Energy expenditure for isometric contractions of right and left ventricular trabeculae over a wide range of frequencies at body temperature

Figures

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  Figure 1. Typical experimental records of simultaneous measurements of stress and heat rate. Upper panel: twitch stress, where the insets depict individual steady-state twitches. Lower panel: rate of heat production, where the dotted line segments were drawn by connecting the silent periods between twitch trains to signify a heat-rate baseline. Peak stress developed during the first twitch at each frequency defines the rested-state contraction following a 2 min pause of stimulation. Data were obtained from a representative LV trabecula (length, 2.5 mm; diameter, 238 μm). For this muscle, the order of presentation of frequencies was 2, 3, 7, 5, 0.1, 0.5, 10, 1 Hz.

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  Figure 2. Twitch characteristics of RV and LV trabeculae at steady-state under isometric contractions. Averaged active stress (A), stress-time integral, STI (B), maximal rates of twitch stress development (+dS/dt, C) and relaxation (−dS/dt, D) are presented as functions of stimulus frequency. The star symbol denotes a significant interaction between ventricle and frequency.

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  Figure 3. Heat outputs of RV and LV trabeculae at steady-state under isometric contractions. Averaged rate of heat output (A) and heat per twitch (heat rate divided by stimulus frequency; B) are plotted as functions of stimulus frequency.

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  Figure 4. Mechanoenergetic indices of RV and LV trabeculae at steady-state under isometric contractions. The ratio of heat to active stress (A), and that of heat to STI (B), of each trabecula in either ventricle group were averaged and the averages presented as functions of stimulus frequency. The star symbol denotes a significant interaction between ventricle and frequency.

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  Figure 5. Effect of time on muscle mechanoenergetic performance. For each dependent variable: active stress (A), stress-time integral (STI) (B), rates of rise and fall of stress (C), and heat (D), all values were averaged across frequencies for each order of stimulation. The ‘+’ symbol denotes a significant effect of time.

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  Figure 6. Steady-state heat per stress and per stress-time integral as functions of time. For each dependent variable, heat per stress (A) and heat per stress-time integral (B), there were no effects of time but there was a difference between the ventricles (denoted by the ‘*’ symbol).

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  Figure 7. Rested-state potentiated stress. The average values of rested-state stress (A) are presented as functions of stimulus frequency. Rested-state stress data, averaged across all eight frequencies, are plotted as a function of time (order), showing a progressive decline with time (as indicated by the ‘+’ symbol) independent of ventricle (B).