Contractions and mechanical properties of dogfish spiral intestine rotator muscle

Influence of intercellular tissue connections on airway muscle mechanics

Contraction of smooth muscle in visceral organs is modified by structures external to the muscle. Within muscle tissue itself, connective tissue plays an important role in force transference among the contractile cells. Connections arranged radially can affect contractile mechanics by limiting tissue expansion at short lengths. Previous work suggests that increased stiffness at extreme shortening is due to such radial constraints. Two approaches to further study of these effects are reported. To increase radial constraints, very thin Silastic bands were placed loosely about strips of canine trachealis muscle at rest length. The strips were allowed to shorten under light afterloads, expanding until restrained by the bands. Subsequent removal of the bands allowed increased shortening, with less increase in stiffness at short lengths. Related isometric effects were observed. To reduce constraints, muscle strips were partially digested with collagenase. Compared with control conditions, this treatment permitted further shortening, with less increase in stiffness at short lengths. These results emphasize the role of extracellular structures in determining mechanical function of smooth muscle.

The pCa-tension and force-velocity characteristics of skinned fibres isolated from fish fast and slow muscles

1. Single fast fibres and small bundles of two to six slow fibres were dissected from the myotomal muscles of the cod, Gadus morhua, and the dogfish, Scyliorhinus canicula. Fibres were chemically skinned with the non-ionic detergent Brij 58.2. The isometric tension properties were investigated. Maximal isometric tensions (mean +/- S.E. of mean) were 18.65+/-1.18 (n = 11) and 8.34+/-0.98 (n = 13) N cm(-2) for cod fast and slow fibres, and 18.34+/-0.88 (n = 28) and 8.24+/-0.39 (n = 12) N cm(-2) for dogfish fast and slow fibres respectively. The values are comparable to those observed in mammalian and amphibian skinned fibres. The lower tensions generated by the slow fibres cannot be fully explained on the basis of their lower myofibrillar fractional volume.3. In common with previous studies, a steep sigmoid relationship between pCa and tension was observed. The threshold for tension generation was around pCa 7.2. Half-maximal pCas were 6.08 and 6.42 for cod fast and slow muscle, and 6.41 and 6.50 for dogfish fast and slow fibres respectively. Cod fibres were maximally activated at around pCa 5.18, and dogfish fibres at pCa 5.62.4. Contraction-induced residual tensions were observed in cod fast fibres after return to relaxing solution. This phenomenon is a feature common to many skinned fibre studies, but the mechanism behind it has yet to be resolved.5. The force-velocity characteristics of fast and slow fibres have been investigated (at 8 degrees C).6. Points below 0.6 P(0) on the P-V curves could be fitted to a linear form of the Hill equation. Extrapolated V(max)s were calculated as follows: cod fast fibre V(max) = 1.01 muscle length sec(-1) (Lsec(-1)) (a = 0.21 P(0); b = 0.21 Lsec(-1)). Slow fibre = 0.53 Lsec(-1) (a = 0.28P(0); b = 0.21 Lsec(-1)). Dogfish fast fibre V(max) = 2.34 Lsec(-1) (a = 0.06 P(0); b = 0.14 Lsec(-1)). Slow fibre = 0.67 Lsec(-1) (a = 0.19 P(0); b = 0.13 Lsec(-1)).7. Contraction velocity in cod slow fibres decreased continuously to produce markedly non-linear velocity transients, similar to those reported for amphibian slow fibres.8. The effect of altering Ca(2+) concentration on the shape of the isotonic velocity curve (at low loads) was studied in dogfish fast fibres (0.5-1 degrees C). Contraction velocity decreased continuously during shortening, at both maximal and half-maximal Ca(2+) concentration. The rate of decay of velocity with shortening was greater at low Ca(2+) concentration.

Dynamic stiffness of rabbit mesotubarium smooth muscle: effect of isometric length

The dynamic stiffness of mesotubarium smooth muscle from nonpregnant adult rabbits was measured continuously during isometric contraction by applying small (0.5 percent of the muscle length) sinusoidal length perturbations and measuring the amplitude and phase of the resulting tension perturbations. Stiffness during contraction was directly proportional to muscle tension; during relaxation stiffness at all tensions was significantly increased as compared to the values encountered during the rise of tension. Peak isometric tension and dynamic stiffness (determined at a common tension level) both decreased at shorter muscle lengths; the relative falloff in stiffness was significantly less than the tension decrease. Varying levels of muscle activation (obtained by changing stimulus strength and by applying quick releases to active muscle) had little effect on the measured elastic modulus. Comparisons of these results with published data on single-cell contractile properties imply a cellular locus for a portion of the measured stiffness.