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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Deficiency of -sarcoglycan differently affects fast- and slow-twitch skeletal muscles

Departments of ¹Human Anatomy and Physiology and ²Biomedical Sciences, University of Padova, Padova; ³Department of Cell Biochemistry, Nencki Institute of Experimental Biology, Warszawa, Poland; and ⁴Neuromuscular Biology and Physiopathology Unit, Consiglio Nazionale delle Ricerca Institute of Neuroscience, Padova, Italy

Submitted 30 September 2004 ; accepted in final form 29 June 2005

-Sarcoglycan (Sgca) is a transmembrane glycoprotein of the dystrophin complex located at skeletal and cardiac muscle sarcolemma. Defects in the -sarcoglycan gene (Sgca) cause the severe human-type 2D limb girdle muscular dystrophy. Because Sgca-null mice develop progressive muscular dystrophy similar to human disorder they are a valuable animal model for investigating the physiopathology of the disorder. In this study, biochemical and functional properties of fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles of the Sgca-null mice were analyzed. EDL muscle of Sgca-null mice showed twitch and tetanic kinetics comparable with those of wild-type controls. In contrast, soleus muscle showed reduction of twitch half-relaxation time, prolongation of tetanic half-relaxation time, and increase of maximal rate of rise of tetanus. EDL muscle of Sgca-null mice demonstrated a marked reduction of specific twitch and tetanic tensions and a higher resistance to fatigue compared with controls, changes that were not evident in dystrophic soleus. Contrary to EDL fibers, soleus muscle fibers of Sgca-null mice distinctively showed right shift of the pCa-tension (pCa is the negative log of Ca²⁺ concentration) relationships and reduced sensitivity to caffeine of sarcoplasmic reticulum. Both EDL and soleus muscles showed striking changes in myosin heavy-chain (MHC) isoform composition, whereas EDL showed a larger number of hybrid fibers than soleus. In contrast to the EDL, soleus muscle of Sgca-null mice contained a higher number of regenerating fibers and thus higher levels of embryonic MHC. In conclusion, this study revealed profound distinctive biochemical and physiological modifications in fast- and slow-twitch muscles resulting from -sarcoglycan deficiency.

limb girdle muscular dystrophy 2D; skeletal muscle fiber types; skeletal muscle contractile properties; caffeine sensitivity of sarcoplasmic reticulum; calcium sensitivity of myofibrillar proteins

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