Glycerol turnover in the nonpregnant and ketotic pregnant sheep

Mammalian aquaglyceroporin function in metabolism

Aquaglyceroporins are integral membrane proteins that are permeable to glycerol as well as water. The movement of glycerol from a tissue/organ to the plasma and vice versa requires the presence of different aquaglyceroporins that can regulate the entrance or the exit of glycerol across the plasma membrane. Actually, different aquaglyceroporins have been discovered in the adipose tissue, small intestine, liver, kidney, heart, skeletal muscle, endocrine pancreas and capillary endothelium, and their differential expression could be related to obesity and the type 2 diabetes. Here we describe the expression and function of different aquaglyceroporins in physiological condition and in obesity and type 2 diabetes, suggesting they are potential therapeutic targets for metabolic disorders.

The efficacy of bamboo charcoal in comparison with smectite to reduce the detrimental effect of aflatoxin B1 on in vitro rumen fermentation of a hay-rich feed mixture

Two commercial materials, a bamboo charcoal (BC) and a smectite clay (SC), were assessed in vitro with aflatoxin B1 (AFB1) in an equilibrium adsorption test. The adsorption capacity and proportion adsorbed (0.381 μg/mg, 0.955) for BC were greater than for SC (0.372 μg/mg, 0.931). The effects of in vitro ruminal fermentation of hay-rich feed incubated with 1.0 μg/mL AFB1 for 0-10 g/L doses of BC and SC were measured at 39 °C for 72 h. The BC and SC binders increased AFB1 loss at dosages ≥1.0 g/L (p < 0.0001). Average AFB1 loss (p < 0.0001) was greater for SC (0.904) than BC (0.881). Both SC and SC addition increased in vitro dry matter loss, and the average dry matter losses were similar. Asymptotic gas volume and volatile fatty acid production were greater for BC than for SC (p < 0.0001). Thus, BC may be as effective as SC in removing aflatoxin B1's detrimental effects on rumen degradability and fermentation under the occurrence of microbial aflatoxin degradation.

Development of a dynamic, mechanistic model of lamb metabolism and growth

A dynamic, mechanistic model of lamb metabolism and growth was developed for the purpose of evaluating hypotheses regarding the mechanisms of action of growth promotants. The model relates tissue growth to DNA accretion and protein turn-over. State variables include circulating amino acids, glucose, lipids and acetate; four protein pools (carcass, viscera, other tissues and wool) and storage triacylglycerol are also included. Equations are mainly of the Michaelis-Menten form, allowing for nutrient utilization patterns to be determined by relative tissue affinities for substrates (ko.5), enzymatic capacities (Vmax) and substrate concentrations ([S]). Protein degradation rates are defined as first-order with respect to protein. The model adequately simulated growth from 20 to 40 kg empty body weight. Simulated changes in nutrient input yielded reasonable energy balance response patterns, although theoretical growth efficiencies were greater than those observed in practice. Variations in volatile fatty acid absorption patterns were accommodated well, with predicted nitrogen retention closely approximating experimental observations. The model also responded appropriately to changes in dietary protein level, with body fat varying inversely with amino acid absorption. In summary, the model was found to perform adequately for the purpose of examining mechanisms responsible for alteration of growth and body composition.

Twin-pregnancy increases susceptibility of ewes to hypoglycaemic stress and pregnancy toxaemia

Pregnancy toxaemia is a metabolic disorder with a high mortality rate and occurs in twin-bearing ewes in late gestation. Maternal hypoglycaemia is a characteristic symptom of the disease and has been attributed to an increase in glucose uptake by the twin-bearing uterus. The possibility that a reduced maternal glucose production rate might cause hypoglycaemia, has received little attention in the past. It was the aim of this study to investigate this explanation as a possible alternative. Six ewes were sequentially subjected to two types of hypoglycaemic stress, firstly by fasting for 14 h and secondly through induction of moderate hyperketonaemia. Glucose kinetics were assessed in each animal during the dry non-gestational period, during late gestation, and during early lactation. Application of these stress factors was associated with variation of plasma glucose concentration from 4.9 to 0.87 mmol L(-1). The plasma glucose concentration was always significantly related to the glucose production rate. The greatest stress-induced reductions in glucose concentration and glucose production rate were seen during late gestation in twin-bearing ewes. The decline in the glucose production rate after an overnight fast and during induced hyperketonaemia was greater in twin-bearing ewes than in single-bearing ewes (59% and 43%, respectively, p<0.05). The stress conditions resulted in the lowest levels of glucose concentration and glucose turnover rates in the stressed, hyperketonaemic, late gestation twin-bearing ewes. This illustrates that the glucose homoeostatic system of ewes bearing twins is significantly more susceptible to hypoglycaemic stress than that of ewes bearing single lambs. These findings also show that the primary cause of hypoglycaemia in late gestation twin-pregnant ewes is an increased susceptibility to a stress related reduction in glucose production rate. This metabolic condition leaves the twin-pregnant ewe predisposed for the development of pregnancy toxaemia.

