Interconversions and production of volatile fatty acids in the sheep rumen

1. Sheep fed at a constant rate were infused intraruminally with [1-(14)C]-acetate, -propionate or -butyrate during 5hr. periods. 2. Volatile fatty acids were estimated in the rumen contents and steady-state conditions were obtained. 3. Of the butyric acid carbon 60% was in equilibrium with 20% of the acetic acid carbon, and 2-3g.atoms of carbon were interconverted/day. 4. Little interconversion took place between propionic acid, acetic acid or butyric acid. 5. The net production rates for acetic acid, propionic acid and butyric acid were 3.7, 1.0 and 0.7moles/day respectively. 6. The production of volatile fatty acids accounted for 80% of the animal's energy expenditure.

Dynamic and static phases of severe dietary obesity in sheep: food intakes, endocrinology and carcass and organ chemical composition

The chronology of changes in body weights, food intakes and plasma concentrations of selected metabolic hormones and metabolites were determined in sheep during the induction (dynamic) and static phases of diet-induced obesity. Lean adult Dorset ewes weighing 47 kg were fed a pelleted hay-grain diet at maintenance (lean; n = 7) or were fed the same diet ad libitum to a maximum intake of 3 kg.sheep-1.d-1 (obese; n = 8) for 78 wk. Body weight of obese sheep doubled (97 vs. 47 kg) by wk 42 of ad libitum intake. Average daily intakes of dry matter (12.8 g/kg) and digestible energy (165 kJ/kg) were comparable in maintenance-fed lean sheep and ad libitum-fed obese sheep consuming maintenance after wk 50, which began the static phase of obesity. Fasting plasma concentrations of insulin in the obese sheep increased steadily from 50 +/- 6 pmol/L at wk 0 to a sustained plateau of 249 +/- 21 pmol/L after wk 30. Plasma levels of glucose, immunoreactive glucagon and thyroid hormones were consistently greater (P less than 0.05) in obese sheep than in lean sheep after wk 2, 3 and 25, respectively, of the experiment. Concentration of lipid (49 vs. 25%) in the carcass stripped of internal fat was greater (P less than 0.01) in obese sheep than in lean sheep, but concentration of protein (10.4 vs. 15.3%) was less in the heavier carcass (58 vs. 24 kg) of the obese sheep. We conclude that hyperinsulinemia and abnormal fuel metabolism are early events during dynamic obesity and these defects persist throughout the static phase of obesity. Maintenance energy requirements relative to unit body weight (W1.0) seem similar in lean and dietary obese sheep.

Energy contributions of volatile fatty acids from the gastrointestinal tract in various species

The VFA, also known as short-chain fatty acids, are produced in the gastrointestinal tract by microbial fermentation of carbohydrates and endogenous substrates, such as mucus. This can be of great advantage to the animal, since no digestive enzymes exist for breaking down cellulose or other complex carbohydrates. The VFA are produced in the largest amounts in herbivorous animal species and especially in the forestomach of ruminants. The VFA, however, also are produced in the lower digestive tract of humans and all animal species, and intestinal fermentation resembles that occurring in the rumen. The principal VFA in either the rumen or large intestine are acetate, propionate, and butyrate and are produced in a ratio varying from approximately 75:15:10 to 40:40:20. Absorption of VFA at their site of production is rapid, and large quantities are metabolized by the ruminal or large intestinal epithelium before reaching the portal blood. Most of the butyrate is converted to ketone bodies or CO2 by the epithelial cells, and nearly all of the remainder is removed by the liver. Propionate is similarly removed by the liver but is largely converted to glucose. Although species differences exist, acetate is used principally by peripheral tissues, especially fat and muscle. Considerable energy is obtained from VFA in herbivorous species, and far more research has been conducted on ruminants than on other species. Significant VFA, however, are now known to be produced in omnivorous species, such as pigs and humans. Current estimates are that VFA contribute approximately 70% to the caloric requirements of ruminants, such as sheep and cattle, approximately 10% for humans, and approximately 20-30% for several other omnivorous or herbivorous animals. The amount of fiber in the diet undoubtedly affects the amount of VFA produced, and thus the contribution of VFA to the energy needs of the body could become considerably greater as the dietary fiber increases. Pigs and some species of monkey most closely resemble humans, and current research should be directed toward examining the fermentation processes and VFA metabolism in those species. In addition to the energetic or nutritional contributions of VFA to the body, the VFA may indirectly influence cholesterol synthesis and even help regulate insulin or glucagon secretion. In addition, VFA production and absorption have a very significant effect on epithelial cell growth, blood flow, and the normal secretory and absorptive functions of the large intestine, cecum, and rumen. The absorption of VFA and sodium, for example, seem to be interdependent, and release of bicarbonate usually occurs during VFA absorption.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of obesity on insulin sensitivity and responsiveness in sheep

