PubMed Health⌕ Search

Biomedical subjects

M C Sugden

Publications and source records attributed to M C Sugden.

At least 37 records · Page 2Linked to original sources
PubMed2000

Fibre-type specific modification of the activity and regulation of skeletal muscle pyruvate dehydrogenase kinase (PDK) by prolonged starvation and refeeding is associated with targeted regulation of PDK isoenzyme 4 expression.

Using immunoblot analysis with antibodies raised against recombinant pyruvate dehydrogenase kinase (PDK) isoenzymes PDK2 and PDK4, we demonstrate selective changes in PDK isoenzyme expression in slow-twitch versus fast-twitch skeletal muscle types in response to prolonged (48 h) starvation and refeeding after starvation. Starvation increased PDK activity in both slow-twitch (soleus) and fast-twitch (anterior tibialis) skeletal muscle and was associated with loss of sensitivity of PDK to inhibition by pyruvate, with a greater effect in anterior tibialis. Starvation significantly increased PDK4 protein expression in both soleus and anterior tibialis, with a greater response in anterior tibialis. Starvation did not effect PDK2 protein expression in soleus, but modestly increased PDK2 expression in anterior tibialis. Refeeding for 4 h partially reversed the effect of 48-h starvation on PDK activity and PDK4 expression in both soleus and anterior tibialis, but the response was more marked in soleus than in anterior tibialis. Pyruvate sensitivity of PDK activity was also partially restored by refeeding, again with the greater response in soleus. It is concluded that targeted regulation of PDK4 isoenzyme expression in skeletal muscle in response to starvation and refeeding underlies the modulation of the regulatory characteristics of PDK in vivo. We propose that switching from a pyruvate-sensitive to a pyruvate-insensitive PDK isoenzyme in starvation (a) maintains a sufficiently high pyruvate concentration to ensure that the glucose-->alanine-->glucose cycle is not impaired, and (b) may 'spare' pyruvate for anaplerotic entry into the tricarboxylic acid cycle to support the entry of acetyl-CoA derived from fatty acid (FA) oxidation into the tricarboxylic acid cycle. We further speculate that FA oxidation by skeletal muscle is both forced and facilitated by upregulation of PDK4, which is perceived as an essential component of the operation of the glucose-FA cycle in starvation.

Animals↗
PubMed2000

Selective modification of the pyruvate dehydrogenase kinase isoform profile in skeletal muscle in hyperthyroidism: implications for the regulatory impact of glucose on fatty acid oxidation.

The pyruvate dehydrogenase kinases (PDK1-4) regulate glucose oxidation through inhibitory phosphorylation of the pyruvate dehydrogenase complex (PDC). Immunoblot analysis with antibodies raised against recombinant PDK isoforms demonstrated changes in PDK isoform expression in response to experimental hyperthyroidism (100 microg/100 g body weight; 3 days) that was selective for fast-twitch vs slow-twitch skeletal muscle in that PDK2 expression was increased in the fast-twitch skeletal muscle (the anterior tibialis) (by 1. 6-fold; P<0.05) but not in the slow-twitch muscle (the soleus). PDK4 protein expression was increased by experimental hyperthyroidism in both muscle types, there being a greater response in the anterior tibialis (4.2-fold increase; P<0.05) than in the soleus (3.2-fold increase; P<0.05). The hyperthyroidism-associated up-regulation of PDK4 expression was observed in conjunction with suppression of skeletal-muscle PDC activity, but not suppression of glucose uptake/phosphorylation, as measured in vivo in conscious unrestrained rats (using the 2-[(3)H]deoxyglucose technique). We propose that increased PDK isoform expression contributes to the pathology of hyperthyroidism and to PDC inactivation by facilitating the operation of the glucose --> lactate --> glucose (Cori) and glucose --> alanine --> glucose cycles. We also propose that enhanced relative expression of the pyruvate-insensitive PDK isoform (PDK4) in skeletal muscle in hyperthyroidism uncouples glycolytic flux from pyruvate oxidation, sparing pyruvate for non-oxidative entry into the tricarboxylic acid (TCA) cycle, and thereby supporting entry of acetyl-CoA (derived from fatty acid oxidation) into the TCA cycle.