Lipid mobilization of long-distance migrant birds in vivo: the high lipolytic rate of ruff sandpipers is not stimulated during shivering

For long migrations, birds must rely on high flux capacities at all steps of lipid metabolism, from the mobilization of adipose reserves to fatty acid oxidation in flight muscle mitochondria. Substrate kinetics and indirect calorimetry were used to investigate key parameters of lipid metabolism in a highly aerobic shorebird: the ruff sandpiper Philomachus pugnax. In this study, we have quantified the effects of cold exposure because such measurements are presently impossible during flight. Lipolytic rate was monitored by continuous infusion of 2-[3H]-glycerol and lipid oxidation by respirometry. Plasma lipid concentrations (non-esterified fatty acids, neutral lipids and phospholipids) and their fatty acid composition were also measured to assess whether cold exposure causes selective metabolism of specific lipids. Results show that shivering leads to a 47% increase in metabolic rate (44.4±3.8 ml O2kg–1min–1 to 65.2±8.1 ml O2kg–1 min–1), almost solely by stimulating lipid oxidation (33.3± 3.3 ml O2 kg–1min–1 to 48.2±6.8 ml O2kg–1 min–1) because carbohydrate oxidation remains close to 11.5± 0.5 ml O2 kg–1min–1. Sandpipers support an unusually high lipolytic rate of 55–60 μmol glycerol kg–1 min–1. Its stimulation above thermoneutral rates is unnecessary during shivering when the birds are still able to re-esterify 50% of released fatty acids. No changes in plasma lipid composition were observed, suggesting that cold exposure does not lead to selective metabolism of particular fatty acids. This study provides the first measurements of lipolytic rate in migrant birds and shows that their capacity for lipid mobilization reaches the highest values measured to date in vertebrates. Extending the limits of conventional lipid metabolism has clearly been necessary to achieve long-distance migrations.

Pregnancy impairs ketone body disposal in late gestating ewes: Implications for onset of pregnancy toxaemia

The impact of pregnancy on ketone body disposal during a hyperketonaemic clamp was examined by tracer isotope dilution techniques in seven 12 h fasted sheep in three reproductive states, in the dry non-gestating period, late in gestation and during early lactation. After a sampling period of 60 min under basal conditions a DL-BHB racemate solution was continuously infused intravenously for 3 h at rates of 14.3-24.3 micromol/(kg min) to elevate the D-BHB concentration in blood plasma to values between 5 and 7 mmol/l. Two separate experiments were carried out with the same sheep in each of the three reproductive states. During pregnancy three ewes were pregnant with a single lamb and four ewes carried twins. Maximal D-BHB turnover rates fell significantly in late gestation by 26% relative to early lactation and by 22% when compared with the dry non-pregnant state. Reduction of maximal D-BHB disposal rate during late gestation was accompanied by a significant 297% (p<0.005) and a non-significant 49% increase in the basal BHB concentration in blood, a non-significant 10% and 4% decrease in the rate constant of D-BHB turnover and a non-significant 24% and 13% rise in the incremented increase of D-BHB concentration per unit D-BHB infusion, relative to the dry and the lactating period, respectively. Induction of hyperketonaemia significantly lowered NEFA and glycerol concentrations in blood by 67% and 57%, respectively, compared to the pre-infusional concentrations. The magnitude of this effect was the same in all reproductive states and was explained as a direct antilipolytic action of D-BHB on adipose tissue. It is concluded that the reduced ability of the late gestating ewe to utilize D-BHB promotes hyperketonaemia. Since hyperketonaemia exerts several adverse effects, e.g. on energy balance and glucose metabolism it appears that the impairment of ketone bodies disposal in late pregnancy facilitates development of pregnancy toxaemia, especially in ewes carrying twins.

