Impact of osmotic compression on sarcomere structure and myofilament calcium sensitivity of isolated rat myocardium

Abstract

Farman, Gerrie P., John S. Walker, Pieter P. de Tombe, and Thomas C. Irving. Impact of osmotic compression on sarcomere structure and myofilament calcium sensitivity of isolated rat myocardium. Am J Physiol Heart Circ Physiol 291: H1847-H1855, 2006. First published June 2, 2006 doi:10.1152/ajpheart.01237.2005.-Changes in interfilament lattice spacing have been proposed as the mechanism underlying myofilament length-dependent activation. Much of the evidence to support this theory has come from experiments in which high-molecular-weight compounds, such as dextran, were used to osmotically shrink the myofilament lattice. However, whether interfilament spacing directly affects myofilament calcium sensitivity (EC50) has not been established. In this study, skinned isolated rat myocardium was osmotically compressed over a wide range (Dextran T500; 0 -6%), and EC50 was correlated to both interfilament spacing and I1,1/I1,0 intensity ratio. The latter two parameters were determined by X-ray diffraction in a separate group of skinned muscles. Osmotic compression induced a marked reduction in myofilament lattice spacing, concomitant with increases in both EC50 and I1,1/I1,0 intensity ratio. However, interfilament spacing was not well correlated with EC50 (r 2 ϭ 0.78). A much better and deterministic relationship was observed between EC50 and the I1,1/I1,0 intensity ratio (r 2 ϭ 0.99), albeit with a marked discontinuity at low levels of dextran compression; that is, a small amount of external osmotic compression (0.38 kPa, corresponding to 1% Dextran T500) produced a stepwise increase in the I1,1/I1,0 ratio concomitant with a stepwise decrease in EC50. These parameters then remained stable over a wide range of further applied osmotic compression (up to 6% dextran). These findings provide support for a "switch-like" activation mechanism within the cardiac sarcomere that is highly sensitive to changes in external osmotic pressure. skinned muscle; trabeculae; X-ray diffraction; dextran; osmotic pressure; myofilament length-dependent activation; regulation MYOFILAMENT LENGTH-DEPENDENT activation (LDA) is a phenomenon found in all striated muscle, but to varying extents, being a prominent feature of cardiac muscle (2,4,7,15,16) and the indirect flight muscle of insects (33) but is less prominent in skeletal muscle (17,26,32). LDA is defined as an increase in the amount of active force developed at longer sarcomere length for a given concentration of activating calcium ions. LDA is the underlying cellular mechanism that underlies the Frank-Starling law of the heart (16). This increase in myofilament calcium sensitivity is commonly measured by the EC 50 parameter, that is, the calcium concentration at which the active developed force is half that observed at a saturating calcium concentration. An understanding of the molecular mechanism(s) underlying LDA has remained elusive. A popular hypothetical mechanism for LDA is that changes in interfilament lattice spacing, usually estimated as changes in fiber width, are directly linke...

Myosin MgADP Release Rate Decreases as Sarcomere Length Increases in Skinned Rat Soleus Muscle Fibers

“…As explained in detail by Tyska and Warshaw (28) ); and 3) the ratio k ÀADP /k þATP , which represents the concentration of MgATP producing half the maximal myosin detachment rate (i.e., [MgATP] 50 ).…”

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

“…One proposed mechanism is that titin strain imposes a radial force on the sarcomere lattice to modulate inter-filament spacing thus leading to altered myosinactin interaction and myofilament Ca 2+ sensitivity [12,19]. However, X-ray diffraction studies on skinned myocardium have questioned the role of interfilament spacing in directly modulating myofilament Ca 2+ sensitivity and proposed that the disposition of myosin heads was a more predictive index [17]. Several other factors, such as phosphorylation of MLC-2 or cMyBP-C are also known to affect the arrangement of myosin heads.…”

“…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]. It now seems more likely that Ca 2+ sensitivity is determined by changes in the structure of myosin filaments [17,36].…”

“…A recent study has shown that interfilament spacing alone cannot explain changes in Ca 2+ sensitivity [17]. It now seems more likely that Ca 2+ sensitivity is determined by changes in the structure of myosin filaments [17,36].…”

Cardiac thin filament regulation

“…We recently found that TnI-Thr144 per se is able to impart length-dependent properties onto the cardiac sarcomere, while a proline at that position virtually eliminates myofilament length dependency [75]. These results suggest that myofilament-length-dependent activation has, at its basis, a signal transduction pathway that includes Tn-Tm-actin signaling rather than a more direct mechanism involving myosin-actin affinity modulated by inter-filament spacing [19,20,40]. The underlying molecular mechanisms, however, are as of yet still largely unknown.…”