Animals↗
PubMed2000

Targeted upregulation of pyruvate dehydrogenase kinase (PDK)-4 in slow-twitch skeletal muscle underlies the stable modification of the regulatory characteristics of PDK induced by high-fat feeding.

In using Western blot analysis with antibodies raised against recombinant pyruvate dehydrogenase kinase (PDK) isoforms PDK2 and PDK4, this study demonstrates selective PDK isoform switching in specific skeletal muscle types in response to high-fat feeding that is associated with altered regulation of PDK activity by pyruvate. The administration of a diet high in saturated fats led to stable (approximately 2-fold) increases in PDK activities in both a typical slow-twitch (soleus [SOL]) muscle and a typical fast-twitch (anterior tibialis [AT]) muscle. Western blot analysis revealed that high-fat feeding significantly increased (approximately 2-fold; P < 0.001) PDK4 protein expression in SOL, with a modest (1.3-fold) increase in PDK2 protein expression. The relative increase in PDK4 protein expression in SOL was associated with a 7.6-fold increase in the pyruvate concentration that was required to elicit a 50% active pyruvate dehydrogenase complex, which indicates a marked decrease in the sensitivity of PDK to inhibition by pyruvate. In AT muscle, high-fat feeding elicited comparable (1.5- to 1.7-fold) increases (P < 0.05) in PDK4 and PDK2 protein expression. Loss of sensitivity of PDK to inhibition by pyruvate was less marked. The data suggest that a positive correlation exists between increases in PDK4 expression and the propensity with which muscles use lipid-derived fuels as respiratory substrates rather than with the degree of insulin resistance induced in skeletal muscles by high-fat feeding. In conclusion, high-fat feeding leads to selective upregulation of PDK4 expression in slow-twitch muscle in response to high-fat feeding in vivo, which is associated with a pronounced loss of sensitivity of PDK activity to acute inhibition by pyruvate. Thus, increased PDK4 expression may underlie the stable modification of the regulatory characteristics of PDK observed in slow-twitch muscle in response to high-fat feeding.

Animals↗
PubMed1999

Current concepts concerning the role of leptin in reproductive function.

Leptin has recently been implicated as having a role in sexual maturation and reproduction. This review describes recent findings regarding the putative reproductive functions of leptin within the context of the attainment of sufficient long-term fuel reserves to sustain and support pregnancy and lactation. The review considers the evidence, within the context of the development of hyperleptinaemia during pregnancy, that leptin has an important function to modulate maternal nutrient partitioning in order to optimise the provision of nutrients for fetal growth and development. It is suggested that, through modulation of maternal insulin secretion and hepatic metabolism, leptin integrates maternal nutrient storage to the nutrient requirements of the fetus. The importance of the placenta as a site of leptin synthesis and the potential role(s) of placentally derived leptin are evaluated in relation to maternal-fetal interactions during intrauterine development. The review also examines whether intrauterine growth retardation due to nutritional restriction reflects dysregulation of such cross-talk. Finally, the review describes emerging evidence for participation of leptin in lactation and neonatal growth.

Animals↗
PubMed1999

Caloric restriction leads to regional specialisation of adipocyte function in the rat.

The study analysed the responses of three metabolic parameters in five distinct adipose tissue depots to caloric restriction (4 weeks) in the rat. The aims were to evaluate whether specific adipose tissue depots were recruited for triacylglycerol (TAG) storage and/or mobilisation, and to determine to what extent specific adipose tissue depots exhibited preferences for the source of fatty acid (FA) for TAG storage. Caloric restriction led to a general enhancement of the response of lipoprotein lipase (LPL), FA synthesis and glucose utilisation to a meal. Effects were particularly marked in the parametrial, perirenal and interscapular depots compared with mesenteric and subcutaneous depots. There was no evidence that individual depots selectively expressed a preference for the pathways concerned with the generation of FA for storage (the exogenous (LPL) and the endogenous (synthesis) pathway). However, the temporal sequence of activation of these pathways differed in a manner consistent with a switch from preponderant use of FA produced via de novo synthesis during the very early phase of feeding towards later use of FA derived from circulating TAG. The overall excursions in insulin levels observed in the calorie-restricted rats were comparable to those found in free-feeding rats, but the magnitude and the rapidity of the individual metabolic responses of the adipocyte were augmented. The data are consistent with a general enhancement of insulin sensitivity and responsiveness in adipose tissue of calorie-restricted rats, together with adaptive regional specialisation of adipocyte function. These adaptations would be predicted to facilitate the immediate conservation of dietary nutrients by promoting their storage as the FA or glycerol moieties of adipose tissue TAG and thereby to ensure the regulated release of FA and glycerol from adipose tissue in accordance with the requirement for glucose conservation and/or production.

