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Initial mechanical efficiency of isolated cardiac muscle

C. J. Barclay^1,*, C. Widén² and L. J. Mellors¹

¹ Department of Physiology, Monash University, Clayton, Victoria 3800, Australia
² School of Physiotherapy and Exercise Science, Griffith University, Gold Coast, Queensland 9726, Australia

View larger version (11K): [in a new window]   Fig. 1. An example of records, averaged over the last 10 cycles (4.54 s) of the contraction protocol, from one papillary muscle (mass, 3.1 mg; Lmax, 5.9 mm). Stimulus was applied at 0 ms. (A) Change in muscle length. The muscle was initially held isometric, was then allowed to shorten at constant velocity through an amplitude of 10% Lmax and was then lengthened at constant velocity. (B) Absolute force output. The horizontal broken line indicates the passive force at Lmax. The force was lower after shortening because the passive force was lower at the short length. The muscle was relaxed during most of the lengthening phase. (C) Time course of work output and total enthalpy output (work + heat). The vertical broken line indicates the time at which shortening ended.

View larger version (15K): [in a new window]   Fig. 2. Records of muscle energy output and illustration of steps in analysis. (A) Cumulative outputs of work, heat and enthalpy (i.e. heat + work) from a rat papillary muscle preparation (mass, 2.97 mg; length, 5.0 mm) during and after a series of 40 twitches delivered at 2.2 Hz. (B) The time course of rate of enthalpy output, calculated by differentiation of the cumulative enthalpy record in A. A single exponential was fitted through the signal recorded after the contractions had ended, indicated by the declining white line from 18.8 s onwards, to determine the time constant for the decline in rate of recovery heat output (=8.4 s). The calculated rate of recovery heat output during the contractions is also shown (white line between 0 s and 18.8 s). The R:I ratio for this preparation was 1.16. (C) The calculated cumulative initial enthalpy output (i.e. initial heat output + work output) for all 40 contractions and recovery heat output. Work output is also shown (broken line).

View larger version (6K): [in a new window]   Fig. 3. The calculated time courses of production of initial and recovery enthalpies. The total enthalpy record is the same as that in Fig. 2C. Records are the averages of those for the last 10 cycles of the contraction protocol. The vertical broken line indicates the time at which shortening ended. Initial enthalpy is the sum of the work output and initial heat output. All the energy from recovery processes appeared as heat. In this example, 75% of the initial enthalpy was produced by the end of shortening. Note that, although there was slightly more initial enthalpy than recovery enthalpy produced in the cycle, recovery enthalpy output continued after the contractions were finished so that the total recovery enthalpy output was greater than the total initial enthalpy output.

View larger version (13K): [in a new window]   Fig. 4. Analysis of O2 supply to isolated papillary muscles. (A) Partial pressure of O2 (PO) profile through the cross-section of a cylindrical muscle. 1 atm=1.013x105 Pa. The thickest line shows the PO profile for a preparation with a radius equal to the mean value (0.41 mm) of those used in the study. The results for the smallest and largest preparations used (upper and lower lines, respectively) are also shown. Analysis was performed using the following parameters: PO at muscle surface, 0.95 atm (horizontal broken line); diffusivity of O2, 2.39x10-5 cm2 atm-1 min-1 at 27°C, adjusted to 30°C using a Q10 of 1.04 (Mahler et al., 1985); steady-state rate of active metabolism, 9.7 mW g-1 (present study; equivalent rate of O2 consumption, 29 µl min-1 g-1); basal metabolic rate, 4.4 mW g-1 at 27°C (Baxi et al., 2000); Q10=1.31 (Loiselle, 1985b). (B) The relationship between net efficiency and the muscle radius for all preparations. The slope of a straight line fitted through the data did not differ significantly from zero.

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