Regulation of peripheral lipogenesis by glucagon. Inability of the hormone to inhibit lipogenesis in rat mammary acini in vitro in the presence or absence of agents which alter its effects on adipocytes

Hormonal regulation of leucine catabolism in mammary epithelial cells

Branched-chain amino acids (BCAA) are actively taken up and catabolized by the mammary gland during lactation for syntheses of glutamate, glutamine and aspartate. Available evidence shows that the onset of lactation is associated with increases in circulating levels of cortisol, prolactin and glucagon, but decreases in insulin and growth hormone. This study determined the effects of physiological concentrations of these hormones on the catabolism of leucine (a representative BCAA) in bovine mammary epithelial cells. Cells were incubated at 37 °C for 2 h in Krebs buffer containing 3 mM D-glucose, 0.5 mM L-leucine, L-[1-14C]leucine or L-[U-14C]leucine, and 0-50 μU/mL insulin, 0-20 ng/mL growth hormone 0-200 ng/mL prolactin, 0-150 nM cortisol or 0-300 pg/mL glucagon. Increasing extracellular concentrations of insulin did not affect leucine transamination or oxidative decarboxylation, but decreased the rate of oxidation of leucine carbons 2-6. Elevated levels of growth hormone dose dependently inhibited leucine catabolism, α-ketoisocaproate (KIC) production and the syntheses of glutamate plus glutamine. In contrast, cortisol and glucagon increased leucine transamination, leucine oxidative decarboxylation, KIC production, the oxidation of leucine 2-6 carbons and the syntheses of glutamate plus glutamine. Prolactin did not affect leucine catabolism in the cells. The changes in leucine degradation were consistent with alterations in abundances of BCAA transaminase and phosphorylated levels of branched-chain α-ketoacid dehydrogenase. Reductions in insulin and growth hormone but increases in cortisol and glucagon with lactation act in concert to stimulate BCAA catabolism for glutamate and glutamine syntheses. These coordinated changes in hormones may facilitate milk production in lactating mammals.

Delineating potential control mechanisms of mammary protein synthesis utilizing atropine

Amino acids (AA) and/or somatotropin (ST) were infused into a jugular vein of lactating cows receiving atropine to determine the effect on milk protein secretion. Atropine decreased milk protein yield by 35%. Plasma alpha-amino nitrogen, which was decreased by 31% due to atropine, was restored to control levels upon AA infusion. Plasma ST concentration was not affected by atropine and, upon ST infusion, increased by 236% in treated animals. Amino acids, ST, or the combination of the two were unable to return milk protein to control levels in atropine-treated animals. Plasma insulin concentration decreased by 37% in atropine-treated animals and was not significantly different for cows receiving atropine as well as ST, AA, or both ST and AA. Similarly, glucagon decreased by 36% in atropine-treated animals but was partially restored in cows receiving AA. Overall, the insulin-to-glucagon ratio was not significantly affected by treatment. Plasma glucose concentration was not affected by treatment. These data lend support to the importance of the insulin-to-glucagon ratio in terms of whole-body metabolism, with the exception of the mammary glands, which are glucagon-insensitive and, therefore, sensitive to the observed decrease in circulating insulin concentration.

The role of pyruvate dehydrogenase, phosphofructo-1-kinase and acetyl-CoA carboxylase in the regulation of fatty acid synthesis in the lactating rat mammary gland during the starved to re-fed transition

Re-feeding 24-h-starved lactating rats resulted in a rapid (within 0.5 h) restoration of glucose uptake by the mammary gland and a slower (within 3 h) restoration of fatty acid synthesis. The rapid reactivation of glucose uptake (82% of fed value within 0.5 h of re-feeding) correlated with a rapid reactivation of 6-phosphofructo-1-kinase (6-PF-1-K) and glycolysis (as determined by a 97% decrease in the [fructose-6-phosphate]/[fructose-1,6-bisphosphate] ratio). This could not be fully explained by a fall (29%) in the tissue concentration of its allosteric inhibitor, citrate. The delayed reactivation of pyruvate dehydrogenase (PDH) correlated very closely with the delayed reactivation of fatty acid synthesis and explained the continued output of pyruvate and lactate within the first 0.5 h of re-feeding. PDH reactivation preceded the reactivation of acetyl-CoA carboxylase (ACC), which did not occur significantly until 1.5 h of re-feeding. ACC reactivation correlated with a decrease in the tissue concentration of citrate and a second late phase of 6-PF-1-K activation. It is clear that the important regulatory steps 6-PF-1-K, PDH and ACC, are reactivated asynchronously in the lactating mammary gland in response to re-feeding starved rats and that PDH is more important than ACC in the regulation of fatty acid synthesis.