To assess the mechanisms of insulin resistance in the ruminant, severe and adult-onset obesity was produced in Dorset ewes by overfeeding a high-energy ration over a 1- to 2-yr period. Body weights increased to 100 kg compared with 50 kg in lean control sheep; significant hyperinsulinemia (40 +/- 4 vs 10 +/- 1 microU/ml) also developed as did a moderate hyperglycemia (62 +/- 2 vs. 52 +/- 1 mg/100 ml). Tissue sensitivity and responsiveness to insulin were then determined in both obese and lean sheep by the euglycemic glucose-clamp technique. Insulin was infused at eight different rates from 0.2 to 50 mU.kg-1.min-1 and [6-3H]-glucose was infused for measurement of glucose kinetics. The mean dose-response curves for glucose utilization and clearance rates were displaced to the right in obese compared with lean sheep. As a result, the half-maximally effective insulin concentrations usually were elevated two- to fourfold, indicating decreased insulin sensitivity in obese sheep, and this is consistent with decreased insulin receptors in peripheral tissues. On the basis of fat-free body weight, the maximal glucose responses, however, were not significantly different between obese and lean sheep, indicating that postreceptor defects do not exist in muscle tissue. Furthermore, lean ruminants are more resistant to insulin than are humans, but this resistance seems only because of the sheep's decreased responsiveness to insulin and thus only because of postreceptor insulin effects in peripheral tissues.

Effects of obesity and ovarian steroids on insulin secretion and removal in sheep

The interactive effects of sex steroids and obesity on glucose metabolism and pancreatic secretion and organ removal of insulin were determined in multicatheterized lean and obese sheep by multiplying venoarterial concentration differences by plasma flows. Ovariectomized lean and dietary obese ewes received implants of progesterone and estradiol-17 beta that produced plasma concentrations of each equivalent to those during either anestrus (low progesterone), diestrus or pregnancy (high progesterone), or estrus (high estradiol). Sheep were exposed to each of the three steroid treatments for 2 days and fasted overnight before blood samples were collected for 5 h before (basal) and 90 min after injecting glucose (200 mg/kg) to simulate an intravenous glucose tolerance test (IVGTT). Regardless of steroid treatment, pancreatic secretory (18 vs. 5 mU/min) and hepatic (10 vs. 2 mU/min) and hindquarters (1.8 vs. 0.5 mU/min) removal rates of insulin in the basal state were greater (P less than 0.005) in obese than lean sheep. Obese sheep had greater (P less than 0.025) basal hepatic glucose output (66 vs. 47 mg/min) and similar hindquarters glucose removal (37 vs. 32 mg/min) as lean sheep even though arterial concentrations of insulin were fourfold higher (25 vs. 6 microU/ml; P less than 0.01) in the obese sheep. High progesterone increased (P less than 0.05) basal hepatic insulin removal in obese sheep. High progesterone and high estradiol increased insulin but decreased (P less than 0.05) glucose removal in hindquarters of obese sheep in the basal state. High progesterone potentiated significantly glucose-induced hyperinsulinemia in obese sheep, whereas high estradiol suppressed hepatic insulin removal but increased the removal of insulin by hindquarters during glucose stimulation in the obese sheep. We concluded that excessive insulin secretion, not decreased insulin removal, maintains the basal hyperinsulinemia in moderately obese sheep and that the progesterone-to-estradiol ratio has marked and divergent effects on insulin and glucose metabolism in individual tissues of sheep both in the basal state and during an IVGTT.

Glucose-dose dependent characteristics of insulin secretion in obese and lean sheep

We previously reported that obesity in sheep and cattle was associated with basal hyperinsulinemia, insulin resistance, and an exaggerated insulin response to a single dose (350 mg/kg) of glucose. In this study, the glucose-dose dependency of insulin secretion in obese and lean sheep was determined by 1) using jugular venous concentrations of insulin (Exp 1) and 2) arteriovenous differences in insulin concentrations across the pancreas together with plasma flow rates in the portal vein (Exp 2). Sheep were injected with glucose doses of 0 (water), 10, 30, 100, and 350 mg glucose/kg body weight in Exp 1 (six sheep per group) and with a low (20 mg/kg) and high (200 mg/kg) dose of glucose in exp 2 (four sheep per group). In Exp 1, mean (+/- SE) pretreatment plasma concentrations of insulin (22.0 +/- 1.7 vs. 9.4 +/- 0.4 microU/ml) and glucose (56.1 +/- 0.5 vs. 52.4 +/- 0.8 mg/dl) were greater (P less than 0.01) in obese than lean sheep fasted for 12 h. The glucose-induced rises in insulin concentrations above pretreatment levels were always greater (P less than 0.05) in obese than lean sheep regardless of glucose dose. Eadie-Scatchard plot analysis of the hyperbolic relationship between the acute insulin and acute glucose response areas (0 to +10 min) indicated that the maximum (Vmax) early phase insulin response was greater (P less than 0.025) in obese than lean sheep (568 +/- 148 vs. 156 +/- 33 microU ml-1 X min). In Exp 2, pretreatment concentrations of insulin (25.1 +/- 3.4 vs. 5.6 +/- 1.2 microU/ml) and glucose (58.3 +/- 1.8 vs. 45.5 +/- 1.1 mg/dl) in arterial plasma were greater (P less than 0.01) in obese than in lean sheep fasted 18 to 22 h. Similarly, pretreatment pancreatic secretion rates of insulin were greater (P less than 0.01) in obese (17.8 +/- 5.8 mU/min) than in lean (4.9 +/- 1.3 mU/min) sheep. Glucose-induced acute (0 to +10 min) increments in pancreatic secretory rates of insulin also were greater (P less than 0.05) in obese than in lean sheep after the low (215 +/- 73 vs. 11 +/- 15 mU) and high (881 +/- 281 vs. 232 +/- 66 mU) doses of glucose. It was concluded that insulin secretion in response to a range of stimulatory concentrations of glucose was greater in obese than in lean sheep because the obese sheep had greater maximum (i.e. Vmax) acute phases of glucose-induced insulin secretion.

