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Myosin-based mechanisms are increasingly recognized as supplementing their better-known actin-based counterparts to control the strength and time course of contraction in both skeletal and heart muscle. Here we use synchrotron small-angle X-ray diffraction to determine the structural dynamics of local domains of the myosin filament during contraction of heart muscle. We show that, although myosin motors throughout the filament contribute to force development, only about 10% of the motors in each filament bear the peak force, and these are confined to the filament domain containing myosin binding protein-C, the "C-zone." Myosin motors in domains further from the filament midpoint are likely to be activated and inactivated first in each contraction. Inactivated myosin motors are folded against the filament core, and a subset of folded motors lie on the helical tracks described previously. These helically ordered motors are also likely to be confined to the C-zone, and the associated motor conformation reforms only slowly during relaxation. Myosin filament stress-sensing determines the strength and time course of contraction in conjunction with actin-based regulation. These results establish the fundamental roles of myosin filament domains and the associated motor conformations in controlling the strength and dynamics of contraction in heart muscle, enabling those structures to be targeted to develop new therapies for heart disease.heart muscle | myosin motor | muscle regulation | myosin-binding protein C T he pumping action of the heart is driven by rhythmic contractions of its muscular walls. The healthy heart continuously optimizes the strength and time course of contraction by modulating the calcium transient that triggers the heartbeat and the phosphorylation levels of multiple proteins, including components of the myosin and actin filaments that drive contraction, and by direct mechanical feedback (1-4). These signaling pathways alter contractility by changing the structures of the contractile filaments through downstream effector mechanisms that remain poorly understood. For many years attention was focused on actin filament-based regulation and its link to intracellular calcium signaling (1); more recently it became clear, partly by extrapolation from studies on skeletal muscle (5-9), that myosin filamentbased regulation also plays an important role. Moreover, myosinbased regulation is perturbed in heart disease (10, 11), and has been increasingly targeted for the development of novel therapies to treat the failing heart (12). Such efforts have however been impeded by limited knowledge about the action of myosin-based regulation on the timescale of the heartbeat: that is, about mechanisms that operate much faster than kinase signaling (3,4). Two leading candidate mechanisms of this type emerged from studies of skeletal muscle: direct mechanosensing by the myosin filaments (6, 7), and interfilament signaling by myosin binding protein-C (6,13). Although several studies have suggested that these mechanisms are ...

18 F-Fluoride PET allows noninvasive evaluation of regional bone metabolism and has the potential to become a useful tool for assessing patients with metabolic bone disease and evaluating novel drugs being developed for these diseases. The main PET parameter of interest, termed K i , reflects regional bone metabolism. The aim of this study was to compare the long-term precision of 18 F-fluoride PET with that of biochemical markers of bone turnover assessed over 6 mo. Methods: Sixteen postmenopausal women with osteoporosis or significant osteopenia and a mean age of 64 y underwent 18 F-fluoride PET of the lumbar spine and measurements of biochemical markers of bone formation (bone-specific alkaline phosphatase and osteocalcin) and bone resorption (urinary deoxypyridinoline) at baseline and 6 mo later. Four different methods for analyzing the 18 F-fluoride PET data were compared: a 4k 3-compartmental model using nonlinear regression analysis (K i-4k ), a 3k 3-compartmental model using nonlinear regression analysis (K i-3k ), Patlak analysis (K i-PAT ), and standardized uptake values. Results: With the exception of a small but significant decrease in K i-3k at 6 mo, there were no significant differences between the baseline and 6-mo values for the PET parameters or biochemical markers. The long-term precision, expressed as the coefficient of variation (with 95% confidence interval in parentheses), was 12.2% (9%-19%), 13.8% (10%-22%), 14.4% (11%-22%), and 26.6% (19%-40%) for K i-3k , K i-PAT , mean standardized uptake value, and K i-4k , respectively. For comparison, the precision of the biochemical markers was 10% (7%-15%), 18% (13%-27%), and 14% (10%-21%) for bone-specific alkaline phosphatase, osteocalcin, and urinary deoxypyridinoline, respectively. Intraclass correlation between the baseline and 6-mo values ranged from 0.44 for K i-4k to 0.85 for K i-3k . No significant correlation was found between the repeated mean standardized uptake value measurements. Conclusion: The precision and intraclass correlation observed for K i-3k and K i-PAT was equivalent to that observed for biochemical markers. This study provided initial data on the long-term precision of 18 F-fluoride PET measured at the lumbar spine, which will aid in the accurate interpretation of changes in regional bone metabolism in response to treatment. The functional imaging technique of 18 F-fluoride PET allows a direct quantitative assessment of bone metabolism at specific sites of the skeleton (1-12). In the early 1990s, when 18 F-FDG PET was evolving as a major tool in the field of oncology, 18 F-fluoride PET was introduced as a technique for quantifying bone metabolism by Hawkins et al., who first described the 3-compartmental kinetic model that can be applied in clinical studies (7). Like 18 F-FDG PET, 18 Ffluoride PET has several methods for quantitation, ranging from simple semiquantitative measures to methods that require complex protocols for scan acquisition (13,14).18 F-Fluoride PET has been validated by direct comparison with the gold st...