CONTRACTILE PROPERTY AND MYOFIBRILLAR PROTEIN ALTERATIONS OF SINGLE FIBERS FOLLOWING MUSCLE STRETCH INJURY

Abstract

The present study investigated changes in contractile properties and myofibrillar protein expression of single skinned skeletal muscle fibers following a controlled stretch injury to the tibialis anterior muscle of an adult rabbit. Specifically, we examined the force-pCa relationship and the Ca2+-dependence of the rate of force redevelopment (ktr) in chemically skinned single fibers dissected from injured and non-injured muscles. Compared to control fibers (n = 8), injured fibers (n = 13) exhibited a significant reduction in sub-maximal forces resulting in decreased Ca2+-sensitivity of force, with no marked effect on maximal Ca2+-activated force (Po). In addition, the steepness of the force-pCa relationship was markedly reduced in the injured fibers, indicating a decrease in the apparent cooperativity of activation. The rate of force redevelopment in skinned fibers varied with the level of activating Ca2+, i.e., ktr increased nearly 10-fold in non-injured fibers and approximately 5-fold in injured fibers, as Ca2+ was increased from submaximal to maximal levels. Furthermore, stretch injury elicited shifts in the expression of tropomyosin and myosin binding protein C relative to the non-injured fibers. The alterations in contractile properties and shift in contractile protein expression may have implications for the study of muscle injury. Traditional mechanical testing of whole muscle preparations has generally shown recovery of muscle function at this time period post-injury when clearly there are deficits in contractile function and alterations in protein expression. These data suggest the importance of single fiber mechanics when assessing muscle following stretch injury and recovery from stretch injury.