Blood flow and nutrient exchange across the liver and gut of the dairy cow. Effects of lactation and fasting

Absorption and delivery of nutrients for milk protein synthesis by portal-drained viscera

The predictability of diet effects on milk composition is limited by the lack of understanding of the metabolic transformations that absorbed nutrients undergo within the portal-drained viscera and liver of high yielding dairy cows. The mass of splanchnic tissues increases dramatically in early lactation, but little is known about the regulation of gut growth and adaptation in early lactation, and further research may provide strategies for optimizing gut adaptation. Glucose is critical for milk synthesis, but portal-drained visceral tissues normally use rather than absorb glucose on a net basis. Dietary starch of low ruminal digestibility increases postruminal starch digestion and decreases net use of glucose by portal-drained viscera slightly, but increases in glucose absorption by portal-drained viscera never account fully for increases in starch disappearance from the small intestine and occur at the expense of VFA absorption. For cows in positive energy balance, greater glucose availability increases tissue energy balance and glucose oxidation, but has little effect on milk or milk protein yield. Similarly, chronic increases in propionate absorption have little effect on milk or milk protein yield. In contrast, casein infusion into the small intestine consistently increases milk and milk protein yield, but the mechanisms responsible remain unclear. There are few data describing the absorption and metabolism of AA by splanchnic tissues of lactating dairy cows, but, as for glucose and VFA, utilization of many AA by portal-drained viscera is substantial. In addition, the contribution of peptides to AA absorption and transport is uncertain and must be clarified. Therefore, measurements of nutrient disappearance from the lumen of the gut cannot be equated with nutrient appearance in the portal vein. Data describing metabolism of nutrients by portal-drained viscera and liver of high yielding dairy cows are needed to improve feeding standards.

Impact of metabolism by extragastrointestinal tissues on secretory rate of milk proteins

This paper reviews the current state of knowledge about the postabsorptive utilization of AA. Data on the duodenal entry of AA and the projected output of these same AA as milk protein demonstrate significant losses. Within the essential AA, these losses range from 70% for Thr to 40% for Met and Lys. For Val, almost half of the loss is due to its use for nonsecretory purposes by the mammary gland; values decrease to 0% for other AA. The liver is the other major organ, apart from the portal-drained viscera, that is responsible for significant AA disposal. Interactions of Met metabolism with other methyl sources and gluconeogenic precursors that can alter the Met requirement also are discussed. Data on the transhepatic and transsplanchnic AA balance in lactating cows, and the coordinated use of these balances with duodenal flow and mammary balance, are needed. Further research also is needed into the functionality of use of AA for purposes other than milk protein synthesis to determine whether excess catabolism of AA occurs; assuming it does, more efforts are needed to identify regulation of AA disposal in the crucial tissues.

Macromineral nutrition by heat stress interactions in dairy cattle: review and original research

This paper examines whether or not responses of dairy cattle to various dietary macromineral elements differed in hot weather compared with thermoneutral conditions. The consequences of heat stress and the interrelationships of macromineral elements on feed intake, digestive function, use of dietary buffers, mineral element uptake from the portal-drained viscera, perturbation of acid-base physiology and related mineral element nutrition, effects of dietary macromineral element concentrations on lactational performance as affected by season, and influence of mineral elements in drinking water are reviewed, and new research results are presented. Dietary mineral buffers aided in alleviation of the decline in DMI and milk yield induced by heat stress. New research results showed that portal plasma flow declined with heat stress or by restriction of DMI in a thermoneutral environment and that uptake of P from the portal-drained viscera by lactating cows was reduced 50% by heat stress compared with that of cows with the same DMI in a thermoneutral environment. Accelerated respiration rate caused respiratory alkalosis and apparently compensated metabolic acidosis, changing the demand for Na and K during heat stress. There is need to characterize more accurately the nyctohemeral pattern of acid-base physiology in the heat-stressed lactating dairy cow and to relate it to macromineral element needs. A large data set (n = 1444 cow period means) was used to compare milk yield and DMI responses to varying dietary concentrations of P, Na, K, Cl, Ca, Mg, and cation-anion difference in summer compared with those in winter. Interactions of dietary concentrations of Cl, Ca, and Mg with season on DMI and K, Ca, and Mg with season on 4% FCM yield were detected. Interactions of Na by Cl, Na by P, and Cl by P with season on DMI and of Cl by P with season on 4% FCM yield were detected. High concentrations of sulfate and chloride in drinking water jeopardized productivity of cows during hot weather.

