Glucose and amino acid transport and metabolism in flat duodenal sheets of dairy cattle at three stages of lactation

Gastrointestinal transport of calcium and glucose in lactating ewes

During lactation, mineral and nutrient requirements increase dramatically, particularly those for Ca and glucose. In contrast to monogastric species, in ruminants, it is rather unclear to which extend this physiological change due to increased demand for milk production is accompanied by functional adaptations of the gastrointestinal tract (GIT). Therefore, we investigated potential modulations of Ca and glucose transport mechanisms in the GIT of lactating and dried‐off sheep. Ussing‐chamber technique was applied to determine the ruminal and jejunal Ca flux rates. In the jejunum, electrophysiological properties in response to glucose were recorded. Jejunal brush‐border membrane vesicles (BBMV) served to characterize glucose uptake via sodium‐linked glucose transporter 1 (SGLT1), and RNA and protein expression levels of Ca and glucose transporting systems were determined. Ruminal Ca flux rate data showed a trend for higher absorption in lactating sheep. In the jejunum, small Ca absorption could only be observed in lactating ewes. From the results, it may be assumed that lactating ewes compensate for the Ca loss by increasing bone mobilization rather than by increasing supply through absorption from the GIT. Presence of SGLT1 in the jejunum of both groups was shown by RNA and protein identification, but glucose uptake into BBMV could only be detected in lactating sheep. This, however, could not be attributed to electrogenic glucose absorption in lactating sheep under Ussing‐chamber conditions, providing evidence that changes in jejunal glucose uptake may include additional factors, that is, posttranslational modifications such as phosphorylation.

Oxidation of glucose, glutamate, and glutamine by isolated ovine enterocytes in vitro is decreased by the presence of other metabolic fuels

The objective of this study was to evaluate oxidative metabolism of glucose, glutamate, and glutamine by isolated ovine enterocytes in the presence of other metabolic fuels in vitro. A mixed mucosal primary cell culture containing enterocytes was isolated from crossbred wether sheep (n = 6) fed a mixed forage-concentrate diet and incubated for 90 min with 1 mM U-14C-glucose, -glutamate, or -glutamine and additional substrates (water as negative control, acetate, propionate, butyrate, glucose, glutamate, or glutamine) at concentrations of 0.1, 1.0, and 10.0 mM. Oxidation of labeled substrates to CO2 and net production of lactate and pyruvate in incubation media were measured. Oxidation of glucose and glutamine to CO2 was decreased (P < 0.05) by 5 to 40% in the presence of additional substrates except acetate. Our observation that glutamine oxidation can be decreased by the presence of additional substrates is contrary to observations in the literature using enterocytes from nonruminants, indicating that ruminant enterocytes might rely on glutamine to a lesser extent as an energy source. Net glucose utilization was decreased (P < 0.05) 16% by propionate (10 mM) compared with control but was not affected by the other additional substrates. Glutamate oxidation to CO2 was decreased 28% (P < 0.05) in the presence of propionate (10 mM) or by 17 and 33% in the presence of glutamine (1.0 and 10 mM, respectively), but not by that of the other additional substrates. Acetate did not affect the oxidation of glucose, glutamate, and glutamine. Propionate decreased (P < 0.05) the oxidation of glucose and glutamate only at the highest concentration (10 mM), indicating that the sparing effects of propionate on substrate oxidation are affected by its concentration in the incubation media. These observations indicate that ruminant enterocytes possess metabolic flexibility for oxidative metabolism of glucose, glutamine, and glutamate depending on the type and concentration of available additional substrates.

Influence of alpha-linked glucose on jejunal sodium-glucose co-transport activity in ruminants

Eight steers and 12 lambs were used in a completely randomized experimental design to determine the effect of partial alpha-amylase starch hydrolysate (SH) on small intestinal sodium-dependent glucose transport activity. Starch hydrolysate was delivered ruminally or abomasally to steers (960 g/day) and sheep (144 g/day) for 7 days. On day 7, the steers were rendered unconscious, exsanguinated and eviscerated. A 1-m section of jejunum was collected starting at the duodenojejunal flexure. Sheep were anaesthetized with pentobarbital and the second meter of small intestine (jejunum) was collected. Brush-border membrane vesicles were prepared and sodium-dependent glucose uptake activity was measured using the rapid uptake/filtration technique. Alkaline phosphatase and maltase activity was enriched by 8.2+/-0.5- and 8.4+/-1.2-fold in the vesicle preparation, respectively, and was not different between treatments. Abomasal SH increased (P=0.03) the Na/glucose co-transport approximately two-fold in both cattle (47.2-114.0+/-31.5 pmol/mgxsec) and sheep (77.4-152.0+/-25.7 pmol mg(-1) s(-1)). We conclude that Na/glucose co-transport activity by enterocytes responds to luminal alpha-linked glucose (from abomasal infusion) in ruminants, compared with controls. Intestinal maltase-specific activity does not respond to alpha-linked glucose in cattle, and decreases slightly in sheep.

