Evidence for cross-bridge order in contraction of glycerinated skeletal muscle

The linear dichroism of iodoacetylrhodamine labels attached to the single reactive thiol groups of myosin beads was measured to determine the spatial orientation of myosin cross-bridges in single glycerinated skeletal muscle fibers. We have shown previously that in rigor the chromophoric labels are well ordered and assume an orientation nearly perpendicular to the fiber axis; in the presence of MgADP, a large fraction of probe remains well ordered but the probe attitude assumes a more slanted orientation; in relaxed muscle, the probe order is largely lost, implying a high degree of cross-bridge disorder. In this paper, we report that during isometric contraction a large fraction of the probe shows a high degree of order, suggesting the attachment of ≃65% of the cross-bridges to actin. These ordered cross-bridges have a probe attitude similar to that of the MgADP-induced static state and hence are in a mechanical state quite distinct from rigor.