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

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Abstract
The linear dichroism of iodoacetylrhodamine labels attached to the single reactive thiol groups of [rabbit] myosin heads was measured to determine the spatial orientation of myosin cross-bridges in single glycerinated skeletal muscle fibers. It was previously shown 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. Here, it is reported that during isometric contraction, a large fraction of the probe shows a high degree of order, suggesting the attachment of .apprxeq. 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.