Leucine and alpha-ketoisocaproate metabolism and interconversions in fed and fasted sheep

Fed and three-day-fasted sheep were infused with [1-14C] alpha-ketoisocaproate (KIC), L-[1-14C] leucine, and [14C] bicarbonate for determination of their whole-body turnovers, interconversions, and oxidation. Protein synthesis (PS), protein degradation (PD), net tissue metabolism, unidirectional utilization, and production rates also were estimated for the portal-drained viscera, liver, and hindquarters. KIC and leucine arterial concentrations (6.5 and 95 mumol X L-1) both increased with fasting. KIC turnover (9 mumol X min-1) also increased but leucine turnover (108 mumol X min-1) decreased. About 40% of KIC and 15% of leucine were oxidized, but they contributed less than 1% of whole-body CO2 production. The portal-drained viscera released KIC and leucine into the blood only in fed sheep. Hepatic net utilization of KIC and leucine (approximately 2 and 12 mumol X min-1) changed only little with fasting; thus, total splanchnic tissues utilized both in fasted sheep. Net metabolism by the hindquarters (representative of skeletal muscle) was always opposite to splanchnic metabolism. Thus, muscle must produce both KIC and leucine during fasting. In fed sheep whole-body PS, expressed as mumol X min-1 of leucine, was 92 +/- 6 and PD was 71 +/- 5. After fasting, PS decreased by 27%. Calculated liver protein metabolism was unaffected by the fast; PS (fixed and plasma) remained at about 25 and PD at about 15 mumol X min-1. However, protein metabolism by the hindquarters was sensitive to fasting; PS decreased from 30 +/- 4 in fed sheep to 20 +/- 3 mumol X min-1 after fasting and PD increased from 27 +/- 2 to 35 +/- 6 mumol X min-1. Thus, hepatic PS was maintained at the expense of muscle. If the total muscle mass of the body is considered, muscle PS contributed more than one half of whole-body PS.

Insulin and glucose responses to glucose injection in fed and fasted obese and lean sheep

Effects of short-term fasting on the insulin and glucose responses to injected glucose were determined in obese (n = 6) and lean (n = 6) Dorset ewes that were fed a maintenance level of energy intake. Sheep were assigned by Latin-square design to be fasted for 0 (fed), 12 or 24 h before glucose (350 mg/kg) was injected via jugular cannula at 2000 h with at least 7 d between successive tests. Insulin and glucose were quantified in jugular plasma samples. Pretreatment concentrations of insulin were affected (P less than 0.005) only by body condition with higher mean values in obese (23.5 +/- 3.3 microU/ml) than in lean (9.4 +/- 1.0 microU/ml) sheep. Pretreatment concentrations of glucose (53.6 +/- 1.8 mg/dl) were unaffected by body condition and fasting. The insulin responses to glucose, whether determined as absolute levels or response areas above base-line levels, were greater (P less than 0.005) in obese than in lean sheep regardless of fasting period. Insulin and glucose concentrations after glucose injection in lean sheep were unaffected by fasting. In contrast, the insulin response to glucose was greater (P less than 0.005) in fed obese than 12- or 24-h fasted obese sheep while glucose levels in the fed sheep were similar to those in the fasted obese sheep. Thus, factors associated with feeding enhanced the insulin response to glucose in obese sheep. In addition, obesity in sheep was associated with insulin resistance because basal hyperinsulinemia coexisted with euglycemia and because fractional removal rates of injected glucose were similar in obese and lean sheep despite much greater concentrations of insulin in obese sheep.

