To investigate the roles of cross-bridge dissociation and cross-bridge-induced thin filament activation in the time course of muscle relaxation, we initiated force relaxation in single myofibrils from skeletal muscles by rapidly (approximately 10 ms) switching from high to low [Ca(2+)] solutions. Full force decay from maximal activation occurs in two phases: a slow one followed by a rapid one. The latter is initiated by sarcomere "give" and dominated by inter-sarcomere dynamics (see the companion paper, Stehle, R., M. Krueger, and G. Pfitzer. 2002. Biophys. J. 83:2152-2161), while the former occurs under nearly isometric conditions and is sensitive to mechanical perturbations. Decreasing the Ca(2+)-activated force preceding the start of relaxation does not increase the rate of the slow isometric phase, suggesting that cycling force-generating cross-bridges do not significantly sustain activation during relaxation. This conclusion is strengthened by the finding that the rate of isometric relaxation from maximum force to any given Ca(2+)-activated force level is similar to that of Ca(2+)-activation from rest to that given force. It is likely, therefore, that the slow rate of force decay in full relaxation simply reflects the rate at which cross-bridges leave force-generating states. Because increasing [P(i)] accelerates relaxation while increasing [MgADP] slows relaxation, both forward and backward transitions of cross-bridges from force-generating to non-force-generating states contribute to muscle relaxation.

In striated muscle, force generation and phosphate (P(i)) release are closely related. Alterations in the [P(i)] bathing skinned fibers have been used to probe key transitions of the mechanochemical coupling. Accuracy in this kind of studies is reduced, however, by diffusional barriers. A new perfusion technique is used to study the effect of [P(i)] in single or very thin bundles (1-3 microM in diameter; 5 degrees C) of rabbit psoas myofibrils. With this technique, it is possible to rapidly jump [P(i)] during contraction and observe the transient and steady-state effects on force of both an increase and a decrease in [P(i)]. Steady-state isometric force decreases linearly with an increase in log[P(i)] in the range 500 microM to 10 mM (slope -0.4/decade). Between 5 and 200 microM P(i), the slope of the relation is smaller ( approximately -0.07/decade). The rate constant of force development (k(TR)) increases with an increase in [P(i)] over the same concentration range. After rapid jumps in [P(i)], the kinetics of both the force decrease with an increase in [P(i)] (k(Pi(+))) and the force increase with a decrease in [P(i)] (k(Pi(-))) were measured. As observed in skinned fibers with caged P(i), k(Pi(+)) is about three to four times higher than k(TR), strongly dependent on final [P(i)], and scarcely modulated by the activation level. Unexpectedly, the kinetics of force increase after jumps from high to low [P(i)] is slower: k(Pi(-)) is indistinguishable from k(TR) measured at the same [P(i)] and has the same calcium sensitivity.

A fter the recent celebrations of the 50 th anniversary of the modern description of hypertrophic cardiomyopathy (HCM) by Teare and Lord Brock, the time is ripe to reflect on what remains to be discovered. [1][2][3] With the full realization that a massive amount of information relating to the disease has already been uncovered, and paying tribute to all those involved in this process, it is essential to concentrate on the gaps in our knowledge that require concerted efforts to advance the field, particularly in relation to patient management, which continues to be perceived as less than optimal. 3 We believe that this is largely due to the partial disconnect between basic research, and an incomplete understanding of the fundamental mechanisms molding a continuously, often insidiously changing phenotype. A thorough comprehension of these processes requires a translational approach based on long-term clinical observation of large HCM cohorts, coupled with basic scientific research, and represents an essential step toward the development of innovative therapies which need to be both disease-and patient-specific. 2,3Traditionally, the focus of HCM literature has been polarized on 2 aspects of indisputable clinical relevance: the pathogenesis, clinical consequences, and management of dynamic left ventricular (LV) outflow obstruction, 1 and the issue of arrhythmic risk stratification and prevention of sudden cardiac death (SCD).4,5 By comparison, limited attention has been devoted to the life-long process of LV remodeling and progressive dysfunction that occur in a substantial proportion of HCM patients and culminates in the rare but dramatic clinical evolution termed as end-stage or burned-out phase.6-9 Consequently, the stages that precede this severe condition are still relatively unknown, representing an important target for research.3 Indeed, because of the slowly evolving nature of HCM, timely identification of patients at risk of developing advanced LV dysfunction and heart failure (HF) may allow effective preventive strategies over a time span of several years before clinical demise. [7][8][9] To aid the characterization of different phases of HCM in individual patients, we propose a simple framework for systematic clinical staging of the disease. To this purpose, 4 clinical stages are identified, with special emphasis on diagnosis, potential mechanisms, challenges for management, and targets for future investigation: these are defined as nonhypertrophic HCM, classic phenotype, adverse remodeling, and overt dysfunction (Figure 1 and Table). 3,6,7,10 Stage I: Nonhypertrophic HCM Definition and DiagnosisNonhypertrophic HCM is a state characterized by the absence of LV hypertrophy in individuals harboring HCM-causing mutations, investigated in the course of systematic family screenings. In most HCM patients, a hypertrophic phenotype is generally absent in newborn or very young children, and tends to manifest during the second decade of life.7,10,11 Due to incomplete penetrance and age-related onset, however, gen...

Rationale High-myofilament Ca2+-sensitivity has been proposed as trigger of disease pathogenesis in familial hypertrophic cardiomyopathy (HCM) based on in vitro and transgenic mice studies. However, myofilament Ca2+-sensitivity depends on protein phosphorylation and muscle length, and at present, data in human are scarce. Objective To investigate whether high-myofilament Ca2+-sensitivity and perturbed length-dependent activation are characteristics for human HCM with mutations in thick- and thin-filament proteins. Methods and Results Cardiac samples from patients with HCM harboring mutations in genes encoding thick (MYH7, MYBPC3) and thin (TNNT2, TNNI3, TPM1) filament proteins were compared with sarcomere mutation-negative HCM and nonfailing donors. Cardiomyocyte force measurements showed higher myofilament Ca2+-sensitivity in all HCM samples and low phosphorylation of protein kinase A (PKA)-targets compared with donors. After exogenous PKA treatment, myofilament Ca2+-sensitivity was either similar (MYBPC3mut, TPM1mut, sarcomere mutation-negative HCM), higher (MYH7mut, TNNT2mut), or even significantly lower (TNNI3mut) compared with donors. Length-dependent activation was significantly smaller in all HCM than in donor samples. PKA treatment increased phosphorylation of PKA-targets in HCM myocardium and normalized length-dependent activation to donor values in sarcomere mutation-negative HCM and HCM with truncating MYBPC3 mutations, but not in HCM with missense mutations. Replacement of mutant by wild-type troponin in TNNT2mut and TNNI3mut corrected length-dependent activation to donor values. Conclusions High-myofilament Ca2+-sensitivity is a common characteristic of human HCM and partly reflects hypophosphorylation of PKA-targets compared with donors. Length-dependent sarcomere activation is perturbed by missense mutations, possibly via post-translational modifications other than PKA-hypophosphorylation or altered protein–protein interactions, and represents a common pathomechanism in HCM.