It is well known that rapid gain of muscle mass in neonatal pigs is highly related to
protein synthesis. However, the role of protein degradation in muscle gain of the neonatal period has not been well established. Calpains and their endogenous inhibitors, calpastatins, play a significant role in early-stage myofibrillar protein degradation. To investigate the role of
calpain-
calpastatin system in
muscle protein accumulation, we studied the expressions of their
mRNA in muscle tissue sampled at days 1, 4, 6, 12, 20 and 28 from a total of 36 neonatal pigs. The steady-state
mRNA levels of calpains 1A, 2 and 3A,
calpastatin types 1, 2 and 3, obtained by quantitative real-time PCR analysis, decreased by 2-4 folds at the age of 4 to 6 days compared to 1-day-old piglets. Then, the relatively low expression level was maintained through 28 days of age. Expressions of calpains 1A, 3A and
calpastatin type 1 were significantly correlated with the measurements of
muscle protein accumulations such as
muscle protein content and
RNA/
protein ratio. Expressions of
calpain 1A,
calpastatin types 1 and 3 were negatively correlated with
birth weight and fractional rate of growth. The levels of calpains 1A and 2
mRNA were correspondent to their
protease activities. In conclusion, decreased levels of
calpain and
calpastatin expressions over development in neonatal pigs are associated with high
protein accumulations, suggesting that dramatic muscle growth during the neonatal period may be partially controlled by down-regulated
calpain-
calpastatin system.