Splanchnic and peripheral uptake of amino acids in relation to the gut

There are many complexities and theoretical aspects to consider for studies of amino acid absorption and metabolism by the gut, liver, and peripheral tissues. The experimental approach must vary depending on the amino acid. Whether to sample whole blood or plasma has to be considered carefully. Also, a specific blood vessel has to be chosen for taking samples. A jugular vein can be the poorest sampling site for many studies. The amounts of individual amino acids appearing in portal blood are different from amounts disappearing from the gut lumen. Some are absorbed in amounts equal to that disappearing but most are absorbed in lesser quantities because of intestinal metabolism. Further, the liver removes absorbed amino acids and synthesizes plasma proteins, urea, and glucose. Peripheral tissues, of course, exchange amino acids with protein for normal turnover but also use amino acids for oxidation and transamination. Alanine, glutamine, glycine, and arginine are important in transporting nitrogen out of peripheral tissues in a nontoxic form. Branched-chain amino acids are removed by both liver and peripheral tissues mainly for plasma protein and ketoacid formation, respectively. During fasting, however, muscle releases branched-chain amino acids while removal by liver is maintained.

Importance of sites of tracer administration and blood sampling in relation to leucine metabolism. Practical considerations

For the same infusion site of L-[1-14C]leucine, sampling downstream of arterial blood underestimates leucine turnover, whereas sampling of venous blood overestimates turnover. Further, the lungs release a small but consistent amount of leucine into the blood. Unlabelled leucine also is produced by the portal-drained viscera, and some is removed immediately by the liver. These sources of leucine should thus be considered in turnover calculations.

Tissue glucose and lactate metabolism and interconversions in pregnant and lactating sheep

Continuous infusions of [14C]glucose and [14C]lactate on separate days, and measurements of blood flow-rate, were used to obtain values for rates of unidirectional metabolism and of interconversion of glucose and lactate in the portal-drained viscera, liver and hind-quarters of ewes during late pregnancy and early lactation. All infusions were made within 5 h after the morning meal, when steady-state conditions appeared to exist. Use was made of ewes that had been appropriately catheterized during pregnancy, and whose catheters remained patent through into lactation. The liver was the main source of glucose production (67-70%) during both pregnancy and lactation. Other sources were the portal-drained viscera (absorbed glucose) and, presumably, the kidneys. Over 80% of the glucose was utilized by the peripheral tissues with approximately 35-40% of utilization being attributable to the hind-quarters. Of the total lactate production, 76% occurred in the peripheral tissues during pregnancy but only 36% during lactation. While the liver utilized 73% of lactate during pregnancy, this value fell to only 42% during lactation, at which time the portal-drained viscera utilized 26% of the lactate. During pregnancy, approximately 80% of the lactate arose from glucose, chiefly in peripheral tissues, while at least 12% of the glucose arose from lactate, chiefly in the liver. During lactation the extent of these interconversions was decreased. Despite the interconversions, whole-body turnover rates for glucose and lactate were under- or overestimated by only 4-10% and 2-5% respectively. Furthermore, a comparison of turnover rates obtained with [U-14C]- and [6-3H]glucose indicated that there was only 6 and 2% recycling of glucose-C during pregnancy and lactation respectively. Under the conditions employed in this study, lactate does not appear to be a major precursor of glucose in the ruminant, and most of the lactate taken up by the liver must be used for purposes other than gluconeogenesis, such as oxidation or alternative anabolic pathways.

Whole-body metabolism of glucose and lactate in productive sheep and cows

Constant infusions of D-[U-14C]glucose, D-[6-3H]glucose and L-[U-14C]lactate were used to determine rates of apparent turnover, de novo production, disposal and interconversions of glucose and lactate, together with total recycling of glucose-C, in ewes and dairy cows during late pregnancy and early lactation. The cows were also examined while being fasted. In the fed animals, infusions were made within 5 h after the morning meal when steady-state conditions appeared to exist. In the ewes, circulating concentrations of glucose and lactate, and magnitudes of apparent turnovers of glucose and lactate, tended to be higher during lactation than during pregnancy, while the extent of interconversions of glucose and lactate tended to be lower. Although the metabolic pattern seen in the cows appeared to be similar to that of the ewes during pregnancy, there were clear differences during lactation. Thus, in the lactating cows, as compared with the lactating ewes, circulating concentrations of glucose and lactate were lower, as was apparent turnover related to metabolic body-weight. Furthermore, the percentage of lactate turnover converted to glucose was higher. In the cows, fasting was characterized by low rates of apparent turnover of glucose and lactate and relatively high rates of interconversion of the two compounds. The results indicated that, under the conditions used in this study and when feeding is to recommended levels, carbohydrate metabolism in ewes is more precarious during late pregnancy than during early lactation, while in dairy cows it is more or less equally precarious in both physiological states. A further conclusion is that the extent of glucose-lactate interconversions, and thus Cori cycle activity, seems to be lower in ruminants than in other species.

