Effects of acetoacetate administration on glucose metabolism in mammary gland of fed lactating rats

Comparison of the flux of carbon to hepatic glycogen deposition and fatty acid and cholesterol synthesis on refeeding rats fed ad libitum or meal-fed rats with a chow-diet meal

Meal-fed rats and rats fed ad libitum had similar rates of hepatic glycogen deposition on refeeding with a chow meal. In contrast, the rate of hepatic lipid synthesis (cholesterol plus fatty acids) was 6-fold higher on refeeding in the meal-fed group compared with the 'ad libitum' group. There were no significant differences in the gastrointestinal or hepatic contents of glucose or lactate between the two groups. It is suggested that in the meal-fed group exogenous glucose may be directly converted into glycogen, whereas the substrate for lipid synthesis is a C3 unit.

Subclinical ketosis in dairy cows

Subclinical ketosis is defined as a preclinical stage of ketosis. The peak prevalence of subclinical ketosis occurs during the fourth week of lactation. Herd-related factors, breed, parity, and season are other important determinants. Subclinical ketosis can be revealed by determining levels of plasma glucose, plasma NEFA and blood, and milk or urine ketone body concentration. There are theoretical and practical advantages of using milk ketone bodies. Most authors are agreed on approximate lower and upper borderlines for subclinical ketosis. The risk of an outbreak of clinical symptoms has been evaluated by some authors. Most authors have found significant negative relationships between energy balance and ketone body concentration. Some disagreement may be attributable to the fact that the diets used in different experiments can have different glucogenic potential, even if the energy content is the same. This affects the relationship between energy balance and ketone body concentration, as the ketone body level is influenced by both the energy balance and plasma glucose. Feeding silage with high butyric acid content increases the risk of subclinical ketosis. There are indications that cows with the highest milk yield directly after calving are at greatest risk for developing ketosis. Increased ketone body level secondarily reduces milk production, a decrease that has been quantified by some authors. Subclinical ketosis causes delayed reproductive functions return to normal after calving, increased intervals from calving to first and last service, and an increased frequency of ovarian cysts. The routine determination of milk acetone levels in control programs can be used to evaluate the status of individual cows, to indicate the energy feeding in early lactation at a herd level, and to evaluate sires for breeding. The heritability and the tendency toward a positive genetic correlation between milk acetone and milk yield have also been discussed, as have aspects of nutritional prevention. Factors such as energy- and protein-rich roughage, tasty high-energy concentrates, suitable feeding during the dry period, and division of the concentrates into at least four meals are considered to be important.

Rapid inhibition by intragastric triolein of the re-activation of glucose utilization and lipogenesis in the mammary gland during the starved-refed transition in lactating rats. Evidence for a direct effect of oral lipid on mammary tissue

1. Oral administration of triacylglycerol (triolein) to starved/chow-refed lactating rats suppressed the lipogenic switch-on in the mammary gland in vivo. 2. A time-course study revealed that triolein, administered at 30 min after the onset of refeeding, had no influence on lipogenic rate in the mammary gland between 30 and 60 min, but markedly decreased it between 60 and 90 min. Glucose uptake by the mammary gland (arteriovenous difference) increased by 30 min of refeeding, as did lactate production. Between 30 and 90 min glucose uptake remained high in the control animals, but glucose uptake and net C3-unit uptake were decreased in the triolein-loaded animals by 90 min. 3. Triolein increased [glucose 6-phosphate] in the gland and simultaneously decreased [fructose 1,6-bisphosphate], indicative of a decrease in phosphofructokinase activity. This cross-over occurred at 60 min, i.e. immediately before the inhibition of lipogenesis, and by 90 min had reached 'starved' values. 4. Triolein had no effect on plasma [insulin] nor on whole-blood [glucose], [lactate] or [3-hydroxybutyrate]; a small increase in [acetoacetate] was observed. 5. Infusion of the lipoprotein lipase inhibitor, Triton WR1339, abolished the suppression of mammary-gland lipogenesis by triolein and the increase in the [glucose 6-phosphate]/[fructose 1,6-bisphosphate] ratio, suggesting a direct influence of dietary lipid on mammary-gland glucose utilization and phosphofructokinase activity.