Glutamine is a potentially limiting amino acid for milk production in dairy cows: a hypothesis

Recently, extensive research has been focused on glutamine because of its key position between energy and protein metabolism. Evidence is growing that glutamine is essential in situations of metabolic stress; practical application of this knowledge can already be found in parenteral nutrition of severely ill patients. Furthermore, glutamine is claimed to increase muscle protein synthesis. Glutamine and its counterpart, glutamic acid, are the most abundant amino acids in milk protein. Nevertheless, the role of nonessential amino acids (NEAA) in milk protein synthesis in high-yielding dairy cows has been practically neglected during the past 20 years. Evaluating current literature on glutamine metabolism in ruminants with emphasis on data related to milk protein production, we conclude the following: (1) Ruminants have a relatively low glutamine synthetase capacity compared with monogastric species, reflected in relatively low plasma glutamine levels; (2) The uptake of glutamine by the mammary gland is effectively 100% of the arterial supply; (3) Milk production in high-yielding dairy cows represents a metabolic stress comparable to fasting or acidosis; and (4) Responses of plasma and tissue glutamine pools in conditions of "metabolic stress," including high milk production, resemble those of most essential amino acids (EAA). Therefore we hypothesize that glutamine, although regarded as a NEAA, limits milk protein synthesis in high-yielding dairy cows.

Effects of various medium formulations and attachment substrata on the performance of cultured ruminant hepatocytes in biotransformation studies

1. A procedure for the isolation and primary culture of hepatocytes from goat and cattle is described. Hepatocyte culture performance was monitored for 51 h by measuring viability, cytochrome P-450 maintenance, dealkylation of scoparone and ethylmorphine, and glucuronidation of phenol red. 2. Culture medium composition is discussed in relation to differences between splanchnic blood composition of ruminant and monogastric animal species. Main differences are in glucose and volatile fatty acid concentrations. Modified Williams' E culture medium did not yield higher culture performance than non-modified Williams' E. 3. Coating of culture dishes with either collagen or fibronectin did not improve culture performance. 4. Williams' E, although developed for rodent cells, proves to be a suitable basal medium for ruminant hepatocytes. In this medium, culture quality is high for at least several days. 5. In cultured goat hepatocytes, biotransformation rate for scoparone amounted to 20 nmol/mg protein per h, for ethylmorphine 96 nmol/mg protein per h and for phenol red 2 nmol/mg protein per h. Biotransformation activity in cow hepatocytes is approximately half that in goat hepatocytes.

Effects of somatotropin and substrates on patterns of liver metabolism in lactating dairy cattle

Objectives of this study were to quantitate metabolite fluxes in ruminant liver and to delineate effects of recombinant bST on patterns of nutrient metabolism by liver. Nineteen multiparous cows ranging in previous lactational performance from 6400 to 13,500 kg per 305-d lactation were treated with either placebo or bST (40 mg/d) from wk 11 to 18 of lactation. Liver tissue was collected at slaughter. Tissue slices were incubated with various 14C-labeled substrates, and rates of conversion of label to CO2 and metabolites were measured. In vivo recombinant bST treatment increased in vitro conversion of [1-14C]propionate and [2-14C]acetate to glucose more than twofold. At 2.5 mM propionate, bST-treated cows converted propionate to glucose at 90% efficiency. Recombinant bST increased [14C]bicarbonate incorporation into glucose five-fold. Overall, bST treatment resulted in greater C flow from propionate and acetate through the TCA cycle. Acetate had only small effects on propionate metabolism and no effects on lactate plus pyruvate metabolism. Unexpectedly, propionate decreased acetate conversion to ketone bodies. Suggested mechanisms for this observation include depletion of coenzyme A and allosteric regulation of carnitine palmitoyltransferase I by methylmalonyl-coenzyme A formed from propionate. In summary, bST treatment resulted in increased rates of gluconeogenesis and oxidation in liver in support of lactation.