Changes of glucose, insulin and glucagon associated with propionate infusion and vitamin B-12 status in sheep

The effect of propionate on hormonal and metabolic events was studied in ewes that were vitamin B-12 depleted (de-B12) and repleted (re-B12). Experiments were conducted before and after hydroxocobalamin resupplementation. De-B12 sheep had greater blood concentrations and total hepatic influx and efflux of glucose. However, rates of net hepatic release of glucose were similar. Comparable glucagon concentrations and fluxes were reduced in de-B12, but insulin values were unaffected by vitamin B-12 status. Intramesenteric infusion of propionate elevated concentrations of glucose, insulin and glucagon at nearly all samplings. Secretion of insulin was elevated at the first sampling only (15 minutes), while glucagon appeared elevated until 30 minutes. Rates of hepatic removal of hormones were not altered during infusion. Net hepatic release of glucose was increased at nearly all samplings, but de-B12 ewes had a greater increment of total hepatic influx and efflux. De-B12 ewes exhibited a diminished glucagon response to propionate infusion, whereas insulin concentrations and hepatic uptakes tended to be greater. Vitamin B-12 status, within the range usually considered normal, thus influences metabolic and hormonal responses to increased rates of propionate entry in the sheep, independent of feed intake.

Influence of vitamin B-12 status on hepatic propionic acid uptake in sheep

Pregnant ewes were fed a depletion diet low in cobalt (0.06 ppm) for 3 1/2 months. Chronic catheters were implanted 8 weeks postpartum and 7 experiments were performed on these nonlactating vitamin B-12-depleted sheep (de-B12: 340 +/- 30 ng vitamin B-12 per gram wet liver) prior to repletion by intramuscular injection of hydroxocobalamin. Six experiments were then repeated after vitamin B-12 repletion (re-B12: 2220 +/- 50 ng vitamin B-12 per gram wet liver). The hepatic extraction ratios (HER) in continuously fed sheep were 0.81 and 0.77 for de-B12 and re-B12 corresponding to net hepatic uptakes of 460 +/- 50 and 440 +/- 40 mumol propionate per minute, respectively. Continuous infusion of unlabeled propionate into a mesenteric vein at 1 mmol/minute reduced the HER, yet this depression was greatest for re-B12 (0.74 vs. 0.63 for de-B12 and re-B12, respectively). Net hepatic uptake of propionate was increased (1145 +/- 100 vs. 985 +/- 95 mumol/minute, respectively), although vitamin B-12 status was without effect. It is concluded that the ability of liver to extract propionate is not affected at vitamin B-12 concentrations greater than 250 ng/g wet liver. However, when propionate entry rate was enhanced by intramesenteric infusion, the livers of de-B12 sheep had a greater capacity to remove propionate suggesting that alternate routes of metabolism may occur.

Cerebral metabolism of amino acids and glucose in fed and fasted sheep

Net cerebral uptake from or release into whole blood of oxygen, carbon dioxide, glucose, amino acids, lactate, pyruvate, ketone bodies, and acetate was estimated in fed, 3-day-fasted, and 6-day-fasted sheep. The respiratory quotient was similar in all three groups of sheep (approximately 0.95). Glucose uptake (35 mumol X min-1 X 100 g-1) was maintained during fasting, and about 94% of the cerebral oxygen consumption could have been accounted for by glucose oxidation in all sheep. A cerebral uptake of the branched-chain amino acids (leucine, isoleucine, and valine) and proline also was observed with a concomitant production of glutamine and asparagine. The brains of fed and 3-day-fasted sheep were in nitrogen balance, but a small net release of nitrogen occurred in 6-day-fasted sheep (2 mumol N. min-1 X 100 g-1). A small amount of pyruvate was always released (1.4 mumol X min-1 X 100 g-1) into the blood, whereas lactate was released (6 mumol X min-1 X 100 g-1) only in 6-day-fasted sheep. Ketone body and acetate utilization always was negligible when compared with that for glucose. The total cerebral nonglucose carbon release found for 6-day-fasted sheep was equivalent to 23% of the glucose carbon taken up, although only 8% could have been derived directly from glucose. Thus, metabolism by the ovine brain seems resistant to prolonged periods of hypoglycemia with only small adaptations occurring after a 6-day fast.

Glutamate interconversions and glucogenicity in the sheep

Simultaneous measurements were made of net and unidirectional glutamate metabolism by the portal-drained viscera, liver, kidneys, and hindquarters of fed, acidotic, and fasted sheep. The contribution of glutamate to glutamine and glucose production also was estimated. Of the glutamate present in whole blood, 45% was in plasma and 55% in the cellular fraction. Acidosis and fasting reduced blood glutamate concentrations, but did not change the plasma:cellular ratio. [14C]glutamate exchanged only little between plasma and blood cells. Clearly, this demonstrates a lack of the alpha-amino acid transport system in blood cells. Net rates of plasma glutamate flux by the portal-drained viscera and kidneys were less than 0.5 mmol/h, but, in fed sheep, the liver released 2--3 mmol/h into the plasma and the hindquarters removed an average of 0.9 mmol/h. Both were reduced by acidosis and fasting. Unidirectional rates were highly significant and greater than net rates. Acidosis and fasting primarily seemed to affect production by the liver, but only utilization by the hindquarters. Plasma glutamate turnover averaged 6--9 mmol/h, but interconversions with glutamine were low; only 12--25% was converted to glutamine and most of this occurred in extrarenal tissues. A similar rate of 20--26% of the glutamate was converted to glucose, which accounted for about 4% of the total glucose produced. The kidneys seemed to play an important gluconeogenic role; whereas the liver possibly could account for 100% of the glucose produced from glutamate in fed sheep, the kidneys accounted for 40--45% during acidosis and fasting.

