CONTRACTILE PROPERTIES AND PROTEIN ISOFORMS OF SINGLE FIBERS FROM THECHICKEN PECTORALIS RED STRIP MUSCLE

1. The contractile properties of single muscle fibres of the red strip region of adult chicken pectoralis major (PM) muscle, some of which a re known to express an embryonic isoform of myosin heavy chain (MHC), were determined and compared with the properties of the fast white fib res of the PM and the slow tonic fibres of the anterior latissimus dor si (ALD) muscle. 2. The red strip fibres could be classified into two groups, fast and slow. The mean velocity of unloaded shortening (V-max ) in fast red strip fibres was approximately half the V-max of fast wh ite fibres. V-max of slow red strip fibres was less than 20% of the va lue for fast red strip fibres and was not different from V-max of ALD fibres. 3. The tension-generating ability i.e. the maximal isometric t ension/fibre cross-sectional area (P-0/CSA), was the same in fast red strip fibres and fast white fibres. P-0/CSA was approximately 30% lowe r in slow red strip fibres compared with fast red strip fibres but was 70% greater in slow red strip fibres compared with ALD fibres. 4. The tension-pCa relation of fast red strip fibres was shifted to lower pC a values, indicating a lower calcium sensitivity compared with fast wh ite fibres, and this difference was associated with a difference in tr oponin T isoform composition. The tension-pCa relation of slow red str ip fibres was not different from that in ALD fibres. 5. The difference in V-max between fast red strip fibres and fast white fibres was asso ciated with different MHC compositions of these fibres. 6. The myofibr illar protein isoform composition of slow red strip fibres was identic al to that of the slow tonic fibres of ALD muscle and these two groups of fibres had very similar contractile properties.