Variations in Mammary Protein Metabolism During the Natural Filling of the Udder with Milk over a 12-h Period Between Two Milkings: Leucine Kinetics

Abstract

To define the temporal variations of whole body and mammary leucine kinetics over a 12-h period between two milkings, we used two groups of four Holstein cows, one in their second and the other in their third or fourth lactation. Cows were infused with L-[1-13C]leucine during the 12-h interval between two milkings. Blood was sampled every 30 min during that period from arterial and mammary sources. Timeafter milking did not affect whole body irreversible loss rate of leucine but affected whole body leucine oxidation, which broadly followed variations in arterial plasma leucine concentration. Similarly, mammary leucine irreversible loss rate and leucine used for protein synthesis were not affected by time after milking. Leucine oxidation by the mammary gland was, however, affected by time after milking. It increased by 15% from the first 2-h period to the following 4-h period and then decreased by 13% over the following 2-h period. A 21% increase in leucine oxidation was observed from 8 to 10 h after milking, and then it decreased by 26% over the last 2-h period. Protein degradation expressed as percentage of mammary leucine flux followed a similar temporal pattern. Leucine used for protein synthesis by the mammary gland was unaltered over time after milking, suggesting that the increased availability of leucine resulting from mammary protein breakdown would increase intracellular concentrations of leucine, which would have favored its catabolism. Overall, these results confirm the high metabolic activity of the mammary gland, as protein synthesis by the mammary gland averaged 43% of whole body protein synthesis.

Long‐chain omega‐3 fatty acids regulate bovine whole‐body protein metabolism by promoting muscle insulin signalling to the Akt–mTOR–S6K1 pathway and insulin sensitivity

“…Hyperinsulinaemic–euglycaemic–euaminoacidaemic clamp procedure Clamps were performed during steady feeding conditions achieved through 98% restricted 2 h feeding (Lapierre et al 1999; Thivierge et al 2002 b ). Hyperinsulinaemic – euglycaemic – euaminoacidaemic clamps were conducted according to the procedures described by Wray‐Cahen et al (1997).…”

Invited review: Nitrogen in ruminant nutrition: A review of measurement techniques

“…Methionine (Davis and Bickerstaffe, 1978), Trp (Davis and Collier, 1985), and Gln (Meijer et al, 1993) have been suggested as suitable AA, but are rarely used. However, the sum of Phe+Tyr, as recommended by Spires et al (1975), has been extensively used by many investigators (Davis et al, 1988;Cant et al, 1993;Thivierge et al, 2002;Doepel and Lapierre, 2010). Assumption of the stoichiometric transfer of Phe+Tyr from plasma into milk protein is the basis of the method.…”

Protein turnover—what does it mean for animal production?

“…In practice, rates of protein synthesis for the whole mammary gland exceed protein output by 25-38% (Bequette et al 1996b), but how this additional synthesis is partitioned between turnover of milk and constitutive proteins in the udder is unknown. Furthermore, although the mammary gland may contribute 40-42% of whole body protein flux (Bequette et al 1996b), the gland only consumes 7-14% of whole body oxygen (from Tyrrell et al 1988;Guinard and Rulquin 1995;Thivierge et al 2002) so, again, the link between protein turnover and energy expenditure is not strong at the tissue level. This is not too surprising as it would be "unwise", in evolutionary terms, for homeothermic reliance on an organ the metabolic activity of which varies considerably with both stage of lactation and nutrient supply.…”

“…At first sight it does not appear to be linked to the protein deposition or secretion. Thus, the ratio of net product formation to protein synthesis appears much greater in peripheral tissues [10-50%, muscle (Ponter et al 1994;Lobley et al 2000); 21-28% fiber (Nash et al 1994;Liu et al 1998); 70-75%, mammary gland (Bequette et al 1996b;Thivierge et al 2002)] than for the liver and GIT, which have high rates of turnover and low rates of net gain. For personal use only.…”

“…Two reasons may account for this. First, extensive selection pressure within the dairy industry has tended to produce animals with high genetic potential and these have been used experimentally (Bequette et al 1996b;Thivierge et al 2002). In contrast, studies with ruminant meat animals (both beef and lamb) have either been on moderate quality animals or under limited intakes , where the proportion of protein turnover needed to maintain N equilibrium "dilutes" the apparent efficiency ratio.…”