Transport of amino acids in whole blood and plasma of sheep

Comparisons of amino acid transport in both whole blood and plasma were made across the portal-drained viscera, liver, kidneys, and hindquarters in sheep. Lyophilized samples could be stored at -20 degrees C for up to 17 wk with little or no changes observed on reanalysis. Only glutamine, taurine, and serine showed any loss (18, 23, and 16%) over a 3.5-yr period. However, there was no concomitant rise in glutamate over the same period, suggesting that glutamate does not simply deamidate. There were no differences in arterial concentrations of alanine, citrulline, isoleucine, and valine between whole blood and plasma. However, glutamate, asparagine, glycine, serine, threonine, taurine, ornithine, leucine, lysine, phenylalanine, and tyrosine were higher in whole blood compared to plasma, and glutamine and arginine were lower. Whole blood and plasma fluxes for all amino acids were positively correlated, and there were no differences in their regression lines among tissues. Plasma, however, underestimated amino acid transport glutamine, glutamate, and taurine at greater rates than plasma. It is concluded that blood cellular transport, per unit volume, was always concomitant to and usually at the same rate as plasma transport over all tissues measured. Plasma therefore reflects amino acid transport but underestimates total transport to about the extent of the packed cell volume and in cases of glutamine, glutamate, and taurine, perhaps even more.

Intestinal disappearance and portal blood appearance of amino acids in sheep

The intestinal disappearance and simultaneous arterial inflow and portal appearance of individual amino acids (AA) were studied in sheep fed closed formula, unrefined high (H.P.) and medium (M.P.) protein diets. Gut contents were sampled through four intestinal cannulae and blood was sampled through portal and arterial catheters. The amount of total and amino N that was fed decreased on passage into the duodenum but increased in the jejunum, and then again decreased steadily towards the terminal ileum. The amounts of AA passing into the duodenum were significantly higher when the H.P. rather than the M.P. diet was fed. No dietary differences in AA were noted at the ileo-cecal junction, however, meaning that greater amounts of AA disappeared from the intestine when the H.P. diet was fed. The amounts of AA appearing in portal blood were 30 to 80% of those disappearing from the intestine and were greater in sheep fed the H.P. diet. The amount of AA passing into the duodenum also significantly affected the concentrations of AA in arterial blood. Less [U-14C]glutamic acid than [U-14C]alanine, that was infused into abomasum, was detected in the digesta passing through the pylorus. The same also was true for the unlabeled free form of glutamic acid. The portal appearance of both unlabeled and labeled glutamic acid was negligible, but that of alanine was considerable. Variable amounts of [14C]citrulline, [14C]arginine, and [14C]urea were detected in the blood following the abomasal infusions of labeled glutamic acid or alanine. The portal appearance of these labeled metabolites was always negative, however, implying that they were utilized, and not formed, by gut tissues.

Glutamine metabolism, interorgan transport and glucogenicity in the sheep

[U-14C]glutamine and [6-3H]glucose were infused into four groups of sheep: fed, NH4Cl acidotic, fasted, and dexamethasone treated. Net and unidirectional plasma glutamine fluxes in the portal-drained viscera (PDV), liver, kidneys, and hindquarters were measured by multiplying venoarterial concentration differences and 14C extraction ratios by the rate of blood flow. In fed sheep, glutamine was released by kidneys and muscle but removed by PDV and liver. In all other sheep, renal glutamine release either decreased or switched over to net removal largely due to increased unidirectional renal utilization. This increased renal glutamine demand was compensated for, during acidosis, by a decreased net hepatic glutamine removal but, during fasting and dexamethasone treatment, by an increased muscle glutamine release. Plasma glutamine and glucose turnover rates averaged 11-12 and 19-24 mmol/h but the percentage of glutamine converted to glucose was higher during fasting and dexamethasone treatment (21%) than in normal fed sheep (17%) perhaps reflecting the increased glutamine removal by the kidneys. Since renal glutamine utilization increases with acidosis and fasting and, since glutamine turnover remains unchanged, glutamine metabolism by other body tissues must be altered to compensate for renal changes.