Adipocytes↗
PubMed1999

In late pregnancy insulin-dependent glucose transport/phosphorylation is selectively impaired and activation of glycogen synthase by insulin facilitated in skeletal muscles of 24-h starved rats.

AIMS/HYPOTHESIS: We investigated the relation between glucose transport/phosphorylation and glycogen synthase activation in individual rat skeletal muscles in response to moderate hyperinsulinaemia in the absence or presence of hyperglycaemia during pregnancy. METHODS: Rats were studied on day 20 of pregnancy after 24-h starvation, with unmated rats as controls. Insulin and glucose were infused into conscious rats through an indwelling cannula as specified. Glucose transport/phosphorylation was assessed in vivo using 2-deoxy-[1-3H]glucose. Glycogen synthase activity was measured in muscle extracts < or = 10 mmol/l glucose-6-phosphate. RESULTS: In unmated rats, stimulation of glucose transport/phosphorylation occurred in response to euglycaemic-hyperinsulinaemia in all muscles studied but activation of glycogen synthase was not observed. Muscle glucose transport/phosphorylation rates during euglycaemic-hyperinsulinaemia after 24-h starvation were lower in pregnant compared with unmated rats, whereas glycogen synthase activation by insulin occurred in two of three fast-twitch muscles of pregnant rats. When insulin and glucose concentrations were matched between the 24-h starved unmated and pregnant groups through variable glucose infusion (15 min), glycogen synthase activities in fast-twitch muscles did not differ between unmated and pregnant groups. The response of glycogen synthase to hyperglycaemia in slow-twitch muscle was, however, greater in the pregnant group. CONCLUSION/INTERPRETATION: Glycogen synthase activation in slow-twitch muscle in response to hyperinsulinaemia after 24-h starvation is enhanced in late pregnancy but the response is critically dependent on glycaemia. Pregnancy is, nevertheless, associated with reduced muscle glucose transport/phosphorylation. This disassociation ensures that available glucose is directed towards the fetus rather than the mother under conditions of moderate hyperinsulinaemia, for example during refeeding after starvation.

Animals↗
PubMed1999

Substrate interactions in the short- and long-term regulation of renal glucose oxidation.

The present study evaluated the substrate competition between fatty acids (FA) and glucose in the kidney in vivo in relation to the operation of the "glucose-FA" and "reverse glucose-FA" cycles. In fed rats, neither inhibition of adipocyte lipolysis by 5-methylpyrazole-3-carboxylic acid (MPCA) nor inhibition of mitochondrial long-chain FA oxidation by 2-tetradecylglycidate (TDG) influenced the renal ratio of free/acylated carnitine or the percentage of total renal pyruvate dehydrogenase complex (PDHC) in the active (dephosphorylated) form (PDHa). The additional provision of glucose, a precursor for the synthesis of malonyl-coenzyme A (coA), did not influence renal PDHa activity or the renal ratio of free to acylated carnitine, implying that FA oxidation is maximally suppressed in the fed state. A reverse glucose-FA cycle may therefore be important in suppressing renal FA oxidation in the fed state. After 48 hours of starvation, MPCA and TDG decreased short- and long-chain acylcarnitine concentrations (40% to 50%, P < .01) and elevated the renal ratio of free/acylated carnitine (2.5-fold, P < .001, and 3.3-fold, P < .001, respectively), indicating that FA oxidation is increased after starvation. Despite suppression of renal FA oxidation, renal PDHa activity in 48-hour starved rats was only partially restored by treatment with MPCA or TDG. The additional administration of glucose did not remedy this. The failure to reverse completely the effects of prolonged starvation in suppressing PDHC activity by acute inhibition of FA oxidation suggests additional regulatory mechanisms that dampen the PDHC response to acute changes in substrate supply. Estimations of PDH kinase (PDK) activity in renal mitochondria showed a significant 1.7-fold stable increase (P < .01) after 48 hours of starvation. Analysis of PDK pyruvate sensitivity in renal mitochondria incubated with respiratory substrate (5 mmol/L 2-oxoglutarate/0.5 mmol/L L-malate) showed that the pyruvate concentration required for 50% activation was substantially decreased by starvation. Enzyme-linked immunosorbent assay (ELISA) analysis over a range of PDHC activities demonstrated that increased PDK activity was concomitant with a significant (at least P < .01) 1.8-fold increase in the protein expression of the ubiquitously expressed PDK isoform, PDK2. We hypothesize that changes in protein expression and activity of individual PDK isoforms may dictate the renal response to incoming FA lesterification v oxidation) through modulation of the relationship between glycolytic flux and PDHC activity, and thus the provision of precursor for malonyl-coA production.

