Calpastatin Expression in Porcine Cardiac and Skeletal Muscle and Partial Gene Structure

The expression in porcine skeletal and cardiac muscle of calpastatin, the specific endogenous inhibitor of the calpain proteolytic system, was examined 16 h after a single dose of a specific β2-agonist. Immunoblotting of extracts indicated that treatment increased skeletal calpastatin 135-kDa band intensity (P < 0.01), while in cardiac combined 145- and 135-kDa band intensity decreased (P < 0.05). Treatment increased skeletal (P < 0.01) but not cardiac calpastatin mRNA steady-state levels. Three types of cardiac calpastatin mRNA transcripts were identified by 5′-RACE. Types I and II encoded a putative XL region that originated either from exon 1xA or exon 1xB, arranged in tandem. Type III predominated in skeletal muscle and originated from exon 1u, which was located 40–50 kb 3′ to exons 1xA and 1xB. The region 5′ to exon 1u may act as an independent promoter regulated by a cAMP-dependent mechanisms, thereby explaining the differential response of calpastatin to adrenergic stimulation in cardiac and skeletal muscle.