Effects of starvation and exercise on concentrations of citrate, hexose phosphates and glycogen in skeletal muscle and heart. Evidence for selective operation of the glucose-fatty acid cycle

Concentrations of citrate, hexose phosphates and glycogen were measured in skeletal muscle and heart under conditions in which plasma non-esterified fatty acids and ketone bodies were physiologically increased. The aim was to determine under what conditions the glucose-fatty acid cycle might operative in skeletal muscle in vivo. In keeping with the findings of others, starvation increased the concentrations of glycogen, citrate and the fructose 6-phosphate/fructose 1,6-bisphosphate ratio in heart, indicating that the cycle was operative. In contrast, it decreased glycogen and had no effect on the concentration of citrate or the fructose 6-phosphate/fructose 1,6-bisphosphate ratio in the soleus, a slow-twitch red muscle in which the glucose-fatty acid cycle has been demonstrated in vitro. In fed rats, exercise of moderate intensity caused glycogen depletion in the soleus and red portion of gastrocnemius muscle, but not in heart. In starved rats the same exercise had no effect on the already diminished glycogen contents in skeletal muscle, but it decreased cardiac glycogen by 25-30%. After exercise, citrate and the fructose 6-phosphate/fructose 1,6-bisphosphate ratio were increased in the soleus of the starved rat. Significant changes were not observed in fed rats. The data suggest that in the resting state the glucose-fatty acid cycle operates in the heart, but not in the soleus muscle, of a starved rat. In contrast, the metabolite profile in the soleus was consistent with activation of the glucose-fatty acid cycle in the starved rat during the recovery period after exercise. Whether the cycle operates during exercise itself is unclear.

Alterations in mammary-gland blood flow and glucose metabolism in the lactating rat induced by short-term starvation and refeeding

Six-hour starvation of lactating rats caused a 75% decrease in mammary-gland lipogenesis. The inhibition of lipogenesis was accompanied by a 45% decrease of blood flow to the mammary gland and a 60% decrease in glucose uptake. Within 2 h of refeeding, lipogenesis and glucose uptake by the gland increased to fed values though blood flow to the gland remained at only 63% of that in the fed state. It is concluded that blood flow, and hence substrate supply, is not a rate-limiting factor for lipogenesis by the lactating mammary gland in the rat.

Monosaccharide transport in the mammary gland of the intact lactating rat

The Michaelis-Menten equation for the utilization of competing substrates was applied to the uptake of 2-deoxy[3H]glucose into the mammary gland of anaesthetized lactating rats. Intracellular water was calculated from total tissue water and sucrose space. Fed rats had a mean transport capacity of 2.2 mumol/min per g of tissue, giving an actual glucose transport in vivo of 1.1 mumol/min per g. Transport decreased by 90% on overnight starvation and returned to normal by 2 h of re-feeding. Similar changes were observed in the 1 min or 5 min transport of circulating 3-O-methylglucose. Transport of 3-O-methylglucose in starved rats was restored towards normal by insulin. In fed rats it increased between parturition and day 12 of lactation. The findings support the proposal that transport is a rate-limiting factor in the mammary utilization of carbohydrate.

Effect of starvation and refeeding on amino acid uptake by mammary gland of the lactating rat. Role of ketone bodies

Arteriovenous differences of amino acids across the mammary glands of lactating rats are diminished when the rats are starved for 24 h. When 24 h-starved rats were refed for 2 1/2 h, the arteriovenous differences of amino acids returned to values similar to those found in well-fed rats. In order to find a possible explanation for these rapid changes, we tested the effect of ketone bodies on amino acid uptake by the gland. At 5 min after injection of acetoacetate to fed rats, when the total concentration of ketone bodies in blood was similar to that found in starvation, the uptake of amino acids by the mammary gland was similar to that found after starvation, i.e. lower than in fed rats. However, 30 min after administration of acetoacetate, when the arterial concentration of ketone bodies had returned to values similar to those in fed rats, the arteriovenous differences of amino acids were similar to those found in fed rats. We conclude that the changes in blood ketone bodies may be responsible, at least in part, for the changes in amino acid uptake that occur in starvation and in the starvation--refeeding transition.

