The effects of feeding and acute cold exposure on the visceral release of volatile fatty acids, estimated hepatic uptake of propionate and release of glucose, and plasma insulin concentration in sheep

Effect of mild cold exposure on whole-body and net hindquarters glucose metabolism in sheep

Isotope dilution using [U-13C]glucose was combined with measurement of hindquarters arteriovenous difference and blood flow to examine the effect of mild cold exposure on whole-body and net hindquarters glucose metabolism in sheep. Determinations were carried out in a thermoneutral environment (20 +/- 1 degrees C) and on day 5 of mild cold exposure (4 +/- 1 degrees C). Whole-body blood glucose turnover rate was 1.56 and 1.91 mg kg-1 min-1 in the thermoneutral environment and during mild cold exposure, respectively. Net hindquarters glucose uptake was 0.79 and 1.19 mg kg-1 min-1, corresponding to 50 and 61% of whole-body turnover rate, respectively. These results suggest that in sheep the contribution of glucose uptake by the hindquarters tended to increase during mild cold exposure, even though whole-body blood glucose turnover rate did not increase significantly.

Glucose metabolism in shorn and unshorn pregnant sheep

1. Whole-body, hind-limb and uterine tissue metabolism of glucose was studied using a combination of isotopic and arterio-venous difference techniques in shorn and unshorn pregnant sheep over the final 4 weeks of pregnancy. This was combined with the measurement of the concentrations of oxygen and carbon dioxide in arterial blood and plasma concentrations of lactate, acetate, non-esterified fatty acids, 3-hydroxybutyrate, glycerol, growth hormone (GH), insulin, glucagon, cortisol, thyroxine and 3,5,3'-triiodothyronine (T3). 2. Glucose entry rate was 28% higher in shorn ewes compared with unshorn controls, even though there was no difference in the arterial plasma concentration of glucose. This effect may have been caused by a decrease in the molar rate, insulin: glucagon (I:G), which was 40% lower in shorn ewes as a result of a significant decrease in the plasma concentration of insulin. There was no difference in the plasma concentration of cortisol or GH. 3. Blood flow across the hind-limb or uterine tissues was not significantly different between shorn and unshorn groups, neither were the net glucose uptake, glucose oxidation rate or contribution of glucose to O2 consumption across these tissues. 4. Insulin-tolerance tests performed on a separate group of shorn and unshorn ewes showed an increased sensitivity to the hypoglycaemic effects of insulin in the shorn group. 5. There was no significant difference between shorn and unshorn animals in the contribution of glucose to CO2 output or in the proportion of glucose entry rate oxidized. CO2 entry rate was 18% higher in shorn ewes compared with unshorn controls which resulted in a 26% higher estimated value for heat production. There was a 47% increase in glucose oxidation rate in shorn ewes but there was no significant difference in the proportion of total heat production which was derived from glucose. The arterial concentrations of O2 and CO2 were significantly higher in shorn ewes, which may be an indication of the higher metabolic rate in these animals. This effect may be mediated via a significant rise in plasma T3 concentration in the shorn group. 6. It is concluded that as a result of long-term cold exposure there is a significant increase in whole-body glucose entry and oxidation rates in the shorn pregnant ewe. The increase in insulin sensitivity at the same time as a decrease in plasma insulin concentration may represent a mechanism to ensure continued glucose supply to insulin-sensitive tissues while the concomitant decrease in plasma I:G stimulates hepatic gluconeogenesis.

Propionate absorption and metabolism in the rabbit hindgut

Propionate disappearance from the loops of the hindgut in the rabbit was evaluated by measuring variations in the concentration of propionate in caecocolonic loops and differences in the arterial and venous plasma. In vivo metabolism in gut and liver tissues was studied after introduction of (1-14C) propionate into the caecocolonic loops. The rate of disappearance from the loops was always quantitatively significant but was greater in the proximal colon. Hindgut tissue metabolised propionate and the intensity of the metabolism varied with the segment studied; the proximal colon showed by far the highest propionate consumption. Radioactivity was found in a certain number of free amino acids, organic acids, sugars, lipid soluble substances and proteins. Propionate is an efficient respiratory fuel for the colonocyte and a good precursor for gluconeogenesis.

