Crossbridge order and orientation in resting single glycerinated muscle fibres studied by linear dichroism of bound rhodamine labels

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Abstract
Linear dichroism of iodoacetyl-rhodamine labels attached to the highly reactive thiol of the myosin heads was measured in order to infer the spatial orientation and the degree of order in myosin crossbridges in single glycerinated rabbit psoas fibres at rest. We have previously shown that in rigor the chromophoric labels are well ordered and that in the presence of MgADP and during isometric contraction a large fraction of probes is also ordered but at an attitude different from that of rigor. Here we show that in relaxed muscle the probe order is dependent on total ionic strength: at and above 0.180m there is little evidence for any preferred probe orientation, implying a high degree of crossbridge disorder. Below 0.160m there is progressively more order with decreasing ionic strength down to 0.100m, below which no measurements could be taken at room temperature (because the fibres would not relax). The dichroism observed under these conditions resembles that of the rigor state in that the dichroism peaks at the same polarization of excitation light, implying that the average probe attitude relative to the fibre axis is larger than 54.7°. Stretching the muscle beyond the point of overlap between actin- and myosin-containing filaments does not affect the ionic strength dependence of the amount of order present in relaxed muscle, suggesting that the observed order is due to ionic interactions of crossbridges with the thick filament surface.