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G K Radda

Publications and source records attributed to G K Radda.

At least 73 records · Page 4Linked to original sources
PubMed1996

Regulation of oxidative and glycogenolytic ATP synthesis in exercising rat skeletal muscle studied by 31P magnetic resonance spectroscopy.

31P magnetic resonance spectroscopy measurements of pH and the concentrations of orthophosphate and phosphocreatine were used to estimate rates of glycogenolytic and oxidative ATP synthesis in rat leg muscle during 6 min sciatic nerve stimulation at different rates (1-4 Hz). To study the regulation of glycogenolysis during exercise, the apparent 'glycogenolytic capacity' (L(MAX)) was calculated from glycogenolytic ATP synthesis rate and orthophosphate concentration as a measure of the Ca2+-dependent activation of glycogen phosphorylase. This was found to be proportional to the total ATP synthesis rate (F), and to decline with time; expressed relative to total ATP turnover rate as L(MAX)/F, its initial value was 2.9+/-0.6, declining with half-time 1.4+/-0.4 min. The apparent 'mitochondrial capacity' (Q(MAX)), calculated from oxidative ATP synthesis rate and [ADP], was independent of ATP turnover rate, but increased with half-time 0.8+/-0.1 min to 29+/-2 mmol kg(-1) min(-1): thus [ADP] was the predominant but not the only influence on oxidative ATP synthesis. Numerical simulation shows that time-dependent changes in L(MAX)/F exert a strong influence on pH and on the concentrations of phosphocreatine and ADP.

Adenosine Triphosphate↗
PubMed1996

Abnormal regulation of hexokinase in insulin-resistant skeletal muscle.

The present study has assessed the potential involvement of hexokinase in the control of insulin-mediated glucose metabolism in insulin-sensitive and -resistant skeletal muscle. Soleus muscle strips from lean (insulin-sensitive) and obese (insulin-resistant) Zucker rats were incubated with 10 or 10,000 microU insulin.ml-1 and then homogenized using a protocol to maintain the location of hexokinase in situ. Hexokinase is inhibited by glucose 6-phosphate, a metabolic intermediate which may have a central role in the regulation of glycogen synthesis. Two separate measurements of hexokinase activity were made on each muscle homogenate: the total hexokinase activity (glucose 6-phosphate was metabolized immediately by glucose 6-phosphate dehydrogenase) and the fractional hexokinase activity (glucose 6-phosphate accumulated so as to regulate the enzyme as in vivo). The total hexokinase activity was equal in insulin-sensitive and -resistant muscle and was unaffected by the extracellular insulin concentration. The fractional hexokinase activity was significantly increased by insulin (10,000 microU.ml-1) in all muscles (lean, 82%; obese, 52%; P < 0.05) although the stimulated fractional hexokinase activity was lower in the muscle from obese Zucker rats compared to lean (P < 0.05). These results provide evidence that insulin decreases the inhibition of hexokinase by glucose 6-phosphate in insulin-sensitive but not in insulin-resistant muscle. This study has revealed short-term regulation of hexokinase by insulin which is defective in insulin-resistant skeletal muscle. Thus, the study has identified hexokinase as a potential regulatory site of insulin action that is abnormal in insulin resistance. The altered regulation of hexokinase may be a major contributing factor to the reduced insulin-mediated glucose fluxes in insulin-resistant skeletal muscle.

Animals↗
PubMed1996

Intracellular [Ca2+] staircase in the isovolumic pressure--frequency relationship of Langendorff-perfused rat heart.

