Myofibrillar ATPase activity, isoletric tension (P0) and unloaded shortening velocity (V0) were determined in single skinned fibres isolated from rat hindlimb muscles during maximal calcium activation at 12°C. In each fibre, myosin heavy chain (MHC) isoforms were identified using electrophoresis and immunocytochemistry. ATPase activity was determined spectrophotometrically from NADH oxidation in a coupled enzyme assay. On the basis of their MHC isoform composition, the fibres (n = 102) were divided into five groups containing the slow isoform, IMHC, or one of the fast isoforms, IIB MHC, IIA MHC, IIX MHC, or a mixture of the latter three. ATPase activity was significantly higher in IIB than in 2X and IIA fibres (0.230 ± 0.010, 0.178 ± 0.023 and 0.168 ± 0.026 mnol mm-3 s-1, respectively). Mixed fibres had intermediate values. ATPase activity in slow fibres was considerably less (0.045 ± 0.006 mnol mm-3 s-1. The ratio between ATPase activity and P0, i.e. tension cost, was found to be 2.90 ± 0.09, 2.56 ± 0.14, 1.89 ± 0.22, 1.52 ± 0.13 and 0.66 ± 0.004 pmol ATP mN-1 mm-1 s-1 in IIB, mixed, IIX, IIA and slow fibres, respectively. All the differences were statistically significant except that between IIA and IIX fibres. Within each group of fibres with the same MHC composition, ATPase activity was found to correlate with P0, but not V0. However, ATPase activity was found to correlate with V0 when all the fibre types were pooled together. In thirty-seven fast fibres the MLC ratio, i.e. the proportion of the fast alkali light chain isoform, MLC(3f), to the amount of the regulatory light chain, MLC(2f), was determined. IIB fibres had the highest proportion of MLC(3f) and IIA fibres, the lowest. A multiple regression analysis, used to distinguish between the effects of MHC and MLC composition, showed that ATPase activity was insensitive to the MLC ratio, whereas it had a significant impact on V0. The results obtained in this study indicate that in rat skeletal muscle fibres: (a) ATPase activity during isometric contractions and tension cost are strongly dependent on MHC isoform composition, and (b) there is no evidence that the alkali MLC ratio is a determinant of ATPase activity.
|Number of pages||13|
|Journal||Journal of Physiology|
|Publication status||Published - 1994|
ASJC Scopus subject areas