Effect of muscle length on cross-bridge kinetics in intact cardiac trabeculae at body temperature

Abstract

Dynamic force generation in cardiac muscle, which determines cardiac pumping activity, depends on both the number of sarcomeric cross-bridges and on their cycling kinetics. The Frank–Starling mechanism dictates that cardiac force development increases with increasing cardiac muscle length (corresponding to increased ventricular volume). It is, however, unclear to what extent this increase in cardiac muscle length affects the rate of cross-bridge cycling. Previous studies using permeabilized cardiac preparations, sub-physiological temperatures, or both have obtained conflicting results. Here, we developed a protocol that allowed us to reliably and reproducibly measure the rate of tension redevelopment (ktr; which depends on the rate of cross-bridge cycling) in intact trabeculae at body temperature. Using K+ contractures to induce a tonic level of force, we showed the ktr was slower in rabbit muscle (which contains predominantly β myosin) than in rat muscle (which contains predominantly α myosin). Analyses of ktr in rat muscle at optimal length (Lopt) and 90% of optimal length (L90) revealed that ktr was significantly slower at Lopt (27.7 ± 3.3 and 27.8 ± 3.0 s−1 in duplicate analyses) than at L90 (45.1 ± 7.6 and 47.5 ± 9.2 s−1). We therefore show that ktr can be measured in intact rat and rabbit cardiac trabeculae, and that the ktr decreases when muscles are stretched to their optimal length under near-physiological conditions, indicating that the Frank–Starling mechanism not only increases force but also affects cross-bridge cycling kinetics.

Decrease in sarcoplasmic reticulum calcium content, not myofilament function, contributes to muscle twitch force decline in isolated cardiac trabeculae

“…In our study, twitch tensions decreased dramatically while contracture tensions were not (muscles from Figures 1 and 2) or only slightly affected (muscles from Figure 3). This is in agreement with our previous report where we showed that the contracture tensions in rat cardiac trabeculae are stable over time at 4 Hz and 37 °C (Milani-Nejad et al 2013). It is thought that a high extracellular K + concentration during this type of contracture induces slow membrane depolarization, activation of calcium channels, and influx of calcium which in turn activates the myofilaments.…”

“…This solution was bubbled with 95% O 2 /5% CO 2 where the buffering system results in a pH of 7.4. Trabeculae were isolated from the right ventricle and mounted horizontally in a custom made setup as previously described(Milani-Nejad et al 2013). Muscle were stimulated at 20% over theshhold voltage, using 2 ms bipolar pulses, typically ~5 V. The perfusion solution during experiments was changed to a modified Krebs-Henseleit solution without (BDM) and containing 2 mM CaCl 2 .…”

“…This solution was also continuously bubbled with 95% O 2 /5% CO 2 resulting in a pH of 7.4. Muscles were gradually stretched until reaching optimal length as previously described (Milani-Nejad et al 2013) and all of the experiments were performed at this length. Muscles for force-frequency relationship and post-rest potentiation experiments were stabilized at a baseline frequency of 4 Hz and 37 °C.…”

“…After these force-frequency relationship and post-rest potentiation measurements were made, muscles were re-stabilized at 4 Hz. The K + contracture, sometimes with rate of tension redevelopment ( k tr ) protocol, was performed as previously described(Milani-Nejad et al 2013). Following this initial set of experiments (experiment 1), muscles were re-stabilized at 4 Hz for about 15 minutes and this sequence of experiments was performed two more times (experiment 2 and experiment 3).…”

“…Afterwards, the temperature was rapidly switched back to 27 °C, the electrical stimulation was resumed, and the muscles were allowed to stabilize for 5 minutes. The K + contracture and k tr protocol were performed as previously described (Milani-Nejad et al 2013) after which the muscles were allowed to re-stabilize at 1 Hz for 15 minutes. Three more rounds (experiments 2,3, and 4) of rapid-cooling contractures and K + protocols were performed as described above.…”