Increased cross-bridge recruitment contributes to transient increase in force generation beyond maximal capacity in human myocardium

Abstract

Cross-bridge attachment allows force generation to occur, and rate of tension redevelopment (k_tr) is a commonly used index of cross-bridge cycling rate. Tension overshoots have been observed briefly after a slack-restretch k_tr maneuver in various species of animal models and humans. In this study, we set out to determine the properties of these overshoots and their possible underlying mechanism. Utilizing human cardiac trabeculae, we have found that tension overshoots are temperature-dependent and that they do not occur at resting states. In addition, we have found that myosin cross-bridge cycle is vital to these overshoots as inhibition of the cycle results in the blunting of the overshoots and the magnitude of the overshoots are dependent on the level of myofilament activation. Lastly, we show that the number of cross-bridges transiently increase during tension overshoots. These findings lead us to conclude that tension overshoots are likely due to a transient enhancement of the recruitment of myosin heads into the cross-bridge cycling, regulated by the myocardium, and with potential physiological significance in determining cardiac output.

News and noteworthy: We show that isolated human myocardium is capable of transiently increasing its maximal force generation capability by increasing cross-bridge recruitment following slack-restretch maneuver. This process can potentially have important implications and significance in cardiac contraction in vivo.

Keywords: Cardiac contraction; Cardiac regulation; Contractile kinetics; Cross-bridge cycling rate.

Figure 1

Schematic definitions of the terms used for tension overshoot. The movement of the motor is shown on top. Note that the overshoot occurred when the motor was stationary. The overshoot amplitude (%) was calculated as the difference between overshoot tension and developed tension during K⁺ contracture tension (pre-k_tr) and normalized to the latter. The Tension Decline Phase t_1/2 was calculated as the time it takes for the overshoot amplitude to decrease by 50% from the peak of the overshoot to the lowest point after the overshoot.

Figure 2

Temperature affects kinetics of tension overshoot. A) Tracings of the Tension Decline Phase at different temperatures. Decreasing temperature resulted in an increase in t_1/2 of the Tension Decline Phase, B) Close relationship between k_tr and t_1/2 of Tension Decline Phase, C) Temperature had no effect on the amplitude of the tension overshoot. n = 4 trabeculae from four non-failing hearts. * indicates P < 0.05 vs. 27 °C. Data is shown as mean ± S.E.M. ** indicates P < 0.05 between 32 and 37 °C.

Figure 3

No tension overshoots without contracture or stimulation. A) k_tr shown is at the plateau of the contracture. B) Stimulation was paused and three consecutive k_trs were performed without a contracture. C) Expansion of the first k_tr maneuver from Panel B shows lack of tension overshoot without a contracture or stimulation. D) Zoom-in of Panel C on a shorter time-scale

Figure 4

High resting tension is not sufficient to cause tension overshoot in a resting trabecula. A) k_tr maneuver during the maximal K⁺ contracture, B) stretching resting trabecula, no stimulation and no contracture, in order to match its resting tension to that of the K⁺ contracture tension in Panel A, C) The trabecular length was reset to that of Panel A and k_tr protocol was performed during the maximal K⁺ contracture.

Figure 5

BDM decreases magnitude of tension overshoot. A) Control (no BDM), B) 10 mM BDM, C) 20 mM BDM, D) Washout (no BDM), E) 50 mM BDM.

Figure 6

Relationship between tension overshoot amplitude and relative K⁺ contracture tension. 1–4 data points per experiment per point shown.

Figure 7

Number of cross-bridges increase during overshoot. A) A representative tracing where we performed a slack-retretch k_tr maneuver as the trabeculae was sinusoidally oscillating. B) Oscillation amplitude increases during overshoot compared to before slack-restretch k_tr maneuver (Prek_tr). n = 4, and * indicates P < 0.05 between Prek_tr and overshoot via paired t-test.