Nutritional control of amino acid supply to the mammary gland during lactation in the pig

The paucity of data relating to lactation physiology of the sow has frustrated researchers in estimating nutrient needs for production and mammary maintenance functions. The nutritional control of amino acid supply for milk synthesis is influenced by factors that have yet to be measured, such as blood flow and amino acid contribution from the body protein pool. The interaction or role of hormones such as insulin, glucagon or prolactin in amino acid dynamics and inter-organ exchange during lactation in the sow, are not well understood. The discrepancy existing between milk and mammary amino acid uptake profiles relative to lysine may be indicative of mammary metabolism and possibly maintenance requirements for specific amino acids. Hence, amino acid metabolism in the mammary gland, regardless of arterial blood substrate supply, may play an important role in a factorial approach to determining requirements. Mammary amino acid uptake ratios rather than milk amino acid ratios should provide a better tool to estimate amino acid requirements relative to lysine. Although lysine has typically been limiting in maize-soyabean-meal-based diets fed to lactating sows, current production trends are bringing a new dimension to the formulation of lactating-sow diets. Other amino acids may become limiting if dietary crystalline lysine is added without concern for the whole essential amino acid profile. Formulations based on an ideal amino acid profile for the lactating sow will, therefore, become critical.

Effects of lactation on the regulation of hepatic metabolism in the rat and sheep: adrenergic receptors and cyclic AMP responses

The number and coupling efficiency of beta-adrenoceptors in liver membranes and intact hepatocytes of lactating and non-lactating female rats were compared to assess whether or not alterations in this signalling system could contribute towards the changed pattern of hepatic metabolism during lactation. In view of the different adaptations of hepatic metabolism to lactation in ruminants, the adrenergic receptor profile of sheep liver membranes was also determined. Post-receptor responses at two stages 'down-stream' of cyclic AMP generation were also evaluated in rat hepatocytes in response to the beta-adrenergic agonist isoprenaline. No changes in the number of affinity of hepatic beta-adrenoceptors were found in sheep or rats when lactating and non-lactating individuals were compared. Sheep liver was found to have a much greater concentration of beta-adrenoceptors than rat liver, and a much higher ratio of beta:alpha 1. The sensitivity and responsiveness of cyclic AMP generation in response to isoprenaline were similar in hepatocytes prepared from lactating and non-lactating rats, although the response to saturating concentrations of glucagon was diminished in hepatocytes from lactating rats. The activity ratio of cyclic AMP-dependent protein kinase (PK-A) also reacted similarly (in respect of both responsiveness and sensitivity) to isoprenaline in these two groups of hepatocytes. Contrastingly, the sensitivity of rat hepatocyte phosphorylase activity to beta-adrenergic stimulation was greatly diminished during lactation.

Regulation of peripheral glucagon concentrations in cyclic, pregnant, and lactating rats

In the rat, peripheral glucagon concentrations were studied throughout pregnancy and lactation. Basal glucose concentrations were decreased during late pregnancy and during lactation, but basal glucagon concentrations were not affected. Infusion of glucose (7.4 mg/min) caused an elevation of the glucose concentrations, which became lower in the course of lactation, and a suppression of the glucagon concentrations which was the same throughout pregnancy and lactation. Ingestion of 336 mg of glucose or 1 g of rat chow throughout pregnancy and lactation induced a transient increase of the glucose concentrations and a biphasic glucagon response: following a short-lasting elevation, the glucagon concentrations became suppressed. The glucagon responses to these tests did not change during pregnancy and lactation. It is concluded that the regulation of the peripheral glucagon concentration is not affected by pregnancy or lactation, and that the response of the glucagon concentration to a metabolic challenge varies with the kind of test (oral or intravenous) used.

Cholesterol ester hydrolysis and hormone-sensitive lipase in lactating rat mammary tissue

Neutral cholesterol esterase activity is expressed in extracts of mammary epithelial cells. The identity of the enzyme catalyzing this hydrolysis was investigated. Anti-hormone-sensitive lipase immunoglobulin elicited the total inhibition of this activity and also immunoprecipitated a single phosphoprotein of Mr 84 kDa from mammary cell extracts previously phosphorylated in vitro with [gamma-32P]ATP and cyclic AMP-dependent protein kinase. It is concluded that mammary cell cholesterol esterase activity results from the presence of hormone-sensitive lipase.