Acylation↗
PubMed1999

The impact of increased dietary lipid on the regulation of glucose uptake and oxidation by insulin in brown- and a range of white-adipose-tissue depots in vivo.

OBJECTIVE: To determine: (a) whether active pyruvate dehydrogenase (PDHa) activity in interscapular brown adipose tissue (IBAT) is acutely regulated by altered insulin status at euglycaemia; (b) the relationship between glucose uptake/phosphorylation and PDHa activities in IBAT in vivo; and (c) the impact of increased dietary lipid on the regulation of glucose uptake and oxidation by insulin in IBAT in comparison with that exerted on various white adipose tissue depots. DESIGN: Rats were provided with either a standard diet (8% fat, 72% carbohydrate, by energy) or a diet moderately high in saturated fat (47%, by energy) and low in carbohydrate (33%, by energy) for four weeks. Rats were studied in the absorptive state, in the post-absorptive state or after 2.5 h euglycaemic hyperinsulinaemia. Tissues sampled included IBAT and four white adipose tissue depots, two abdominal (parametrial (PM) and perirenal (PR)) and two superficial (subcutaneous (SC) and interscapular (IS)). MEASUREMENTS: Whole-body glucose disposal was estimated using [3-3H]glucose. Glucose uptake/phosphorylation in vivo was estimated using 2-deoxy[1-3H]-D-glucose. Insulin action was evaluated using the euglycaemic-hyperinsulinaemic clamp technique. PDHa activity was assayed spectrophotometrically in freeze-clamped tissue extracts. PDH kinase (PDK) activities were assayed in mitochondrial extracts by rates of ATP-dependent PDHa inactivation. RESULTS: Whole-body glucose disposal was decreased by high-fat-feeding in the post-absorptive state (P < 0.05) and during euglycaemic hyperinsulinaemia (P < 0.01). Glucose transport/phosphorylation in IBAT was decreased by high-fat feeding in the absorptive (P < 0.001) and post-absorptive states (by 84%, P < 0.05) and during steady-state euglycaemic hyperinsulinaemia (by 73%, P < 0.001). IBAT PDHa activities were suppressed by high-fat feeding. Euglycaemic hyperinsulinaemia in post-absorptive control rats increased PDHa activities in IBAT (P < 0.001) to values comparable to those found in the absorptive state. Although IBAT PDHa activities were increased by euglycaemic hyperinsulinaemia in post-absorptive high-fat-fed rats, they remained lower (by 55%; P < 0.01) than those of controls. The failure of hyperinsulinaemia to normalise IBAT pyruvate dehydrogenase complex (PDHC) activities in high-fat-fed rats was associated with a stable 1.4-fold increase in IBAT PDK activity. High-fat feeding decreased glucose utilisation rates in the post-absorptive state in IS, but not in SC, PM or PR white adipose tissue depots. Euglycaemic hyperinsulinaemia significantly increased glucose utilisation in three out of the four depots of the control rats, but did not elicit statistically-significant changes in high-fat-fed rats. High-fat feeding influenced PDHa activity in PR, but was without significant effect on PDHa activity in PM, SC and IS. CONCLUSIONS: The results demonstrate that PDHa activity in the IBAT of rats maintained on standard diet responds to changes in insulin concentrations over the low physiological range, and that inactivation of PDHa in IBAT after feeding a diet moderately high in saturated fat is a consequence of the induction of tissue insulin resistance. Effects of this dietary regime on PDHa activity are paralleled by changes in insulin-stimulated glucose uptake/phosphorylation by IBAT in vivo, and IBAT is specifically targeted, with only moderate effects in white adipose tissue. The finding of impaired activation of BAT glucose transport/phosphorylation and PDHa activity in response to insulin may contribute to impaired thermogenesis in rats maintained on diets containing a relatively high proportion of saturated fat.

