Mechanism of adrenergic stimulation of hepatic ketogenesis

Current Developments on the Role of α1-Adrenergic Receptors in Cognition, Cardioprotection, and Metabolism

The α₁-adrenergic receptors (ARs) are G-protein coupled receptors that bind the endogenous catecholamines, norepinephrine, and epinephrine. They play a key role in the regulation of the sympathetic nervous system along with β and α₂-AR family members. While all of the adrenergic receptors bind with similar affinity to the catecholamines, they can regulate different physiologies and pathophysiologies in the body because they couple to different G-proteins and signal transduction pathways, commonly in opposition to one another. While α₁-AR subtypes (α_1A, α_1B, α_1C) have long been known to be primary regulators of vascular smooth muscle contraction, blood pressure, and cardiac hypertrophy, their role in neurotransmission, improving cognition, protecting the heart during ischemia and failure, and regulating whole body and organ metabolism are not well known and are more recent developments. These advancements have been made possible through the development of transgenic and knockout mouse models and more selective ligands to advance their research. Here, we will review the recent literature to provide new insights into these physiological functions and possible use as a therapeutic target.

The role of α1-adrenergic receptors in regulating metabolism: increased glucose tolerance, leptin secretion and lipid oxidation

The role of α₁-adrenergic receptors (α₁-ARs) and their subtypes in metabolism is not well known. Most previous studies were performed before the advent of transgenic mouse models and utilized transformed cell lines and poorly selective antagonists. We have now studied the metabolic regulation of the α_1A- and α_1B-AR subtypes in vivo using knock-out (KO) and transgenic mice that express a constitutively active mutant (CAM) form of the receptor, assessing subtype-selective functions. CAM mice increased glucose tolerance while KO mice display impaired glucose tolerance. CAM mice increased while KO decreased glucose uptake into white fat tissue and skeletal muscle with the CAM α_1A-AR showing selective glucose uptake into the heart. Using indirect calorimetry, both CAM mice demonstrated increased whole body fatty acid oxidation, while KO mice preferentially oxidized carbohydrate. CAM α_1A-AR mice displayed significantly decreased fasting plasma triglycerides and glucose levels while α_1A-AR KO displayed increased levels of triglycerides and glucose. Both CAM mice displayed increased plasma levels of leptin while KO mice decreased leptin levels. Most metabolic effects were more efficacious with the α_1A-AR subtype. Our results suggest that stimulation of α₁-ARs results in a favorable metabolic profile of increased glucose tolerance, cardiac glucose uptake, leptin secretion and increased whole body lipid metabolism that may contribute to its previously recognized cardioprotective and neuroprotective benefits.

Suppression of VLDL associated triacylglycerol secretion by both alpha- and beta-adrenoceptor agonists in isolated rat hepatocytes

The signal transduction pathways of intracellular calcium and adenosin 3',5'-cyclic monophosphate (cAMP) participate in the regulation of intrahepatic metabolism of very low density lipoproteins (VLDL). The adrenoceptors are linked to calcium and cAMP signal transduction pathways so it is proposed that they may be involved in the regulation of VLDL secretion. The current study is designed to test the effects of alpha- and beta-adrenoceptor agonists and antagonists on triacylglycerol secretion in freshly isolated rat hepatocytes. The inhibitory effect of epinephrine appeared at concentrations of more than 1 microM and reached a plateau at 100 microM. Epinephrine concentration for the half of the maximal bio-effect (EC(50)) was about 10 microM. Epinephrine at a concentration of 10 microM suppressed the secretion of triacylglycerol by 33% (P<or=0.01) and increased cellular content of triacylglycerol (18%, P<or=0.05) and total phospholipids (20%, P<or=0.05). Time course experiments for triacylglycerol secretion exhibited a linear relationship with a slope of 8.2+/-0.6 mug triacylglycerol/3 h mg cell protein. In the presence of epinephrine, cellular triacylglycerol and total phospholipids were slightly but significantly higher than the respective control at all points of time examined. The inhibitory effect elicited by epinephrine (10 microM) was abolished by the inclusion of the general alpha-adrenoceptor antagonist phentolamine (10 microM) and the specific alpha(1)-antagonist prazosin (1 microM) but not with the nonselective beta-antagonist propranolol (10 microM). Trifluoperazine an alpha-adrenoceptor antagonist and anticalmodulin agent, concealed the inhibitory effect of epinephrine in a concentration dependent manner, whereas theobromine a cAMP-phosphodiestrase inhibitor did not have any significant effect. The secretion of triacylglycerol was decreased not only by the alpha-adrenoceptor agonist phenylephrine (10 microM) but also by the beta-agonist isoproterenol (10 microM). Dibutyryl-cAMP (0.1 mM) also inhibited the secretion of triacylglycerol by 30% (P<or=0.01). The results suggest that epinephrine inhibits the secretion of triacylglycerol from rat hepatocytes via the alpha(1)-adrenoceptor while stimulation of beta- as well as alpha-adrenoceptors can also exert a similar effect.