Cholesterol ester hydrolase activity in mammary tissue of the lactating rat

Cholesterol ester hydrolase activity has been studied in mammary glands of rats. Subcellular fractionation of the glands obtained in mid-lactation indicated that around 80% of the recovered activity was associated with particulate fractions. Two distinct cholesterol ester hydrolase activities were identified, one with an optimum pH of 7.5-9.0 and the second (approximately 5% of the total activity) with a more acidic pH optimum. Although the neutral cholesterol ester hydrolase had some properties in common with the lipoprotein lipase in mammary tissue, it was shown to be a separate entity by several criteria. Its activity could be increased following treatment with Mg-ATP and cAMP-dependent protein kinase, suggesting identity with the hormone sensitive lipase of adipose tissue. The cholesterol ester hydrolase activity in mammary glands just after parturition was greater than in glands obtained either from late-pregnant or midlactating animals. The subcellular distribution of the neutral cholesterol ester hydrolase suggested that it may have a different function to the neutral cholesterol ester hydrolase of adrenals and other tissues. Nevertheless the fact that the activity of the enzyme can be modulated by cAMP-dependent protein kinase suggests the possibility that hormonal control of this enzyme may be involved in the regulation of cholesterol metabolism in the mammary gland.

Amino acid metabolism and protein synthesis in lactating rats fed on a liquid diet

1. Amino acid metabolism was studied in control virgin rats, lactating rats and virgin rats protein-pair-fed with the lactating rats (high-protein virgin rats). 2. Urinary excretion of nitrogen and urea was higher in lactating than in control virgin rats, and in high-protein virgin rats it was higher than in lactating rats. 3. The activities of urea-cycle enzymes (units/g) were higher in high-protein virgin than in lactating rats, except for arginase. In lactating rats the activities of carbamoyl-phosphate synthase, ornithine carbamoyltransferase and argininosuccinate synthase were lower than in control virgin rats. When the liver size is considered, the activities in lactating rats were similar to those in high-protein virgin rats, except for arginase. 4. N-Acetylglutamate content was higher in high-protein virgin rats than in the other two groups. 5. The rate of urea synthesis from precursors by isolated hepatocytes was higher in high-protein virgin rats than in the other two groups. 6. The flooding-dose method (L-[4-3H]phenylalanine) for measuring protein synthesis was used. The absolute synthesis rates of mammary gland, liver and small-intestinal mucosa were higher in lactating rats than in the other two groups, and in high-protein virgin rats than in control virgin rats 7. These results show that the increased needs for amino acids during lactation are met by hyperphagia and by a nitrogen-sparing mechanism.

Reciprocal changes in amino acid metabolism in mammary gland and liver of the lactating rat on starvation and refeeding as indicated by the tissue accumulation of alpha-amino[1-14C]isobutyrate

Measurements of the tissue accumulation in vivo and in vitro by hepatocytes and mammary-gland acini of alpha-amino[1-14C]isobutyrate ([1-14C]AIB) were compared in virgin and lactating rats. The results indicate the existence of a reciprocal relationship between mammary gland and liver for AIB accumulation that is dependent on the lactational and the nutritional state of the rat. This suggests that amino acids are preferentially directed to the mammary gland during active lactation.

Cyclic AMP-dependent protein kinase in mammary tissue of the lactating rat. Activity ratio and responsiveness of the target enzymes acetyl-CoA carboxylase and glycogen phosphorylase to beta-adrenergic stimulation

The role of cyclic AMP in acute regulation of the metabolism of mammary tissue in the lactating rat was examined by measuring the activity ratio of cyclic AMP-dependent protein kinase (A-kinase) and by examining the properties of this enzyme in its two major isoenzymic forms. Isoenzyme II is the major form in soluble extracts of rat mammary tissue. A-kinase activity ratio in such extracts is unaffected by starvation of the lactating rat. Treatment of the intact rat with isoprenaline, or addition of isoprenaline to incubations in vitro of mammary acini, resulted in a major increase in the activity ratio of A-kinase. These treatments equally affected isoenzymes I and II. The treatment in vitro lead to a rapid depletion of A-kinase as subsequently measured in extracts of acini. The degree of activation of the enzymes acetyl-CoA carboxylase and glycogen phosphorylase in extracts of mammary tissue and of acini was assessed as a function of these treatments. The increased activation of A-kinase induced by isoprenaline was unaccompanied by significant changes in the activity of acetyl-CoA carboxylase in acini, although we previously showed that this agent activates acetyl-CoA carboxylase in intact mammary tissue. Contrastingly, isoprenaline-induced enhancement of A-kinase activity was accompanied by an increase in the activity ratio of phosphorylase in acini. These results indicate that: (a) a normal response of expressed A-kinase activity to cyclic AMP operates in mammary acini and mammary tissue from lactating rats; (b) rapid modulation of the total amount of soluble A-kinase is mediated in mammary epithelial cells by cyclic AMP; (c) phosphorylase, an ultimate target of the protein phosphorylation cascade initiated by A-kinase, is activated in acini under conditions where A-kinase activity is enhanced; and (d) mechanisms other than that of the A-kinase phosphorylation/inhibition model for acetyl-CoA carboxylase regulation must operate in mammary tissue preparations and in vivo to account for the response of this enzyme to enhanced A-kinase activity.