The effect of shearing on the energy metabolism of the pregnant ewe

1. Metabolizable energy (ME) intakes, heat production, non-protein respiratory quotient (NPRQ) and the plasma concentrations of glucose, non-esterified fatty acids (NEFA), 3-hydroxybutyrate, insulin, growth hormone (GH) and cortisol were measured in shorn and unshorn pregnant ewes. 2. Lamb birth-weight was 17% higher from shorn ewes despite similar ME intakes in the two groups. Shearing resulted in a significant decrease in the digestibility of dry matter and energy. 3. Both shorn and unshorn ewes were found to be in positive nitrogen balance and negative energy balance. Heat production was 28% higher in shorn ewes. This increase in heat production in the shorn group could be completely accounted for by an increase in the oxidation of fatty acids as measured using the NPRQ values. 4. Despite an apparent increase in the use of fat as an energy source there were no effects of shearing on the mean plasma concentrations of NEFA, 3-hydroxybutyrate, GH and cortisol. 5. Measurements made at 1 h intervals for 24 h indicated a tendency for the concentrations of glucose to be increased and insulin decreased in shorn ewes, particularly, between 6 and 11 h after feeding. 6. It is concluded that shearing pregnant ewes at 8 weeks before lambing results in a chronic increase in energy requirements which are met by oxidizing body fat depots. The cold stress induced by shearing may also inhibit insulin secretion resulting in increased plasma glucose concentrations. The effects of shearing on energy metabolism in the ewe are discussed in relation to the nutrient supply for the developing fetus.

Insulin response to glucose and glucose tolerance following feeding in sheep

Sheep offered a roughage diet for 4 h daily were injected intravenously with glucose before and at various times after feeding. The insulin secretory response to glucose and the rate of disappearance of injected glucose were determined. While the basal concentration of plasma insulin was unchanged, the base-line plasma glucose concentration tended to decrease during the meal. The glucose load brought about an increase in the plasma insulin concentration at each injection, but the insulin response to glucose and the rate of glucose disposal were increased during the meal. On varying the time of feeding between 08.00 and 16.00 hours, the increase in the insulin response to glucose and in the rate of glucose disposal always appeared to be related to the giving of food, independent of the time food was offered. It is concluded that feeding increases the insulin response to an intravenous glucose load even when the increase in the basal level of plasma insulin on feeding is very modest in sheep given a roughage diet. The increased insulin response and glucose disposal rate following feeding did not appear to be related to diurnal rhythms in insulin secretory activity or glucose metabolism.

Effects of feeding frequency on glucose concentration, glucose turnover, and insulin concentration in steers

Patterns of diurnal variation of glucose production, utilization, and concentration and of variation of insulin concentration in blood plasma were studied. Four Holstein steers (200 kg) were in a switchback design. Each received 4.6 kg of 70% chopped alfalfa hay and 30% dry cracked corn daily, either as two equal meals at 12-h intervals or 12 equal meals at 2-h intervals. None of mean daily glucose production, utilization, and concentration nor mean insulin concentration was affected by feeding frequency. In response to feeding, insulin was released simultaneously with increased glucose production, suggesting that glucose production and insulin release are regulated by a common effector system. Feeding twice daily resulted in a sinusoidal pattern of glucose specific radioactivity. On the basis of changes of glucose concentration, glucose utilization closely followed production. Within-day variation of glucose production for steers fed twice daily was greater than indicated by variation of glucose concentration. Such variation of production will lead to errors in estimates of glucose production from short (less than 12 h) isotope infusion experiments with steers fed twice daily. Conversely, daily glucose production was not different for steers fed twice or 12 times daily; therefore, frequent feeding is both necessary and appropriate for investigations of glucose kinetics.