Adipose Tissue↗
PubMed1999

Hyperthyroidism facilitates cardiac fatty acid oxidation through altered regulation of cardiac carnitine palmitoyltransferase: studies in vivo and with cardiac myocytes.

The aim was to establish whether increased cardiac fatty acid oxidation in hyperthyroidism is due to direct alterations in cardiac metabolism which favour fatty acid oxidation and/or whether normal regulatory links between changes in glucose supply and fatty acid oxidation are dysfunctional. Euthyroid rats were sampled in the absorptive state or after 48 h starvation. Rats were rendered hyperthyroid by injection of tri-iodothyronine (1000 microg/kg body wt. per day; 3 days). We evaluated the regulatory significance of direct effects of hyperthyroidism by measuring rates of palmitate oxidation in the absence or presence of glucose using cardiac myocytes. The results were examined in relation to the activity/regulatory characteristics of cardiac carnitine palmitoyltransferase (CPT) estimated by measuring rates of [3H]palmitoylcarnitine formation from [3H]carnitine and palmitoyl-CoA by isolated mitochondria. To define the involvement of other hormones, we examined whether hyperthyroidism altered basal or agonist-stimulated cardiac cAMP concentrations in cardiac myocytes and whether the effects of hyperthyroidism could be reversed by 24 h exposure to insulin infused subcutaneously (2 i. u. per day; Alzet osmotic pumps). Rates of 14C-palmitate oxidation (to 14CO2) by cardiac myocytes were significantly increased (1.6 fold; P< 0.05) by hyperthyroidism, whereas the percentage suppression of palmitate oxidation by glucose was greatly diminished. Cardiac CPT activities in mitochondria from hyperthyroid rats were 2-fold higher and the susceptibility of cardiac CPT activity to inhibition by malonyl-CoA was decreased. These effects were not mimicked by 48 h starvation. The decreased susceptibility of cardiac CPT activities to malonyl-CoA inhibition in hyperthyroid rats was normalised by 24 h exposure to elevated insulin concentration. Acute insulin addition did not influence the response to glucose in cardiac myocytes from euthyroid or hyperthyroid rats and basal and agonist-stimulated cAMP concentrations were unaffected by hyperthyroidism in vivo. The data provide insight into possible mechanisms by which hyperthyroidism facilitates fatty acid oxidation by the myocardium, identifying changes in cardiac CPT activity and malonyl-CoA sensitivity that would be predicted to render cardiac fatty acid oxidation less sensitive to external factors influencing malonyl-CoA content, and thereby to favour fatty acid oxidation. The increased CPT activity observed in response to hyperthyroidism may be a consequence of an impaired action of insulin but occurs through a cAMP-independent mechanism.

Animals↗
PubMed1999

Antecedent protein restriction exacerbates development of impaired insulin action after high-fat feeding.

The study investigated whether a persistent impairment of insulin secretion resulting from mild protein restriction predisposes to loss of glucoregulatory control and impaired insulin action after the subsequent imposition of the diabetogenic challenge of high-fat feeding. Offspring of dams provided with either control (20% protein) diet (C) or an isocaloric restricted (8%) protein diet (PR) were weaned onto the maintenance diet with which their mothers had been provided. At 20 wk of age, protein restriction enhanced glucose tolerance despite impaired insulin secretion and an augmented and sensitized lipolytic response to norepinephrine in adipocytes. C and PR rats were then transferred to a high-fat diet (HF, 19% protein, 22% lipid, 34% carbohydrate) and sampled after 8 wk. These groups are termed C-HF and PR-HF. Glucose tolerance was impaired in PR-HF, but not C-HF, rats. Insulin-stimulated glucose disposal rates were significantly lower (by 30%; P < 0.01) in the PR-HF group than in the C-HF group, and a specific impairment of antilipolytic response of insulin was unmasked in adipocytes from PR-HF, but not C-HF, rats. The study demonstrates that antecedent protein restriction accelerates and augments the development of impaired glucoregulation and insulin resistance after high-fat feeding.

