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).

~ ~~~Whole-body protein synthesis, estimated by the irreversible loss rate procedure, and hind-leg protein metabolism determined by arterio-venous techniques were monitored in response to three nutritional conditions (approximately 0.6, 1 2 and 1.8 x energy maintenance (M)) in ten wether lambs (33 kg average live weight). In all lambs and treatments measurements were based on radiolabelled phenylalanine, but the terminal procedures (five a t 0.6 x M and five at 1.8 x M) also included infusion of[ I-'3Clleucine ; this permitted comparison of amino acids catabolized (leucine) and non-metabolized (phenylalanine) by the hind-limb tissues. Whole-body protein synthesis increased with intake and the relationship with energy expenditure was slightly lower than that reported previously for pigs and cattle. The efficiency of protein retention: protein synthesis did not exceed 0.25 between the two intake extremes. Effects of intake on amino acid oxidation were similar to those observed for cattle. Hind-limb protein synthesis also increased significantly (P < 0001) in response to intake. Estimates of protein gain, from net uptake values, indicated that the tissues made a greater proportional contribution to total protein retention above M and to protein loss below M, emphasizing the role played by muscle tissue in providing mobile protein stores. The rates of protein synthesis calculated depended on the selection of precursor (blood) metabolite, but rates based on leucine always exceeded those based on phenylalanine when precursor from the same pool was selected. The incremental efficiency of protein retained: protein synthesis was apparently unity between 0.6 and 1.2 x M but 0.3 from 1.2 to 1.8 x M. Blood flow through the iliac artery was also proportional to intake. Leucine and 0x0-acid catabolism to carbon dioxide increased with intake such that the metabolic fate of the amino acid was distributed in the proportion 2: I between protein gain and oxidation. The rates of oxidation were only 1-3% the reported capacity of the rate-limiting dehydrogenase enzyme in muscle, but sufficient enzyme activity resides in the hindlimb adipose tissue to account for such catabolism. 1987) and, thus, small changes in the balance of the two processes can produce marked effects on net anabolism and production efficiency. One effective modulator of protein turnover is intake and for a variety of commercial species, including pig (Reeds et ul. 1980),

A new method for measuring albumin synthesis in humans with stable isotopes is presented. This can readily be applied in most clinical conditions, even when albumin losses are occurring or when repeated assessment is required. After rapid intravenous injection of a large dose of [13C]leucine (57 mg/kg body wt, 19.4 atoms%), plasma samples were taken at intervals up to 90 min. The enrichment of free leucine in plasma measured by gas chromatography-mass spectrometry rose to a peak at 10 min and then fell slowly, whereas that in liver biopsies (from surgical patients) ranged from 101.5 to 80.5% of the plasma value between 10 and 90 min after injection. The fractional synthesis rate (FSR) was calculated by dividing the increase in enrichment of leucine in albumin, measured by gas isotope ratio mass spectrometry, by the area under the plasma free leucine enrichment vs. time curve after allowing for the period between synthesis of the protein and its secretion into the plasma. The FSR in healthy postabsorptive males was 7.2 +/- 1.3%/day, and the absolute synthesis rate was 157 +/- 39 mg.kg body wt-1.day-1. These rates are comparable to those obtained by other methods.

1. The effects of progressive reduction in food intake from 1.6 x maintenance (1.6 M) to approximately maintenance (M′) and then to zero (fasting) on energy expenditure and leucine kinetics were examined in Hereford x Friesian finishing beef steers.2. Estimates of whole body protein synthesis and protein oxidation were obtained from the specific radioactivity of free leucine in blood and of exhaled carbon dioxide during continuous infusions of [I-14C]leucine. Protein synthesis contributed a minimum of 0.19 to total heat production across all three intakes.3. The apparent efficiencies with which synthesized protein was retained were 0.28 between 1.6 M and M′ and 1.04 between M′ and fasting. The greater efficiency below M′ reflected probable use of amino acids as energy sources during fasting, which would be spared as soon as feed was available.4. Nitrogen derived from protein oxidation made a minor contribution to urine N at both 1.6 M (0.45) and M′ (0.36) but provided a significant proportion to the increment in urine N between intakes (0.68).5. Amino acid absorption, estimated indirectly as the sum of protein oxidation and protein retention, represented only 0.28 of N intake at M′ and 0.38 at 1.6 M but the contribution to the increment in N intake between these two diet levels was greater (0.56).6. The estimated efficiency of utilization of absorbed amino acids between M′ and 1.6 M was 0.45.

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.