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P J Randle

Publications and source records attributed to P J Randle.

At least 19 recordsLinked to original sources
PubMed1998

Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years.

Competition for respiration between substrates in animal tissues has been known for at least 80 years. The most important interaction, quantitatively is between glucose and fatty acids. The starting point in 1963 for the so called Glucose Fatty Acid Cycle was the realisation that the metabolic relationship between glucose and fatty acids is reciprocal and not dependent. Glucose provision promotes glucose oxidation and glucose and lipid storage, and inhibits fatty acid oxidation. Provision of free fatty acids promotes fatty acid oxidation and storage, inhibits glucose oxidation and may promote glucose storage if glycogen reserves are incomplete. This review is concerned predominantly with evidence in man in vivo. In the authors opinion the evidence for inhibitory effects of fatty acids on whole body glucose utilization ad oxidation (predominantly muscles) is decisive and enzyme mechanisms mediating these effects are well established. There is also much evidence that fatty acid oxidation inhibits glucose oxidation and stimulates glucose formation in liver and again enzyme mechanism are known. A permissive role for fatty acids in the insulin secretory response of islet beta-cells has now been firmly established and can be visualised as a mechanism to protect continuing provision of respiratory substrate. Longer term exposure of islet beta-cells to fatty acids impairs the insulin secretory response to glucose and mechanisms are known. There is compelling evidence that fatty acid oxidation may impair glucose oxidation in uncontrolled Type 1 and Type 2 diabetes, but no convincing evidence that fatty acids have a role in diminished glucose storage (glycogen deposition) in Type 2 diabetes. The inhibition of glucose storage which may follow prolonged elevation of plasma FFA in man and experimental animals is associated with glycogen repletion whereas the inhibition of glucose storage in Type 2 diabetes is associated with glycogen depletion. The precise role of fatty acids in disturbed carbohydrate metabolism in Type 2 diabetes is an area where future progress is confidently predicted.

Animals↗
PubMed1996

Fasting and decreased B cell sensitivity: important role for fatty acid-induced inhibition of PDH activity.

Fasting inhibits glucose-induced insulin secretion. We investigated the role of a glucose fatty acid cycle for such inhibition and its molecular basis in pancreatic islets from 48-h fasted rats. The fasting-impaired insulin response to 27 mM glucose was restored by 41% with a carnitine palmitoyltransferase I inhibitor, etomoxir. Etomoxir also restored (by 50%) impaired glucose oxidation in islets from fasted rats and increased the ratio of oxidation to glycolytic flux from 33 to 43%. Fasting decreased total pyruvate dehydrogenase (PDH) activity (active, unphosphorylated plus inactive, phosphorylated form) by 29%, as well as the percentage of active form (54 +/- 5 vs. 79 +/- 2% in fed rats, P < 0.001). Fasting increased islet PDH kinase activity as follows: PDH-bound activity by 36% and free (not PDH bound) PDH kinase by 70%. Fasting failed to affect PDH kinase content when assayed by an enzyme-linked immunoabsorbent assay with antibodies raised against 45 kDa PDH kinase alpha-chain. We conclude that fasting impairs B cell function to a major extent through the operation of a glucose fatty acid cycle and that decreased PDH activity resulting from increased specific activity of PDH kinase constitutes an important molecular mechanism.

Animals↗
PubMed1994

Role of protein synthesis and of fatty acid metabolism in the longer-term regulation of pyruvate dehydrogenase kinase.

Antibodies were raised in rabbits to free rat liver pyruvate dehydrogenase (PDH) kinase alpha-chain and shown to react with PDH kinase alpha-chain in rat heart and liver PDH complexes, in purified pig heart PDH complex and in bovine kidney dihydrolipoamide acetyltransferase-protein X-PDH kinase subcomplex. E.l.i.s.a for PDHE1 (pyruvate dehydrogenase) and PDH kinase have been developed and applied to assays of these proteins in extracts of rat liver and rat heart mitochondria; the measured immunoreactivities for PDHE1 (heart > liver) and for PDH kinase alpha-chain (liver > heart) paralleled known differences in PDH complex and PDH kinase activities respectively. The results of e.l.i.s.a of PDH kinase alpha-chain in extracts of rat liver mitochondria showed that the effects of starvation to increase PDH kinase activity in vivo, and the effects of dibutyryl cyclic AMP or palmitate to increase PDH kinase activity in hepatocytes cultured in vitro, are due largely (> 90%) to an increase in the specific activity of PDH kinase. The effect, in cultured hepatocytes, of dibutyryl cyclic AMP to increase PDH kinase activity was blocked by cycloheximide; the effect of palmitate was blocked by an inhibitor of carnitine palmitoyltransferase I (Etomoxir), but not by cycloheximide.

Animals↗
PubMed1994

Glucose fatty acid interactions and the regulation of glucose disposal.

