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

Abstract

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.