If one accepts the view that non-mammary use of AA is a function of mammary use, then it becomes critical to understand what regulates mammary removal and use. Net removal of AA by the udder has often been correlated with arterial concentrations of AA (e.g. Hanigan et al., 1992; Guinard and Rulquin, 1994). However, this relation has not always been observed. For example, when dairy cows were fed three levels of crude protein and where milk protein yield increased by 50 g day-1 (Metcalf et al., 1996), mammary uptakes of only lysine and valine increased, despite the fact that concentrations of most essential AA, except for methionine, phenylalanine and threonine, were increased by infusion.
The intragastric infusion studies of Rulquin and colleagues are among the few studies that have allowed a comparison of the partition and utilization by the mammary gland of controlled known rates of AA supply. Total removal of essential AA by the mammary gland increased in response to incremental duodenal infusions of casein, although marginal removals of the essential AA declined from 0.81 of the casein infused at the lowest rate to 0.50 at the highest level of infusion (Guinard and Rulquin, 1994). The efficiency of converting the extracted essential AA into milk protein was also reduced considerably (from 0.88 to 0.49) with nearly all AA extracted far in excess of net requirements for milk protein synthesis. These observations underscore the importance of understanding the mechanisms regulating AA metabolism by mammary secretory cells.
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