Effect of site and source of energy supplementation on milk yield in dairy cows

The effect of infusing similar energy equivalents of starch into the rumen, or starch or oil into the abomasum was studied in four midlactation cows in a 4 x 4 Latin square design experiment; controls were ruminally infused with water. Cows were fitted with cannulas in the rumen, abomasum, and ileum, and nutrient digestion in the rumen and small intestine was evaluated with Cr as a digesta marker. Ruminal infusions of starch, or abomasal infusions of starch or oil, were associated with a decrease in voluntary feed organic matter intake. Overall energy intake was reduced in oil-infused, but not in starch-infused cows. Nonstructural carbohydrate digestibility in the rumen and in the small intestine was similar among treatments. In abomasally infused cows 3.4 kg/d of nonstructural carbohydrates was apparently digested in the small intestine. Milk production was reduced in oil-infused cows, but the efficiency of milk energy and protein yield was unaffected by treatments. Plasma glucose, insulin, and IGF-1 concentration, mammary glucose extraction rate, rumen ammonia and plasma urea, and arterial and mammary extraction rate of amino acids were all similar among treatments. Large quantities of starch can be digested in the rumen or small intestine of dairy cows. There appear to be no metabolic advantage to increasing the supply of starch to the rumen or the abomasum of mid-lactation dairy cows maintained on highly concentrated diets and exhibiting a positive energy balance.

Invited review: summary of steam-flaking corn or sorghum grain for lactating dairy cows

Nineteen lactation trials (43 grain processing comparisons) are summarized, in addition to digestibility and postabsorptive metabolism studies. The net energy for lactation (NEL) of steam-flaked corn or sorghum grain is about 20% greater than the NEL for dry-rolled corn or sorghum. Based on lactational performance, steam-flaked sorghum grain is of equal value to steam-flaked corn, and steam-flaked corn is superior to steam-rolled corn. Steam-flaking of corn or sorghum compared to steam-rolling of corn or dry-rolling of corn or sorghum consistently improves milk production and milk protein yield. This result is because of a much greater proportion of dietary starch fermented in the rumen, enhanced digestibility of the smaller fraction of dietary starch reaching the small intestine, and increased total starch digestion. Steam-flaking increases cycling of urea to the gut, microbial protein flow to the small intestine, and estimated mammary uptake of amino acids. Steam-rolling compared to dry-rolling of barley or wheat did not alter total starch digestibilities in two trials, one with each grain source. Lactation studies with these processing comparisons have not been reported. Most cited studies have been with total mixed rations (TMR) and alfalfa hay as the principal forage. Additional studies are needed with lactating cows fed steam-flaked corn or sorghum in TMR containing alfalfa or corn silage. Optimal flake density of steam-processed corn or sorghum grain appears to be about 360 g/L (approximately 28 lb/bu).

Regulation of the gene expression of glucose transporter in liver and kidney of lactating cows by bovine growth hormone and bovine growth hormone-releasing factor

Our objective was to study the effects of recombinant bovine growth hormone and bovine growth hormone-releasing factor on gene expression of glucose transporters in the liver and kidney of lactating cows. Primiparous cows received 29 mg/d of bovine growth hormone or 12 mg/d of bovine growth hormone-releasing factor by continuous intravenous infusion or no treatment (control) from 118 to 181 d postpartum. Growth hormone or growth hormone-releasing factor did not influence the mRNA abundance of the liver-type glucose transporter in the liver or kidney. Similarly, neither growth hormone-releasing factor nor growth hormone altered the mRNA abundance of the erythrocyte-type or the intestinal-type glucose transporter in the kidney, but large individual differences were observed in the mRNA abundance of the intestinal-type glucose transporter in liver.

Influence of stage of lactation on glucose and glutamine metabolism in isolated enterocytes from dairy cattle

Pathways of glutamine and glucose metabolism in early-, mid-, and late-lactation dairy cows were evaluated by in vitro incubations of enterocytes for 2 hours with [U-14-C]glutamine and [U-14C]glucose. Enterocytes from early-lactation cows produced greater amounts of CO2 from glutamine in concentrations that ranged from 2 to 8 mmol/L than enterocytes from either mid- or late-lactation cows. Enterocytes from early-lactation cows also produced greater amounts of CO2 from 4 and 6 mmol/L glucose than enterocytes from either mid- or late-lactation cows. Glutamine was metabolized via glutaminolysis mainly to ammonia, alanine, aspartate, glutamate, and CO2, and more of these products were produced in enterocytes from early-lactation cows than from pooled mid- and late-lactation (PML) cows. Glucose was metabolized mainly to lactate, as compared with pyruvate and CO2. Lactate and CO2 production were both greater in enterocytes from early-lactation cows than from PML cows. Glutamine as the sole substrate accounted for all the energy requirements of enterocytes from early-lactation cows but contributed only 31% in the presence of glucose. Similarly, glucose accounted for all the energy requirements of enterocytes from early-lactation cows and contributed 69% in the presence of glutamine. In enterocytes from all cows, the rate of adenosine triphosphate (ATP) production was greater in the presence of both glucose and glutamine compared with that in the presence of either substrate alone.(ABSTRACT TRUNCATED AT 250 WORDS)