Factors influencing the ascending limb of the sarcomere length‐tension relationship in rabbit skinned muscle fibres.

Abstract

SUMMARY1. The length dependence of Ca2+ -activated tension within the ascending limb of the length-tension relationship, corresponding to sarcomere lengths below about 2-25 ptm, was investigated in skinned fibres from rabbit psoas muscle. At high [Ca2+] a shallow phase and then a steep phase of tension decline were observed as sarcomere length was reduced, while at low [Ca2+] tension decreased monotonically with decreases in sarcomere length. The sarcomere length at which the ascending limb intersected zero tension was greater for lower concentrations of Ca2+.2. The length-tension relationship from maximally activated fibres changed when filament lattice spacing was reduced by osmotic compression. Relationships obtained in the presence of 5% (w/v) dextran T500 more distinctly demonstrated both the shallow and steep portions of the ascending limb than did relationships fr'om untreated fibres.3. As striation spacing was decreased a progressive decline in the Ca2+ sensitivity of tension development was observed. Tension-pCa relationships from both control and dextran-treated fibres underwent a rightward shift (i.e. to a higher [Ca2+]) by 023 pCa units as sarcomere length was reduced between 2-46 and 1'54 ,um.4. Fibre stiffness was studied by applying a 3-3 kHz sinusoidal length change at one end of the fibre and measuring the resultant tension change. At submaximal activation (pCa 5-8), stiffness increased relative to tension as sarcomere length was decreased below -2-4 /tm, suggesting that there is an activation-related internal load at low [Ca2+]. At maximal activation, a significant increase in this ratio occurred only at sarcomere lengths less than -1-8 ,um, and presumably involved collision of the thick filaments with the Z-lines.5. Length-dependent changes in the Ca2+ sensitivity of tension development do not appear to be the result of shortening-induced dissociation of Ca2+ from troponin-C, the Ca2+ binding subunit of troponin. Fibres activated in the absence of Ca2+, by the partial removal of whole troponin complexes, produced length-tension relationships similar to those observed in the same fibres before troponin removal at a submaximal [Ca2+] yielding similar active tensions.

C‐protein limits shortening velocity of rabbit skeletal muscle fibres at low levels of Ca2+ activation.

“…GREASER AND R. L. MOSS C-protein extraction has no effect on maximum shortening velocity at pCa 4-5. In our model, this result would be expected since there appear to be no long-lived crossbridges in maximally activated fibres (Allen & Moss, 1987). Thus, as shortening proceeds, normally-cycling cross-bridges would detach before straining of S-2 occurs due to the C-protein tether.…”

“…The internal load may be a population of cross-bridges that detach from actin more slowly than normal, ultimately imposing a load on the muscle as it shortens. Evidence for long-lived cross-bridges during partial activation was obtained by Allen & Moss (1987) who found that stiffness was virtually unchanged and tension decreased in skinned psoas fibres when sarcomere length was shortened to a new isometric value. Slowly-cycling crossbridges have been suggested to arise in zones of transition between active and inactive functional groups on the thin filament (Moss, 1986).…”

Dissociation of force from myofibrillar MgATPase and stiffness at short sarcomere lengths in rat and toad skeletal muscle.

“…4B) and demonstrate clearly that there is very little change in stiffness as the sarcomere length decreases from 2-70 4tm to where the myosin filaments begin to collide with the Z-lines at 1 61 /sm. In a recent study, Allen & Moss (1987) also demonstrated an increase in the ratio between stiffness and tension in maximally activated skinned mammalian fibres at sarcomere lengths shorter than 1-9 ,tm. The dissociation between MgATPase activity and isometric force at short sarcomere lengths is contrary to other early observations of whole tetanized muscles (Infante et al 1964;Sandberg & Carlson, 1966) and glycerinated fibres (Hayashi & Tononura, 1968) which suggested that myofibrillar MgATPase activity decreased at short sarcomere lengths similarly to isometric force.…”

Myofilament Calcium Sensitivity in Skinned Rat Cardiac Trabeculae

“…It has been suggested that changes in Ca 2ϩ sensitivity with length involve modulation of cooperative crossbridge binding to the thin filament. 27 Although sarcomere geometry and filament overlap may contribute to the availability of additional crossbridges along the ascending limb of the length-tension relationship, 28 it alone cannot account for the magnitude of the increase in force development. 3,7,29 It has been suggested that thin filament isoform proteins regulate the response to changes in length by some as of yet undetermined mechanism.…”