CURRENT CONCEPTS OF AMINO-ACID AND PROTEIN-METABOLISM IN THE MAMMARY-GLAND OF THE LACTATING RUMINANT

Milk protein responses to protein nutrition are typically poor and, in part, may be due to the low efficiency (similar to 25 to 30%) of conv erting dietary N into milk. Posthepatic availability of amino acids (A A) is not limited, yet only similar to 30% is converted into milk. The poor capture of AA by the mammary gland may relate to the imbalanced and uncoordinated timing of nutrient delivery to the gland. The infusi on of essential AA improves the efficiency of utilization (0.31); howe ver, further catabolism of AA within the mammary gland suggests that A A transport is not a major limitation. These losses may serve ancillar y or functional roles, but mammary oxidation of some AA occurs only wh en AA extraction exceeds the stoichiometric requirements for milk prot ein synthesis. Intracellular substrate supply may be more limiting tha n is the appartus for protein synthesis. Studies utilizing isotope lab eling and conducted in vitro and in vivo now suggest that circulating peptides and proteins can serve as sources of perhaps all AA for casei n synthesis, but the source of these remains elusive. Constitutive pro tein and casein turnover contribute significantly (42 to 72%) to mamma ry protein synthesis. All AA are extensively channeled through an inte rmediary protein pool or pools that have rapid turnover rates. The AA are then incorporated into casein, which appears to be fixed in associ ation with protein turnover. The mammary gland is a major controller o f its metabolism, and the mechanisms of AA extraction and conversion i nto milk protein are linked to secretion events. Blood flow may be a k ey point of regulation whereby mechanisms sense and respond to nutrien t supply and balance to the gland via alterations in hemodynamics.