Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC250
The Energetics of Cardiac Trabeculae Undergoing Quasi-Realistic Work-Loops
A. Taberner1,3, J. Han1, P. Nielsen1,3, D. Loiselle1,2
1. Auckland Bioengineering Institute, Auckland, New Zealand. 2. Physiology, University of Auckland, Auckland, New Zealand. 3. Engineering Science, The University of Auckland, Auckland, New Zealand.
An unresolved issue in the field of cardiac energetics is the want of explanation for the well-documented linear relationship between cardiac energy expenditure and pressure-volume-area (PVA)1,2. PVA is given by the sum of the pressure-volume-time ‘work loop’ and the triangular region lying to its left between the end-systolic and end-diastolic pressure-volume relations. In order to address this issue, we have developed a unique flow-through micro-mechano-calorimeter that is capable of measuring, simultaneously, both the force and the heat produced by actively contracting ventricular trabeculae3,4. Adult rats were deeply anaesthetised with isoflurane and their hearts removed. A geometrically-uniform, free-running, right-ventricular trabecula was dissected and mounted in the calorimeter. Mechanical and thermal measurements were made at room temperature in response to various stimulus frequencies. The superfusate was a modified Krebs-Henseleit solution containing 1.5 mM [Ca2+]o. Trabeculae underwent both isometric and fixed-end contractions, as well as quasi-realistic (‘rectangular’) force-length loops designed to mimic pressure-volume loops generated by the heart in vivo. We found the force-length relationship of trabeculae to be non-linear in response to both fixed-end and isometric contractions. The same non-linearity prevailed when preparations underwent force-length (work) loops, whether under variable pre-load or variable after-load. Despite this non-linearity, the heat versus force-length-area (FLA) relation was linear, in accord with VO2-FLA results from ferret papillary muscles5 and VO2-PVA results from canine whole-hearts1. There can be little doubt that the phenomenon is a characteristic of cardiac myocytes per se and is not an emergent property of the three-dimensional whole-heart. We are currently developing mathematical models with the aim of understanding its cellular origin.
Where applicable, experiments conform with Society ethical requirements