Quantification of Cross-Bridge Cycling between the Different Physicochemical Conformations by Optical Means

Abstract

The transmitted light polarization is determined by sarcomere alignment, sarcomere length (SL) and cross-bridge (XB) distribution between the various physicochemical conformations. Aims: To quantify the changes in cardiac muscle optical properties during contraction and to differentiate between the effects of changes in SL and those of XB cycling. Methods: Thin trabeculae (n=6) were isolated from rat right ventricles. SL was measured by laser diffraction technique and controlled by a fast servomotor. The direction of the incident polarized HeNe laser light was set to 450 relative to the fiber axis. The changes in the transmitted light polarization were measured during rest, twitch contraction, sarcomere control isometric contractions (1.97µm), and at different calcium concentrations (0.75, 1.5, 4.5 [mM]). Results: The time to peak change in the transmitted light intensity preceded (50±10msec) the peak force (120msec), and the transition back to baseline intensity lagged behind force relaxation. There was a tight correlation between the rate of changes in the light intensity and the rate of force development. The degree of polarization decreased during force development, reaching a minimal value close to the peak force. These observations could not be attributed to changes in the SL since similar phenomena were observed during isometric sarcomere contractions, and the variations in the degree of polarization were even greater (13%) during sarcomere isomeric contractions. The role of the XBs was furtherer validated by imposing isometric contractions at the same SL but different force levels with different extra-cellular calcium concentrations. Significance: optical measurements provide additional information about XB dynamics that differ from the observed dynamics of force or stiffness measurements, and it can be used for quantifying cardiac muscle activation and XB cycling.