Amino acid supply and metabolism by the ruminant mammary gland.

Metabolism in the gastrointestinal tract (MacRae et al. 1997) 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 (MacRae et al. 1995). 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.