The Effects of Force Inhibition by Sodium Vanadate on Cross-Bridge Binding, Force Redevelopment, and Ca2+ Activation in Cardiac Muscle
Strongly bound, force generating myosin crossbridges play an important role as allosteric activators of cardiac thin filaments. Sodium vanadate (Vi) is a phosphate analog that inhibits force by preventing crossbridge transition into force producing states. This study characterizes the mechanical state of crossbridges with bound Vi as a tool to examine the contribution of crossbridges to cardiac contractile activation. The K{sub i} of force inhibition by Vi was {approx} 40 {mu}M. Sinusoidal stiffness was inhibited with Vi, although to a lesser extent than force. We used chord stiffness measurements to monitor Vi induced changes in crossbridge attachment/detachment kinetics at saturating [Ca{sup 2+}]. Vi decreased chord stiffness at the fastest rates of stretch, while at slow rates chord stiffness actually increased. This suggests a shift in crossbridge population towards low force states with very slow attachment/detachment kinetics. Low angle X-ray diffraction measurements indicate that with Vi crossbridge mass shifted away from thin filaments, implying decreased crossbridge-thin filament interaction. The combined X-ray and mechanical data suggest at least two crossbridge populations with Vi; one characteristic of normal cycling crossbridges, and a population of weak-binding crossbridges with bound Vi and slow attachment/detachment kinetics. The Ca{sup 2+}-sensitivity of force (pCa{sub 50}) and force redevelopment kinetics (k{sub TR}) were measured to study the effects of Vi on contractile activation. When maximal force was inhibited by 40% with Vi pCa{sub 50} decreased, but greater force inhibition at higher [Vi] did not further alter pCa{sub 50}. In contrast, the Ca{sup 2+}-sensitivity of k{sub TR} was unaffected by Vi. Interestingly, when force was inhibited by Vi k{sub TR} increased at sub-maximal levels of CaS{sup 2+}-activated force. Additionally, kTR is faster at saturating Ca{sup 2+} at [Vi] that inhibit force by more than {approx}70%. The effects of Vi on k{sub TR} imply that k{sub TR} is determined not only by the intrinsic properties of the crossbridge cycle, but also by crossbridge contribution to thin filament activation.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- Doe - Office Of Science
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 930553
- Report Number(s):
- BNL-80669-2008-JA; BIOJAU; TRN: US200904%%585
- Journal Information:
- Biophysical Journal, Vol. 92, Issue 12; ISSN 0006-3495
- Country of Publication:
- United States
- Language:
- English
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