Influence of rumen volatile fatty acids and insulin, on the metabolism of glucose and protein in lambs

Abstract

The effects of manipulating the rumen fermentation pattern on the blood metabolites and insulin levels, glucose metabolism, protein turnover, body composition, and the energetic efficiencies of growing lambs were investigated in this work.

Two groups of lambs, with an average LBW of 20 kg, were fed on a barley based, all-concentrate diet consisting of 85% barley, 7% lucerne meal, 6% fish meal, 1% molasses and 1% minerals and vitamins. One group was fed on whole loose barley with the rest of the ingredients being pelleted. The diet of the other group was ground and pelleted with 2% of a mixture of buffering minerals consisting of 35% Na₂HP0₄, 45% NaHCO₃, 15% CaCO₃ and 5% KCl. The food consumption and body weight changes were measured weekly. Rumen VFA concentration and the levels of plasma glucose, urea, amino acids and insulin were also determined weekly. Food digestibility was determined during the experiment, and the parameters of glucose kinetics were studied by, firstly, the glucose tolerance test, and secondly the continuous intravenous infusion of D-[U-¹⁴ C] glucose. With the latter method, the measurements of glucose irreversible loss (IL), blood CO₂ derived from glucose and the incorporation of glucose carbon into fatty tissues, were possible.

Protein synthesis in the whole body and in different tissues were also determined, simultaneously, by the continuous infusion of L-[side chain, 2, 3,-³H] tyrosine. The fractional synthetic rates of protein in the skin, skeletal muscle, brain, heart, rumen, abomasum, small intestines, liver and kidney were measured, and the whole body protein synthesis and turnover were estimated from the flux of plasma tyrosine, the fractional synthetic rates of different tissues and the body protein gain rates.

The animals were killed at a predesigned LBW; i.e. after gaining 10 - 15 kg LBW, and the chemical composition of the whole body was determined.

The dietary treatments resulted in a marked difference in the patterns of rumen VEA between the two groups. The whole loose barley diet produced a pattern dominated by propionic acid; i.e. (P) group, whilst the ground pelleted with minerals diet produced a pattern of VFA dominated by acetic acid ((A) group). The alterations in the rumen VFA pattern were attributed to a difference in the rumen dilution rate induced by the addition of the minerals. The difference in the VFA patterns was reflected in the glucogenic to nonglucogenic VFA ratio, being more than 2-fold higher in the (P) group. The apparent digestibility of the diet and the AA flow from the rumen to the small intestines were higher in the (P) group. This was attributed to the dilution rate and the mean retention time of the rumen.

The changes of the VFA patterns were also associated with changes at the blood and the tissue levels. Plasma glucose and insulin levels were significantly higher in the (P) group (82 mg% vs. 75 mg% and 14 µU/ml vs. 5 µU/ml respectively). By contrast the (A) group showed higher levels of plasma urea ,(58 mg% vs. 32 mg%), alanine (2.3 mg% vs. 1.7 mg%), methionine (0.31 mg% vs. 0.18 mg%), the branched chain AA; i.e. valine (3.0 mg% vs. 1.7 mg%), isoleucine (0.9 mg% vs. 0.5 mg%) and leucine (1.9 mg% vs. 1.2 mg%). All the differences were statistically significant.

Exogenous glucose loads were cleared significantly faster by the (P) group (19 min. vs. 38 min) and that was associated with higher insulin release (29 µU/ml/min. vs. 15 µU/ml/min.).

Glucose IL tended to be higher in the (P) group (20 mg/kg W ⁰. ⁷⁵ / min. vs. 17 mg/kg W ⁰.⁷⁵/min.), and so were the proportion of CO₂ derived from glucose (31% vs. 25%), and the radioactive glucose incorporated into adipose tissue.

The analysis of the body composition showed that per kilogram EBW there was significantly more dry matter in the (P) group. This was mainly due to significantly more fat and, to a lesser extent, protein. Consequently the total amounts of energy stored in the body were greater and accordingly the efficiency of the dietary energy utilization, since the energy consumptions were similar in both groups.

In most of the examined tissues, protein fractional synthetic rates were slightly, but not significantly, higher in the (P) group, with the exception of skeletal muscle where protein synthesis rate was significantly higher in the (P) group.

Whole body protein synthesis tended to be slightly higher in the (A) group, but on the other hand total gain (kg) and gain rate (g/d) of body protein tended to be higher in the (P) group, indicating that the rate of protein breakdown may be lowered in the (P) group.

It was concluded that the (P) group was supplied with more propionate and hence glucose and that this was associated with an increase in the secretion of insulin. Both glucose and insulin stimulated the uptake and utilization of nutrients for fat and protein synthesis and hence the efficiency of utilization of energy.

It was also concluded that the animals in the (A) group utilized alternative precursors to propionate for glucose synthesis and that this influenced the utilization of those precursors for anabolic processes in the tissues. Finally, insulin appears to play important roles in regulating the metabolism of nutrients in growing ruminant lambs.