Calmodulin antagonist W-7 inhibits de novo synthesis of cholesterol and suppresses secretion of de novo synthesized and preformed lipids from cultured hepatocytes

The effects of a calmodulin antagonist W-7 were studied on the synthesis and secretion of lipids in primary rat hepatocytes and McArdle-RH7777 cells. In time course experiments, W-7 (20 microM) inhibited secretion of newly synthesized triacyl[(3)H]glycerol by 35%. When the cells were pre-treated overnight with W-7 (20 microM), followed by incubation with [(3)H]oleate, a significant decrease in the secretion of triacylglycerol (TG) and cholesteryl ester (CE) was observed. De novo synthesis of cholesterol from acetate or mevalonolactone was inhibited by W-7, but not glycerolipid synthesis from glycerol and oleic acid precursors. Concentration-response curves for the effects of overnight pre-incubation with W-7 followed labeling with [(3)H]glycerol and [(14)C]mevalonolactone revealed that: (1). the inhibitory effect of W-7 was concentration-dependent and appeared even at the lowest concentration examined (1 microM). W-7 at a concentration of 20 microM suppressed secretion of TG by 60% (P<or=0.002), phosphatidylcholine (PC) by 31% (P<or=0.05), CE by 59% (P<or=0.002) and cholesterol by 64% (P<or=0.002). (2). The incorporation of [(14)C]mevalonolactone into cellular cholesterol and CE was decreased significantly, while W-7 did not have any significant effect upon incorporation of [(3)H]glycerol into glycerolipids, except at the highest concentration examined (50 microM), where synthesis of both TG and PC was significantly suppressed. (3). While the percentage of secreted de novo synthesized glycerolipids and CE decreased proportionally with increasing concentration of W-7, the percentage of secreted newly made cholesterol remained unaffected at any concentration of W-7. In the absence of W-7, about 19% of newly formed cholesterol became esterified into CE, whereas W-7 increased cholesterol esterification in a concentration-dependent manner. (4) W-7 (20 microM) also suppressed the secretion of preformed cholesterol by 24% and CE by 55% but did not affect the recruitment of preformed cholesterol for esterification. About 6.5% of pre-labeled cholesterol and 20% of CE were directed to secretion, which was suppressed in the presence of W-7 by 17% (P<or=0.09) and 48% (P<or=0.001), respectively. These results suggest that, W-7 in the range of 1-20 microM inhibited de novo synthesis of cholesterol and the secretion of both de novo synthesized and preformed lipids.

