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.