Cardiac muscle contracts more efficiently at lower contraction frequencies

Figures

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  FIGURE 1. Experimental records and post‐experimental analysis. Twitch stress (a) and rate of heat production (b) of a representative trabecula contracting at 5 Hz and undergoing progressively decreasing afterloaded work‐loop contractions (labelled b–f), bounded by isometric contractions at L o (labelled a). The first twitch illustrates the rested‐state contraction immediately following the commencement of electrical stimulation. Steady‐state twitch stress profiles at various afterloads under work‐loop contractions (c) and at various muscle lengths under isometric contractions (d). (e) A parametric plot of stress and length (relative to L o) of the work‐loop protocol to illustrate after‐loaded work‐loops (labelled b–f) and the isometric contraction (labelled a). (f) A parametric plot of stress and length (relative to L o) of the isometric length‐change protocol.

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  FIGURE 2. Energetics of work‐loop contractions at steady state. Work output, change of enthalpy and mechanical efficiency plotted as functions of relative afterload of a single trabecula contracting at 5 Hz (a, d, g) or the mean relations of all trabeculae (b, e, h) respectively). Data were fitted using second‐order polynomials for change of enthalpy and third‐order polynomials for the work and efficiency as functions of relative afterloads. *P < 0.05. Bar plots present means ± SEM, n = 7.

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  FIGURE 3. Shortening‐related parameters arising from the work‐loop protocol. (a, d) The extent of shortening and velocity of shortening plotted as functions of relative afterload from a representative trabecula contracting at 5 Hz, from which peak values were computed. Data were fitted using third‐order polynomials. (b, e) The relations averaged across all trabeculae. (c, f) Peak values of velocity of shortening (c) and extent of shortening (f). Peak values were predicted from the fitted curves at the afterload where active stress is zero, i.e., in the vicinity of passive stress. *P < 0.05. Bar plots present means ± SEM, n = 7.

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  FIGURE 4. Energetics of isometric contractions at steady state. (a) Linear regression fitted to the heat–stress data from the isometric length‐change protocol at 5 Hz (filled circles) for a representative trabecula. (b) The heat–stress relations averaged across all trabeculae. (c, d) Peak values of active stress (obtained at L o) (c) and peak values of active heat (obtained at L o) (d). (e, f) The heat‐intercept (‘activation heat’) (e) and the active heat–stress relation slope (f) averaged across all trabeculae. All parameters were not significantly different among stimulus frequencies. Bar plots present means ± SEM, n = 7.

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  FIGURE 5. Kinetics of isometric twitches at steady state. (a, d, g) Linear regression of steady‐state isometric twitch duration (a) and maximum rates (dS/dt) of rise (d) and fall (g) of twitches as functions of active stress of a representative trabecula contracting at 5 Hz. (b, e, h) The corresponding mean relations across all trabeculae. (c, f, i) Mean peak values. *P < 0.05. Bar plots present means ± SEM, n = 7.

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  FIGURE 6. Correlations among mean peak values of mechano‐energetic parameters. The dashed line in each panel is fitted to the three data points denoting 2 Hz (red circle), 3.5 Hz (blue circle) and 5 Hz (black circle). Symbols are means ± SEM, n = 7.