Effects of mesenteric vein L-alanine infusion on liver metabolism in beef heifers fed on diets differing in forage:concentrate ratio

Effects of 24 h mesenteric vein infusion of L-alanine (75 mmol/h) and diet forage:concentrate ratio on net nutrient metabolism by portal-drained viscera (PDV) and liver were measured in four Hereford-Angus heifers. Heifers were fed on diets containing approximately 750 g lucerne (Medicago sativa) or 750 g concentrate/kg every 2 h at similar metabolizable energy intakes in a balanced single reversal design with 6 weeks for diet adaptation. Measurements of blood flow and net metabolism of nutrients were obtained at 30 min intervals starting immediately before (four measurements) and after (five measurements) beginning, immediately before ending (four measurements) and 1.5 h after ending (three measurements) alanine infusion. Only values obtained immediately before beginning and ending alanine infusion were compared statistically. Net total splanchnic (PDV plus liver) release of glucose was greater when heifers were fed on the 750 g concentrate/kg v. 750 g lucerne/kg diet due to greater net PDV absorption. Net PDV absorption of ammonia-nitrogen, beta-hydroxybutyrate (BOHB) and lactate, and net liver and total splanchnic release of urea-N (UN) were greater when heifers were fed on the 750 g lucerne/kg v. 750 g concentrate/kg diet. Infusion of alanine increased net liver removal of alanine and alpha-amino-N, net liver release of UN and net total splanchnic release of alanine, UN and lactate. Infusion of alanine decreased net liver release of BOHB and removal of lactate. Mesenteric vein infusion of alanine altered the net availability of precursors for liver glucose synthesis, but did not affect net glucose release by liver in these beef heifers.

Site of digestion of starch in the gastrointestinal tract of dairy cows and its effect on milk yield and composition

Physical and chemical processing of feed ingredients and feeding management strategies are major instruments of manipulating amount and site of starch digestion in the gastrointestinal tract. Generally, as rumen escape of starch increases, postruminal starch digestion increases, and there does not appear to be a limitation to intestinal starch digestion. However, the efficiency with which postruminal starch is digested decreases, which represents a limitation that warrants investigation. Even though digestible dietary starch is presented to the intestine, there is no net glucose absorption at the portal vein, and plasma glucose levels remain relatively unaffected. This result may be associated with the large metabolic requirement for postruminally absorbed glucose, which is preferentially used for oxidative metabolism at the visceral tissue level. In addition, peripheral glucose concentration is highly regulated. A possible implication is that the exogenous glucose supply may spare endogenously synthesized glucose for gut metabolism, allowing more to be directed to the mammary gland. Amino acids also may be spared (less metabolism of dietary and tissue amino acids in the gut). Current production studies yield no clear evidence as to the benefits of postruminal digestion of starch to enhance milk yield or to change its composition. However, studies suggest that starch digested postruminally is used more efficiently for milk synthesis than that digested in the rumen.

Lipid nutrition

Cows in early lactation or producing more than 80 lb of milk per day need supplemental fat and can benefit from it. Fat should be added to the diet over a period of several weeks to allow the cows to become accustomed to it. Feed intake should be monitored because additional fat may decrease feed intake and offset the benefit of the fat. Supplemental fat should not exceed 4 to 5% of the dry matter intake. The first 2% of added fat should be supplied by oilseeds under most circumstances. The next 1 or 2% can come from commodity fat if availability and handling ability permits its use. If the last increment of fat is needed, it should be supplied by specialty fats that have been processed to improve ruminal inertness. Extra calcium, magnesium, and ruminally undegraded protein should be added to the diet when fat is added. Fat is a more expensive source of energy than feed grains in most of the world and should not be used beyond needs.

