Protein Metabolism in Lactating Goats Subjected to the Insulin Clamp

Abstract

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.

Responses in mammary and splanchnic metabolism to altered lysine supply in dairy cows

“…As a primary outcome of Lys degradation, Glu then acts as a transamination precursor for Asp, Ala and Ser in particular for the NEAA and for the BCAA from the EAA. Indeed, in ruminants, Leu has been shown to be a major intermediate in N-transfers linked to the high transamination rate in many tissues including muscle (Holtrop et al, 2004) and the MG (Bequette et al, 2002). Indeed, in goats injected with a single dose of […”

Insulin regulation of amino-acid metabolism in the mammary gland of sheep in early lactation and fed fresh forage

“…Insulin plays an important role in regulating the partitioning of nutrients to the mammary gland (McGuire et al, 1995;Mackle et al, 2000;Bequette et al, 2002). These studies showed that a hyperinsulinaemic euglycaemic clamp (HEC) increased milk protein output in concentrate-fed ruminants, suggesting that amino acid (AA) partitioning to and/or AA utilisation within the mammary gland increased under HEC conditions.…”

“…There is evidence that the nutritional status of the animals might also affect their response to insulin (Bequette et al, 2002). This is largely untested in early lactating ruminants fed fresh forage: the few existing studies reported no increase in milk protein concentration or yield in lactating dairy cows (Back et al, 2003) and sheep with the HEC alone, or with supplemental protein (Back et al, 1998).…”

“…A common observation in both concentrate-and grass-fed lactating ruminants is that the HEC decreased circulating AA concentrations (Back, 2002;Bequette et al, 2002). Despite this, Bequette et al (2002) showed, in well fed goats, an increase in mammary blood flow (MBF) and transport activity of essential AA (EAA) with the branched chain AA (BCAA) the most affected, as well as a 38% reduction in mammary leucine oxidation (LO).…”

“…Despite this, Bequette et al (2002) showed, in well fed goats, an increase in mammary blood flow (MBF) and transport activity of essential AA (EAA) with the branched chain AA (BCAA) the most affected, as well as a 38% reduction in mammary leucine oxidation (LO). These data suggest that insulin might exert a differential effect on AA metabolism in the mammary gland, at least in concentrate-fed goats.…”