A CROSS-BRIDGE MECHANISM CAN EXPLAIN THE THIXOTROPIC SHORT-RANGE ELASTIC COMPONENT OF RELAXED FROG SKELETAL-MUSCLE

1. The passive tension and sarcomere length of relaxed frog skeletal m uscle fibres were measured in response to imposed length stretches. Th e tension response to a constant-velocity stretch exhibited a clear di scontinuity. Tension rose more rapidly during the initial similar to 0 .4% L-o of the stretch than during the latter stages (where L-o is the resting length of the fibre). This initial tension response is attrib uted to the short-range elastic component (SREC). 2. The use of paired triangular stretches revealed that the maximum tension produced durin g the SREC response of the second stretch was significantly reduced by the first stretch. This history-dependent behaviour of the SREC refle cts thixotropic stiffness changes that have been previously described in relaxed muscle. 3. The biphasic nature of the SREC tension response to movement was most apparent during the first imposed length change after a period at a fixed length, irrespective of the direction of mov ement. 4. If a relaxed muscle was subjected to an imposed triangular l ength change so that the muscle was initially stretched and subsequent ly shortened back to its original fibre length, the resting tension at the end of the stretch was reduced relative to its initial pre-stretc h value. Following the end of the stretch, tension slowly increased to wards its initial value but the tension recovery was not accompanied b y a contemporaneous increase in sarcomere length. This finding suggest s that the resting tension of a relaxed muscle fibre is not entirely d ue to passive elasticity. The results are compatible with the suggesti on that a portion of the resting tension - the filamentary resting ten sion (FRT) - is produced by a low level of active force generation. 5. If a second identical stretch was imposed on the muscle at a time whe n the resting tension was reduced by the previous stretch, the maximal tension produced during the second stretch was the same as that produ ced during the first, despite the second stretch commencing from a low er initial resting tension. 6. In experiments using paired triangular length changes, an inter-stretch interval of zero did not produce a su bstantially greater thixotropic reduction in the second stretch elasti c limit force than an inter-stretch interval in the range 0.5-1 s. 7. A theoretical model was developed in which the SREC and FRT arise as m anifestations of a small number of slowly cycling cross-bridges linkin g the act,in and myosin filaments of a relaxed skeletal muscle. The pr edictions of the model are compatible with many of the experimental ob servations. If the SREC and FRT are indeed due to cross-bridge activit y, the model suggests that the cross-bridge attachment rate must incre ase during interfilamentary movement. A mechanism (based on misregistr ation between the actin binding sites and the myosin cross-bridges) by which this might arise is presented.