MYOFIBRILLAR ATPASE ACTIVITY DURING ISOMETRIC CONTRACTION AND ISOMYOSIN COMPOSITION IN RAT SINGLE SKINNED MUSCLE-FIBERS

1. Myofibrillar ATPase activity, isometric tension (P-o) and unloaded shortening velocity (V-o) were determined in single skinned fibres iso lated from rat hindlimb muscles during maximal calcium activation at 1 .2 degrees C. In each fibre, myosin heavy chain (MHC) isoforms were id entified using electrophoresis and immunocytochemistry. ATPase activit y was determined spectrophotometrically from NADH oxidation in a coupl ed enzyme assay. 2. On the basis of their MHC isoform composition, the fibres (n = 102) were divided into five groups containing the slow is oform, I MHC, or one of the fast isoforms, IIB MHC, IIA MHC, IIX MHC, or a mixture of the latter three. ATPase activity was significantly hi gher in IIB than in 2X and IIA fibres (0.230 +/- 0.010, 0.178 +/- 0.02 3 and 0.168 +/- 0.026 nmol mm(-3) s(-1). respectively). Mixed fibres h ad intermediate values. ATPase activity in slow fibres was considerabl y less (0.045 +/- 0.006 nmol mm(-3) s(-1)). 3. The ratio between ATPas e activity and P-o, i.e. tension cost, was found to be 2.90 +/- 0.09, 2.58 +/- 0.14, 1.89 +/- 0.22, 1.52 +/- 0.13 and 0.66 +/- 0.004 pmol AT P mN(-1) mm(-1) s(-1) in IIB, mixed, IIX, IIA and slow fibres, respect ively. All the differences were statistically significant except that between IIA and IIX fibres. 4. Within each group of fibres with the sa me MHC composition, ATPase activity was found to correlate With P-o, b ut not V-o. However, ATPase activity was found to correlate with V-o w hen all the fibre types were pooled together. 5. In thirty-seven fast fibres the MLC ratio, i.e. the proportion of the fast alkali light cha in 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. 6. A multiple regression analysis, used to distinguish between the effects of MHC and MLC composition, showed th at ATPase activity was insensitive to the MLC ratio, whereas it had a significant impact on V-o. 7. The results obtained in this study indic ate that in rat skeletal muscle fibres: (a) ATPase activity during iso metric contractions and tension cost are strongly dependent on MHC iso form composition, and (b) there is no evidence that the alkali MLC rat io is a determinant of ATPase activity.