Uptake and output of various forms of choline by organs of the conscious chronically catheterized sheep

The net uptake and output of plasma unesterified choline, glycerophosphocholine, phosphocholine and lipid choline by organs of the conscious chronically catheterized sheep were measured. There was significant production of plasma unesterified choline by the upper- and lower-body regions and the alimentary tract and uptake by the liver, lungs and kidneys. The upper- and lower-body regions drained by the venae cavae provided the bulk (about 82%) of the total body venous return of plasma unesterified choline. Production of plasma unesterified choline by the alimentary tract was approximately balanced by the plasma unesterified choline taken up by the liver, and was almost equal to the amount of choline secreted in the bile. There was a considerable amount of glycerophosphocholine in the liver and there was production of plasma glycerophosphocholine by the liver and uptake by the lungs and kidneys. Glycerophosphocholine was higher in the plasma of sheep than in that of rats. Plasma phosphocholine was produced by the alimentary tract and kidneys. There was production of plasma lipid choline by the upper- and lower-body regions drained by the venae cavae. The results suggest that the sheep synthesizes substantial amounts of choline in ectrahepatic tissues and has the capacity for extensive retention and recycling of bile choline. These observations, coupled with a slow turnover of the endogenous choline body pool, explain the low requirement of sheep for dietary choline in contrast with non-ruminant species.

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

The rate of blood flow in the portal and hepatic veins, and the net exchange across the gut and liver of volatile fatty acids (VFA), glucose, lactate, pyruvate, amino acids, ketone bodies, glycerol, non-esterified fatty acids (NEFA) and oxygen, were measured in lactating and non-lactating cows (a) in the normal, fed state and (b) before, during and after 6 d of fasting. Blood flow rate through the liver was 52% higher in normal, fed, lactating cows as compared with non-lactating cows, and was decreased by fasting in both groups of cows. Portal blood flow rate increased with an increase in metabolizable energy (ME) intake. Lactating, as compared with non-lactating, cows exhibited lower arterial concentrations of glucose and lactate, higher net portal outputs of VFA and ketone bodies, a higher net hepatic output of glucose, and higher net hepatic uptake of propionate and lactate. The splanchnic outputs of acetate, glucose and hydroxybutyrate were all apparently greater in the lactating cows. Fasting caused a rapid decrease in the blood concentrations of the VFA and an increase in those of glycerol and NEFA. The portal, i.e. gut, outputs of VFA, lactate, ketone bodies, alanine and (serine + threonine), and the portal uptake of O2, were all decreased by fasting. Fasting for 6 h also decreased the hepatic output of glucose and acetate by 77 and 95% respectively, increased the hepatic uptake of pyruvate, glycerol and NEFA, and doubled hepatic ketone-body output. The splanchnic output of acetate and glucose and the splanchnic uptake of O2 were also decreased by fasting. The net portal outputs of VFA, lactate and hydroxybutyrate, and the net hepatic output of glucose, were all correlated with ME intake in fed and fasted cows. Hepatic glucose output was also correlated with milk yield. The net hepatic uptake of gluconeogenic precursors measured in this study could account for net hepatic glucose output in the fasted cows, but not in the fed cows. The net hepatic uptake of the ketogenic precursors butyrate and NEFA was sufficient to account for the hepatic output of ketone bodies in both fed and fasted cows, but it is unlikely that the hepatic uptake of ketogenic precursors could also account for the observed hepatic output of acetate.

Metabolic limits to milk production, especially roles of growth hormone and insulin

Potential of the bovine mammary gland to synthesize milk probably is determined fully at parturition. Realization of that potential depends on the gland receiving adequate amounts of nutrients in the correct proportions. Provided the cow is fed adequately in relation to her potential yield, supply to the body of those nutrients that are absorbed directly from the gut is not likely to limit milk synthesis. Glucose and long-chain fatty acids, however, are not absorbed from the gut in adequate amounts, and metabolic factors (primarily hormonal regulation) determining the supply of these may be important limits to milk synthesis. Even when amounts and proportions of metabolites entering circulation are adequate for maintenance and maximum milk synthesis, the cow's potential cannot be realized fully unless nutrients are channeled selectively towards the mammary gland. This occurs naturally in the high-yielding cow in early lactation and probably can be maintained artificially into midlactation by use of growth hormone. Consequences of this on milk output, health, and reproductive performance are considered.

