Compliant realignment of binding sites in muscle: transient behavior and mechanical tuning

The presence of compliance in the lattice of filaments in muscle raises a number of concerns about how one accounts for force generation in the context of the cross-bridge cycle--binding site motions and coupling between cross-bridges confound more traditional analyses. To explore these issues, we developed a spatially explicit, mechanochemical model of skeletal muscle contraction. With a simple three-state model of the cross-bridge cycle, we used a Monte Carlo simulation to compute the instantaneous balance of forces throughout the filament lattice, accounting for both thin and thick filament distortions in response to cross-bridge forces. This approach is compared to more traditional mass action kinetic models (in the form of coupled partial differential equations) that assume filament inextensibility. We also monitored instantaneous force generation, ATP utilization, and the dynamics of the cross-bridge cycle in simulations of step changes in length and variations in shortening velocity. Three critical results emerge from our analyses: 1) there is a significant realignment of actin-binding sites in response to cross-bridge forces, 2) this realignment recruits additional cross-bridge binding, and 3) we predict mechanical behaviors that are consistent with experimental results for velocity and length transients. Binding site realignment depends on the relative compliance of the filament lattice and cross-bridges, and within the measured range of these parameters, gives rise to a sharply tuned peak for force generation. Such mechanical tuning at the molecular level is the result of mechanical coupling between individual cross-bridges, mediated by thick filament deformations, and the resultant realignment of binding sites on the thin filament.     

Calcium regulation of tension redevelopment kinetics with 2-deoxy-ATP or low [ATP] in rabbit skeletal muscle

The correlation of acto-myosin ATPase rate with tension redevelopment kinetics (k(tr)) was determined during Ca(+2)-activated contractions of demembranated rabbit psoas muscle fibers; the ATPase rate was either increased or decreased relative to control by substitution of ATP (5.0 mM) with 2-deoxy-ATP (dATP) (5.0 mM) or by lowering [ATP] to 0.5 mM, respectively. The activation dependence of k(tr) and unloaded shortening velocity (Vu) was measured with each substrate. With 5.0 mM ATP, Vu depended linearly on tension (P), whereas k(tr) exhibited a nonlinear dependence on P, being relatively independent of P at submaximum levels and rising steeply at P > 0.6-0.7 of maximum tension (Po). With dATP, Vu was 25% greater than control at Po and was elevated at all P > 0.15Po, whereas Po was unchanged. Furthermore, the Ca(+2) sensitivity of both k(tr) and P increased, such that the dependence of k(tr) on P was not significantly different from control, despite an elevation of Vu and maximal k(tr). In contrast, lowering [ATP] caused a slight (8%) elevation of Po, no change in the Ca(+2) sensitivity of P, and a decrease in Vu at all P. Moreover, k(tr) was decreased relative to control at P > 0.75Po, but was elevated at P < 0.75Po. These data demonstrate that the cross-bridge cycling rate dominates k(tr) at maximum but not submaximum levels of Ca(2+) activation.     

Pre-evaluation and system optimization of the Elecsys thyroid electrochemiluminescence immunoassays

Aortic atherosclerosis has early been recognized as a potential source of embolism. The histological finding of cholesterol clefts in small end-arteries characterized the entity of cholesterol embolism. The clinical picture was extremely variable and the diagnosis was frequently established post-mortem or by means of invasive although insensitive procedures including biopsy and angiography. Therefore, cholesterol embolism was thought to be rare. With the routine use of transesophageal echocardiography for the diagnostic workup of arterial embolism, aortic atherosclerosis was shown to be the source of otherwise unexplainable embolism. Cross-sectional studies demonstrated an independent association between prominent plaques of more than 4 to 5 mm of thickness or plaques with mobile components in the aortic arch. In follow-up studies, the risk of embolic events in patients with this kind of lesions exceeded 10% per patient-year. The results of pathological studies were consistent with these findings showing that ulcerated complex plaques carry an independent risk for embolic events. Apart from spontaneous embolism, atherosclerosis of the proximal aorta was shown to be a cause of embolic complications during cardiac surgery and catheterization procedures which involve the aorta. Medical treatment for the prevention of embolism in atherosclerotic disease of the aorta has not been studied systematically. In a variant form of aortic atherosclerosis consisting of mobile pedunculated thrombi inserting on relatively small plaques, anticoagulant therapy has proved to be useful in small numbers of patients. Recurrent embolic events could be prevented and regression of the thrombotic masses has been observed.     

Adaptive evolution of highly mutable loci in pathogenic bacteria

Bacteria have specific loci that are highly mutable. We argue that the coexistence within bacterial genomes of such 'contingency' genes with high mutation rates, and 'housekeeping' genes with low mutation rates, is the result of adaptive evolution, and facilitates the efficient exploration of phenotypic solutions to unpredictable aspects of the host environment while minimizing deleterious effects on fitness.