Cross-Bridge Cycling Kinetics in Intact Multicellular Cardiac Muscle at Physiological Temperature: Impact of Muscle Length

Abstract

The contractility of the heart is physiologically determined by load, frequency and β-adrenergic stimulation. It has been shown that these regulatory mechanisms involve post-translational modifications of myofilament proteins that can potentially influence the rate of cross-bridge cycling, an important determinant of cardiac output. We set out to develop a method for measuring cross-bridge cycling rate in intact cardiac muscle preparations where the cascades of post-translational signaling are functionally intact. With the use of a K+ contracture protocol, we were able to induce a steady-state tension in intact trabeculae and measure the rate of tension redevelopment (ktr), an index for cross-bridge cycling rate. We utilized this technique in order to investigate the effect of load on cross-bridge cycling rate. In cardiac trabeculae isolated from Brown Norway rats (n=11), the rate of tension redevelopment was measured twice at Lopt (optimal length) and at L90 (corresponding to 90% of optimal length) in each muscle. The ktr for the L90 was 45.1 ± 7.6 s−1 and it was significantly decreased to 27.7 ± 3.3 s−1 as the muscles were stretched to their Lopt (P < 0.05). The ktr for each length was measured a second time in order to show the reproducibility of the system. There was no significant difference between the duplicate measurements of each length (P = 0.84). In addition, we were able to apply these experiments in mammals that more closely reflect the human situation (such as the rabbit and dog) and muscle preparations isolated from explanted human hearts. This technique permits the studying of cross-bridge cycling kinetics in intact muscles in a reproducible and reliable manner, where the impact of signaling cascades leading to post-translational modifications can be studied.