Regulation of fatty acid metabolism and gluconeogenesis by growth hormone and insulin in sheep hepatocyte cultures. Effects of lactation and pregnancy

Primary monolayer hepatocyte cultures derived from non-mated, pregnant and lactating sheep were used to investigate the interactions between the effects of growth hormone and insulin on (i) the partitioning of fatty acid metabolism between oxidation and esterification, and (ii) the rate of gluconeogenesis. In hepatocytes from lactating sheep the rates of gluconeogenesis, ketogenesis and very-low-density lipoprotein secretion were approx. 2-fold higher than in cells from non-mated or pregnant animals. There was no apparent difference in the rates of fatty acid uptake between the three groups of sheep cells. Growth hormone stimulated gluconeogenesis only in hepatocytes from non-mated sheep. It has no effect on the flux of fatty acid towards ketone body formation. Growth hormone inhibited intracellular accumulation of acylglycerol from exogenous fatty acid. Insulin alone had no such effect, but it blunted the effect of growth hormone when the two hormones were present together. The data suggest that major differences may exist between ruminants and non-ruminants in the response of liver metabolism both to lactation per se and to the effects of growth hormone and insulin.

Effect of lactation on gluconeogenesis and ketogenesis in ovine hepatocytes

1. Rates of glucose synthesis from radioactive precursors and ketogenesis were determined in hepatocytes from control and lactating sheep. 2. Gluconeogenesis from propionate was the same in both groups. Gluconeogenesis from lactate + pyruvate was three-fold higher in hepatocytes from lactating sheep. Palmitate stimulated gluconeogenesis from lactate + pyruvate in both groups. 3. Rates of ketogenesis from palmitate but not butyrate were slightly higher in hepatocytes from lactating sheep. No other differences in the metabolism of palmitate or butyrate were seen in the two groups. Exogenous carnitine stimulated ketogenesis from palmitate. Propionate inhibited ketogenesis from palmitate and butyrate. Lactate + pyruvate also inhibited ketogenesis slightly but stimulated oxidation and esterification. 4. It is concluded that the major changes in glucose and ketone production seen in the lactating ruminant are not the result of long-term changes within the hepatocyte but occur because of the changes in substrate supply to the liver and changes in intracellular concentrations of metabolites.

Tissue and whole-body oxygen uptake in fed and fasted steers

The effect of feeding v. fasting, on tissue blood flow, oxygen uptake and proportional contributions of the portal drained viscera (PDV), liver (Expts 1 and 2) and hindquarters (HQ; Expt 2) to whole-body O2 uptake were studied in beef steers. The combined techniques of indirect calorimetry and net tissue flux, the latter being the product of arterio-venous concentration difference and blood flow, were used in the experiments. In response to fasting, whole-body O2 consumption decreased as did O2 uptake by all measured tissues except the liver (trend only in Expt 1). Blood flow to all measured tissues decreased during fasting and fractional uptake of O2 decreased in PDV and increased in liver and HQ (Expt 2). Proportional contribution of specific tissues to whole-body O2 uptake changed when animals were switched from the fed to the fasted state. The percentage consumed by PDV decreased from 25.4 to 19.9, by liver increased from 20.5 to 26.4 and by HQ was unchanged (9.6 and 10.5) in Expt 2. These significant responses in Expt 2 were observed as trends in Expt 1. The changes in proportional contribution of tissues to whole-animal O2 uptake reflect the changing metabolic role of specific tissues to lack of food supply. These findings emphasize the central role of the liver in metabolism and indicate that fasting (catabolic) measurements may not reflect the previous fed (anabolic) physiological state.

Effect of insulin on the utilization of propionate in gluconeogenesis in sheep

The effects of insulin on the utilization of propionate in glucose synthesis were studied in fed and fasted sheep. Insulin was infused at 0.40 microU/h into the mesenteric vein. Glucose was infused to prevent hypoglycaemia. The rate of incorporation of [2-14C]propionate into glucose was determined before and during insulin infusion. After 150 min of insulin infusion endogenous glucose synthesis was about 70% of control values, whereas the incorporation of [14C]propionate into plasma glucose was 94% of control values. In contrast, the incorporation of other glucose precursors into glucose was decreased 30-50% by insulin. Therefore, insulin does not appear to decrease the utilization of propionate in gluconeogenesis. These results are consistent with the proposition that insulin differentially affects the rate of incorporation of glucose precursors into glucose in ruminant animals.

