J. Anim Sci., doi: 10.2527/jas.2007-0799
©Copyright, 2008, The American Society of Animal Science
Changes of activity and mRNA expression of urea cycle enzymes in the liver of developing Holstein calves
M. Takagi 1,
T. Yonezawa 1,
S. Haga 2,
H. Shingu 2,
Y. Kobayashi 2,
T. Takahashi 1,
Y. Ohtani 1,
Y. Obara 1,
K. Katoh 1*
1 Department of Animal Physiology, Graduate School of Agricultural Science, Tohoku University, 1-1 Amamiyamachi, Sendai, Miyagi, 981-8555, Japan
2 National Institute of Livestock and Grassland Science, 2Ikenodai, Tsukuba, Ibaraki, 305-0901, Japan
* To whom correspondence should be addressed. E-mail: kato{at}bios.tohoku.ac.jp.
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Abstract |
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Urea is an important re-utilizable nitrogen-source for the ruminant and is mainly synthesized through the urea cycle in the liver. The cycle is undertaken by five enzymes: carbamoyl phosphate synthetase (CPS), ornithine transcarbamoylase (OTC), argininosuccinate synthetase (ASS), argininosuccinate lyase (ASL) and arginase (ARG). The purpose of this study was to investigate changes in the activity of the enzymes and mRNA expression from the point of view of an increase in the plasma urea concentrations with age in Holstein calves. First, the plasma concentrations of metabolites and hormones were determined in calves at 1, 3, 8, 13 and 19 wk of age (n = 4, weaned at 6 wk of age). The plasma concentration of urea drastically increased after weaning (P < 0.001). The plasma concentration of glucose was lowest at 8 wk. The plasma concentration of IGF-I gradually increased with age, although that of NEFA, glucagon and cortisol decreased (P < 0.001). Concentrations of triglyceride, 
-amino N, growth hormone and insulin did not significantly change throughout the experiment. Next, using the liver tissues taken from calves at 2, 13 and 19 wk of age (n = 4 to 6 at each time point, weaned at 6 wk of age), the activity and mRNA expression of the enzymes were measured by biochemical methods and quantitative RT-PCR, respectively. The activities of CPS (P < 0.001), OTC (P = 0.001) and ASS (P = 0.015) increased with age whereas ASL (P = 0.003) decreased. Although the mRNA expression was decreased with age in ASL (P = 0.002) and ARG (P = 0.007), no significant change was observed in CPS, OTC and ASS. We conclude that the increased urea production in the liver may be explained not only by an increase in the activities of the urea cycle enzymes, but also by an increased ammonia production by rumen fermentation and gluconeogenesis from amino acids around weaning time.
Key Words:
calf, development, liver, mRNA, urea cycle enzymes, weaning
