To the heart of activation heat

Cardiac muscle functions as a molecular machine that converts an electrical stimulus into a Ca²⁺‐dependent mechanical contraction. At the microscopic level, this is determined by cross‐bridge cycling, as well as the combined efforts of several Ca²⁺‐handling proteins, while at the macroscopic level contraction results in the generation of force and heat. Research in the field of cardiac physiology predominantly focuses on how force development is modulated by different (patho)physiological conditions. However, how heat generation is affected is often neglected, usually because of the difficulty of making accurate measurements. In cardiac muscle, heat arises from two distinct sources. Firstly, the detachment of actin–myosin cross‐bridges generates heat as a metabolic by‐product of enzyme‐dependent ATP hydrolysis. Secondly, metabolic recovery of the cardiomyocyte, mediated by sarco(endo)plasmic reticulum Ca²⁺‐ATPase (SERCA) and the sarcolemmal Na⁺/Ca²⁺ exchanger (NCX), utilises ATP hydrolysis to lower the cytosolic Ca²⁺ concentration, thereby terminating the ‘active state’ of the cell and allowing relaxation. The sum of the heat generated by the direct and indirect ATP hydrolysis of SERCA and NCX is collectively known as ‘activation heat’, a term originally described more than four decades ago (Chapman & Gibbs, 1972).

In an experimental setting, accurate measurement of cardiac activation heat, independent of other heat sources, has long been contentious. Despite the absence of macroscopic active force development, heat generated from residual actin–myosin cross‐bridge cycling is difficult to dismiss. Because of this, the activation heat required to restore Ca²⁺ homeostasis, and thus establish an ionic and electric equilibrium within a cardiomyocyte, has often been thought to be overestimated. Historically, the concept of activation heat measurement was developed in skeletal muscle where the muscle can be reversibly stretched to a point at which macroscopic active force development is negligible. However, this stretch‐dependent principle cannot be applied to cardiac muscle, as extension of the cardiac sarcomere beyond maximal stretch is functionally irreversible. To overcome this, Gibbs et al. developed a protocol that incrementally shortens the muscle, instead of lengthening it, to the point where macroscopic active force is negligible. By doing so, it was assumed that any subsequent heat generated by the muscle would be activation heat independent of other heat sources (Gibbs et al. 1967). Despite this methodology, the controversy around residual cross‐bridge detachment heat contamination has remained.

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. This ‘flow‐through’ microcalorimeter, originally described by the same group in 2011 (Taberner et al. 2011), allows heat generation to be accurately determined by measuring small differences in superfusate temperature between up‐stream and down‐stream thermophile arrays. The sensitivity of the developed microcalorimeter is superior to previous measurements. By combining the selective myosin II ATPase inhibitor blebbistatin with the microcalorimeter parallel, real‐time measurement of length‐dependent active force generation and activation heat can be attained, which was previously not possible.

Under physiological stimulation frequency Pham et al. determined, in right and left rat ventricular trabeculae, the muscle length at which maximum active force was produced. From this they stepwise reduced the muscle length to the length at which zero active force was produced, while at each step, both force and heat were simultaneously measured. Introduction of blebbistatin into the superfusate kept heat output constant, while active force decreased to zero at all muscle lengths, which could thereby be attributed to activation heat. Moreover, in the presence of blebbistatin the activation heat estimated from the intercept of the heat–stress relation was not different. From these data the authors made two important conclusions. Firstly, the energy cost of the activation processes is length independent, and secondly, because the residual heat produced at zero force was negligible, the intercept of the heat–force relation provides an accurate estimate of the activation heat in cardiac muscle.

This new approach creates an opportunity to explore other important physiological and pathophysiological cardiac energetic issues. The relative contributions of the Ca²⁺‐lowering structures (SERCA and NCX) to Ca²⁺ homeostasis are species dependent (Bers, 2002), and therefore it is likely that different species have different levels of activation heat, as suggested by the authors (Pham et al. 2017). Based on data from other conditions – adrenaline (epinephrine), elevated extracellular Ca²⁺ 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. 2017). 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. Future simultaneous and accurate measurements of activation heat and force under pathophysiological conditions such as obesity, type 2 diabetes and heart failure might provide valuable new insights into the underlying myocardial energetics of the increasing cohort of these patients. Thus, Pham et al. address a long‐standing dispute in the field of cardiac energetics with a novel approach that could also be used to resolve important (patho)physiological energetic issues in the heart.

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Competing interests

None declared.