Effects of norepinephrine on the metabolism of fatty acids with different chain lengths in the perfused rat liver

The effects of norepinephrine on ketogenesis in isolated hepatocytes have been reported as ranging from stimulation to inhibition. The present work was planned with the aim of clarifying these discrepancies. The experimental system was the once-through perfused liver from fasted and fed rats. Fatty acids with chain lengths varying from 8-18 were infused. The effects of norepinephrine depended on the metabolic state of the rat and on the nature of the fatty acid. Norepinephrine clearly inhibited ketogenesis from long-chain fatty acids (stearate > palmitate > oleate), but had little effect on ketogenesis from medium-chain fatty acids (octanoate and laureate). With palmitate the decrease in oxygen uptake was restricted to the substrate stimulated portion; with stearate, the decrease exceeded the substrate stimulated portion; with oleate, oxygen uptake was transiently inhibited. Withdrawal of Ca2+ attenuated the inhibitory effects. 14CO2 production from [1-14C]oleate was inhibited. Net uptake of the fatty acids was not affected by norepinephrine. In livers from fed rats, oxygen uptake and ketogenesis from stearate were only transiently inhibited. The conclusions are: (a) in the fasted state norepinephrine reduces ketogenesis and respiration by means of a Ca2+-dependent mechanism; (b) the degree of inhibition varies with the chain length and the degree of saturation of the fatty acids; (c) norepinephrine favours esterification of the activated long-chain fatty acids in detriment to oxidation; (d) in the fed state the stimulatory action of norepinephrine on glycogen catabolism induces conditions which are able to reverse inhibition of ketogenesis and oxygen uptake.

Epinephrine's ketogenic effect in humans is mediated principally by lipolysis

To quantify epinephrine's effects on acetoacetate and beta-hydroxybutyrate kinetics, we infused subjects with 0.3 and 2.5 micrograms/min epinephrine, either alone or with a concomitant somatostatin infusion with insulin, glucagon, and growth hormone replaced at postabsorptive levels (islet clamp). Additional subjects received no epinephrine but sequential infusions of heparin plus 10% Intralipid at rates of 0.5 and 3.0 ml/min. Both epinephrine and Intralipid increased ketone body appearance (unaffected by islet clamp), augmented the interconversion rates between ketone bodies and, during the 2.5 micrograms/min infusion, caused a marked increase in beta-hydroxybutyrate appearance. The fraction of plasma free fatty acid (FFA) flux appearing as plasma ketones increased from 6 to 7% in the basal state to 11% at the high-epinephrine infusion. This fraction was also unaffected by the islet clamp and was not different from values obtained at similar Intralipid plus heparin-induced elevations in plasma FFA levels. We conclude that epinephrine's ketogenic effect in humans is primarily the result of its lipolytic effect, is accompanied by a significantly increased rate of ketone body interconversion, is manifest largely as an increase in plasma beta-hydroxybutyrate appearance at high plasma epinephrine values, and is not limited by portal insulin at post-absorptive levels.

Hypoglycemic effects of a beta-agonist, Ro 16-8714, in streptozotocin-diabetic rats: decreased hepatic glucose production and increased glucose utilization in oxidative muscles

Streptozotocin (STZ)-induced diabetic rats are glycosuric, hyperglycemic, hyperketonemic, overproduce glucose, and have a decreased glucose utilization in oxidative muscles. Treatment with a beta-agonist, Ro 16-8714, decreases the glycosuria, hyperglycemia, hyperketonemia, and hepatic glucose production. Tissue glucose utilization was unchanged, except in oxidative muscles, where it was increased.