A comparison of amino acid arteriovenous differences across the liver and placenta of the fetal lamb

Amino acid and ammonia concentrations as well as oxygen content were measured in either the right or left hepatic vein, the umbilical vein, and the umbilical artery in 13 fetal lambs in late gestation. There was an uptake of all of the essential and most of the nonessential amino acids by both lobes of the fetal liver. The umbilical venous-hepatic venous amino acid concentration differences were similar in the two hepatic lobes. While glutamine and glycine were taken up by both hepatic lobes, their metabolically related amino acids, glutamate and serine, were released by the fetal liver into the systemic circulation. There was a reciprocal net placental uptake from the umbilical circulation of glutamate and serine and a net fetal of glutamine and glycine, suggestive of interorgan cycling of these amino acids between the placenta and fetal liver. Total fetal umbilical nitrogen uptake was 0.91 g N.kg-1.day-1. The umbilical venous-hepatic venous differences of ammonia were positive and not significantly different in the two lobes. There was a significant umbilical uptake of ammonia (12.8 +/- 1.8 microM; 0.0078 microM NH3/microM O2). However, 0 the ratios of NH3 to O2 were much higher in each lobe (right, 0.060; left, 0.079; each P less than 0.01) than in the umbilical circulation.

Effect of level of nutrition on splanchnic blood flow and oxygen consumption in sheep

The objective of the present study was to measure changes in splanchnic blood flow and oxygen consumption in sheep fed on a high-concentrate diet ad lib. (ADLIB) or an amount sufficient to maintain body-weight (MAINT) for 21 d. Eleven ram lambs were surgically implanted with chronic indwelling catheters in the portal, hepatic and mesenteric veins and mesenteric artery to measure blood flow and net O2 flux through the liver and portal-drained viscera (PDV). During the 21 d period, PDV (P less than 0.05) and liver (P less than 0.01) blood flow increased in ADLIB and decreased in MAINT lambs (treatment x day, linear). After 21 d, O2 consumptions in PDV and liver of MAINT lambs were 37 and 63% lower than in ADLIB lambs. In the control period, total splanchnic tissues represented an average of 52% of whole body O2 consumption. After 21 d, the relative contributions of PDV and liver to whole-body O2 consumption were 28 and 41% in ADLIB and 19 and 22% in MAINT lambs respectively. Allometric regression variables indicate that liver O2 consumption responds more rapidly to changes in metabolizable energy intake than portal O2 consumption. These results indicate that blood flow and O2 consumption in both PDV and liver are related to level of nutrition. Furthermore, splanchnic tissues represent a significant component of whole-body O2 consumption that is subject to manipulation by level of nutrition.

Net metabolism of volatile fatty acids, D-beta-hydroxybutyrate, nonesterifield fatty acids, and blood gasses by portal-drained viscera and liver of lactating Holstein cows

Net flux of VFA, D-beta-hydroxybutyrate, nonesterified fatty acids, and blood gasses across portal-drained viscera and liver was measured in four lactating Holstein cows fed a 60:40 corn silage: concentrate diet ad libitum and milked at 12-h intervals. Twelve consecutive hourly measurements of net flux (venous-arterial concentration difference times blood flow) were obtained during wk 4 and 8 postpartum for each cow. Milk yield and DM intake averaged 32.2 and 15.6 kg/d. On a net basis, hepatic tissues produced acetate and removed 63 to 101% of other VFA absorbed by portal-drained viscera. Hepatic and portal-drained visceral tissues produced 60 and 40%, respectively, of D-beta-hydroxybutyrate produced by splanchnic tissues. Hepatic tissues removed 9.3% of nonesterified fatty acids in portal vein and hepatic arterial blood. Oxygen use was greater by liver than for portal-drained viscera (3062 vs. 2394 mmol/h). Net portal-drained visceral flux of VFA, D-beta-hydroxybutyrate, alpha-amino nitrogen, L-lactate, and oxygen together accounted for 84.9% of calculated metabolizable energy intake. Net hepatic removal of propionate, L-lactate, and alpha-amino nitrogen maximally accounted for 55.1, 17.4, and 16.5% of carbon in glucose produced by hepatic tissues.