Glucose and insulin responses during mixed meals or infusion of glucose in pregnant and lactating rats

We studied the glucose tolerance in freely moving rats throughout pregnancy and lactation and during the first week after weaning. Dioestrous virgin rats served as controls. Basal glucose and insulin levels were determined after a 2-hr fasting period. Subsequently, the changes of the insulin and the glucose levels were determined during ingestion of a mixed ad lib meal or a 2 g mixed test meal, or during infusion of glucose (7.4 mg/min for 20 min) into the vena cava. Basal glucose levels were high during early pregnancy, low during late pregnancy, and in the normal range throughout lactation and after weaning. Basal insulin levels were decreased at the end of lactation. The results of the ad lib meal and test meal experiments were essentially the same. Glucose tolerance during meals was somewhat decreased early in pregnancy. The corresponding insulin responses greatly increased during the last week of pregnancy. Glucose tolerance during IV infusion of glucose was normal during pregnancy, but increased during lactation. Insulin responses to the infusion were increased during pregnancy and decreased during lactation. We concluded that glucose tolerance is hardly affected by pregnancy and even increases in the course of lactation. This is effected by an increased responsiveness of the B-cells to glucose during late pregnancy and by an increased turnover of glucose during lactation. We discuss to what extent the actions of progesterone, placental lactogen and prolactin may explain these adaptions of maternal metabolism.

Re-examination of the putative roles of insulin and prolactin in the regulation of lipid deposition and lipogenesis in vivo in mammary gland and white and brown adipose tissue of lactating rats and litter-removed rats

1. The effects of various treatments to alter either plasma prolactin (bromocryptine administration or removal of litter) or the metabolic activity of the mammary gland (unilateral or complete teat sealing) on the disposal of oral [14C]lipid between 14CO2 production and [14C]lipid accumulation in tissues of lactating rats were studied. In addition, the rates of lipogenesis in vivo were measured in mammary gland, brown and white adipose tissue and liver. 2. Bromocryptine administration lowered plasma prolactin, but did not alter [14C]lipid accumulation in mammary gland or in white and brown adipose tissue. 3. In contrast, complete sealing of teats results in no change in plasma prolactin, but a 90% decrease in [14C]lipid accumulation in mammary gland and a 4-fold increase in white and brown adipose tissue. The rate of lipogenesis in mammary gland was decreased by 95%, but there was no change in the rate in white and brown adipose tissue. Unilateral sealing of teats resulted in a decrease in [14C]lipid accumulation in white adipose tissue. 4. Removal of the litter for 24 h (low prolactin) produced a similar pattern to complete teat sealing, except that there was a 6-fold increase in lipogenesis in white adipose tissue. Re-suckling for 5 h increased plasma prolactin, but did not alter the response seen in litter-removed lactating rats. 5. Changes in lipoprotein lipase activity and in plasma insulin paralleled the reciprocal changes in [14C]lipid accumulation in white and brown adipose tissue and in mammary gland. 6. It is concluded that the plasma insulin is more important than prolactin in regulating lipid deposition in adipose tissue during lactation, and that any effects of prolactin must be indirect.

Changes in the sensitivity to glucagon of lipolysis in adipocytes from pregnant and lactating rats