Regulation of lipolysis and ketogenesis by norepinephrine in conscious dogs

The lipolytic and ketogenic effects of norepinephrine (NE) at levels present in the circulation or the synaptic cleft during stress were examined in the overnight-fasted conscious dog. Insulin and glucagon were maintained at basal levels while NE, at a rate of either 0.04 (n = 6) or 0.32 micrograms.kg-1.min-1 (n = 5), or saline (n = 6) was infused for 3 h. NE rose from 129 +/- 17 to 442 +/- 85 pg/ml (P less than 0.05) and 100 +/- 24 to 3,244 +/- 807 pg/ml (P less than 0.05) with the low and high infusion rates, respectively (unchanged with saline infusion). There were no significant changes in lipolysis or ketogenesis with saline infusion. Both low and high NE infusion produced sustained increases in glycerol (from 72 +/- 20 to 119 +/- 24 microM and 59 +/- 19 to 248 +/- 32 microM, respectively, both P less than 0.05), while nonesterified fatty acids (NEFA) rose from 609 +/- 85 to 952 +/- 100 and 767 +/- 140 to 2,054 +/- 199 microM (both P less than 0.05). Ketone levels and net hepatic production rose significantly only with the high NE infusion (from 88 +/- 10 to 266 +/- 46 microM and 1.30 +/- 0.26 to 7.62 +/- 1.48 mumol.kg-1.min-1, respectively, both P less than 0.05). The ratio of net hepatic ketone production to NEFA uptake rose 54% with high NE infusion. In conclusion, at circulating levels seen during stress, NE stimulates lipolysis but does not directly influence ketogenesis. At circulating levels projected to exist in the synaptic cleft during stress, NE has a potent lipolytic effect and stimulates ketogenesis.

Alpha 1-adrenergic regulation of ketogenesis in isolated rat hepatocytes

Studies were conducted to clarify the effects of noradrenaline on oleate metabolism in isolated hepatocytes from fed rats. Noradrenaline caused an inhibition of ketogenesis from oleate along with a stimulation of glucose release through alpha 1-adrenergic receptors. Anti-ketogenic action of noradrenaline was confirmed by the suppression of the formation of radioactive acid-soluble products from [1-14C]oleate in response to this agent. Noradrenaline increased the conversion of [1-14C]oleate into 14CO2 but failed to affect [1-14C]oleate esterification. When hepatocytes were incubated in a medium containing 1 mM EGTA but no Ca2+, the effects of noradrenaline on oleate oxidation were negated. On the other hand, noradrenaline-induced increase in glucose release remained unchanged even in the absence of Ca2+ in the incubation medium. Decrease in ketogenesis and increase in glucose release produced by vasopressin was completely abolished by calcium depletion. Noradrenaline caused a significant increase in cAMP levels in both the presence and absence of Ca2+, although the effect was more marked in the latter. Vasopressin did not affect it. The noradrenaline-induced increase in cAMP and glucose release in the absence of Ca2+ was also mediated by alpha 1-adrenergic receptors. These data are discussed and it is suggested that alpha 1-adrenergic agonists may control hepatic ketogenesis and glycogenolysis through two separate signal transduction mechanisms, i.e., a calcium-mobilizing system which is common with vasopressin, and a cAMP generation system which vasopressin lacks.

The regulation of the matrix volume of mammalian mitochondria in vivo and in vitro and its role in the control of mitochondrial metabolism

The purpose of this article is to describe briefly the methods by which the intra-mitochondrial volume may be measured both in vitro and in situ, to summarise the mechanisms thought to regulate the mitochondrial volume and then to review in more detail the evidence that changes in the intra-mitochondrial volume play an important part in the regulation of liver mitochondrial metabolism by glucogenic hormones such as glucagon, adrenaline and vasopressin. It will be shown that these hormones cause an increase in matrix volume sufficient to produce significant activation of fatty acid oxidation, respiration and ATP production, pyruvate carboxylation, citrulline synthesis and glutamine hydrolysis. These are all processes activated by such hormones in vivo. I will go on to demonstrate that the increase in matrix volume is brought about by an increase in mitochondrial [PPi]. This is able to stimulate K+ entry into the matrix, perhaps through an interaction with the adenine nucleotide translocase. The rise in matrix [PPi] is a consequence of an increase in cytosolic and hence mitochondrial [Ca2+] which inhibits mitochondrial pyrophosphatase. In the final section of the review I provide evidence that changes in mitochondrial volume may be important in the responses of a variety of tissues to hormones and other stimuli. I write as a metabolist with a working knowledge of bioenergetics rather than the converse, and this will certainly be reflected in the approach taken. If I cause offence to any dedicated experts in the field of bioenergetic by my ignorance or lack of understanding of their studies I can only offer my apologies and ask to be corrected.