Net portal-drained visceral and hepatic metabolism of glucose, L-lactate, and nitrogenous compounds in lactating holstein cows

Net portal-drained visceral and hepatic flux of glucose, L-lactate, alpha-amino N, NH3N, urea N, glutamate, and glutamine were measured in four Holstein cows. Cows were fed a 60:40 corn silage: concentrate diet ad libitum and milked at 12-h intervals. Six to 16 d postpartum chronic catheters were established in hepatic portal, hepatic, and mesenteric veins and a carotid artery was elevated. Twelve Measurements of net flux, the mathematical product of blood flow (measured by p-aminohippurate dilution) and venous-arterial concentration difference, were obtained for each cow at hourly intervals during 1 d of wk 4 and 8 postpartum. Dry matter, N, and energy digestion trials began 1 to 2 d after blood sampling. Dry matter intake and milk yield averaged 15.6 and 32.2 kg/d. Portal-drained visceral blood flow averaged 80% of hepatic blood flow (2041 L/h). Net flux of NH3N, urea N, and alpha-amino N across portal-drained viscera represented 68, 54, and 51% of N apparently digested. There was net use of glucose by portal-drained viscera. Hepatic glucose production (3.1 kg/d) exceeded calculated mammary glucose requirements. Net hepatic removal of L-lactate, alpha-amino N, and NH3N represented 115, 43, and 101%, respectively, of their net absorption by portal-drained viscera. Net hepatic L-lactate and alpha-amino N removal could account maximally for 17.4 and 16.5% of glucose produced.

Net portal absorption of volatile fatty acids and L(+)-lactate by lactating Holstein cows

Net absorption of L-lactate and VFA from the portal drained viscera of first lactation Holstein cows was measured at 4, 8, (four cows), 12, 16, and 20 wk (two cows) of lactation. Chronic indwelling catheters were installed 7 to 14 d postpartum in appropriate vessels to measure blood flow and net nutrient absorption. Cows were fed a completely mixed, 60:40 (dry basis) corn silage:supplement diet and milked every 12 h. Average metabolizable energy intake was 2.8X maintenance and mean milk production was 24.0 kg. Net absorption of lactate, any of the VFA, or their total was not affected by week postpartum. Net absorption of L-lactate, VFA, and alpha-amino N accounted in sum for 53.6% of metabolizable energy intake; contributions of each component to energy in absorbed nutrients were acetate and propionate, 29.5% each; alpha-amino N, 23.4%; L-lactate, 5.4%; n-butyrate, 5.3%; 2-methylbutyrate, 2.8%, and i-butyrate, i-valerate, and n-valerate, 1.2 to 1.6% each. Comparison of paired samples of blood and plasma showed that blood cells contribute to the transport of acetate, propionate, i-butyrate, and 2-methyl-butyrate but not of n-butyrate, i-valerate, or n-valerate.

Studies of gut and hepatic metabolism in conscious rabbits

The present study was designed to develop the techniques for chronic catheterization of the hepatic and portal venous circulation in conscious rabbits and to apply these techniques to a study of hepatic metabolism in this species. Experiments were made after an 18-h fast and for 4 h after the initial feeding. Measurements of arteriovenous differences of substrates were combined with measurements of hepatic and gastrointestinal blood flow. Hepatic glucose production was suppressed by 60% at 1 h and had returned to control levels by 4 h. The hepatic uptake of lactate declined slightly at 1 h and had returned to control level 2 h after the meal. There was a marked and rapid fall in hepatic ketone body output after refeeding. Although amino acid concentrations displayed a transient increase 1 h after the meal, only the arterial concentration of branched-chain amino acids remained significantly elevated for 4 h. The total hepatic uptake of the gluconeogenic amino acids (alanine, serine, threonine) remained constant. Refeeding resulted in a doubling of arterial insulin concentrations at 1 h followed by a progressive decline over the next 3 h. It is concluded that in rabbits fed a mixed meal partial suppression of hepatic glucose output is mainly due to a decline in glycogenolysis rather than a decrease in gluconeogenesis, shortly after refeeding the liver is able to virtually shut off its ketone body production, the major gluconeogenic precursors (lactate, alanine, glycine, serine, and threonine) may contribute to approximately 40% of the glucose release.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of fatty acids and starvation on the metabolism of gluconeogenic precursors by isolated sheep liver cells

