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

Abstract

Single fast fibers and small bundles of 2-6 slow fibers were dissected from the myotomal muscles of the cod, Gadus morhua, and the dogfish, Scyliorhinus canicula. Fibers were chemically skinned with the non-ionic detergent Brij 58. The isometric tension properties were investigated. Maximal isometric tensions (mean .+-. [standard error of the mean]) were 18.65 .+-. 1.18 (n = 11) and 8.34 .+-. 0.98 (n = 13) N cm-2 for cod fast and slow fibers, and 18.34 .+-. 0.88 (n = 28) and 8.24 .+-. 0.39 (n = 12) N cm-2 for dogfish fast and slow fibers, respectively. The values are comparable to those observed in mammalian and amphibian skinned fibers. The lower tensions generated by the slow fibers cannot be fully explained on their lower myofibrillar fractional volume. In common with previous studies, a steep sigmoid relationship between pCa [-log [H+]] and tension was observed. The threshold for tension generation was .apprx. pCa 7.2. Half-maximal pCa were 6.08 and 6.42 for cod fast and slow muscle, and 6.41 and 6.50 for dogfish fast and slow fibers, respectively. Cod fibers were maximally activated at .apprx. pCa 5.18, and dogfish fibers at pCa 5.62. Contraction-induced residual tensions were observed in cod fast fibers after return to relaxing solution. This phenomenon is a feature common to many skinned fiber studies, but the mechanism behind it has yet to be resolved. The force-velocity characteristics of fast and slow fibers were investigated (at 8.degree. C). Points below 0.6 P0 on the P-V curves could be fitted to a linear form of the Hill equation. Extrapolated Vmax were calculated as follows: cod fast fiber Vmax = 1.01 muscle length s-1 (Ls-1) (a = 0.21 P0; b = 0.21 Ls-1). Slow fiber = 0.53 Ls-1 (a = 0.28 P0; b= 0.21 Ls-1). Dogfish fast fiber Vmax = 2.34 Ls-1 (a = 0.06 P0; b = 0.14 Ls-1). Slow fibers = 0.67 Ls-1 (a = 0.19 P0; b = 0.13 Ls-1). Contraction velocity in cod slow fibers decreased continuously to produce markedly non-linear velocity transients, similar to those reported for amphibian slow fibers. The effect of altering Ca2+ concentration on the shape of the isotonic velocity curve (at low loads) was studied in dogfish fast fibers (0.5-1.degree. C). Contraction velocity decreased continuously during shortening, at both maximal and half-maximal Ca2+ concentration. The rate of decay of velocity with shortening was greater at low Ca2+ concentration.