Glucose is essential for the energy metabolism of some cells and conservation of glucose is obligatory for survival during starvation. The principal site of this glucose conservation is the mitochondrial pyruvate dehydrogenase (PDH) complex, which is regulated by reversible phosphorylation (phosphorylation is inactivating). In cells in which glucose oxidation is switched off during starvation, fatty acids are used as fuel, and acetyl CoA and NADH formed by beta-oxidation promote phosphorylation of PDH complex by activation of PDH kinase. A longer-term mechanism further increases PDH kinase activity in response to cAMP and products of beta-oxidation of fatty acids. Coordinated inhibition of glycolytic flux mediated by effects of citrate on PFK1 and PFK2 in muscles and liver results in an associated inhibition of glucose uptake. Similar mechanisms lead to impaired glucose oxidation in diabetes.

Animals↗
PubMed1994

Mechanisms modifying glucose oxidation in diabetes mellitus.

The Glucose Fatty Acid Cycle as formulated 30 years ago and reviewed in the Minkowski lecture in 1966 described short term effects of fatty acids (minutes) to decrease uptake, glycolysis and oxidation of glucose in heart and skeletal muscles. Such short term effects have since been extended to include inhibition of glucose uptake and glycolysis and stimulation of gluconeogenesis in liver and these effects have also been convincingly demonstrated in man in vivo. More recently a longer term effect of fatty acid metabolism to decrease glucose oxidation (hours) has been shown in heart and skeletal muscle and liver. This effect increases the specific activity of pyruvate dehydrogenase kinase, which in turn results in enhanced phosphorylation and inactivation of the pyruvate dehydrogenase complex. Activity of the pyruvate dehydrogenase complex is the major determinant of glucose oxidation rate. It seems likely that longer term effects of fatty acids on this and other aspects of glucose metabolism could be important in the development of insulin resistance in diabetes mellitus in man.

Animals↗
PubMed1992

Purification and partial characterization of rat liver pyruvate dehydrogenase kinase activator protein (free pyruvate dehydrogenase kinase).

Rat liver pyruvate dehydrogenase (PDH) kinase activator protein (KAP), a free PDH kinase readily separable from PDH complex and its intrinsic kinase, has been purified to apparent homogeneity from liver mitochondria of fed and 48-h starved rats. On SDS-PAGE an apparently single band of M(r) 45 kDa was obtained. N-Terminal amino acid sequence analyses (8-10 cycles) confirmed the presence of a single peptide in each case. The specific activity of the purified KAP from 48-h starved rats (14,413 U/mg protein) was 4.5-fold greater than that from fed rats.

Amino Acid Sequence↗
PubMed1992

Cyclic AMP and free fatty acids in the longer-term regulation of pyruvate dehydrogenase kinase in rat soleus muscle.

Starvation increased pyruvate dehydrogenase (PDH) kinase activity in extracts of freshly excised rat soleus 2.2-fold (from 0.6 min-1 in fed rats to 1.31 min-1 in 48-h-starved rats). In fed rats, activities were unchanged following 24 h of culture in medium 199, but increased 2.1-fold on 24 h of culture with 50 microM dibutyryl cAMP plus 1 mM n-octanoate and 1.6-1.7-fold with either agent alone. Approx. 70% of the increase in PDH kinase induced by starvation was lost following 24 h of culture in medium 199; the loss was prevented by 50 microM dibutyryl cAMP plus 1 mM n-octanoate. cAMP concentrations in fresh soleus muscle were 1 nmol/g (fed rats) and 1.6 nmol/g (starved rats). After 20-60 min of culture the fed-starved difference disappeared and [cAMP] fell to 0.4 nmol/g. Calcitonin-gene-related peptide (CGRP) increased cAMP 3-fold; the increase was maintained throughout 24 h of culture, but was readily reversed at 30 min or 24 h of culture by 60-min incubation with CGRP-free medium. Starvation of the rat (48 h) had no effect on the sensitivity of soleus towards the [cAMP]-increasing effect of CGRP. It is concluded that culture may reverse effects of starvation on PDH kinase activity by lowering cAMP and by removal from the in vivo effects of circulating free fatty acids; and that starvation and CGRP had no detectable long-term effects on the cAMP system in soleus muscle.

Adenosine Deaminase↗
PubMed1991

Evidence that rat liver pyruvate dehydrogenase kinase activator protein is a pyruvate dehydrogenase kinase.

It is shown here that rat liver pyruvate dehydrogenase (PDH) kinase activator protein (KAP) catalyses ATP-dependent inactivation and [32P]phosphorylation of pig heart PDHE1 and of yeast (Saccharomyces cerevisiae) PDH complex devoid of PDH kinase activity, that fluorosulphonylbenzoyladenosine inactivates rat liver KAP and the intrinsic PDH kinase of rat liver PDH complex, and that KAP, like PDH kinase, is inactivated by thiol-reactive reagents. It is concluded that KAP is a free PDH kinase.

