Energetics of rat papillary muscle during contractions with sinusoidal length changes.
ABSTRACT The mechanical efficiency of rat cardiac muscle was determined using a contraction protocol involving cyclical, sinusoidal length changes and phasic stimulation at physiological frequencies (1-4 Hz). Experiments were performed in vitro (27 degrees C) using rat left ventricular papillary muscles. Efficiency was determined from measurements of the net work performed and enthalpy produced by muscles during a series of 40 contractions. Net mechanical efficiency was defined as the percentage of the total, suprabasal enthalpy output that appeared as mechanical work. Maximum efficiency was approximately 15% at contraction frequencies between 2 and 2.5 Hz. At lower and higher frequencies, efficiency was approximately 10%. Enthalpy output per cycle was independent of cycle frequency at all but the highest frequency used. The basis of the high efficiency between 2 and 2.5 Hz was that work output was also greatest at these frequencies. At these frequencies, the duration of the applied length change was well matched to the kinetics of force generation, and active force generation occurred throughout the shortening period.
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ABSTRACT: We compare the energetics of right ventricular (RV) and left ventricular (LV) trabeculae carneae isolated from rat hearts. Using our work loop calorimeter, we subjected trabeculae to stress length work (W), designed to mimic the pressure volume work of the heart. Simultaneous measurement of heat production (Q), allowed calculation of the accompanying change of enthalpy (ΔH = W + Q). From the mechanical measurements (i.e. stress and change of length), we calculated work, shortening velocity and power. In combination with heat measurements, we calculated activation heat (Q(A)), crossbridge heat (Q(xb)) and two measures of cardiac efficiency: 'mechanical efficiency' (ε(mech)= W/ΔH) and 'crossbridge efficiency' (ε(xb) = W/(ΔH-Q(A))). With respect to their LV counterparts, RV trabeculae have higher peak shortening velocity, and higher peak mechanical efficiency, but with no difference of stress development, twitch duration, work performance, shortening power, or crossbridge efficiency. That is, the 35% greater maximum mechanical efficiency of RV than LV trabeculae (13.6% versus 10.2%) is off-set by the greater metabolic cost of activation (Q(A)) in the latter. When corrected for this difference, crossbridge efficiency does not differ between the ventricles.The Journal of Physiology 11/2012; · 4.38 Impact Factor
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ABSTRACT: 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 modi- fied 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.  Suga H (1990) Ventricular energetics. Physiol. Rev. 70: 247-277.  Loiselle DS, Crampin EJ, Niederer SA, Smith NP and Barclay CJ (2008) Energetic consequences of mechanical loads. Prog. Biophys. Molec. Biol. 97: 348-366.  Han J-C, Taberner AJ, Kirton RS, Nielsen PMF, Smith NP and Loiselle DS (2009) A unique micromechanocalorimeter for simultaneous measurement of heat rate and force production of cardiac trabeculae carneae. J. Appl. Physiol. 107: 946-951.  Taberner AJ, Hunter IW, Kirton RS, Nielsen PMF and Loiselle DS (2005) Characterization of a flow-through microcalorimeter for measuring the heat production of cardiac trabeculae. Review of Scientific Instruments 76: 104902: 1-7.  Hisano R and Cooper G (1987) Correlation of force-length area with oxygen consumption in ferret papillary muscle. Circ. Res. 61: 318-328.Physiology 2011;
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ABSTRACT: The aim of this study was to determine whether the net efficiency of mammalian muscles depends on muscle fibre type. Experiments were performed in vitro (35°C) using bundles of muscle fibres from the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles of the mouse. The contraction protocol consisted of 10 brief contractions, with a cyclic length change in each contraction cycle. Work output and heat production were measured and enthalpy output (work + heat) was used as the index of energy expenditure. Initial efficiency was defined as the ratio of work output to enthalpy output during the first 1 s of activity. Net efficiency was defined as the ratio of the total work produced in all the contractions to the total, suprabasal enthalpy produced in response to the contraction series, i.e. net efficiency incorporates both initial and recovery metabolism. Initial efficiency was greater in soleus (30 ± 1%; n= 6) than EDL (23 ± 1%; n= 6) but there was no difference in net efficiency between the two muscles (12.6 ± 0.7% for soleus and 11.7 ± 0.5% for EDL). Therefore, more recovery heat was produced per unit of initial energy expenditure in soleus than EDL. The calculated efficiency of oxidative phosphorylation was lower in soleus than EDL. The difference in recovery metabolism between soleus and EDL is unlikely to be due to effects of changes in intracellular pH on the enthalpy change associated with PCr hydrolysis. It is suggested that the functionally important specialization of slow-twitch muscle is its low rate of energy use rather than high efficiency.The Journal of Physiology 09/2004; 559(2). · 4.38 Impact Factor