Energetics of rat papillary muscle during contractions with sinusoidal length changes.

Department of Physiology, Monash University, Clayton, Victoria 3168, Australia.
AJP Heart and Circulatory Physiology (Impact Factor: 4.01). 06/2000; 278(5):H1545-54.
Source: PubMed

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.
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