We investigated the roles of insulin and amino acid (AA) in regulating milk production and the uptake of AA and blood flow (BF) by the mammary gland and hind-leg of goats (n = 4). During two periods, either saline or AA (65 g/d) was infused i.v. for 7.5 d, and, beginning on d 5, goats were subjected to a hyperinsulinemic-euglycemic clamp. The insulin clamp elevated plasma insulin levels threefold and insulin-like growth factor-1 by 27%, and euglycemia was maintained by the infusion of glucose. Arterial, mammary, and tarsal vein blood samples were obtained on d 4 and 8 of each period, and blood flow was monitored continuously by probes. Insulin and insulin plus AA infusions increased the yields of milk by 13 to 18% and protein by 10 to 21%, but AA infusion alone had no effect. The insulin clamp reduced milk fat content by 21 to 31% and yield by 8 to 19%, and reduced the yields of milk fatty acids >C16. The insulin clamp increased mammary blood flow by 42%, but insulin and AA infusions both increased hind-leg BF by 29 to 52% and by 25%, respectively. Net uptakes of most plasma AA by the udder were reduced by insulin, whereas AA infusion had no effect. For the leg, the uptake of His and Thr were decreased by insulin, whereas the infusion of AA stimulated the uptake of total essential AA. Insulin increased the uptake of glucose by the udder but not by the leg. This study suggests that the udder and leg tissues respond differently to infusions of insulin and AA; the udder was more responsive to insulin, while the leg was more responsive to AA concentralion (supply), at least in terms of AA uptake and net anabolism (protein gain or secretion).

The contribution to casein biosynthesis of peptides derived from blood was examined in late lactation goats (254 to 295 d in milk). Ratios of mammary uptake of free amino acids (AA) in blood to output of AA in milk protein and ratios of the enrichments of Phe, Tyr, Met, and Lys at isotopic plateau in secreted milk casein to the free AA in arterial and mammary vein blood were monitored during the last 5 h of a 30-h continuous i.v. infusion of [1-13C]Phe, [2H4]Tyr, [5-13CH3]Met, and [2-15N]Lys on two occasions: before (control) and on d 6 of an i.v. infusion of Phe (6 g/d). During the control, uptakes of free Phe and Met were less than their output in milk. This result was comparable with the labeling kinetic results, suggesting that vascular peptides contributed 5 to 11% of Phe and 8 to 18% of Met. Free Tyr and Lys uptakes during the control were sufficient for milk output; however, the labeling kinetics indicated that 13 to 25% of the Tyr and 4 to 13% of the Lys were derived from peptides. Infusion of Phe increased the uptake of free AA but reduced the contribution of peptides toward Phe (0 to 3%) and Tyr (8 to 14%) supply for casein synthesis. Whole body hydroxylation of Phe to Tyr increased from 10 to 18% with the infusion of Phe; within the mammary gland, this conversion was lower (3 to 5%). Results suggest that the mammary utilization of peptides containing Phe and Tyr appears to depend on the supply of free AA in blood.

A model of Leu and protein metabolism by the mammary gland and hind leg of lactating goats was constructed and evaluated from data collected by using [15N, 1-13C]Leu kinetics measured during amino acid (AA) infusion and a hyperinsulinemic-euglycemic clamp (IC). Goats were given continuous intravenous infusions of either saline or AA (65 g/d) for 7.5 d and from d 5 to 7.5 goats were subjected to IC. Arteriovenous kinetics were monitored on d 4 and 8 by continuous infusion (8 h) of [15N, 1-13C]Leu. Milk protein yield was increased by IC (+10%) and IC +AA (+21%), whereas AA infusion had no effect. The data were used to construct model equations that describe rates of protein synthesis and degradation, and from these equations, milk and muscle net protein synthesis were described. The model was unable to describe the observed responses in milk protein synthesis. Similar to observations in the literature, net protein gain by the hind leg increased with AA, IC, and IC + AA infusion, primarily through stimulation of protein synthesis by AA. For both tissues, IC depressed Leu oxidation, but only in the absence of AA infusion. Although the IC appears to regulate the ability of the mammary gland to coordinate blood flow and Leu catabolism in support of protein synthesis, our ability to construct a precise model describing mammary protein anabolism is still limited. In contrast, the response in protein anabolism of the hind-leg tissues of these midlactation goats was predicted well by the model, which indicate that the leg tissues were more sensitive to AA supply than the mammary gland.

An arteriovenous kinetics technique was used to monitor mammary gland lysine and protein metabolism in goats (n = 4) at two stages of lactation (80 +/- 17 vs. 233 +/- 14 DIM) in response to an i.v. infusion of lysine (Lys) plus methionine (Met). At each stage of lactation [2-15N] and [1-13C; 6,6-2H2] Lys kinetics were performed on the last day of 5-d i.v. infusion of saline followed by Lys (370 mg/h) plus Met (84 mg/h, LM). Milk and protein yields and dry matter intake were higher in early than in late lactation, but LM infusion did not affect these variables. Regardless of stage of lactation, the absolute and fractional oxidation rates of Lys by the mammary gland increased in response to LM infusion. When corrected for Lys oxidation, net uptake of Lys by the gland was less than milk protein Lys secretion. However, correction for the contribution of peptides (15.8%) to Lys uptake brought net Lys uptake close into balance with milk Lys secretion. The present data suggests that when Lys is in excess of requirements, the mammary gland appears to dispose of the extra supply via the oxidative mechanism.

An arteriovenous technique, combined with a 30-h i.v. infusion of [5-(13)CH3]Met and [5,5,5-(2)H]Leu, was used to monitor mammary uptake of free amino acid (AA) and to estimate the proportion of casein synthesized from circulating peptides in goats in early and late lactation. At both stages, kinetics was performed on the last day of consecutive 5.5-d periods. The first period was an i.v. infusion of saline and the second an i.v. infusion of lysine (8.9 g/h) plus methionine (2 g/h). Net uptake of essential AA and protein yields were higher in early than in late lactation. Uptake of free Met, His, and Pro was less than, uptake of Tyr and Lys was equal to, and uptake of Arg, Leu, Val, and Ile was greater than milk protein synthesis. Peptide uptake, estimated from the difference in casein and plasma free AA enrichment, accounted for a larger fraction of casein-Met (17 vs. 8%) and casein-Leu (27 vs. 12%) in late than in early lactation. Small decreases in mammary blood flow, AA transport activity, and AA concentrations accounted for the lower uptake of AA in late compared with early lactation. Based on our studies of several AA, the utilization of circulating peptides for casein synthesis appears to be a general phenomenon.