Ks. Campbell et Rl. Moss, A thixotropic effect in contracting rabbit psoas muscle: prior movement reduces the initial tension response to stretch, J PHYSL LON, 525(2), 2000, pp. 531-548
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.