Fluorescence and 31P magnetic resonance spectroscopy have been used to monitor simultaneously, the [Ca2+]i staircase and high energy phosphate metabolism in isolated Langendorff-perfused rat heart paced at 2, 4 and 6 Hz. In order to investigate further the relationship between high energy phosphate metabolism and the calcium staircase we perturbed the intracellular phosphocreatine (PCr)/creatine concentration with dietary beta-guanidinopropionic acid (beta-GPA). We have observed that: (a) At 2 Hz stimulation, the ventricular -Ca2+-i-dependent fluorescence decay is biexponential and continues to decay throughout the interstimulus interval; (b) at 4 Hz and 6 Hz, the [Ca2+]i decay is monoexponential; (c) end-diastolic [Ca2+]i is elevated at higher stimulation frequencies; (d) net [Ca2+]i flux per cycle is reduced at higher stimulation frequencies and is therefore correlated inversely with stimulation frequency and end-diastolic [Ca2+]i; (e) "heart rate * [Ca2+]i flux product" which is a measure of the work done in cycling calcium, is directly proportional to stimulation frequency; (f) the hysteresis between peak ventricular isovolumic pressure and peak fluorescence is decreased at higher stimulation frequencies; (g) no correlation was detected between the PCr/ATP ratio and stimulation frequency; (h) despite a 60% decrease in the myocardial PCr/ATP ratio after beta-GPA feeding, rat heart is able to maintain the end-diastolic [Ca2+]i-dependent fluorescence, and therefore the [Ca2+]i staircase relationship, similar to that of normal rat heart. In conclusion, using a physiological stimulation range and substrate supply we have observed a negative staircase of both [Ca2+]i and isovolumic pressure in whole heart which is not hypoxic. We propose that the inability of the sarcoplasmic reticulum to sequester sufficient cytosolic calcium at high stimulation frequencies leads to an elevation in end-diastolic [Ca2+]i, decreased net calcium flux per cycle resulting in a negative [Ca2+]i staircase and thus a negative isovolumic pressure-frequency relationship. We did not detect any correlation between steady-state high energy phosphate metabolism and stimulation frequency.

Adenosine Triphosphate↗
PubMed1996

Modulation of bryostatin 1 muscle toxicity by nifedipine: effects on muscle metabolism and oxygen supply.

Bryostatin 1, an anti-neoplastic agent and protein kinase C activator, has dose-limiting toxicity manifesting as myalgia. Studies in vivo have suggested that this myalgia may be caused by impairment of oxidative metabolism as mitochondrial capacity, muscle reoxygenation and proton washout from muscle are reduced by bryostatin, possibly as a result of vasoconstriction. To investigate these mechanisms further, and to enable use of bryostatin for prolonged periods, the effect of a vasodilator on the established effects of bryostatin on calf metabolism was studied using 31P magnetic resonance spectroscopy and near infrared spectroscopy. Six patients with disseminated melanoma were examined on four occasions: before and 1 week after initiation of long-term nifedipine (10 mg twice daily) treatment and then 4 and 48 h after bryostatin infusion (25 micrograms m(-2)). Nifedipine impaired muscle oxidative metabolism but had no effect on proton efflux or muscle reoxygenation rate. In the presence of nifedipine, two of the effects of bryostatin, impaired reoxygenation rate and reduced proton efflux, were abolished, but the impaired mitochondrial activity remained. These results show that nifedipine counteracted the vasoconstrictive effect of bryostatin 1. However, because nifedipine itself had an unexpected effect on mitochondrial metabolism, it was not possible to assess whether nifedipine modified bryostatin's effect on this variable. There was no additive detrimental effect of bryostatin on mitochondrial metabolism and nifedipine did not reduce the clinical toxicity of bryostatin 1, which cannot therefore be due to vasoconstriction.

Adult↗
PubMed1996

Increased oxidative and delayed glycogenolytic ATP synthesis in exercising skeletal muscle of obese (insulin-resistant) Zucker rats.

1. To examine metabolic correlates of insulin resistance in skeletal muscle, we used 31P magnetic resonance spectroscopy to study glycogenolytic and oxidative ATP synthesis in leg muscle of lean and obese Zucker rats in vivo during 6 min sciatic nerve stimulation at 2 Hz. 2. The water content of resting muscle was reduced by 21 +/- 7% in obese (insulin-resistant) animals compared with lean animals, whereas the lipid content was increased by 140 +/- 70%. These results suggest that intracellular water content was reduced by 17% in obese animals. 3. During exercise, although twitch tensions were not significantly different in the two groups, rates of total ATP synthesis (expressed per litre of intracellular water) were 48 +/- 20% higher in obese animals, suggesting a 50 +/- 8% reduction in intrinsic "metabolic efficiency'. Changes in phosphocreatine and ADP concentration were significantly greater in obese animals than in lean animals, whereas changes in intracellular pH did not differ. 4. These results imply that oxidative ATP synthesis during exercise is activated earlier in obese animals than in lean animals. This difference was not fully accounted for by the greater increase in the concentration of the mitochondrial activating signal ADP. Neither the post-exercise recovery kinetics of phosphocreatine nor the muscle content of the mitochondrial marker enzyme citrate synthase was significantly different in the two groups. The increased oxidative ATP synthesis in exercise must therefore be due to altered kinetics of mitochondrial activation by signals other than ADP. 5. Thus, the insulin-resistant muscle of obese animals may compensate for its decreased efficiency (and consequent increased need for ATP) by increased reliance on oxidative ATP synthesis.

