Amino acid exchange by the mammary gland of lactating goats when histidinelimits milk production

The aim of this study was to monitor amino acid (AA) exchange kinetics of t he mammary gland in response to an imposed limitation on His supply for mil k production. Lactating goats (n = 4, similar to 120 DIM) were fed a low pr otein ration that provided only 77% of metabolizable protein and 100% of en ergy requirements for milk production. The protein deficiency was alleviate d by infusion into the abomasum of an AA mixture (67 g/d) including (+H; 4. 4 g/d) or excluding (-H) His. Goats were assigned to treatments (6 to 7 d) according to a switchback design. On the last day of the first two periods, [U-C-13]AA were continuously infused i.v. for 7 h and arterial and mammary vein blood was withdrawn to determine plasma AA concentration and enrichme nt. Flow probes monitored mammary blood flow. The secretion and enrichments of AA in milk casein were monitored each hour. A three-pool model of the g land was used to derive bi-directional rates of plasma AA exchange. Arteria l plasma His concentration was lower during -H infusion (8 vs. 73 mu M), bu t those of other AA changed little. Responses to low levels of plasma His w ere: 1) mammary blood flow increased by similar to 33%; 2) the gland's capa city to remove plasma His increased 43-fold, whereas the gland's capacity f or other AA declined by two- to threefold; and 3) influx and efflux of His by the gland decreased. Thus, as the reduction in His efflux was insufficie nt to offset the reduced influx, milk protein yield decreased from 118 to 9 7 g/d.