Age-related changes of long-chain fatty acid metabolism in rat liver

Palmitate oxidation activity and the activities of several enzymes involved in long-chain fatty acid metabolism were examined in the liver of young adult (2-month-old) and senescent (32-month-old) female rats. Palmitate oxidation activity in rat liver mitochondria showed age-related decrease, as judged by the rates of both 14CO2 production and formation of radioactive acid-soluble products from [1-14C]palmitate. In addition, long-chain acyl-coenzyme A synthetase activity was found to be decreased in liver mitochondria and increased in liver microsomes in senescent rats. These results suggest that, in the rat liver, preferential channeling of long-chain fatty acids through the triacylglycerol synthetic pathway may increase with age, and as a result, energy production by their oxidation may decrease.

Short-term regulation of carnitine palmitoyltransferase activity in isolated rat hepatocytes

An assay procedure for carnitine palmitoyltransferase is described which allows rapid measurement of the overt activity of this enzyme in isolated rat hepatocytes. In a one-step procedure digitonin permeabilizes the plasma membrane and at the same time carnitine palmitoyltransferase activity is measured. The use of the present procedure shows that carnitine palmitoyltransferase activity is regulated on the short term by different types of agonists. Thus, insulin, epidermal growth factor, vasopressin and the phorbol ester PMA inhibit carnitine palmitoyltransferase activity, whereas glucagon treatment renders the enzyme more active. These changes in enzyme activity coincide with corresponding changes in the rate of fatty acid oxidation.

Alpha-adrenergic suppression of very-low-density-lipoprotein triacylglycerol secretion by isolated rat hepatocytes

The effect of adrenaline on triacylglycerol synthesis and secretion was examined in isolated rat hepatocytes. Cells were incubated with 0.5 mM-[1-14C]oleate, and the accumulation of triacylglycerol and [14C]triacylglycerol was measured in the incubation medium. Triacylglycerol appearing in the medium was present in a form with properties similar to very-low-density lipoproteins. Triacylglycerol, [14C]triacylglycerol and [14C]phospholipid contents of hepatocytes were also determined. Addition of 10 microM-(-)adrenaline decreased accumulation of glycerolipid in the incubation medium and also decreased cellular [14C]phospholipid content. Prazosin abolished these effects, whereas propranolol did not. The hormone did not affect cellular triacylglycerol content or rates of incorporation of [1-14C]oleate into cell triacylglycerol. The effect of adrenaline on the removal of newly secreted triacylglycerol and the secretion of synthesized glycerolipid was also examined. The catecholamine did not affect rates of removal of newly secreted triacylglycerol. Adrenaline did inhibit the secretion of pre-synthesized lipid by the cells, as assessed by the appearance of radiolabelled triacylglycerol from hepatocytes that had been preincubated with [1,2,3-3H]-glycerol. Adrenaline did not affect rates of fatty acid uptake by hepatocytes, but did stimulate oxidation of [1-14C]oleate, principally to 14CO2.

Alpha 1-adrenergic stimulation of ketogenesis and fatty acid oxidation is associated with inhibition of lipogenesis in rat hepatocytes

The effect of norepinephrine on fatty acid synthesis (3H2O incorporation into fatty acids), on fatty acid oxidation to CO2 and on ketogenesis was studied in isolated hepatocytes of fed rats. After incubation with norepinephrine (50 microM), lipogenesis was lower (5.7 +/- 1.1 nmoles 3H2O incorporated into fatty acids/mg dry weight/30 min) than in controls (7.5 +/- 1.7; n = 6, p less than 0.02). In contrast, (1-14C) palmitate conversion into total ketone bodies was increased to 10.9 +/- 1.8 nmoles/mg/30 min with norepinephrine, vs 8.5 +/- 1.6 in controls (p less than 0.05), and more (1-14C) palmitate was converted to 14CO2 with norepinephrine than in controls (1.48 +/- 0.10 nmoles/mg/30 min vs 1.06 +/- 0.11, p less than 0.05). The inhibitory effect of norepinephrine on lipogenesis was abolished by addition of the alpha 1-receptor blocker prazosin, but not by alpha 2 or beta-blockers. The results demonstrate that the ketogenic effect of norepinephrine is coupled with an inhibitory effect on lipogenesis which may be explained by diminished activity of acetyl-CoA carboxylase, diminished formation of malonyl-CoA and decreased activity of carnitine palmitoyl transferase I.

