2012 Volume 55 Issue 1 Pages 49-53
The effect of high ambient temperature on myofibrillar proteolysis in high-yielding lactating cows was investigated. In a 2 x 2 crossover design, four multiparous lactating Holstein cows were maintained in a chamber under conditions of constant moderate ambient temperature (MT) or high ambient temperature (HT). Dry matter intake (P < 0.10) and milk yield (P < 0.10), and milk protein yield (P < 0.05) were lower in HT than MT. Urinary 3-methylhistidine excretion were 1.87 mmol/day in MT and 1.92 mmol/day in HT, but no statistically significant difference was found. Plasma 3-methylhistidine concentrations were higher (P < 0.05) in HT (6.9 nmol/ml) than MT (4.1 nmol/ml), indicating that high ambient temperature increased the myofibrillar proteolysis. The expression of muscle calpain SS (small subunit (28kDa)), -1, and -3, and calpastatin mRNA in HT were increased 1.4, 1.1, 1.6, and 1.4-fold of MT, respectively, but no statistically significant difference was found. Meanwhile, the correlation between the level of plasma 3-methylhistidine and the level of muscle calpain 3 mRNA was significant (P < 0.05) and its correlation coefficient was the highest in calpain family. In conclusion, high ambient temperature affected the plasma index of myofibrillar proteolysis related to the expression of muscle calpain 3 and calpastatin mRNA in high-yielding lactating dairy cows.