Hyperketonemia Impairs Glucose Metabolism in Pregnant and Nonpregnant Ewes

The present study was undertaken to test the hypothesis that high ketone body concentrations suppress endogenous production of glucose and in pregnant sheep facilitate development of pregnancy toxemia. Rates of endogenous glucose production [mmol.min(-1)], and rate constants of glucose turnover [min(-1)] were measured in seven 12-h fasted sheep in the presence of normo- and hyperketonemia by use of D-2-[(3)H]-glucose. The measurements were carried out in the same sheep during the nonpregnant nonlactating state, during late pregnancy (10 +/- 7 d antepartum) and during lactation (19 +/- 6 d postpartum). Hyperketonemia (5 to 7 mmol.L(-1)), similar to that present in spontaneous ovine pregnancy toxemia, was induced by continuous intravenous 4-h infusions of DL-beta-hydroxybutyrate (DL-BHB). Glucose turnover [mmol.min(-1)] in the same 7 nonpregnant nonlactating, late pregnant, and lactating sheep was significantly greater during normoketonemia (0.80, 1.16, 1.76) than during hyperketonemia (0.66, 0.92, 1.16, respectively). The rate constants of glucose turnover were not altered by elevation of the BHB concentration. The results demonstrated that high BHB concentrations significantly suppressed endogenous glucose production but showed no effect on glucose utilization. The suppressive effect of hyperketonemia on hepatic glucose production resulted in a significant reduction of plasma glucose concentration and was qualitatively the same in all three reproductive states. The results indicate that hyperketonemia, which is regularly present in late twin pregnant hypoglycemic sheep contributes significantly to the reduction of available glucose. This effect of hyperketonemia may invoke sustained hypoglycemia and may render the ewe into a vicious cycle that probably makes the animal refractory to treatment in most cases.

Non-esterified fatty acid and glycerol kinetics and fatty acid re-esterification in goats during early lactation

Non-esterified fatty acid (NEFA) and glycerol kinetics were studied in lactating goats to gain insight into the mechanisms by which animals in early lactation can initially mobilize and later replenish body fat reserves. Kinetic measurements were made at days 10, 38 and 76 post-partum in ten multiparous lactating does. Plasma NEFA concentrations and NEFA entry rate decreased as lactation advanced, being significantly higher at day 10 than at either days 38 or 76 of lactation. Both plasma NEFA concentrations and NEFA entry rate were negatively correlated with calculated energy balance. In contrast, glycerol entry rate was significantly higher at day 76 than at day 10 of lactation and was positively related to both calculated energy intake and energy balance. Apparent intracellular fatty acid re-esterification was lower at day 10 than at later stages of lactation and was positively related to calculated energy balance. It is suggested that during early lactation, substantial shifts in adipose tissue fat reserves can occur via altering rates of fatty acid re-esterification and de novo lipogenesis, without major changes in the rate of lipolysis.

Lipid metabolism in shorn and unshorn pregnant sheep

The aim of the present study was to determine how the long-term metabolic adaptations to winter shearing of the pregnant ewe result in significant changes in the rates of lipid mobilization and utilization of non-esterified fatty acids (NEFA) in comparison with unshorn controls. Continuous infusions of [1-14C]palmitic acid, [2-3H]glycerol and NaH14CO3 were used to measure whole-body lipid metabolism in fed (estimated metabolizable energy (ME) intake 9.54 MJ/d) and under-fed (estimated ME intake of 3 MJ/d), shorn and unshorn sheep over the final 4 weeks of pregnancy. Whole-body carbon dioxide, estimated heat production, total NEFA entry and oxidation rates were all significantly higher in fed shorn ewes compared with unshorn controls, even though there was no difference in the arterial plasma NEFA concentration. These differences may be mediated via an increase in the plasma concentrations of thyroid hormones in shorn animals. As a result of under-feeding any significant differences in lipid metabolism between shorn and unshorn groups were removed. In all sheep the mean total NEFA entry rate as measured using [1-14C]palmitic acid was 3.4 times the value obtained using [2-3H]glycerol. It is concluded that when sheep are fed on a diet from which no more than half the required ME for late pregnancy is obtained, then lipolysis of body fat depots occurs via the incomplete breakdown of adipose tissue triglycerides. This effect is significantly greater in the fed shorn pregnant ewe which exhibits higher entry and oxidation rates of NEFA.