Adenosine Triphosphate↗
PubMed1996

Brain metabolism is abnormal in the mdx model of Duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is an X-linked genetic disorder primarily affecting young boys, often causing mental retardation in addition to the well-known progressive muscular weakness. Normal dystrophin expression is lacking in skeletal muscle and the CNS of both DMD children and the mdx mouse model. To date, 31P-magnetic resonance spectroscopy (MRS) has shown in vivo several abnormalities within skeletal muscle of mdx mice and DMD boys. In this study, we determined whether similar abnormalities occur in mdx brain in vivo by using 31P-MRS in addition to metabolite and enzyme analysis to study cerebral metabolism. An increased inorganic phosphate (P(i))/phosphocreatine (PCr) and pH was found in vivo for mdx brain compared with controls, and biochemical analysis showed a reduction in total creatine, an increased extracellular and decreased intracellular volume in mdx brain. No differences were found in any glycolytic or mitochondrial maximal enzyme activities. These changes are discussed with respect to the biochemical changes found in muscle from DMD patients and mdx mice. It is proposed that these biochemical changes may be a factor in the reduced cognitive capacity of mdx mice and some DMD children.

Animals↗
PubMed1996

Wortmannin-mediated inhibition of phosphatidylinositol 3-kinase activity triggers apoptosis in normal and neoplastic B lymphocytes which are in cell cycle.

The B cell functional response following ligation of surface (s) IgM is dependent upon the differentiation stage of the population studied: cross-linking sIgM promotes proliferation of resting tonsillar follicular mantle (FM) B lymphocytes but induces apoptosis in the susceptible Epstein-Barr virus genome-negative Burkitt lymphoma (BL) cell line Ramos (Ramos-BL). This study investigates whether phosphatidylinositol-3-kinase (Pl3-kinase), which has been reported to be intimately involved in the regulation of cellular growth, plays a role in the regulation of these sigpromoted B cell responses, and uses the selective and irreversible inhibitor of Pl3-kinase activity, wortmannin (Wm). In Ramos-BL B cells, at 8 h post-treatment, Wm triggers a transient increase in apoptosis of 16 +/- 6.9% with a concomitant cellular loss of 16 +/- 6.1% from the G1 phase of cell cycle; [3H]thymidine incorporation also decreases by 33 +/- 5.0%, from 37,274 c.p.m. +/- 10% to 25,127 c.p.m. +/- 4.0%. Moreover, at 72 h culture, Wm inhibits anti-IgM-induced FM B lymphocyte levels of [3H]thymidine incorporation typically by 47% and triggers 80% apoptosis from the G0G1 phase of cell cycle. Ramos-BL B cells exhibit high basal levels of Pl3-kinase activity, as determined by immunoprecipitation with antibody to the p85 regulatory subunit of Pl3-kinase and 32P incorporation into phosphatidylinositol, which is not significantly affected by anti-IgM stimulation; by contrast, anti-IgM stimulates significant Pl3-kinase activity over negligible basal levels in FM B lymphocytes. Pre-treatment with Wm inhibits Pl3-kinase activity in both cell types. Taken together these data indicate that in Ramos-BL B cells sigM-triggered growth arrest and apoptosis is Pl3-kinase independent, whereas Pl3-kinase activity is critical for sIgM-triggered mitogenesis of FM B lymphocytes. Thus Pl3-kinase plays a pivotal role in the regulation of both normal and neoplastic B lymphocyte progression through the cell cycle, such that if this Pl3-kinase-dependent pathway is inhibited these cells default to apoptosis.

1-Phosphatidylinositol 4-Kinase↗
PubMed1996

Effect of creatine on aerobic and anaerobic metabolism in skeletal muscle in swimmers.

