Effects of Sustained Length-Dependent Activation on In Situ Cross-Bridge Dynamics in Rat Hearts

Abstract

The cellular basis of the length-dependent increases in contractile force in the beating heart has remained unclear. Our aim was to investigate whether length-dependent mediated increases in contractile force are correlated with myosin head proximity to actin filaments, and presumably the number of cross-bridges activated during a contraction. We therefore employed x-ray diffraction analyses of beat-to-beat contractions in spontaneously beating rat hearts under open-chest conditions simultaneous with recordings of left ventricle (LV) pressure-volume. Regional x-ray diffraction patterns were recorded from the anterior LV free wall under steady-state contractions and during acute volume loading (intravenous lactate Ringers infusion at 60 ml/h, <5 min duration) to determine the change in intensity ratio (I(1,0)/I(1,1)) and myosin interfilament spacing (d(1,0)). We found no significant change in end-diastolic (ED) intensity ratio, indicating that the proportion of myosin heads in proximity to actin was unchanged by fiber stretching. Intensity ratio decreased significantly more during the isovolumetric contraction phase during volume loading than under baseline contractions. A significant systolic increase in myosin head proximity to actin filaments correlated with the maximum rate of pressure increase. Hence, a reduction in interfilament spacing at end-diastole ( approximately 0.5 nm) during stretch increased the proportion of cross-bridges activated. Furthermore, our recordings suggest that d(1,0) expansion was inversely related to LV volume but was restricted during contraction and sarcomere shortening to values smaller than the maximum during isovolumetric relaxation. Since ventricular volume, and presumably sarcomere length, was found to be directly related to interfilament spacing, these findings support a role for interfilament spacing in modulating cross-bridge formation and force developed before shortening.

Synchrotron Radiation Imaging for Advancing Our Understanding of Cardiovascular Function

“…By exposing a finely collimated SR X-ray beam (dimensions, 0.2×0.2 mm) to the whole heart, it is now possible to record the diffraction peaks in a myocardial region without averaging over multiple heart beats at a time resolution of 15 to 20 ms. [78][79][80] The time resolution is not limited by the available flux but mainly by radiation damage. Studies of a whole heart started with the isolated rat heart.…”

“…This initial study showed that the interfilament spacing does not differ significantly at different layers of the wall. 80 Studies of an isolated rat heart have expanded to more physiological studies of an open-chest in situ rat heart 78,79 and a closed-chest mouse heart. 81 The open-chest heart preparation allows the researcher to investigate regional differences in cross-bridge dynamics, because the myocardial regions exposed to the beam can be determined.…”

“…Perhaps the greatest limitation to X-ray diffraction studies of cardiac muscle is that it is not directly possible to distinguish between strong and weakly bound cross-bridge formation from the routinely obtained equatorial reflections, 64 because cardiac muscle generally produces poor meridional reflections. 72,82 Hence, indirect correlation with force or pressure development 79 and diffraction recordings in the relaxed and rigor states are required to make inferences regarding strong cross-bridge formation. 82 Finally, at the moment there is still no adequate method to measure the sarcomere length in situ and, therefore, estimates of sarcomere length can be derived only from established interfilament spacing-sarcomere length relations.…”

Differential contribution of cardiac sarcomeric proteins in the myofibrillar force response to stretch

“…Part of this effect may be due to reduced interfilament lattice spacing with stretch, which may increase the probability of myosin binding to actin to yield more forcegenerating cross-bridges that activate the thin filament [31,39]. A recent study has shown that interfilament spacing alone cannot explain changes in Ca 2+ sensitivity [17].…”

Resusitasi Cairan: dari Dasar Fisiologis hingga Aplikasi Klinis

“…Pada tingkat molekular, peningkatan end-diastolic volume berkorelasi dengan peningkatan cross-bridges antar filamen tipis. 13 Secara klinis, karakteristik intrinstik miokardium ini diaplikasikan dengan meningkatkan preload untuk meningkatkan isi sekuncup. 16 Brierley dkk 6 menganjurkan penggunaan bolus cairan isotonik atau albumin 5% 20 ml/kg berat badan sebagai langkah awal resusitasi pada anak.…”