Isolated liver cells prepared from fed sheep synthesize glucose from propionate at twice the rate observed with cells from starved animals. Addition of palmitate or palmitate + carnitine to incubations of liver cells from starved animals inhibited the rate of glucose synthesis with lactate as a precursor, but had little effect when propionate and pyruvate were substrates. Liver cells from fed and starved sheep synthesized lactate and pyruvate when incubated with propionate. Fatty acids inhibited this formation of lactate and pyruvate from propionate. It is proposed that the different responses of gluconeogenic precursors to fatty acids can be explained by the effect of reducing equivalents on the transport of carbon atoms across the mitochondrial membrane.

Effect of previous nutrition on body composition and maintenance energy costs of growing lambs

1. Forty-eight intact male lambs (30 kg) were fed to gain 16 (H), 5 (M) or -6 (L) kg during a 42 d interval (period 1). Lambs from each of the H and M groups were fed to gain either 16 (HH, MH), 5 (HM, MM) or -6 (HL, ML) kg and lambs from the L group were fed to gain 27 (LS), 16 (LH) or 5 (LM) kg during the ensuing 42 d (period 2). 2. Fasting heat production (FHP) of four lambs from each treatment was determined at the end of period 2. 3. Weights and compositions of the carcass, offal and digesta-free body as well as weights of major internal organs were determined for four lambs of each treatment at the end of periods 1 and 2. 4. Within groups of lambs of similar weight at the end of period 2, body composition was, in general, similar, but FHP was greater in lambs that had been on higher planes of nutrition during period 2. 5. Within groups of lambs of similar weight, lambs that were fed at higher planes of nutrition during period 2 had greater weights or proportions of liver, small intestine, large intestine and stomach. 6. Neither weight of the liver, kidney, stomach, small intestine, large intestine nor daily fasting heat production were constant functions of body-weight. Relations of these traits to body-weight changed with rate of gain. 7. Regression analysis indicated that the feeding of lambs at higher planes of nutrition during period 1 resulted in higher maintenance requirements of those lambs during period 2.

Control of bovine hepatic fatty acid oxidation

Fatty acid oxidation by bovine liver slices and mitochondria was examined to determine potential regulatory sites of fatty acid oxidation. Conversion of 1-[14C]palmitate to 14CO2 and total [14C]acid-soluble metabolites was used to measure fatty acid oxidation. Oxidation of palmitate (1 mM) was linear in both liver slice weight and incubation time. Carnitine stimulated palmitate oxidation; 2 mM dl-carnitine produced maximal stimulation of palmitate oxidation to both CO2 and acid-soluble metabolites. Propionate (10 mM) inhibited palmitate oxidation by bovine liver slices. Clofenapate, an inhibitor of fatty acid esterification, alone increased palmitate oxidation and was able to prevent the propionate-induced inhibition of palmitate oxidation by liver slices. Propionate (.5 to 10 mM) had no effect on palmitate oxidation by mitochondria, but malonyl Coenzyme A, the first committed intermediate of fatty acid synthesis, inhibited mitochondrial palmitate oxidation (inhibition constant = .3 microM). Liver mitochondrial carnitine palmitoyltransferase (EC 2.3.1.21) exhibited Michaelis constants for palmitoyl Coenzyme A and l-carnitine of 11.5 microM and .59 mM, respectively. Long-chain fatty acid oxidation in bovine liver is regulated by mechanisms similar to those in rats but adapted to the unique digestive physiology of the bovine.

