Effect of Amount and Profile of AA Supply on Mammary AA Metabolism

Abstract Text:

Amino acids function not only as protein building blocks but also as signaling molecules that affect milk protein synthesis and mammary metabolism. In an attempt to expose the mammary gland to different amounts and profile of AA in the arterial blood, ten Holstein cows were blocked by DIM into two 5x5 Latin squares and fed 5 treatments: (1) positive control (16% CP), formulated to meet metabolizable protein requirements; 14.9% CP with (2) or without (3) EAA infusion; or 13.5% CP diet with (4) or without (5) EAA infusion. The AA solutions were prepared according to AminoCow to provide all limiting EAA in each diet and were infused continuously into the abomasum. On the last day of each 14-d period, blood samples were taken from the coccygeal vessel and the mammary vein simultaneously, at four time-points equally spaced between two milkings. Plasma samples were analyzed for AA by GC-MS. Data was analyzed using Proc Mixed, significance was declared at P<0.10 and trend at P<0.20. Contrasts and LS-means are reported in the table. All AA are analyzed and responded differently to the treatments. Methionine and Lysine are presented as the most limiting AA for milk protein synthesis and as examples of the two groups of AA according to their post-absorptive metabolism. The changes in AA arterial concentrations were smaller than we attempted to achieve. Nonetheless, some effects were observed. Fractional removal of lysine decreased with the decrease of CP levels in the diet, despite the similar lysine arterial concentrations. For methionine, fractional removal decreased with AA infusions, even though methionine arterial concentrations were increased. Mammary uptake to milk output ratio was affected by CP levels for methionine, suggesting responses within the mammary gland. 

	Treatments					Contrasts
Variable	16%CP	14.9%CP +EAA	14.9%CP	13.5%CP +EAA	13.5%CP	16 vs 14.9	16 vs 13.5	14.9 vs 14.9 + EAA	13.5 vs 13.5+ EAA
Lysine
Artery, uM	93.14	67.17	99.82	96.83	94.55	NS	NS	0.1313	NS
Fractional removal1, %	46.90	32.57	37.18	41.23	37.93	0.0888	0.1278	NS	NS
Mammary uptake:milk output	1.28	0.64	1.21	0.76	1.37	NS	NS	NS	NS
Methionine
Artery, uM	25.92	37.94	23.11	49.08	23.98	NS	NS	0.0003	<.0001
Fractional removal, %	43.38	27.32	43.32	24.24	40.43	NS	NS	0.0011	0.0014
Mammary uptake:milk output	1.14	1.01	1.01	0.97	0.96	0.0657	0.0171	NS	NS

¹Fractional removal = arterio-venous difference/arterial concentration

Keywords:

amino acid

mammary gland