Beta-adrenergic blockade restores glucose's antiketogenic activity after exercise in carbohydrate-depleted athletes

1. The development of post-exercise ketosis is not abolished by the ingestion of glucose immediately after exercise, despite inducing high insulin/glucagon ratios in the peripheral (and therefore by implication in the portal) blood. 2. To investigate the possibility of autonomic control of the liver influencing its sensitivity to the major counter-regulatory hormones, we administered 50 g glucose, either on its own, or together with 0.5 mg prazosine, 40 mg propranolol, or 15 mg propantheline, to forty-seven 48 h carbohydrate-starved athletes who had just run 25 km. 3. The blood 3-hydroxybutyrate concentration rose from 0.30 +/- 0.05 (mean +/- S.E. of mean) to 0.52 +/- 0.08 mmol/l with exercise, and then to 1.32 +/- 0.40 mmol/l at 6 h after exercise in subjects who had ingested only glucose after exercise. 4. The effects of prazosine and propantheline on the blood ketone body concentration at 2 h after exercise was not statistically significant. Propranolol, on the other hand, significantly lowered the blood 3-hydroxybutyrate concentration (compared with controls) to 0.09 +/- 0.03 mmol/l at 3 h (P less than 0.01), and 0.35 +/- 0.08 mmol/l at 6 h (P less than 0.01) after exercise. 5. The plasma insulin, glucagon, glucose and free fatty acid concentrations were unaffected by propranolol, indicating that the antiketogenesis was the result of a direct effect on ketone body metabolism. 6. Since beta-adrenergic blockade has not previously been shown to have antiketogenic activity, except in somatostatin-induced hyperketonaemia, it is concluded that its effectiveness in post-exercise ketosis can probably be ascribed to a functional hepatic insulin and glucagon deficiency.

Suppression of triglyceride secretion by epinephrine in isolated rat hepatocytes

The effect of epinephrine on triglyceride synthesis and secretion was examined in isolated rat hepatocytes. Epinephrine potently inhibited triglyceride secretion but did not affect cellular triglyceride content or the rate of incorporation of radiolabelled glycerol into cell triglyceride. The inhibitory effect of epinephrine was abolished by inclusion of the alpha-adrenergic antagonist prazosin but not the beta-antagonist propranolol.

Intramitochondrial regulation of fatty acid beta-oxidation occurs between flavoprotein and ubiquinone. A role for changes in the matrix volume

Rat liver mitochondria were incubated in media of different osmolarities and in the presence of various substrates. Rates of oxygen consumption and mitochondrial matrix volumes were measured in the presence and absence of ADP and uncoupler. Duroquinol oxidation was insensitive to matrix volume, whereas other substrates tested showed increased rates of oxidation when the matrix volume increased from 1.0 to 1.5 microliter/mg of protein; this is the range of values measured in situ [Quinlan, Thomas, Armston & Halestrap (1983) Biochem. J. 214, 395-404]. Palmitoylcarnitine, octanoate and butyrate oxidations were particularly sensitive to the matrix volume, increasing from negligible rates to maximal rates within this range. Swelling induced by K+ uptake also stimulated palmitoylcarnitine oxidation. A similar effect of volume on substrate oxidation was seen when ferricyanide in the presence or absence of ubiquinone-1 replaced oxygen as terminal electron acceptor. Measurement of flavoprotein reduction (A 460-480) demonstrated that the locus of the effect of matrix volume is between the electron-transfer flavoprotein and ubiquinone. It is suggested that volume-mediated regulation of fatty acid and proline oxidation may be an important component of the hormonal stimulation of their oxidation.

