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Three cows within their 26th week of lactation (average milk yield: 15.7 kg/day) were used to determine the effects of starvation on 19 free amino acids (AA) of plasma, uremia and glycemia. For three successive days, blood samples were collected at hourly intervals from 09.00 am to 03.00 pm. From 1 month before up to the ist day of the sampling period, the animals were overfed in two equal meals (06.15 am and 03.30 pm) with a hay-based diet. On the 2nd and the 3rd day, only water was available. On the Ist day of fasting, muscle protein catabolism increased as suggested by an increase (P < 0.05) in mean plasma levels of 3-methylhistidine (MeHis, index of muscle protein catabolism in cattle), Asp, Gly, His, Lys and Ser. On the same day, mean plasma concentrations of Glu and Pro decreased (P < 0.05), possibly through an enhanced utilization of those AA for gluconeogenesis and/or energy needs as suggested by the maintainance of glycemia and the increase (P < 0.05) in uremia. On the 2nd day of fasting, further changes of metabolic parameters occurred: mean plasma levels of MeHis, urea, Arg and Leu still increased (P < 0.05); those of Asn, Asp, Glu, Met and Tyr decreased (P < 0.05); plasma Gly, His, Lys and Ser plateaued at high and plasma Pro at low levels. Unlike results reported in dry cow, plasma Ala, Gin, lie and Val did not increase during the fasting period. Furthermore, despite an enhanced protein mobilization, glycemia and milk yield dropped (P < 0.05) on the 2nd day of fasting. The fall in plasma glucose probably resulted from a continuous glucose uptake by the mammary gland for milk lactose synthesis which was not entirely compensated by gluconeogenesis. Within-day variations in plasma metabolites were observed, but these variations were generally lower during fasting than during feeding, especially for plasma glucose which showed variation with time of day during both fasting days. It was concluded that, as in dry cows, a 2-day fasting enhances muscle proteolysis in lactating cows. Contrary to dry cows, however, gluconeogenesis mechanism which attempts to maintain glucose homeostasis becomes insufficient in lactating cows to avoid a drop in glycemia from the 2nd day of fasting. The decrease in milk yield might be, at least partly, the result of that drop in glycemia. Finally, the time course from the last meal is a factor affecting within-day variations of plasma metabolites

Influence de la mise à jeun sur l'aminoacidémie libre, l'urémie et la glycémie chez la vache laitière