Abstract
Background: In the past thirty years different experimental techniques
have been explored in attempt to elucidate the structural dynamic changes of
the contractile proteins inside the sarcomere during muscle contraction. We
hypothesized that the temporal transitions of the cross-bridge (XB) motors
between the different conformations during muscle contraction can be further
characterized by analyzing the changes in the transmitted light polarization. Methods: Thin trabeculae (n=6) were isolated from rat right
ventricle (K-H solution, 25°C). Sarcomere length (SL) was measured by laser
diffraction technique and controlled by a fast servomotor. The elliptical
polarization changes were quantified by utilizing a linear polarized incident
light that was aligned at an angle of 45° relative to the longitudinal fiber
axis. The transmitted light passed though linear polarizer towards the photo
detector. The changes in the transmitted light polarization were measured at
all the polarizer angles during rest, muscle and sarcomere isometric
contraction (1.95[?m]), and at different extracellular calcium concentrations
(0.75, 1.5, 4.5 [mM]). Results:
Passive length oscillations had mild effects on the transmitted light
polarization (1.70?1.70). However, large changes in the
polarization were observed during active contraction. Controlled sarcomere
isometric contractions had larger effects on the transmitted elliptical
polarization (17.00?10.60, n=6, p<0.01) compared to
muscle isometric contraction (10.80?6.40). The kinetics
of the polarization changes differed from the kinetics of force development.
The changes in the polarization preceded the force development at the beginning
of the contraction (51.7?18.9 [msec]) and lagged behind force relaxation at the
end of the contraction (50?41 [msec]). The rate of changes in the polarization
depended on the extracellular calcium level. The increase in the extracellular
calcium (from 0.75 to 1.5 and 4.5 [mM]) was associated with an increase in the
force (from 36?14.5 to 106.2?51.6 and 152.7?42.2 [
],
respectively) and a larger shift in the polarization at the same SL (
,
and
, n=3, p<0.01). Conclusions: The plausible explanation to the observations is that the
movement of myosin heads toward actin filament and XB attachment is the
dominant mechanism that modifies the transmitted light polarization under
physiological conditions. Significance: optical measurements provide additional information about
XB dynamics that differs from the observed dynamics of force or stiffness
measurements, and can be used for quantifying cardiac muscle activation and XB
cycling.