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

Abstract

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.

Experimental and modelling evidence of shortening heat in cardiac muscle

“…The simulations predict that extrapolation of the work-loop heat-afterload relation to the heat intercept would overestimate Q A because of contamination by shortening heat. Our recent study, using blebbistatin (a cross-bridge inhibitor), has demonstrated the stress (length) independence of Q A (Pham et al 2017), providing further evidence that any sources of heat in excess of Q A are unrelated to activation processes.…”

“…Our recent study, using blebbistatin (a cross‐bridge inhibitor), has demonstrated the stress (length) independence of Q A (Pham et al . ), providing further evidence that any sources of heat in excess of Q A are unrelated to activation processes.…”

To the heart of activation heat

“…Based on data from other conditions – adrenaline (epinephrine), elevated extracellular Ca 2+ and lower temperature, all of which exhibited similar slopes of heat–stress relations – the authors tentatively inferred that activation heat generation is stress independent (see Pham et al . ). However, under these conditions the intercept of the heat–stress relation modestly increased, suggesting that activation heat might not be fully independent of other forms of physiological stress.…”

“…In this issue of The Journal of Physiology, Pham et al have attempted to circumvent the risk of residual actin-myosin cross-bridge heat contamination by selectively inhibiting the myosin II ATPase with blebbistatin and, in doing so, they obtained elegant results that address this contentious issue (Pham et al 2017). They have expanded on traditional cardiac pre-shortening techniques by using a highly sophisticated microcalorimeter that allows simultaneous force and heat measurements from isolated trabeculae.…”

Heart heat separation

“…In this issue of The Journal of Physiology , Pham et al . () report important progress: they demonstrate how activation heat can be determined at different sarcomere lengths in isolated myocardial trabeculae of the rat.…”