The effects of cold exposure of pregnant sheep on foetal plasma nutrients, hormones and birth weight

1. Five pregnant sheep, with indwelling catheters positioned for withdrawal of maternal right-atrial and foetal arterial blood, were exposed for 2 h to neutral and to cold (-10 and wool-clipped) environmental temperatures. Acute maternal cold exposure increased the concentrations of glucose, glycerol and non-esterified fatty acids in maternal plasma and increased the concentration of glucose in foetal plasma, but not glycerol or non-esterified fatty acids. The concentration of corticosteroids in maternal plasma increased. The concentration of corticosteroids in foetal plasma did not change but the concentration of insulin increased. 2. Thirteen pregnant sheep were housed at thermoneutrality and thirteen similar sheep, fed the same amount of food, were clipped and exposed to cold during the last 5-6 weeks of pregnancy. Chronic maternal cold exposure increased the birth weight of both single and twin lambs. 3. It is concluded that exposure of pregnant sheep to cold can alter the partition of some nutrients between mother and foetus in favour of the foetus, and it is suggested how this may be brought about.

Insulin secretion in sheep exposed to cold

1. Cold exposure caused a marked decrease in insulin response to intravenous injection of glucose, with a sharply declining response over the first 4 days of cold exposure followed by a constant low response up to 13 days of the experimental cold period. 2. The glucose-induced insulin response was unaffected by concomitant infusion of phentolamine in the warm environment. In contrast, the low response of insulin secretion to glucose during cold exposure was so augmented by concomitant infusion of phentolamine as to exceed the response observed in the warm environment. 3. Intravenous infusion of phentolamine caused an increase in the concentration of plasma insulin in the cold but not in the warm environment. 4. Adrenaline completely abolished the insulin response to glucose in the warm environment. 5. Exposure to cold environment brought about an increase in urinary excretion of adrenaline and noradrenaline and in heart rate, but rectal temperature was unchanged. 6. It is concluded that cold exposure insufficient to cause hypothermia produces a marked decrease in insulin secretion by the pancreas of sheep, mediated through adrenergic alpha-receptors stimulated by augmented sympatho-adrenomedullary activity.

Feeding in sheep during intraportal infusions of short-chain fatty acids and the effect of liver denervation

1. Castrated male sheep were prepared with cannulae in the hepatic portal vein and jugular vein through which infusions lasting for 3 hr were made. Animals had free access to a pelleted feed the weight of which was continuously recorded so that feeding behaviour could be studied.2. Infusion into the portal vein of a mixture of salts of short-chain fatty acids (acetate, propionate, butyrate: 55, 30, 15) caused a dose-dependent depression in food intake, feeding stopping completely with 4.0 m-mole/min of the mixture. Jugular infusion depressed intake slightly, compared with controls.3. Separate infusions of salts of the three acids showed that the effect of the mixture was due almost entirely to its propionate content; 1.2 m-mole/min of propionate into the portal vein almost completely prevented feeding (39 g eaten per 3 hr) compared with jugular infusion at the same rate (210 g) or no infusion (205 g).4. Surgical sectioning of the hepatic nerve plexus around the wall of the hepatic artery was attempted. Of seven animals which recovered normal food intake, three continued to eat during portal vein infusions of propionate at 1.2 m-mole/min; these sheep were subsequently shown to have been at least 95% denervated. One animal was 50% denervated and ate normally during some infusions but not others. In the remaining three, feeding was suppressed by portal vein infusion of propionate, and these were less than 75% denervated.5. It was concluded that there are receptors in the liver which are sensitive to propionate and which have afferent fibres in the hepatic plexus.