1. Rates of lipolysis were measured at different concentrations of glucagon in adipocytes prepared from parametrial adipose tissue of fed or starved rats in different reproductive states. All experiments were performed in the presence of a high concentration of adenosine deaminase (1 unit/ml). 2. Maximal rates of lipolysis (elicited by 25 nM-glucagon in each instance) were higher in adipocytes from peak-lactating rats than those from pregnant animals in both the fed and starved states. 3. Of adipocytes from fed animals, those from peak-lactating rats were the most sensitive to glucagon, whereas those from late-pregnant and early-lactating rats were 1-2 orders of magnitude less sensitive. 4. Adipocytes from 24 h-starved rats showed a much smaller stimulation of lipolysis by glucagon, making the assessment of sensitivity difficult. Therefore, rates of lipolysis were also measured in the presence of a maximally anti-lipolytic dose of insulin. The presence of insulin did not alter the relative sensitivities to glucagon of adipocytes from fed animals in different reproductive states, although all dose-response curves were shifted to the right. When lipolysis in adipocytes from starved animals was measured in the presence of insulin, it became evident that starvation for 24 h markedly increased the sensitivity of adipocytes from late-pregnant rats to glucagon, but did not affect that of cells from animals in the other reproductive states. 5. It is concluded that the large changes in sensitivity to glucagon that occurred during the reproductive cycle may enable the modulation of adipose-tissue lipolysis in vivo to satisfy the different metabolic requirements of the animal in the transition from pregnancy to peak lactation.

An 'in situ' perfusion system suitable for investigating mammary-tissue metabolism in the lactating rat. Hormonal regulation of acetyl-CoA carboxylase

A technique is described for the non-recirculating perfusion of inguinal/abdominal mammary tissue in situ in anaesthetized lactating rats. Tissue viability was maintained, without resort to infusion of vasoactive chemicals which may also be effectors of cellular metabolism, for at least 90 min. Total tissue adenine nucleotides (per mg of DNA) were somewhat decreased in perfused relative to non-perfused mammary tissue. DNA content (per g wet wt. of tissue) was diminished after 90 min of perfusion to approx. 65% of its value in control tissue. Adenylate energy-charge ratios were lower in perfused tissue in the absence of hormones than in control tissue. They were increased to control values by the presence of either insulin or isoprenaline in the perfusate. No changes occurred in flow rate of the perfusate that might account for these increases. In mammary tissue perfused without addition of hormones, acetyl-CoA carboxylase activities were similar to those measured in control tissue samples, although activity-ratio measurements implied some increase in the phosphorylation of this enzyme. Insulin or isoprenaline increased the activity of acetyl-CoA carboxylase, especially when this was measured at low concentrations of citrate. Confirming conclusions from previous experiments with mammary acini and explant preparations, insulin activated acetyl-CoA carboxylase in mammary tissue, but inhibition of its activity was not mediated by cyclic AMP.

Alterations in the activities of rat tissue hexose monophosphate dehydrogenases in response to premature weaning and dietary restriction at mid-lactation

The activities of the hexose monophosphate dehydrogenases increased in adipose tissue, remained unchanged in liver and decreased in mammary gland following the weaning of rats at mid-lactation (day 14). When dietary intake was restricted at mid-lactation, the activities of the hexose monophosphate dehydrogenases increased in adipose tissue, decreased in liver, but were unaltered in mammary gland. Premature weaning on day 14 postpartum resulted in maternal increases in both plasma insulin and glucose, which peaked at day 16. The plasma insulin levels decreased from day 14 to day 18 postpartum in the normal lactating rat, and a similar trend was observed for animals on a restricted dietary intake. Daily food consumption in the lactating rat decreased from 50 g to 20 g after premature weaning. The live weight of pups raised on dams given a restricted food intake from day 14 had decreased by day 17 postpartum, whereas an increase in daily live weight gain was recorded for the litters from the lactating controls. The results demonstrate that the activities of the hexose monophosphate dehydrogenases are regulated differentially between tissues of the lactating rat.

Insulin activation of lipogenesis in isolated mammary acini from lactating rats fed on a high-fat diet. Evidence that acetyl-CoA carboxylase is a site of action

Feeding lactating rats on high-fat cheese crackers in addition to laboratory chow increased the dietary intake of fat from 2 to 20% of the total weight of food eaten and decreased mammary-gland lipogenesis in vivo by approx. 50%. This lipogenic inhibition was also observed in isolated mammary acini, where it was accompanied by decreased glucose uptake. These inhibitions were completely reversed by incubation with insulin. Insulin had no effect on the rate of glucose transport into acini, nor on pyruvate dehydrogenase activity as estimated by the accumulation of pyruvate and lactate, suggesting that these are not the sites of lipogenic inhibition. Insulin stimulated the incorporation of [1-14C]acetate into lipid in acini from high-fat-fed rats. In the presence of alpha-cyanohydroxycinnamate, a potent inhibitor of mitochondrial pyruvate transport, and with glucose as the sole substrate, neither [1-14C]glucose incorporation into lipid nor glucose uptake were stimulated by insulin. Insulin did stimulate the incorporation of [1-14C]acetate into lipid in the presence of alpha-cyanohydroxycinnamate, and this was accompanied by an increase in glucose uptake by the acini. This indicated that increased glucose uptake was secondary to the stimulation of lipogenesis by insulin, which therefore must occur via activation of a step in the pathway distal to mitochondrial pyruvate transport. Insulin stimulated acetyl-CoA carboxylase activity measured in crude extracts of acini from high-fat-fed rats, restoring it to values close to those of chow-fed controls. The effects of insulin on acetyl-CoA carboxylase activity and lipogenesis were not antagonized by adrenaline or dibutyryl cyclic AMP.