Net absorption of glucose, L-lactate, volatile fatty acids, and nitrogenous compounds by bovine given abomasal infusions of starch or glucose

Three experiments were conducted to evaluate effects of abomasal infusion of glucose or starch on absorption of metabolites (glucose, L-lactate, volatile fatty acids, and nitrogenous compounds) from portal-drained viscera of a nonlactating Holstein cow and two Hereford X Angus heifers. Portal blood and plasma flow were determined by dilution of para-aminohippuric acid. Net absorption was portal-arterial concentration difference times portal plasma flow. Plasma concentration and portal-arterial difference of glucose increased rapidly and directly in response to increased rates of abomasal glucose infusion in the cow (Experiment 1). In the cow (Experiment 2), net absorption of L-lactate was greater with carbohydrate than with water infusion; net absorption of L-lactate was greater and of n-butyrate was less with starch than with glucose infusion. These responses were not evident in the heifers (Experiment 3). Net absorption of alpha-amino-N in the heifers was greater with starch than with glucose infusion. Increased concentrate intake by the heifers did not interact with responses to abomasal infusions. Recovery of infused glucose as absorbed glucose was similar for the cow and the heifers (65%). However, heifers absorbed (as glucose) 35% of infused starch and the cow only 8%.

Aspects of the regulation of long-chain fatty acid oxidation in bovine liver

Factors involved in regulation of bovine hepatic fatty acid oxidation were examined using liver slices. Fatty acid oxidation was measured as the conversion of 1-[14C] palmitate to 14CO2 and total [14C] acid-soluble metabolites. Extended (5 to 7 d) fasting of Holstein cows had relatively little effect on palmitate oxidation to acid-soluble metabolites by liver slices, although oxidation to CO2 was decreased. Feeding a restricted roughage, high concentrate ration to lactating cows resulted in inhibition of palmitate oxidation. Insulin, glucose, and acetate inhibited palmitate oxidation by bovine liver slices. We suggest the regulation of bovine hepatic fatty acid oxidation may be less dependent on hormonally induced alterations in enzyme activity as observed in rat liver and more dependent upon action of rumen fermentation products or their metabolites on enzyme systems involved in fatty acid oxidation.

Effects of insulin on net hepatic metabolism of acetate and beta-hydroxybutyrate in sheep (Ovis aries)

The net productions of acetate and beta-hydroxybutyrate by the liver and portal-drained viscera were determined before and during insulin plus glucose infusions. Glucose was infused at rates to maintain euglycemia. During insulin infusion the net hepatic productions of beta-hydroxybutyrate and acetate were reduced to a greater extent than could be accounted for by a reduction in provision of free fatty acids to the liver. It is concluded that insulin has a direct anti-ketogenic effect on the ovine liver.

Net absorption of glucose and nitrogenous compounds by lactating Holstein cows

Net absorptions from portal-drained viscera of glucose, ammonia nitrogen, alpha-amino nitrogen, and urea nitrogen were measured in four Holstein cows in first lactations. Cows were fed for ad libitum consumption a completely mixed, 60:40 corn silage:supplement diet (dry basis) in two equal feedings at 12-h intervals daily. Portal blood flow (dye dilution) and net absorption (portal-arterial difference times portal blood flow) were measured at hourly intervals for 12 h for all cows 4 and 8 wk postpartum and for two cows 4, 8, 12, 16, and 20 wk postpartum. Apparent digestibilities of dry matter and nitrogen were measured immediately after net absorption. Milk yield and composition were measured with absorption and digestion. Mean daily dry matter intake and milk yield were 14.1 +/- .6 and 23.8 +/- 1.8 kg. Portal blood flow was not affected by time of day or weeks postpartum; mean was 1371 +/- 123 liters/h. Net absorption of glucose was negative, indicating no uptake of glucose from dietary sources. Means (g/day) for nitrogenous sources were: nitrogen intake 387, fecal nitrogen 121, urinary nitrogen 146, milk nitrogen 116, net ammonia nitrogen absorption 149, net alpha-amino nitrogen absorption 137, and net loss of urea nitrogen from blood to the gut 93. Net absorption of amino nitrogen and alpha-amino nitrogen in blood was greater than in plasma; corresponding losses of urea nitrogen and glucose to the gut were greater from blood than plasma.