Adipocytes↗
PubMed1999

Protein restriction during early development enhances insulin responsiveness but selectively impairs sensitivity to insulin at low concentrations in white adipose tissue during a later pregnancy.

Poor early nutrition may elicit long-term detrimental effects on adult health, including susceptibility to non-insulin-dependent diabetes mellitus. We investigated the impact of moderate maternal protein restriction during pregnancy and lactation on the action of insulin on adipocyte glucose uptake in female offspring during their own pregnancies. Offspring of dams provided with diets containing either 200 g protein/kg or 80 g protein/kg during pregnancy and lactation (termed C and EPR groups respectively) were weaned on to 200 g protein/kg diet at 24 d of age. At 9-12 weeks of age both groups were time-mated and studied at day 19 of gestation. Rates of glucose utilization (assessed using the 2-deoxy-D-[1-3H]glucose technique) measured in five distinct adipose tissue depots (parametrial (PM), mesenteric (MES), perirenal (PR), subcutaneous (s.c.), interscapular (IS)) in vivo in the post-absorptive state were consistently lower in early-protein-restricted (EPR) pregnant rats compared with control (C) pregnant rats. In C pregnant rats, insulin significantly increased glucose utilization only in the IS depot. In contrast, significantly increased glucose utilization rates in response to hyperinsulinaemia were evident in all five adipose-tissue depots of the EPR pregnant group. Consequently, glucose utilization rates in PM and s.c. depots during hyperinsulinaemia were significantly higher in EPR pregnant rats compared with C pregnant rats. Adipocytes were isolated from PM and MES depots to determine whether altered responses to insulin in vivo were retained in vitro. Rates of insulin-stimulated glucose uptake at sub-maximal (15 microU/ml) and maximal (15 mU/ml) insulin concentrations were significantly higher in both MES and PM adipocytes from EPR pregnant rats, but the sensitivity of glucose uptake to insulin at low concentrations was blunted compared with adipocytes from C pregnant rats. The results demonstrate that early protein restriction enhances the capacity for adipocyte glucose uptake at high insulin concentrations, but dampens the response to insulin at low physiological concentrations.

Adipocytes↗
PubMed1998

Moderate protein restriction during pregnancy modifies the regulation of triacylglycerol turnover and leads to dysregulation of insulin's anti-lipolytic action.

Moderate protein restriction throughout pregnancy in the rat leads to relative hyperlipidaemia and blunted insulin responsiveness of lipid fuel supply, and impairs foetal growth. The present study examined the basis for these changes. Isocaloric 8% (vs 20%) protein diets were provided throughout pregnancy. Rats were sampled at 19-20 days of gestation. Protein restriction enhanced triacylglycerol (TAG) secretion rates (estimated using Triton WR 1339) 1.6-fold (P < 0.05) in the post-absorptive state. Insulin infusion (4.2 mU/kg per min) decreased plasma TAG concentrations by 33% (P < 0.05) and 48% (P < 0.05) in control (C) and protein-restricted (PR) pregnant groups, an effect associated with suppression of TAG secretion by 42% (P < 0.05) and 51% (P < 0.01) respectively, in the C and PR groups. Since TAG concentrations decline more rapidly, while TAG secretion is enhanced, TAG utilisation during hyperinsulinaemia is enhanced in the PR group. We evaluated whether these changes were associated with dysregulation of lipolysis using adipocytes from two abdominal depots (mesenteric and parametrial). Noradrenaline-stimulated glycerol release was enhanced in parametrial adipocytes (by 40%; P < 0.05) from PR pregnant rats. The anti-lipolytic action of insulin at low concentrations (< or = 15 microU/ml) was impaired by protein restriction (adipocytes from both depots). There was no evidence for altered intra-hepatic regulation of fatty acid (FA) disposal at the level of carnitine palmitoyltransferase. Our results demonstrate increased post-absorptive production of non-carbohydrate energy substrates (TAG and FA) as a consequence of mild protein restriction during pregnancy. These adaptations contribute to a homeostatic strategy to reduce the maternal requirement for gluconeogenesis from available amino acids, optimising the foetal protein supply. Protein restriction also enhances TAG turnover during hyperinsulinaemia. This effect is not a consequence of abnormal regulation of hepatic lipid metabolism by insulin.