Relations between plasma non-esterified fatty acid metabolism and body tissue mobilization during chronic undernutrition in goats

1. Eleven mature goats were offered 140 kJ metabolizable energy/kg per d (M) of lucerne (Medicago sativa) hay-oaten grain (1:1, w/w) for at least 1 month before plasma non-esterified fatty acid (NEFA) kinetics and tritiated water space (TS) were determined. 2. Goats were then fed at M, 0.5 M or 0.25 M for 34 (SE 6) d, at which time the experimental procedures were repeated. 3. Chronic undernutrition resulted in elevated NEFA concentrations and NEFA entry rate, with a tendency for the ratio plasma NEFA:glycerol to increase, suggesting that body-fat mobilization during prolonged underfeeding is due more to decreased lipogenesis and intracellular NEFA re-esterification rather than to increased lipolysis. 4. Plasma NEFA concentrations and NEFA entry rate, as well as being highly correlated with each other, were significantly related to calculated energy balance and body fat losses estimated from changes in live weight and TS. 5. Increases in NEFA entry rate were highly correlated with, and of the same magnitude as, body fat losses, confirming that NEFA kinetics do quantitatively reflect lipid mobilization.

Measurement of glycerol turnover by infusion of nonisotopic glycerol in normal and injured subjects

A new technique for measuring glycerol turnover in human subjects is presented using a staged infusion of unlabeled glycerol. A total of 105 measurements was made in normal subjects and postoperative or acutely ill patients. The relation between glycerol turnover rate (R tau) and concentration (C) (R tau = 31 + 2.07C) from 80 staged infusion measurements in 21 normal subjects did not differ significantly from observations of others made by isotope dilution. In 11 patients, glycerol turnover was also measured by dilution of isotopic glycerol. Mean values did not differ significantly between the two methods. The average difference, regardless of sign, for individual values was 17%. This agreement tends to validate the assumptions of both methods. Advantages of the staged infusion method are that it is quicker and easier to perform and does not entail a risk due to administration of radioactive materials. Evidence obtained in this study from a three-stage procedure indicates that satisfactory information may be obtained from a two-stage infusion at successive rates of 250 and 600 mumol/min for 70 min each, with a prime of 2 mmol given as a bolus at the start of each stage.

Effect of glycemic changes on lipolysis in sheep in vivo

The purpose of this study was to evaluate the effects of glycemic changes on lipolysis in sheep. To prevent hyperglycemia from altering insulin concentrations and hypoglycemia from altering adrenomedullary hormonal secretion, the experiments were conducted in insulin-treated alloxan diabetic sheep whose adrenal glands were denervated. Hyperglycemia was associated with a reduction in glycerol concentrations and hypoglycemia with elevated glycerol. These changes in glycerol concentrations were independent of changes in plasma insulin, glucagon, catecholamine, and cortisol concentrations. It appears that change in glucose concentrations per se can alter lipolysis in sheep.

Glucose production and utilization in non-pregnant, pregnant and lactating ewes

By using continuous infusions of 3H- and 14C-labelled substrates, three-pool models, incorporating rumen propionate, plasma glucose and blood carbon dioxide were constructed to determine the contribution of propionate to glucose in non-pregnant, pregnant (mid and late) and lactating hill ewes. Although the intakes of non-pregnant and pregnant ewes were the same (1200 g dried grass/d) and resulted in similar levels of propionate production (33 g C/d), glucose production rate (GPR) increased from 44 g C/d in the non-pregnant ewes to 62 g C/d in the ewes carrying twins in late pregnancy. In lactating ewes given 2500 g dried grass/d, propionate production increased to 56 g C/d and GPR increased to 93 and 104 g C/d in ewes suckling single and twin lambs respectively. There was an increase in the percentage of the propionate resource which was diverted to glucose, from 37% in the non-pregnant ewes and ewes in mid-pregnancy, to 55% in late pregnancy and 60% in lactation. In spite of this apparent metabolic adaptation to the additional requirements for glucose, approximately 55% of the glucose-C was supplied by metabolites other than propionate and CO2. From the determination of plasma glycerol concentrations it was estimated that the maximum possible contribution of glycerol-C to glucose was only 8-12 g C/d. The remaining 40% of the glucose-C could not be accounted for and could have been derived from non-essential amino acids (NEAA). In the non-pregnant and pregnant ewes only 62% of the GPR was oxidized to CO2. In the lactating ewes only 49 and 30% of the GPR was oxidized to CO2 in the ewes suckling single and twin lambs respectively. In the majority of cases there was a marked similarity between the amounts of glucose-C apparently derived from NEAA and the amount of glucose-C which was not oxidized to CO2.

