“Anti-rigor” cross bridges may reflect altered state of actin

We reported earlier that in insect flight muscle, myosin heads could form either rigor bridges or “anti-rigor” bridges, with reversed angle. When rigor develops at constant fiber length, the bridge angle is typically 45°—the rigor bridge. When rigor develops isotonically, so that the specimen can shorten, regularly arranged anti-rigor bridges are also observed. To investigate anti-rigor bridges further, bumblebee-ilight muscle fibers were prepared as described previously. Fresh fiber samples were stretched to 5-15% beyond their natural length, and then chemically skinned by a two-hour soak in a relaxing solution containing 0.5% Triton X-100. The stretched muscles were then alternately transferred and kept for 20 minutes in relaxing and rigor solutions until some of the fibers broke as a result of the developing rigor tension. Samples were then fixed in rigor, and the broken fibers were separated from the intact fibers. Micrographs of the intact fibers revealed the conventional rigor-bridge angles (Figure. 1). “Chevrons” formed by bridge pairs pointed away from the Z-line in a regular manner.