β-agonists are known to enhance muscle growth in livestock species, although sometimes at the expense of increased meat toughness. In pigs, administration of porcine somatotropin (pST) produces muscle hypertrophy without detrimental effects on toughness. Calpastatin, a specific inhibitor that regulates the calpain proteinases responsible for cleavage of myofibrillar proteins, has a key role in growth and the rate at which meat tenderises postmortem. In the porcine calpastatin gene there are at least three calpastatin promoters (upstream of exons 1xa, 1xb and 1u respectively), of which 1u is used predominantly in skeletal muscle. All three promoters contain transcription factor binding motifs suggesting sensitivity to β-agonist/cyclic AMP (cAMP)/protein kinase A (PKA) and IGF-1/Ca2+/calcineurin-mediated signalling pathways (Parr et al., 2004). It was previously shown that expression of calpastatin mRNA from all three promoters can be increased in pigs treated with clenbuterol and suppressed in pST-treated animals (Sensky et al., 2004), suggesting differential regulation by the two pathways. In this study, the effect of manipulating these pathways is evaluated in vitro in order to determine if there is a specific element within the porcine 1u promoter that is responsive to both pathways, using rat muscle cells transfected with a series of 5’-deleted porcine 1u promoter constructs and treated with cAMP analogues and the calcium ionophore calcimycin.