Protein phosphorylation in rat mammary acini and in cytosol preparations in vitro. Phosphorylation of acetyl-CoA carboxylase is unaffected by cyclic AMP

Phosphorylation of soluble proteins in rat mammary acinar cells was investigated. When phosphorylation proceeded in intact cells, in the presence of [32P]Pi, the major non-casein phosphoproteins, including acetyl-CoA carboxylase, were unresponsive to incubation conditions that caused major increases in the intracellular concentration of cyclic AMP. The overall 32P specific radioactivity (c.p.m./microgram of protein) of acetyl-CoA carboxylase, assessed after affinity purification of the enzyme with avidin-Sepharose, was unchanged by incubation under such conditions. Furthermore, the distribution of 32P among tryptic phosphopeptides of the enzyme, resolved by reversed-phase h.p.l.c., was not altered by cyclic AMP-increasing treatments of the acinar cells. When cytosol fractions were incubated with [gamma-32P]ATP, some phosphoproteins responded to the addition of micromolar concentrations of dibutyryl cyclic AMP or cyclic AMP by undergoing an enhancement of phosphate incorporation. In these experiments in vitro, protein phosphatase activity did not make a major contribution to the net phosphorylation of individual phosphoproteins, and acetyl-CoA carboxylase was not prominent among the phosphoproteins identified after short (less than 1 min) incubations of cytosols with [gamma-32P]ATP. The resistance of protein phosphorylation to variations in the cyclic AMP concentration in intact mammary epithelial cells, demonstrated by this work, is one of several mechanisms that ensure the pleiotropic refractoriness of those cells to agents which normally cause a stimulation of adenylate cyclase activity in hormone-sensitive cells.

Modulation of intracellular cyclic AMP content and rate of lipogenesis in mammary acini in vitro

Relationships between the cyclic AMP content, the rate of lipogenesis and the activity of acetyl-CoA carboxylase in acini prepared from lactating rat mammary tissue were investigated by exposing them to agents that increase their cyclic AMP content in the presence or absence of insulin. The dose-dependent inhibition of lipogenesis by theophylline in acini isolated from fed rats was highly correlated with the induced increases in acinar cyclic AMP content. Cyclic AMP of acini from 24 h-starved lactating rats was more sensitive in its response to theophylline than that in acini from fed animals. Neither forskolin nor a mixture of isoprenaline and Ro 7-2956 were able significantly to change either the rate of lipogenesis or the activity of acetyl-CoA carboxylase in acini from fed rats when added to incubations in vitro, in spite of the large increases in cyclic AMP concentration produced by these agents. Insulin was without effect on the activity of acetyl-CoA carboxylase and on either the basal or isoprenaline-stimulated cyclic AMP content of acini. These results are discussed in terms of the possibility that the rate of lipogenesis and the cyclic AMP content in mammary acini can vary independently of one another and of the activity of acetyl-CoA carboxylase.

The role of acetyl-CoA carboxylase phosphorylation in the control of mammary gland fatty acid synthesis during the starvation and re-feeding of lactating rats

Activation of acetyl-CoA carboxylase during incubation of crude extracts of lactating rat mammary gland with Mg2+ and citrate can be blocked by NaF, suggesting that it represents a dephosphorylation of the enzyme. The greater extent of activation in extracts from 24 h-starved rats (200%) compared with fed controls (70%) implies that the decrease in acetyl-CoA carboxylase activity in response to 24 h starvation may involve increased phosphorylation of the enzyme. Acetyl-CoA carboxylase was purified from the mammary glands of lactating rats in the presence of protein phosphatase inhibitors by avidin-Sepharose chromatography. Starvation of the rats for 24 h increased the concentration of citrate giving half-maximal activation by 75%, and decreased the Vmax. of the purified enzyme by 73%. This was associated with an increase in the alkali-labile phosphate content from 3.3 +/- 0.2 to 4.5 +/- 0.4 mol/mol of enzyme subunit. Starvation of lactating rats for 6 h, or short-term insulin deficiency induced by streptozotocin injection, did not effect the kinetic parameters or the phosphate content of acetyl-CoA carboxylase purified from mammary glands. The effects of 24 h starvation on the kinetic parameters and phosphate content of the purified enzyme were completely reversed by re-feeding for only 2.5 h. This effect was blocked if the animals were injected with streptozotocin before re-feeding, suggesting that the increase in plasma insulin that occurs on re-feeding was responsible for the activation of the enzyme. The effects of re-feeding 24 h-starved rats on the kinetic parameters and phosphate content of acetyl-CoA carboxylase could be mimicked by treating enzyme purified from 24 h-starved rats with protein phosphatase-2A in vitro. Our results suggest that, in mammary glands of 24 h-starved lactating rats, insulin brings about a dephosphorylation of acetyl-CoA carboxylase in vivo, which may be at least partly responsible for the reactivation of mammary lipogenesis in response to re-feeding.

