Amino acid supply and metabolism by the ruminant mammary gland

Abstract

Metabolism in the gastrointestinal tract ) and liver (Lobley & Milano, 1997) has been shown to be important in determining the amount and pattern of amino acids (AA) available post-hepatically for mammary gland utilization as well as for muscle, skin and, indeed, the gastrointestinal tract. High rates of metabolism by the non-mammary tissues have been suggested as one reason for the low efficiency (approximately 20 %) of conversion of dietary N into milk protein . Thus, ways to reduce these losses could lead to a larger proportion of AA being partitioned to the mammary gland for milk protein synthesis. Recent studies (Guinard & Rulquin, 1994; Bequette et al. 1996b), however, now suggest that once AA are extracted by the mammary gland considerable losses in efficiency of conversion of these AA into milk protein also occur (i.e. net mammary AA uptake greater than milk AA outputs), but little is known of the factors controlling intracellular metabolism. Thus, a better understanding of how catabolic and alternative routes of metabolism within the gland are regulated, and how these might be avoided or reduced, could lead to an increase in the efficiency of milk production and a reduction in non-mammary tissue N losses. The present paper will discuss what is currently known about the processes controlling arterial AA delivery (supply) to the mammary gland, and the mechanisms and metabolic pathways the mammary gland employs to balance the supply of AA at the site of milk protein synthesis during times of inadequacy and excess. PARTITION OF AMINO ACIDS TO THE MAMMARY GLANDAt the onset of lactation in the dairy goat, whole-body protein synthesis is considerably enhanced, with a substantial proportion attributable to the increased metabolism of the mammary gland. This appears to occur at the expense of other tissues as protein synthesis is relocated from the carcass tissues to the mammary gland, changing from 0.28 and 0.01-0.03 of whole-body protein synthesis respectively in the non-lactating, non-pregnant animal to 0.08 and 0.34-0.43 respectively during early (week 2) lactation (Champredon et al. 1990;Baracos et al. 1991). Hence, it has often been said that during full lactation the body of the dairy cow is an appendage to the mammary gland. The various factors responsible for initiating and maintaining this dramatic shift are poorly understood, but probably involve ontological regulatory mechanisms and changes in the recognition and sensitivity of nutrient-hormonal signals at the different organ, tissue and cellular levels. These types of regulatory mechanisms will not be discussed in the present paper, which will instead concentrate on sources and delivery of AA to the gland, and subsequently their extraction and metabolism within the mammary gland in support of casein biosynthesis.In support of protein synthesis in the mammary gland, an increase in the metabolic partition of AA to the gland is required, and when the partition of individual AA to the mammary gland are considered, a different picture emerges compared wi...

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