Adipocytes↗
PubMed1998

Studies of the long-term regulation of hepatic pyruvate dehydrogenase kinase.

The administration of a low-carbohydrate/high-saturated-fat (LC/HF) diet for 28 days or starvation for 48 h both increased pyruvate dehydrogenase kinase (PDHK) activity in extracts of rat hepatic mitochondria, by approx. 2.1-fold and 3.5-fold respectively. ELISAs of extracts of hepatic mitochondria, conducted over a range of pyruvate dehydrogenase (PDH) activities, revealed that mitochondrial immunoreactive PDHKII (the major PDHK isoform in rat liver) was significantly increased by approx. 1.4-fold after 28 days of LC/HF feeding and by approx. 2-fold after 48 h of starvation. The effect of LC/HF feeding to increase hepatic PDHK activity was retained through hepatocyte preparation, but was decreased on 21 h culture with insulin (100 micro-i.u./ml). A sustained (24 h) 2-4-fold elevation in plasma insulin concentration in vivo (achieved by insulin infusion via an osmotic pump) suppressed the effect of LC/HF feeding so that hepatic PDHK activities did not differ significantly from those of (insulin-infused) control rats. The increase in hepatic PDHK activity evoked by 28 days of LC/HF feeding was prevented and reversed (within 24 h) by the replacement of 7% of the dietary lipid with long-chain omega-3 fatty acids. Analysis of hepatic membrane lipid revealed a 1.9-fold increase in the ratio of total polyunsaturated omega-3 fatty acids to total mono-unsaturated fatty acids. The results indicate that the increased hepatic PDHK activities observed in livers of LC/HF-fed or 48 h-starved rats are associated with long-term actions to increase hepatic PDHKII concentrations. The long-term regulation of hepatic PDHK by LC/HF feeding might be achieved through an impaired action of insulin to suppress PDHK activity. In addition, the fatty acid composition of the diet, rather than the fat content, is a key influence.

Animals↗
PubMed1998

Impact of protein restriction on the regulation of cardiac carnitine palmitoyltransferase by malonyl-CoA.

Using a rat model of isocaloric protein restriction (8 v 20% protein diet), the study tested the hypothesis that growth retardation in utero, induced by maternal protein malnutrition, influences cardiac carnitine palmitoyltransferase (CPT) activity and regulation by malonyl-CoA in the newborn period, as well as in the offspring's adult life. The susceptibility of cardiac CPT to inhibition by malonyl-CoA was greater in adulthood than in hearts of 4-day-old neonatal rats, consistent with decreased expression of the L-CPT I isoform and increased expression of the M-CPT I isoform in adulthood. Maternal protein restriction during pregnancy resulted in reduced foetal growth and significantly (P < 0.05) lower rates of cardiac glucose utilization in vivo in the adult offspring, suggesting a switch to the use of substrates other than glucose. Maternal protein restriction did not affect CPT activity in hearts of 4-day-old neonatal offspring and, furthermore, the relative sensitivity of CPT activity to malonyl-CoA inhibition was unchanged by maternal protein restriction. It is therefore unlikely that maternal protein malnutrition has any major impact on cardiac mitochondrial fatty acid oxidation in the offspring during early postnatal development through altered regulatory characteristics of CPT. Transfer of rats previously maintained on 8% protein diet to 20% protein diet at weaning did not influence age-dependent changes in cardiac CPT activity or increase the susceptibility of cardiac CPT to inhibition by malonyl-CoA. Cardiac CPT activities and the susceptibility of cardiac CPT activities to malonyl-CoA inhibition in adulthood did not differ significantly between rats maintained on 8 or 20% protein throughout. Palmitate oxidation was suppressed to a similar extent by glucose in cardiac myocytes from adult rats maintained on 20% protein diet or 8% protein diet throughout and from rats transferred from 8 to 20% protein diet at weaning. The results exclude cardiac CPT activity as a direct target for the metabolic programming of cardiac function and cardiovascular disease associated with early growth retardation.