Acute change in the cyclic AMP content of rat mammary acini in vitro. Influence of physiological and pharmacological agents

The cyclic AMP content of acini, freshly prepared from mammary tissue of lactating rats, was measured during incubation in vitro. Neither adrenergic agonists nor cyclic AMP phosphodiesterase inhibitors alone caused a change of more than 2-fold in the basal cyclic AMP content of acini. Together, however, these agents provoked increases of around 20-fold in acini cyclic AMP content. Forskolin caused similar effects. The relative potency of adrenergic agonists in increasing cyclic AMP in acini, together with the ability of selective antagonists to oppose such rises, indicated that beta 2-adrenergic receptors were involved in mediating the effects. Receptor-binding experiments using [3H]dihydroalprenolol and selective beta-antagonists confirmed the predominant presence of beta 2-adrenergic receptors on acini membranes and on membranes prepared from purified mammary secretory epithelial cells. These results elucidate some previous findings [Robson, Clegg & Zammit (1984) Biochem. J. 217, 743-749; Williamson, Munday, Jones, Roberts & Ramsey (1983) Adv. Enzyme Regul. 21, 135-145], questioning the role of cyclic AMP in the regulation of lipogenesis in mammary acini.

Evidence that glucagon-mediated inhibition of acetyl-CoA carboxylase in isolated adipocytes involves increased phosphorylation of the enzyme by cyclic AMP-dependent protein kinase

The kinetic parameters and phosphorylation state of acetyl-CoA carboxylase were analysed after purification of the enzyme by avidin--Sepharose chromatography from extracts of isolated adipocytes treated with glucagon or adrenaline. The results provide evidence that the mechanism of inhibition of acetyl-CoA carboxylase in adipocytes treated with glucagon [Zammit & Corstorphine (1982) Biochem. J. 208, 783-788] involves increased phosphorylation of the enzyme. Hormone treatment had effects on the kinetic parameters of the enzyme similar to those of phosphorylation of the enzyme in vitro by cyclic AMP-dependent protein kinase. Glucagon treatment of adipocytes led to increased phosphorylation of acetyl-CoA carboxylase in the same chymotryptic peptide as that containing the major site phosphorylated on the enzyme by purified cyclic AMP-dependent protein kinase in vitro [Munday & Hardie (1984) Eur. J. Biochem. 141, 617-627]. The dose--response curves for inhibition of enzyme activity and increased phosphorylation of the enzyme were very similar, with half-maximal effects occurring at concentrations of glucagon (0.5-1 nM) which are close to the physiological range. In general, the patterns of increased 32P-labelling of chymotryptic peptides induced by glucagon or adrenaline were similar, although there were quantitative differences between the effects of the two hormones on individual peptides. The results are discussed in terms of the possible roles of cyclic AMP-dependent and -independent protein kinases in the regulation of acetyl-CoA carboxylase activity and of lipogenesis in white adipose tissue.

Short-term regulation of acetyl CoA carboxylase: is the key enzyme in long-chain fatty acid synthesis regulated by an existing physiological mechanism?

Acetyl CoA carboxylase, the rate-limiting enzyme in regulating fatty acid synthesis, is thought to be controlled by allosteric effectors, its state of aggregation, covalent modulation and protein inhibitors. It is still obscure whether citrate, a positive allosteric effector, and long-chain fatty acyl CoA esters, negative allosteric effectors, function physiologically to regulate acetyl CoA carboxylase activity. New evidence from several laboratories reveals that the covalent phosphorylation may not involve regulation of acetyl CoA carboxylase activity. Protein inhibitors from liver cytosol and a peptide from fat cells were found to regulate acetyl CoA carboxylase both in vivo and in vitro. Coenzyme A, guanosine 5-monophosphate and phosphatidylinositol 4,5-bisphosphate may have an indirect effect, but certainly no direct involvement, on carboxylase activity.