Effect of catecholamines on the metabolic fate of nonesterified fatty acids in isolated hepatocytes from newborn rats

Isolated hepatocytes from newborn rats are able to produce ketone bodies from added medium-chain and long-chain fatty acids. Carnitine enhances the rate of ketone body synthesis from palmitate as well as from caprinoate. The 3-OHB/AcAc ratios indicate a highly reduced state of the mitochondrial redox carriers in the presence of both fatty acids and carnitine. Ketogenesis from palmitate accounts for about 90% of the total beta-oxidation. At recovery of 95% of the radioactivity two thirds of totally fatty acid uptake are channeled into esterification, whereas the remainder is oxidized. alpha- and beta-agonists stimulate glycogen degradation and glucose release and reduce net lactate production in hepatocytes from newborn rats. The (1-14C)-palmitate uptake is not altered by alpha- and beta-agonists. Phenylephrine significantly enhances 14CO2 production from (1-14C)-palmitate. Neither of the agonists affects the rate of esterification or of ketone body production with palmitate as substrate. Isoproterenol, however, stimulates ketogenesis from caprinoate even in the presence of optimal carnitine concentrations.

Effects of vasopressin, angiotensin II and phenylephrine on hepatic ketogenesis and fatty acid synthesis

Studies were conducted to clarify the effects of vasopressin, angiotensin II and phenylephrine on hepatic ketogenesis and fatty acid synthesis. Hepatocytes from fed rats were incubated with oleate or octanoate. Vasopressin stimulated fatty acid synthesis as well as lactate and pyruvate accumulation in the presence of oleate. In accordance with this action, vasopressin caused a marked decrease in ketogenesis from oleate. When octanoate was added as a substrate, vasopressin failed to inhibit ketogenesis. Neither angiotensin II nor phenylephrine affected ketogenesis or fatty acid synthesis. The results in the present study show that there are vasopressin-mediated reciprocal changes in ketogenesis from oleate and fatty acid synthesis in isolated hepatocytes.

A suppressive role of c-kinase for the stimulation of hepatic ketogenesis by glucagon and epinephrine

The regulatory mechanism of hepatic palmitate oxidation into ketone bodies by c-kinase has been studied in isolated hepatocytes. Glucagon and epinephrine stimulated [U-14C]palmitate oxidation to ketone bodies by 60 and 25% as early as at 1 h. The stimulatory effects were almost totally prevented by the simultaneous presence of vasopressin, phorbol 12-tetradecanoate 13-acetate (TPA), or diacylglycerol (1-oleoyl-2-acetylglycerol). When hepatocytes were treated with glucagon or epinephrine, carnitine palmitoyltransferase (CPT), a key regulatory enzyme of palmitate oxidation, was activated. This hormone-induced activation of CPT was not observed in the presence of TPA. These observations suggest that c-kinase inhibits glucagon- or epinephrine-stimulated palmitate oxidation to ketone bodies, and that this inhibition may be mediated through a covalent modification of CPT.

Phosphorylation of carnitine palmitoyltransferase and activation by glucagon in isolated rat hepatocytes

Effects of glucagon and forskolin on the phosphorylation and changes of activity of carnitine palmitoyltransferase (CPT) have been studied in isolated rat hepatocytes using anti-CPT immunoglobulin. When the activity was determined in lysed hepatocytes after glucagon or forskolin treatment, it was found to be stimulated 30-80% mainly through increased affinity for palmitoyl-CoA. By SDS electrophoresis of the immunoprecipitates, CPT subunit (Mr 69000) was noted to be phosphorylated 4-5-fold with glucagon (1.2 X 10(-7) M) and forskolin (0.1 mM) over control. These results indicate that hepatic ketogenesis is regulated with glucagon by phosphorylation of CPT through cAMP-dependent protein kinase.