Plasticity in skeletal, cardiac, and smooth muscle - Selected contribution: Skeletal muscle focal adhesion kinase, paxillin, and serum response factor are loading dependent

Selected Contribution: Skeletal muscle focal adhesion kinase, paxillin, and serum response factor are loading dependent. J Appl Physiol 90: 1174-1183, 2001.-This investigation examined the effect of mechanical loading state o n focal adhesion kinase (FAK), paxillin, and serum response factor (SRF) in rat skeletal muscle. We found that FAK concentration and tyrosine phosphor ylation, paxillin concentration, and:SRF concentration are all lower in the lesser load-bearing fast-twitch plantaris and gastrocnemius muscles compar ed with the greater load-bearing slow-twitch soleus muscle. Of these three muscles, 7 days of mechanical unloading via tail suspension elicited a decr ease in FAK tyrosine phosphorylation only in the soleus muscle and decrease s in FAK and paxillin concentrations only in the plantaris and gastrocnemiu s muscles. Unloading decreased SRF concentration in all three muscles. Mech anical overloading (via bilateral gastrocnemius ablation) for 1 or 8 days i ncreased FAK and paxillin concentrations in the soleus and plantaris muscle s. Additionally, whereas FAK tyrosine phosphorylation and SRF concentration were increased by less than or equal to1 day of overloading in the soleus muscle, these increases did not occur until somewhere between 1 and 8 days of overloading in:the: plantaris muscle. These data indicate that, in the s keletal muscles of rats, the focal adhesion complex proteins FAK and paxill in and the transcription factor SRF are generally modulated in association with the mechanical loading state of the muscle. However, the somewhat diff erent patterns of adaptation of these proteins to altered loading in slow- vs. fast-twitch skeletal muscles indicate that the mechanisms and time cour se of adaptation may partly depend on the prior loading state of the muscle .