Effects of dexamethasone on muscle protein homeostasis and on calpain and calpastatin activities and gene expression in rabbits.

The objectives were to investigate the mechanisms by which glucocorticoids control proteolysis in muscle cells and the relationship between the calpain:calpastatin system and proteolysis in muscle. Female rabbits were treated with 1 mg dexamethasone (Dex)/kg body weight per day for 0, 1, 2 or 4 days after which animals were killed and muscle samples taken for analyses. Dex reduced urinary N tau-methylhistidine (NMH) 48% (day 4 versus day 1 of Dex treatment) and muscle NMH concentrations by 49% (day 1) to 40% (day 2) respectively, suggesting that protein degradation was reduced. To investigate whether the changes in apparent proteolysis were related to calpains, we examined the effects of Dex on muscle calpain and calpastatin activities. These were unaffected by Dex. This implies that Dex-dependent changes in degradation are not mediated by changes in muscle calpain or calpastatin activities. We studied the effects of Dex on calpain and calpastatin gene expression as a means of clarifying the relationships between proteinase gene expression and proteinase activities. mu-Calpain mRNA concentration was unaffected by Dex but m-calpain mRNA and calpastatin mRNA concentrations were reduced by 42-55% and 40% respectively. Dex had a similar effect on beta-actin mRNA. Although calpain and calpastatin genes behaved as house-keeping genes, changes in their expression mimicked apparent changes in proteolysis. The observation that calpain and calpastatin activities were unchanged indicates that additional regulation of the calpain:calpastatin system exists at other sites in muscle cells. To determine whether Dex-dependent changes in proteolysis were mediated indirectly, we assayed the effects of Dex on plasma thyroid hormone concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)