Short-term dietary regulation of lipogenesis in the lactating mammary gland of the rat

Short-term (6 hr) withdrawal of chow diet from lactating rats decreases the rate of lipogenesis in mammary gland by 87%. This inhibition is in part explained by a 60% decrease in the extraction of glucose (the major lipogenic precursor) by the mammary tissue. These changes are not accompanied by any significant alteration in the arterial concentrations of glucose, lactate or insulin; the concentration of acetoacetate did increase by about 30%. Removal of food for 6 hr did not alter the activation state of acetyl-CoA carboxylase or the total activity of the enzyme. Glucose utilization by mammary gland acini from short-term starved rats was not depressed although a higher proportion of the glucose appeared as lactate in the medium and consequently less glucose was converted to lipid. Insulin was able to reverse these changes. Glucagon, adrenaline or cAMP did not inhibit glucose utilization or lipogenesis in isolated acini. It is concluded that the inhibition of lipogenesis in mammary gland after short-term withdrawal of food is mainly due to decreased extraction of glucose. The signal for this change does not appear to be an alteration in plasma insulin and it is postulated that there may be an intestinal factor(s) which acts synergistically with insulin.

Primary ketosis in the high-producing dairy cow: clinical and subclinical disorders, treatment, prevention, and outlook

Bovine ketosis typically occurs in early lactation. Clinical signs include diminished appetite, decreased milk production, loss of weight, hypoglycemia, and hyperketonemia. Susceptibility to ketosis is probably due to the combination of appetite limitation and a high degree of precedence given to the demand of the mammary gland for nutrients, in particular glucose. The precipitating cause is likely to be development of a marked imbalance between glucose supply and glucose requirement. This imbalance then leads to decreased carbohydrate status, decreased insulin secretion, increased fat mobilization, and increased hepatic ketogenesis. Hepatic ketogenesis may be augmented by the diminished carbohydrate status. The role of hormones other than insulin in the etiology of ketosis, although probably important, has not yet been elucidated satisfactorily. Treatment of ketosis involves increasing glucose supply relative to glucose demand. Incidence of clinical ketosis can be minimized by correct nutrition and management as outlined in recommended guidelines. Besides decreasing milk field, clinical ketosis may affect productivity adversely in other ways, for example, by impairing fertility. Subclinical ketosis is important because it may remain undetected and yet have effects on productivity which parallel those elicited by clinical ketosis. Future research should be directed toward understanding mechanisms conferring priority on milk production and regulating appetite.

Metabolic significance of milk glucose

The free glucose concentration in the aqueous phase of samples of goat, sheep, cow, rat and rabbit milk was about 0.1-0.3 mM, while that in human milk was about 2mM. During starvation the glucose concentration of goat milk fell considerably (by about 80% in 2 d) in parallel with the decreased rate of lactose production. With rats fed ad lib., glucose concentration in the milk was greater at 12.00 h than at 18.00 h, when lactose synthesis has been shown to decrease. 3-O-Methyl-D-glucose injected into the goat mammary gland via the teat canal specifically entered the blood. These findings support the idea that glucose equilibrates across the apical membrane of mammary secretory cells, so that milk concentrations reflect intracellular glucose concentratioins.

