A thixotropic effect in contracting rabbit psoas muscle: prior movement reduces the initial tension response to stretch

1. Paired ramp stretches and releases ('triangular length changes', typical ly 0.04 +/- 0.09 L-o, s(-1) mean +/- S.E.M.) were imposed on permeabilised rabbit psoas fibre segments under sarcomere length control. In actively con tracting fibres, the tension response to stretch was biphasic; tension rose more rapidly during the first 0.005 L-o of the imposed stretch than therea fter. Tension also dropped in a biphasic manner during shortening, and at t he end of the length change was reduced below the steady state. 2. If a second triangular length change was imposed shortly after the first , tension rose less sharply during the initial phase of lengthening, i.e. t he stiffness of the muscle during the initial phase of. the response was re duced in the second stretch. This is a thixotropic effect. If a third trian gular length change was imposed on the muscle, the response was the same as that to the second. 3. The time required to recover the original. tension response was measured by varying the interval between triangular length changes. Recovery to ste ady state occurred at a rate of similar to 1 s(-1). 4. The stiffness of the muscle during the initial phase of the response sca led with the developed tension in pCa (= -log(10)[Ca2+]) solutions ranging from 6.3 (minimal activation) to 4.5 (saturating effect). The relative thix otropic reduction in stiffness measured using paired length changes was ind ependent of the pCa of the activating solution. 5. The thixotropic behaviour of contracting skeletal muscle can be explaine d by a cross-bridge model of muscle contraction in which the number of atta ched cross-bridges is temporarily reduced following an imposed movement.