Amino Acid Exchange by the Mammary Gland of Lactating Goats when Histidine Limits Milk Production

Abstract

The aim of this study was to monitor amino acid (AA) exchange kinetics of the mammary gland in response to an imposed limitation on His supply for milk production. Lactating goats (n = 4, approximately 120 DIM) were fed a low protein ration that provided only 77% of metabolizable protein and 100% of energy requirements for milk production. The protein deficiency was alleviated 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day of the first two periods, [U-13C]AA were continuously infused i.v. for 7 h and arterial and mammary vein blood was withdrawn to determine plasma AA concentration and enrichment. 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 gland was used to derive bi-directional rates of plasma AA exchange. Arterial plasma His concentration was lower during -H infusion (8 vs. 73 microM), but those of other AA changed little. Responses to low levels of plasma His were: 1) mammary blood flow increased by approximately 33%; 2) the gland's capacity to remove plasma His increased 43-fold, whereas the gland's capacity for 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 insufficient to offset the reduced influx, milk protein yield decreased from 118 to 97 g/d.

The effect of jugular or abomasal infusion of amino acids on milk yield in lactating cows fed a protein deficient diet

“…The present results also show that the arterial limitation in methionine did not affect mammary blood flow as observed when histidine severely limited milk protein synthesis [6]. Such mammary arterial blood flow adjustment has also been observed when methionine was limiting milk protein synthesis [22].…”

“…This contrasts with observations made from the comparison of separate experiments, which used cows at similar lactation status and similar protein deficient diets. These results suggest that milk protein yield in response to additional essential amino acid supply would be greater with jugular than 6 results in a modification in milk yield without any alteration in milk protein concentration.…”

“…Such mammary arterial blood flow adjustment has also been observed when methionine was limiting milk protein synthesis [22]. Comparison between experiments suggests that the udder adjusts blood flow according to the extent of the arterial limitation, requiring mammary arterial blood flow adjustment with severe histidine limitation [6] and adapting to less severe limitation in histidine by increasing mammary extraction rate [25]. 10 when intestinal metabolism is first bypassed.…”

Responses of the bovine mammary glands to absorptive supply of single amino acids

“…Subtraction of 30 g d -1 His caused a decrease of 186 g d -1 in protein yield and an increase of 181 g d -1 in fat yield. Bequette et al (2000) induced very low circulating concentrations of His in lactating goats by subtracting approximately 40% of the His from the total duodenal supply. Mammary blood flow rate was 36% higher at 8 µM plasma His compared to 73 µM.…”

“…Correction of a His deficiency has repeatedly been shown in subtraction experiments to increase milk and milk protein yields and decrease milk fat yield (Fraser et al 1991;Kim et al 1999;Bequette et al 2000;Weekes and Cant 2000;Cant et al 2001). In early-lactation cows fed a 9%-protein diet plus 1.2 kg d -1 abomasal AA, Weekes and Cant (2000) found that improvements in each of Met, Lys and His supplies caused protein yields to increase, while fat yields dropped, but the effect was most pronounced for His.…”