Heat production in quiescent cardiac muscle is length-, velocity-, and muscle-dependent: Implications for active heat measurement.

Abstract

<h4>New findings</h4>What is the central question of this study? Intracellular energetic processes in quiescent cardiac muscle release 'basal' heat; during contraction, a much larger amount of 'active' heat is also produced. Measurement challenges have previously constrained researchers to assume that basal heat rate remains constant during contraction and shortening. Is this assumption correct? What is the main finding and its importance? Our results overturn convention: basal heat rate is modulated by the extent and velocity of muscle shortening. Their relative contributions are muscle-specific. We apply a method with which researchers can now disentangle, during each experiment, changes in basal heat from active heat production, providing more precise measures of the individual energetic processes underlying cardiac muscle contraction.<h4>Abstract</h4>Separating the variations in cardiac basal heat rate from variations in active heat rate is necessary to accurately determine cardiac muscle energy consumption during the performance of active work. By developing a model of cardiac muscle basal heat rate, we aim to investigate changes in basal heat rate when cardiac muscle performs work. Experiments were conducted on ten isolated rat cardiac trabeculae subjected to both active (work-loops) and quiescent (length-change and velocity) interventions. Muscle force, length and heat rate output were simultaneously measured in a flow-through work-loop calorimeter. Quiescent muscle characteristics were utilized to parameterize muscle-specific models of change in basal heat rate, and thereby to predict dynamic changes in basal heat rate during active work-loop contraction. Our data show that the quiescent heat characteristics of cardiac muscle varied between samples, displaying dependence on both the extent and the rate of muscle length change. We found a moderate correlation between muscle dimensions (cross-sectional area and volume) and the length-dependent basal heat parameter (p-value = 0.0330 and p-value = 0.0242, respectively), but no correlation with the velocity-dependent parameter. These findings lead us to conclude that the heat output of cardiac muscle at quiescence varies, with both extent and velocity of shortening, to an extent that is muscle-specific, and that this variation must be measured, and accounted for, in each specimen, when assessing active energetics. This article is protected by copyright. All rights reserved.