Animals↗
PubMed1998

The impact of genetic hypertension on insulin secretion and glucoregulatory control in vivo: studies with the TGR(mRen2)27 transgenic rat.

OBJECTIVE: To examine the associations among hypertension, insulin secretion, glucose tolerance and insulin resistance in vivo. DESIGN: Glucose tolerance and insulin secretion during an intravenous glucose tolerance test and action of insulin on whole-body glucose kinetics in the post-absorptive state and during hyperinsulinaemia were examined in conscious, unrestrained TGR(mRen2)27 rats and age-matched transgene-negative controls. METHODS: Glucose tolerance and insulin secretion were examined after intravenous administration of 500 mg glucose/kg body weight. Endogenous glucose production and whole-body glucose disposal were estimated using [3-3H]-glucose during euglycaemic-hyperinsulinaemic clamping. Muscle glucose utilization was estimated using 2-deoxy-[1-3H]-glucose. RESULTS: Despite there being higher insulin levels, whole-body glucose turnover was significantly lower in post-absorptive TGR(mRen2)27 rats than it was in transgene-negative controls. This was associated with significant suppression of glucose uptake/phosphorylation by oxidative skeletal muscles. TGR(mRen2)27 rats also exhibited significantly lower blood glucose levels, higher plasma insulin levels and higher rates of disappearance of glucose after intravenous administration of glucose. During hyperinsulinaemia, steady-state glucose infusion rates required to maintain euglycaemia in TGR(mRen2)27 rats were significantly greater, indicating that an increase in whole-body action of insulin had occurred. This was due to significantly greater suppression of endogenous production of glucose: insulin-stimulated glucose disposal rates did not differ significantly between the two groups. CONCLUSIONS: The results indicate that TGR(mRen2)27 rats have an enhanced and sensitized insulin-secretory response to glucose, together with a greater than normal hepatic action of insulin. Insulin-mediated glucose disposal was not impaired. The results therefore do not support the hypothesis that hypertension plays a primary role in the development of insulin resistance.

Animals↗
PubMed1998

Cardiac protein kinase C expression in two models of cardiac hypertrophy associated with an activated cardiac renin-angiotensin system: effects of experimental hyperthyroidism and genetic hypertension (the mRen-2 rat).

There is evidence for a role of protein kinase C (PKC) in the development of cardiac hypertrophy. We examined the expression of individual PKC isoforms in the adult rat heart in two distinct, well-characterised in vivo models of cardiac hypertrophy associated with an activated cardiac renin-angiotensin system, namely experimental hyperthyroidism and the TGR(mRen2)27 rat. The cardiac expression of a range of PKC isoforms (PKC-alpha, PKC-omega, PKC-epsilon, PKC-gamma, and PKC-tau) was examined by immuno-blotting. Our work demonstrates that the expression of total cardiac nPKC-omega and nPKC-epsilon relative to protein is selectively and differentially modified in these models. A consistent up-regulation of nPKC-omega in conjunction with overall down-regulation of nPKC-epsilon was observed in both models. The expression of other PKC isoforms was unaffected. The divergent responses of the expression of the two nPKC isoforms to an activated cardiac renin-angiotensin system in vivo in adulthood suggest that these individual nPKC isoforms subserve specific roles in the response.

Animals↗
PubMed1997

Different mechanisms underlie the long-term regulation of pyruvate dehydrogenase kinase (PDHK) by tri-iodothyronine in heart and liver.

Antibodies to purified recombinant PDHKII were used for ELISAs of PDHKII in mitochondrial extracts. In liver, hyperthyroidism elicited a 2.3-fold increase in PDHK activity (P < 0.01) which was accompanied by a significant 1.5-fold (P < 0.001) increase in the amount of mitochondrial immunoreactive PDHKII. In contrast, despite a stable 2.0-fold increase in cardiac PDHK activity (P < 0.001), the amount of mitochondrial immunoreactive PDHKII in heart was unaffected by hyperthyroidism. The mechanisms for long-term regulation of PDHK activity by thyroid hormones therefore differ fundamentally between heart and liver.

Animals↗