Regulation of lactating-rat mammary-gland lipogenesis by insulin and glucagon in vivo. The role and site of action of insulin in the transition to the starved state

Starvation for 6h and 24h caused an 80% and 95% decrease in the rate of mammary-gland lipogenesis respectively in conscious lactating rats. 2. Plasma insulin concentrations decreased and circulating ketone-body concentrations increased with the length of starvation. 3. The inhibition of lipogenesis after 24h starvation was accompanied by increased concentrations of glucose, glucose 6-phosphate and citrate in the mammary gland. Qualitatively similar changes were observed after 6h starvation. 4. Infusion of insulin at physiological concentrations caused a 100% increase in the rate of lipogenesis in fed animals and partially reversed the inhibition of lipogenesis caused by starvation. 5. Infusion of insulin tended to reverse the changes seen in intracellular metabolite concentrations. 4. Infusion of glucagon into fed rats caused no change in the rates of lipogenesis in mammary gland, liver or white adipose tissue. 7. It is concluded that (a) insulin acts physiologically to regulate lipogenesis in the mammary gland, (b) hexokinase and phosphofructokinase are important regulatory enzymes in the short-term control of lipogenesis in the mammary gland, which are under the influence of insulin, and (c) the unresponsiveness of mammary-gland lipogenesis in vivo to infusions of glucagon is consistent with an adaptive mechanism which diverts substrate towards the lactating mammary gland and away from other tissues.

Physiological significance of altered insulin metabolism in the conscious rat during lactation

Uptake of radioactively labelled insulin by the mammary gland of the rat increased 12-fold in lactation compared with non-lactating controls. This uptake was decreased by the presence of unlabelled insulin, indicating that it occurred via insulin receptors. The plasma half-life of insulin is decreased in lactation from 9.4 min to 4.8 min, and the metabolic clearance rate for insulin increased from 7.26 to 13.03 ml/kg body wt. per min. The basal insulin and glucose concentrations in the plasma were decreased in lactation. Infusion of insulin at a dose which led to a small physiological rise in plasma insulin concentration increased lipogenic rates in the mammary gland by 100% without causing marked hypoglycaemia. It is concluded that the lactating mammary gland is a highly insulin-sensitive tissue and that the lower plasma insulin during lactation occurs primarily as a result of this sensitivity increasing extraction of glucose by the gland and thus producing a decrease in the plasma glucose concentration. It is suggested that a secondary result of the fall in plasma insulin concentration is the preferential direction of substrates (glucose and non-esterified fatty acids) towards the lactating mammary gland and away from adipose tissue and the liver.

Cyclic AMP phosphodiesterase and cyclic GMP phosphodiesterase activities of rat mammary tissue

Cyclic nucleotide phosphodiesterase activity in mammary tissue from rats in midlactation was resolved by DEAE-cellulose chromatography into three functionally distinct fractions: a Ca2+/calmodulin-stimulated cyclic GMP phosphodiesterase, a cyclic GMP-stimulated low-affinity cyclic nucleotide phosphodiesterase, and a high-affinity cyclic AMP-specific phosphodiesterase. The absolute activities and relative proportions of high- and low-affinity enzymes resemble those found, for example, in liver, as distinct from those in excitable tissues. Three functional characteristics are described which are peculiar to mammary-tissue phosphodiesterases. Firstly, the concentration of free Ca2+ required to achieve half-maximal activation of the Ca2+/calmodulin-stimulated phosphodiesterase is somewhat higher than for the analogous enzyme in other tissues; secondly, the activity of this enzyme towards cyclic AMP relative to that towards cyclic GMP is unusually low, and thirdly, the low-affinity cyclic nucleotide phosphodiesterase is inhibited by low concentrations of free Ca2+.

Insulin-stimulated high affinity cyclic AMP phosphodiesterase in rat mammary acini

High affinity cyclic AMP phosphodiesterase activity in preparations of acini isolated from mammary tissue of lactating rats is shown to be stimulated by the addition of physiological concentrations of insulin to incubations of acini in vitro. This effect is expressed specifically on membrane-associated phosphodiesterase and occurs in the absence of concurrent protein synthesis. The possible functional role of this aspect of insulin's action on mammary tissue is discussed and compared with the well-known reversal by this hormone of the effects of lipolytic agents in adipose tissue and liver.