Adenosine Triphosphate↗
PubMed1990

Long term culture of rat soleus muscle in vitro. Its effects on glucose utilization and insulin sensitivity.

Rat soleus muscle strips cultured for 24 h in medium 199 were well preserved in terms of electron microscopy; ATP and creatine phosphate concentrations; rates of glucose utilization, glycogen and protein synthesis, and effects of insulin thereon. Culture led to modest changes in fluid spaces and intracellular (K+); increased basal glucose utilization up to two-fold; had no effect on the maximum response to insulin; and had no effect on sensitivity to insulin except in the presence of adenosine deaminase. Thus in vitro neither denervation nor absence of insulin had any marked effects in 24 h to decrease responses to insulin.

Adenosine Deaminase↗
PubMed1990

Longer-term regulation of pyruvate dehydrogenase kinase in cultured rat cardiac myocytes.

The increased activity of pyruvate dehydrogenase (PDH) kinase induced in hearts of rats by starvation for 48 h was maintained following preparation of cardiac myocytes, and it was also maintained, though at a decreased level, after 25 h of culture in medium 199. This loss of PDH kinase activity was not prevented by n-octanoate, dibutyryl cyclic AMP or glucagon. The PDH kinase activity of myocytes from fed rats was increased to that of starved rats after 25 h of culture with n-octanoate, dibutyryl cyclic AMP or both agents together.

Animals↗
PubMed1989

Longer-term regulation of pyruvate dehydrogenase kinase in cultured rat hepatocytes.

The activities of pyruvate dehydrogenase (PDH) kinase and of PDH kinase activator protein (KAP) were increased 2-2.4-fold during 25 h of culture of hepatocytes from fed rats with glucagon plus n-octanoate. PDH kinase activity in hepatocytes from starved rats (initially 2.2 x fed control) fell during 25 h of culture in medium 199 (to 1.5 x fed control), but was maintained by glucagon plus octanoate. Dibutyryl or 8-bromo cyclic AMP increased PDH kinase activity 2-2.2-fold in hepatocytes from fed rats, but phenylephrine and isoproterenol (isoprenaline) were without effect. Insulin blocked the action of glucagon to increase PDH kinase activity and decreased the effect of octanoate and octanoate plus glucagon. It is suggested that the effects of starvation to increase activities of PDH kinase and of KAP in liver are mediated by alterations in circulating concentrations of glucagon, fatty acids and insulin and in hepatic cyclic AMP.

Animals↗
PubMed1989

Longer-term regulation of branched-chain-2-oxoacid dehydrogenase complex studied in rat hepatocytes in culture.

The effect of protein-free diet to decrease liver activity of branched-chain (-2-oxoacid) dehydrogenase (BCD) complex (active form) and increase BCD kinase activity was unaffected by preparation of hepatocytes, but partially reversed by 25 h of culture of hepatocytes in medium 199. Activation of BCD complex preceded loss of BCD kinase. The effect of culture on BCD complex was completely prevented by omission of branched-chain amino acids and partially prevented by 1 mM-alpha-cyano-4-hydroxycinnamate or 0.2 mM-pyruvate/2 mM-lactate. Protein-free diet decreased plasma branched-chain amino and oxo ('keto') acids and increased plasma pyruvate and lactate. It is concluded: (1) that branched-chain amino acids are involved directly in regulation of activities of BCD complex and BCD kinase; (2) that mitochondrial uptake of branched-chain oxo acids is necessary for regulation of BCD complex activity; and (3) that the stable increase in BCD kinase may function as a hysteresis mechanism.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗
PubMed1988

Activity of branched-chain 2-oxo acid dehydrogenase complex in rat liver mitochondria and in rat liver.

Four mitochondrial marker enzymes were used to show that: (1) high-protein (24%) diet increased the rat liver concentration and content of total branched-chain 2-oxo acid dehydrogenase complex (BCDC) by 31% by increasing mitochondrial specific activity of BCDC; (2) starvation increased the liver concentration of BCDC by 25% by decreasing liver weight; the liver content of mitochondria and the mitochondrial specific activity of BCDC were unchanged; (3) protein-free diet decreased rat liver BCDC concentration and content by 20%, by decreasing the liver concentration and content of mitochondria. Protein-free diet increased liver mitochondrial specific activities of L-glutamate, 2-oxoglutarate and NAD-isocitrate dehydrogenases. The validity of a mitochondrial method for the determination of the liver concentration of BCDC and the percentage in the active form in vivo is confirmed, and improvements are described. The experimental basis of criticisms of its use in this regard by Zhang, Paxton, Goodwin, Shimomura & Harris [(1987) Biochem. J. 246, 625-631] was not confirmed. The finding by Harris, Powell, Paxton, Gillim & Nagae [(1985) Arch. Biochem. Biophys. 243, 542-555], that starvation has no effect on the percentage of BCDC in the active form in rat liver, is confirmed.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