The distribution and metabolism of glycerol in the rabbit

1. The effective volume of distribution of labelled glycerol was studied in conscious young adult rabbits provided with in-dwelling cannulae in the femoral blood vessels. This could be estimated after sampling arterial blood throughout an intravenous infusion of [2-3H]glycerol. The volume was calculated by using an algebraic method of graphical area analysis over 100 min of equilibration, and is symbolized 100V e or 100V e%. It occupied 34.1 +/- 2.2% (mean +/- S.E.M.; n = 13) of the body weight. The pool of endogenous glycerol occupying this space is distinguished in the present paper by calling it the transit pool, symbolized 100Me. 2. The median time of transit of glycerol through this pool was approx. 6 min in these conscious rabbits with normal (less than 0.2 mM) blood glycerol concentrations. 3. The metabolism of glycerol was also studied in rabbits while anaesthetized with urethane or while conscious. On average, half of the change in glycerol concentration that occurred on overnight starvation could be attributed to a decrease in clearance, whereas half was due to an increase in lipolysis. 4. The correlation between the reciprocal of glycerol concentration and clearance showed that in these animals about a quarter of the variation in concentration was due to an association with clearance. The remainder of the variation was attributed to variations in the rate of glycerol formation (lipolysis). 5. The regression of glycerol turnover rate on concentration implied that turnover was positive at zero glycerol concentration. This confirms previous findings from studies on other species. The explanation offered for this phenomenon is that the well-known physiological changes induced by feeding (decreased lipolysis, increased splanchnic blood flow) may independently decrease the glycerol concentration by both decreasing its release into the blood and simultaneously increasing its clearance.

Studies on the control of gluconeogenesis in sheep: effect of glucose infusion

1. Effect of glucose infusions on the rate of gluconeogenesis, as indicated by changes in the irreversible loss of plasma glucose, synthesis of glucose from ruminal propionate or fixation of blood bicarbonate into glucose, has been examined in sheep.

Sheep, feeding at hourly intervals, received intravenous infusions of [U-14C]glucose, NaH14CO3, or [6-3H]glucose, infused simultaneously with an intraruminal infusion of [2-14C]propionate. Glucose (4–144 mg/min) was infused intravenously at a single rate or at two different successive rates of equal duration. The glucose infusions were over a 6 h period and followed estimates of pre-infusion kinetic measurements.

2. The infusion of glucose suppressed endogenous glucose production in direct proportion to that infused for sheep given diets of lucrene or wheat. The maximum rate of suppression recorded was equivalent to about 50–60% of the endogenous production rate of glucose.

Transfer rates of carbon-14 from blood bicarbonate or ruminal propionate to plasma glucose were reduced during glucose loadings. The results indicate that administration of glucose was more effective in suppressing the synthesis of glucose from substrates other than ruminal propionate.

3. The percentage of glucose carbon derived from blood bicarbonate was lower in sheep given wheat than in sheep given lucerne. It was suggested that this difference may have been due to absorption of glucose from the alimentary canal of sheep on the former ration. Approximately 8% of the wheat starch may have escaped fermentation and been absorbed as glucose in these animals.

Glycerol turnover and oxidation in man

The oxidation and turnover of plasma glycerol has been studied in lean and obese, fed and starving man by means of a long-term infusion of glycerol-(14)C, and the participation of glycerol in gluconeogenesis has been determined. Under none of the experimental conditions did glycerol contribute more than 10% of the total respiratory CO(2). Glycerol turnover in fed lean subjects was 106 mmoles/min. Glycerol levels and turnover were higher in the obese subjects and with all subjects after starvation. There was a direct correlation between plasma levels and turnover values for which a regression equation was derived: y = 1556 x + 33.1, when y = turnover in micromoles per minute and x = glycerol level in micromoles per milliliter. Whereas a direct relation was established between glycerol and FFA levels, the FFA/glycerol turnover ratio was 4.7:1 in the lean group indicating incomplete hydrolysis of adipose tissue triglycerides.During starvation plasma glycerol is nearly or completely converted to glucose in the lean and obese groups, respectively. Of the new glucose formed from protein and glycerol 38% is derived from glycerol in the lean and 79% in the obese. Protein and glycerol have been shown to be adequate as precursors to supply at least as much glucose as is being oxidized per day.

Glycerol metabolism in the neonatal rat

1. The possible role of glycerol as a precursor in neonatal gluconeogenesis in the rat was investigated by recording the activities of glycerol kinase and l-glycerol 3-phosphate dehydrogenase in the liver, kidney and other tissues around birth and during the neonatal period. 2. Blood glycerol concentrations in the neonatal rat are high. 3. There is a marked increase after birth in the ability of both liver and kidney slices to convert glycerol into glucose plus glycogen that correlates with the increase in glycerol kinase activity. 4. High hepatic and renal l-glycerol 3-phosphate dehydrogenase activities are also found in the neonatal period. 5. The marked capacity for neonatal gluconeogenesis from glycerol thus demonstrated and the role of glycerol kinase in its control are discussed.