Renal function studies in normal and toxemic pregnant sheep

Renal blood and plasma flow, glomerular filtration rate (GFR) and maximal tubular transport of PAH (TmPAH) were measured in nonpregnant and twin-pregnant sheep. Twin-pregnant animals were studied during normal pregnancy as well as during ovine pregnancy toxemia artificially produced by starvation. All animals were surgically prepared with aortic, post caval and renal vein cannulas at least one week prior to experimentation. Total renal blood and plasma flow was found to be elevated during pregnancy, but if expressed on the basis of body weight no changes were noted. Starvation and the resultant development of hypoglycemia and hyperketonemia caused a 25-30% decline in renal blood and plasma flow. GFR in pregnant fed sheep (193 ml/min or 2.7 ml/kg.min) was significantly higher (P less than .001) than that of nonpregnant ewes (118 or 2.3 ml/kg min). During ovine pregnancy toxemia the GFR was significantly (P less than .001) diminished (142 ml/min or 2.0 ml/kg min). TmPAH also was significantly higher (179 mg/min or 2.5 mg/kg min) in pregnant animals when compared to nonpregnant ewes (98 mg/min or 1.9 mg/kg min.), but starvation had no effect on Tm PAH in pregnant sheep. It thus appears that a functional renal hypertrophy occurs during pregnancy which is similar to that which follows unilateral nephrectomy or renal disease. During ovine pregnancy toxemia the diminution of renal function probably results from the metabolic derangements and is thus not comparable to human preeclampsia.

Quantitative aspects of secretion and hepatic removal of glucagon in sheep

The secretion of immunoreactive glucagon (IRG) into the portal blood and its removal by the liver were determined in conscious-fed sheep by simultaneous measurement of venoarterial plasma concentration differences and portal and hepatic plasma flows. IRG was determined using Manns' antiserum and Unger's 30K antiserum, the latter being highly specific for pancreatic glucagon. In 21 experiments in which Manns' antiserum was used the IRG secretory rate was 7.1 +/- u.4 mug/h. The value using 30K antiserum was lower (5.5 +/- 1.3, n=6), but not significantly different. Although the hepatic extraction ratio (hepatic removal - total IRG presented to the liver) was only 7%, the hepatic removal of 2.4 +/- 0.5 mug/h was equivalent to 31-35% of the portal IRG secretory rate. Since during steady-state conditions, glucagon secretion equals glucagon removal, the liver must account for approximately one-third of the glucagon degraded by the entire body.

Single-column chromatographic analysis of labeled glutamine and other amino acids in whole blood

A method is presented for single-column analysis of the concentrations and specific activities of the free amino acids in both whole blood and plasma. Interference from glutathione in whole blood was eliminated by the use of sodium sulfite although losses of about one-half of the cystine and methionine occurred. Seventy-five percent +/- 1 of the glutamine was recovered consistently in both whole blood and plasma so that corrections for this loss readily could be made. Elimination of the baseline shift due to ammonia was accomplished by passing the buffers through ion-exchange columns before entering the sample loops. There were several significant differences between amino acid concentrations and specific activities in whole blood and in plasma, indicating that care should be taken in interpreting data on metabolism of amino acids.

Studies of glucose production in sheep using (6-3H)glucose and (U-14C)glucose

Simultaneous primed-continuous intravenous infusions of [6-3H]glucose and [U-14C]glucose were performed on 13 fed, 4 fasted, and 4 dexamethasone-treated sheep. In 10 of the experiments on fed sheep, glucagon or insulin was infused intraportally for 2 h after control values were obtained. The 3H-labeled glucose gave glucose production values that were only 4.4 +/- 0.5, 5.4 +/- 1.0, and 5.8 +/- 0.8% higher than 14C-labeled glucose in the normal fed, fasted, and dexamethasone-treated sheep, respectively. Glucagon or insulin infusions did not significantly alter this recycling. It is condluced that a recycling of glucose carbon through metabolic intermediates is minimal in the sheep as compared with other species and also that it is not significantly altered by fasting or by hormones that affect glucose production.

Quantitative aspects of insulin secretion and its hepatic and renal removal in sheep

The secretion of insulin into the portal blood and its removal by the liver and kidneys in conscious fed sheep were determined by simultaneously measuring venoarterial plasma concentration differences and portal, hepatic, and renal plasma flows. The basal secretory rate of insulin was 0.43 +/- 0.03 U/h or 7.8 mU/kg-h. The secretory rate of insulin and the amount of insulin presented to the liver also were altered by 2-h intraportal infusions of glucagon (150 mug/h), insulin (1.17 U/h), and insulin (1.17 U/h) lus glucose (2.2 g/h). Hepatic removal under all conditions was about 50% of the insulin secretory rate, although the extraction ratio was only 0.08. Renal removal was 35% of the insulin secretory rate. The renal extraction ratio was 0.35. During insulin-induced hypoglycemia and also during starvation, the hepatic extraction ratio of insulin increased significantly, but the removal as a percentage of insulin secretion did not change. It appears that in sheep on a maintenance diet the basal secretory rate of insulin is less than that of nonruminant species and that, within physiological limits, the liver disposes of about one-half and the kidney about one-third of the insulin. Other tissues, presumably, remove the remaining 10--20%.

