THE EFFECT OF HYPERTONICITY ON FORCE GENERATION IN TETANIZED SINGLE FIBERS FROM FROG SKELETAL-MUSCLE

1. We compared the tension transient that follows a step change in sar comere length in normal Ringer solution with that in Ringer solution m ade hypertonic by the addition of 98 mM sucrose. Steps were applied on tetanized single muscle fibres during either the isometric plateau or the steady force response to lengthening at low speed. Sarcomere leng th was controlled on selected fibre segments by a striation follower. analysis is limited to phase 1 (the tension change simultaneous with t he length step, mainly due to cross-bridge elasticity) and phase 2 (th e quick phase of tension recovery, a manifestation of the cross-bridge elementary force-generating process). 2. At the isometric tetanus pla teau the steady force is reduced by 19 % in hypertonic solution, and t he stiffness is slightly increased. During slow lengthening both stead y force and stiffness are similar in normal solution and in hypertonic solution. In hypertonic solution the tension-to-stiffness ratio, a me asure of the mean cross-bridge extension before the step, is markedly reduced in isometric conditions (-23 %), but not during lengthening (- 2 %). 3. The plots of instantaneous tension versus the length change d uring the step show that in hypertonic medium the elasticity of the fi bre is almost undamped. Thus the increase in stiffness cannot be attri buted to an increase in viscosity. 4. In isometric conditions (T-2 - T -1)/(T-1 - T-1), the proportion of the initial tension drop recovered at the end of phase 2, is not affected by hypertonicity for releases o f moderate and large size (> 2 nm) and is reduced for small releases ( < 2 nm) and for stretches. The abscissa intercept of the relation (T-2 - T-1)/(T-1 - T-1) versus step amplitude is the same in both media. D uring lengthening, for releases of small and moderate size, (T-2 - T-1 )/(T-1 - T-1) is 20 % lower in hypertonic solution. For large releases the slope of the relation is lower so that the abscissa intercept is not changed. 5. The speed of quick tension recovery following a step l ength change imposed in isometric conditions is slightly depressed in hypertonic solution. The relation between speed of recovery and step a mplitude maintains its shape and is shifted downwards. During lengthen ing, the speed of quick tension recovery in hypertonic solution is les s dependent on step amplitude than in normal solution, as if a more li near viscoelasticity is responsible for a large fraction of residual r ecovery. 6. The results indicate that during steady force response to slow lengthening, when the mean cross-bridge extension is similar in b oth media, the quick tension recovery is depressed in hypertonic mediu m, revealing that the reduction in isometric force by hypertonicity is due to impairment of the elementary force-generating process. The res ults are simulated with a model of contraction originally used to pred ict the mechanical behaviour during lengthening. The only change neede d to simulate the effects of hypertonicity is a 43 % reduction of the rate constants governing the transitions of the attached cross-bridges to higher force-generating states.