Effect of cross-linking on the contractile behavior of myofibrils

When rabbit psoas myofibrils in rigor are cross-linked with DMS (dimethyl suberimidate) for various periods of time, they contract on activation to a final sarcomere spacing of 1.3-1.5 microns. This behavior is observed out to 100 min cross-linking time (2 mg/ml DMS; 10 degrees C). Over the next 100 min of cross-linking, the sarcomere spacing, following activation and contraction, gradually increases and finally plateaus near its initial (rigor) value. We also determined the unloaded shortening velocity of the cross-linked myofibrils using an inverted microscope equipped with a video camera. Following photo-activation of caged ATP, the fast contracting process observed in control (untreated) myofibrils decreases in rate and magnitude with increasing cross-linking time. When taken together with earlier cross-linking studies, our present results suggest that the suppression of contraction may result from two distinct cross-linking reactions: (1) Cross-linking of myosin rods in the filament core which immobilizes the S-2 subunit and acts to decrease the isometric force (approximately 90% at 100 min). (2) Cross-linking within the S-1 subunit. This latter reaction is believed to account for the continuous decay in the rate and magnitude of the unloaded shortening process.