Effects of glucagon and insulin on net hepatic metabolism of glucose precursors in sheep

The net hepatic metabolism of amino glycerol, lactate, and pyruvate was determined in conscious fed sheep by multiplying the venoarterial concentration differences by the hepatic blood or plasma flow. In each experiment several sets of control blood samples were taken; glucagon or insulin then was infused intraportally for 2 h during which additional samples were taken. Four types of experiments were performed: 1) glucagon infusion (150 mug/h) into normal sheep, 2) glucagon infusion (100 mug/h) into insulin-treated alloxanized sheep, 3) insulin infusion (1.17 U/h) into normal sheep, and 4) insulin plus glucose infusion (12.3 mmol/h) into normal sheep. The second group of experiments was performed to prevent reflex hyperinsulinemia, and the fourth was performed to prevent reflex hyperglucagonemia. Glucagon directly stimulated the net hepatic uptake of alanine, glycine, glutamine, arginine, asparagine, threonine, serine, and lactate. Glucagon also stimulated lipolysis in adipose tissue. Insulin, on the other hand, appeared to have a lipogenic effect on adipose tissue and to stimulate directly the uptake of valine, isoleucine, leucine, tyrosine, lysine, and alanine only at extrahepatic sites. The study showed that, in sheep, the effects of glucagon primarily are on liver, and insulin's effects primarily are on skeletal muscle and adipose tissue where it promotes protein and lipid synthesis.

Effect of glucagon on plasma alanine and glutamine metabolism and hepatic gluconeogenesis in sheep

Net hepatic uptakes of plasma alanine (Ala), glutamate (Glu), and glutamine (Gln) were measured before and during intraportal glucagon infusions in five normaland four insulin-and alloxan-treated (ITA), conscious, fed sheep. Since hyperinsulinemia is associated with glucagon administration, ITA sheep were used so that constant plasma insulin levels could be maintained. Glucose turnover was determined by a vena caval infusion of glucose-6-'3H. In addition, in ITA sheep, Ala-'14C wasinfused for measurement of plasma Ala turnover, its unidirectional organ metabolism, and contribution to glucose synthesis. During infusion of glucagon, the net hepatic uptake of Ala increased significantly (P is less than 0.01) from control values of 3.8 plus or minus 0.5 and 2.7 plus or minus 0.6 mmol/h to 5.9 plus or minus 1.0 and 5.5 plus or minus 0.8 mmol/h in normal and ITA sheep, respectively. Similarly, Gin uptake increased from 4.3 plus or minus 1.4 and 1.6 plus or minus 0.5 to 5.5 plus or minus1.6 and 3.7 plus or minus 1.0 mmol/h, respectively. The conversion of Ala to glucose increased from control values of 1.7 plus or minus 0.5 to 3.0 plus or minus 0.5 mmol/h. Arterial plasma Ala and Gin concentrations decreased about 25% during glucagon administration, presumably as a result of their increased hepatic uptakes. A decreasein utilization of plasma Ala, but no change in production was calculated for the nonhepatic tissues, indicating that glucagon increased gluconeogenesis from Ala at the expense of muscle protein synthesis. Glucagon thus has a direct effect on the liver butonly an indirect effect on other tissues.

Quantitative studies of the metabolism of chylomicron triglycerides and cholesterol by liver and extrahepatic tissues of sheep and dogs

Unanesthetized sheep and dogs, previously fitted with indwelling catheters in the aorta, lower vena cava, mesenteric, portal, left hepatic and jugular veins, were given constant intravenous infusions of lymph in which the chylomicron lipids were variously labeled with (3)H or (14)C. Para-aminohippuric acid was infused into the mesenteric venous catheter for measurement of portal and hepatic venous blood flow. In some animals, alternately labeled free fatty acids bound to albumin were mixed with the lymph to be infused. In both species, chylomicron triglyceride fatty acids were taken up in the region drained by the lower vena cava and portal vein and free fatty acids derived from hydrolysis of these triglycerides were extensively recycled in the blood. Direct uptake of triglyceride fatty acids also occurred in liver and accounted for about 10% of the total triglyceride fatty acids removed from the blood in sheep and 22% in dogs. In sheep, 10% and, in dogs, about 40% of these triglyceride-fatty acids were released into the blood as free fatty acids. The free fatty acids recycled from various regions accounted for a substantial fraction of the chylomicron fat eventually deposited in each tissue. Uptake of chylomicron cholesterol from the blood of sheep occurred primarily in liver and to a small extent in certain tissues drained by the portal vein. The results obtained, together with other available data, demonstrate that chylomicron triglycerides are removed primarily in extrahepatic tissues of both species, while the liver removes cholesterol contained in chylomicron "skeletons" from which most of the triglycerides have been removed. The quantitative differences between transport of chylomicron lipid in sheep and dogs may be related to known differences in the structure of their hepatic sinusoids.