and performed experiments at 37°C and 5 Hz stimulus frequency to emulate conditions of the rat heart in vivo. Each trabecula was subjected to two experimental protocols: (i) isotonic work-loop contractions at a variety of afterloads, and (ii) isometric contractions at a variety of preloads. Velocity of shortening was calculated from the former protocol during the isotonic shortening phase of the contraction. Simultaneous measurements of force-length work and heat output allowed calculation of mechanical efficiency. The shortening-dependent thermal component was quantified from the difference in heat output between the two protocols. Our results show that both extent of shortening and velocity of shortening were higher in trabeculae from the right ventricle. Despite these differences, trabeculae from both ventricles developed the same stress, performed the same work, liberated the same amount of heat, and hence operated at the same mechanical efficiency. Shortening heat was also ventricle independent. The interventricular differences in velocity of shortening and extent of shortening of isolated trabeculae were not manifested in any index of energetics. These collective results underscore the absence of any mechano-energetic advantage or disadvantage conferred on right-ventricular trabeculae arising from their superior velocity of shortening.

“…In these experiments, series of work-loop contractions were executed commencing at optimal muscle length and two suboptimal muscle lengths: 0.95L o and 0.90L o . These experiments were performed at 37°C and 4 Hz stimulation frequency in the same flow-through microcalorimeter but upgraded to have a 10-fold higher thermal resolution Taberner et al 2015).…”

Section: Methodsmentioning

confidence: 99%

Experimental and modelling evidence of shortening heat in cardiac muscle

Tran¹,

Han²,

Crampin

et al. 2017

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When a muscle shortens against an afterload, the heat that it liberates is greater than that produced by the same muscle contracting isometrically at the same level of force. This excess heat is defined as 'shortening heat', and has been repeatedly demonstrated in skeletal muscle but not in cardiac muscle. Given the micro-structural similarities between these two muscle types, and since we imagine that shortening heat is the thermal accompaniment of cross-bridge cycling, we have re-examined this issue. Using our flow-through microcalorimeter, we measured force and heat generated by isolated rat trabeculae undergoing isometric contractions at different muscle lengths and work-loop (shortening) contractions at different afterloads. We simulated these experimental protocols using a thermodynamically constrained model of cross-bridge cycling and probed the mechanisms underpinning shortening heat. Predictions generated by the model were subsequently validated by a further set of experiments. Both our experimental and modelling results show convincing evidence for the existence of shortening heat in cardiac muscle. Its magnitude is inversely related to the afterload or, equivalently, directly related to the extent of shortening. Computational simulations reveal that the heat of shortening arises from the cycling of cross-bridges, and that the rate of ATP hydrolysis is more sensitive to change of muscle length than to change of afterload. Our results clarify a long-standing uncertainty in the field of cardiac muscle energetics.