Effects of fasting on tissue amino acid concentrations and urea-cycle enzymatic activities in young pigs.

Weanling pigs were fed a 30% crude protein (CP) diet for 18 d and then assigned to one of three regimens for either 4 or 8 d: 1) fasting, 2) 3% CP, i.e., maintenance or 3) 30% CP after which they were bled, then sacrificed for tissue assessment. Relative to pigs fasted or fed 30% CP, feeding 3% CP resulted in decreased urea-N and NH3-N excretion in the urine. Arginase and ornithine transcarbamoylase (OTC) activities in liver, and arginase activity in kidney cortex were markedly lower in pigs fed 3% CP compared with those either fasted or fed 30% CP. Hepatic arginase and OTC, however, were higher in fasted pigs than in those fed 30% CP. Pigs fed 3% CP had much lower levels of free threonine, tyrosine, cystathionine, taurine and branched-chain amino acids in plasma, liver, kidney, muscle and brain than fasted pigs or those fed 30% CP. Threonine concentration in brain, liver, muscle and plasma increased as length of the fast increased. Fasted pigs had decreased free alanine levels in plasma, and decreased free serine levels in plasma and liver when compared with fed pigs. Inter-organ comparisons provided evidence that both alanine and serine were important gluconeogenic amino acids during fasting. In general, free amino acid levels in brain were similar between fasted pigs and those fed 30% CP. Fasting for 8 d caused a 10-fold elevation in urinary taurine excretion relative to that observed for 4-d fasted pigs.