Lactose synthesis in the rat, and the effects of litter size and malnutrition

1. The rate of lactose synthesis per g of mammary tissue, measured in vivo by a radioisotopic technique, rose 13-fold between parturition and day 16 of lactation in the rat, but was unaffected by wide variation in litter size. 2. The increase reflected a greater tissue content of galactosyltransferase (EC 2.4.1.22), and was augmented by a rise in the total weight of mammary tissue. Superimposed on this were unpredictable changes in the functional efficiency of the enzyme. 3. Lactose synthesis in 14-day-lactating rats, permitted only 76% of the food intake of paired control rats over the previous 3 weeks, showed a pronounced diurnal variation at an overall rate markedly below that in control rats. 4. Such nutritional deficiency did not affect the tissue content of galactosyltransferase, but impaired its functional efficiency in a manner reversed by renewed feeding or by the preparation and incubation of acini in vitro. 5. Plasma insulin concentrations decreased at parturition and with increasing litter size, and remained relatively unchanged during lactation and malnutrition.

Utlization of D-3-hydroxy[3-14C]butyrate for lipogenesis in vivo in lactating rat mammary gland

Incorporation of D-3-hydroxy[3-14C]butyrate into lipid in vivo suggests that lactating mammary gland is a major site of ketone-body utilization. The incorporation decreases in short-term insulin deficiency (2h) and on starvation (24h), but increases again on refeeding (2h). The activity of cytosolic acetoacetyl-CoA synthetase parallels the changes in nutritional state, but is not affected by short-term insulin deficiency.

Diurnal variation and response to food withdrawal of lactose synthesis in lactating rats

1. The incorporation of radiolabelled plasma glucose into mammary lactose was used to measure the rate of lactose synthesis in lightly anaesthetized lactating rats. 2. Lactose synthesis showed a diurnal variation with a minimum at 18:00h 3. Food withdrawal for 6h did not affect lactose synthesis in the early morning but greatly decreased it in the afternoon or evening. 4. Plasma glucose, milk sugars and total galactosyltransferase activity (EC 2.4.1.22) did not show the above changes. 5. Measurements of plasma insulin, which varies diurnally, and experiments with injected insulin suggested that variations of insulin within the physiological range do not account for the changes in lactose synthesis described.

Control of glucose metabolism in isolated acini of the lactating mamary gland of the rat. Effects of oleate on glucose utilization and lipogenesis

Oleate (1mM) had only small inhibitory effects on glucose utilization and lipogenesis in acini isolated from rat mammary gland. Esterification of [1-14C]oleate was unaffected by insulin but were decreased by 60% by acetoacetate (2mM). Glycerol (1mM), but not insulin, relieved this inhibition. These experiments provide further support for the role of acetoacetate in regulating substrate utilization by the gland.

Control of glucose metabolism in isolated acini of the lactating mammary gland of the rat. The ability of glycerol to mimic some of the effects of insulin

Inhibition of glucose uptake by acetoacetate and relief of this inhibition by insulin found previously in slices of rat mammary gland [Williamson, McKeown & Ilic (1975) Biochem. J. 150. 145-152] was confirmed in acini, which represent a more homogeneous population of cells. Glycerol (1mM) behaved like insulin (50 minuits/ml) in its ability to relieve the inhibition of glucose (5 mM) utilization caused by acetoacetate (2 mM) in acini. Both glycerol and insulin reversed the increase in [citrate] and the decrease in [glycerol 3-phosphate] and the [lactate]/[pyruvate] ratio in the presence of acetoacetate. Lipogenesis from 3H2O, [3-14C] acetoacetate, [1-14C]- and [6-14C]-glucose was stimulated, whereas 14CO2 formation from [3-14C]acetoacetate was decreased. Neither insulin nor glycerol relieved the acetoacetate inhibition of glucose uptake when lipogenesis was inhibited by 5-(tetradecyloxy)-2-furoic acid. From measurements of [3-14C]acetoacetate incorporation into lipid in the various situations it is suggested that a cytosolic pathway for acetoacetate utilization may exist in rat mammary gland. In the absence of acetoacetate, glycerol inhibited glucose utilization by 60% and increased both [glycerol 3-phosphate] and the [lactate/[pyruvate] ratio. Possible ways in which glycerol may mimic the effects of insulin are discussed.