Energetics of stress production in isolated cardiac trabeculae from the rat

Abstract

The heat liberated upon stress production in isolated cardiac muscle provides insights into the complex thermodynamic processes underlying mechanical contraction. To that end, we simultaneously measured the heat and stress (force per cross-sectional area) production of cardiac trabeculae from rats using a flow-through micromechanocalorimeter. In a flowing stream of O2-equilibrated Tyrode solution (ϳ22°C), the stress and heat production of actively contracting trabeculae were varied by 1) altering stimulus frequency (0.2-4 Hz) at optimal muscle length (Lo), 2) reducing muscle length below Lo at 0.2 and 2 Hz, and 3) changing extracellular Ca 2ϩ concentrations ([Ca 2ϩ ]o; 1 and 2 mM). Linear regression lines were adequate to fit the active heat-stress data. The active heat-stress relationships were independent of stimulus frequency and muscle length but were dependent on [Ca 2ϩ ]o, having greater intercepts at 2 mM [Ca 2ϩ ]o than at 1 mM [Ca 2ϩ ]o (3.5 and 2.0 kJ·m Ϫ3 ·twitch Ϫ1 , respectively). The slopes among the heat-stress relationships did not differ. At the highest experimental stimulus frequency, pronounced elevation of diastolic Ca 2ϩ resulted in incomplete twitch relaxation. The resulting increase of diastolic stress, which occurred with negligible metabolic energy expenditure, subsequently diminished due to the time-dependent loss of myofilament Ca 2ϩ -sensitivity.cardiac thermodynamics; heat-stress relationships; dynamic stiffness; diastolic calcium, myofilament calcium sensitivity WHEN AN ISOLATED CARDIAC MUSCLE, held fixed at both ends, is electrically stimulated, twitch force is produced. A consequence of this mechanical contraction is the simultaneous liberation of heat. Thus, the relationship between heat and stress (force per cross-sectional area) production provides insights into the complex thermomechanical processes occurring within the muscle. In selecting suitable isolated multicellular tissue preparations for in vitro experiments, two important criteria have to be met. First, for an unambiguous interpretation of stress production, the myocytes should be aligned in parallel with the direction of force measurement. Second, to avoid the risk of tissue anoxia, the preparation should be sufficiently small in radial dimension for O 2 to diffuse into the muscle core under high rates of O 2 demand. The first criterion can be achieved using either papillary muscles or cardiac trabeculae. Compared with papillary muscles, cardiac trabeculae have cross-sectional areas an order of magnitude smaller. Thus, to satisfy the second criterion, cardiac trabeculae are preferable, owing to their minute radial dimensions (about that of a human hair), which greatly facilitates the diffusion of O 2 .Using a flow-through micromechanocalorimeter (11), we simultaneously measured the heat and stress production of cardiac trabeculae excised from the right ventricles of rat hearts. Optimized in vitro metabolic conditions were achieved by continuous provision of O 2 and removal of waste products, enabling measureme...

A high-resolution thermoelectric module-based calorimeter for measuring the energetics of isolated ventricular trabeculae at body temperature

“…Following the methods outlined in Han et al (17), a free-running trabecula was dissected out of the right ventricle, mounted in the calorimeter, and positioned within the measurement chamber (Fig. 1).…”

“…Studies of force production simultaneous with heat output of cardiac muscle require the use of isolated tissue preparations such as small papillary muscles (2-4, 12, 26, 34) or trabeculae carneae (17)(18)(19)(20)(21) as they are sufficiently small to avoid muscle anoxia (14). The use of these tissue preparations, which have approximately one-dimensional arrangements of myocytes, simplifies the interpretation of measurements.…”

Muscle heat: a window into the thermodynamics of a molecular machine

“…The first version (Fig. 8) used thin-film infra-red thermopile sensors in "conduction mode" (64) and allowed the heat output of rat right-ventricular trabeculae undergoing fixed-end contractions to be made (65). Subsequent modifications to both hardware and software (132) achieved both truly isometric contractions as well as isotonic "workloop" contractions at various afterloads.…”

An innovative work-loop calorimeter for in vitro measurement of the mechanics and energetics of working cardiac trabeculae

“…Extirpation of trabeculae has been described in detail previously (8,9). Briefly, a suitable preparation was dissected from either ventricle of a rat heart under a dissection solution comprising (in mM): 130 NaCl, 6 KCl, 1 MgCl 2, 0.5 NaH2PO4, 0.3 CaCl2, 10 HEPES, 10 glucose, and 20 BDM (2,3-butanedione monoxime).…”