OBJECTIVE: To examine the effect of a relatively low dose of creatine on skeletal muscle metabolism and oxygen supply in a group of training athletes. METHODS: 31P magnetic resonance and near-infrared spectroscopy were used to study calf muscle metabolism in a group of 10 female members of a university swimming team. Studies were performed before and after a six week period of training during which they took either 2 g creatine daily or placebo. Calf muscle metabolism and creatine/choline ratios were studied in resting muscle, during plantar flexion exercise (10-15 min), and during recovery from exercise. RESULTS: There was no effect of creatine on metabolite ratios at rest or on metabolism during exercise and recovery from exercise. Muscle oxygen supply and exercise performance were not improved by creatine if compared to placebo treated subjects. CONCLUSIONS: Oral creatine supplementation at 2 g daily has no effect on muscle creatine concentration, muscle oxygen supply or muscle aerobic or anaerobic metabolism during endurance exercise.

Administration, Oral↗
PubMed1996

Magnetic resonance spectroscopy in congenital heart disease.

OBJECTIVE: To determine the feasibility of studying myocardial and skeletal muscle bioenergetics using 31P magnetic resonance spectroscopy (MRS) in babies and young children with congenital heart disease. SUBJECTS: 16 control subjects aged 5 months to 24 years and 18 patients with CHD, aged 7 months to 23 years, of whom 11 had cyanotic CHD, five had cardiac failure, and two had had a Senning procedure. DESIGN: 31P MRS was carried out using a 1.9 Tesla horizontal 65 cm bore whole body magnet to study the myocardium in 10 patients and skeletal muscle (gastrocnemius) in 14 patients, eight of whom were exercised, together with appropriate controls. RESULTS: In hypoxaemic patients, in skeletal muscle at rest intracellular pH (pHi) was abnormally high [7.06 (SEM 0.04) v 7.04 (0.05), P < 0.01] and showed a positive correlation with haemoglobin (P < 0.03). On exercise, hypoxaemic patients fatigued more quickly but end-exercise pHi and phosphocreatine recovery were normal, implying that an equivalent but smaller amount of work had been performed. End-exercise ADP concentration was lower. On recovery, the initial rate of phosphocreatine resynthesis was low. Skeletal muscle bioenergetics were within normal limits in those in heart failure. In the myocardium, the phosphocreatine/ATP ratio was similar in controls and hypoxaemic subjects, but low in those in heart failure. CONCLUSIONS: In heart failure, the myocardial phosphocreatine/ATP ratio was reduced, as in adults, while resting skeletal muscle studies were normal. By contrast, hypoxaemic children had normal myocardial bioenergetics, but showed skeletal muscle alkalinity, and energy reserves were more readily depleted on exercise. On recovery, the initially slow phosphocreatine resynthesis rate reflects a low rate of mitochondrial ATP synthesis, probably due to an inadequate oxygen supply. 31P MRS offers a safe, non-invasive method of studying myocardial and skeletal muscle bioenergetics in children as young as 5 months.

Adenosine Triphosphate↗
PubMed1996

Abnormalities in exercising skeletal muscle in congestive heart failure can be explained in terms of decreased mitochondrial ATP synthesis, reduced metabolic efficiency, and increased glycogenolysis.

OBJECTIVE: To distinguish between the effects of reduced oxidative capacity and reduced metabolic efficiency on skeletal muscle bioenergetics during exercise in patients with congestive heart failure. DESIGN AND PATIENTS: Patients were studied by 31P magnetic resonance spectroscopy during aerobic exercise and recovery, and results compared with controls. RESULTS: In flexor digitorum superficialis muscle (26 patients) there was a 30% decrease in oxidative capacity compared with control (mean (SE) 36 (2) v 51 (4) mM/min) and also a 40% decrease in "effective muscle mass" (5 (1) v 9 (1) arbitrary units), probably at least partly the result of reduced metabolic efficiency. Both contribute to increased phosphocreatine depletion and intracellular acidosis during exercise. However, an increased concentration of ADP (an important mitochondrial regulator) during exercise permitted near-normal rates of oxidative ATP synthesis. Results were similar in gastrocnemius muscle (20 patients), with a 30% decrease in maximum oxidative capacity (29 (4) v 39 (3) mM/min) and a 65% decrease in effective muscle mass (5 (1) v 13 (2) arbitrary units). Exercise training improved maximum oxidative capacity in both muscles, and in gastrocnemius effective muscle mass also. CONCLUSIONS: Skeletal muscle exercise abnormalities in patients with congestive heart failure results more from decreased metabolic efficiency than from the abnormalities in mitochondrial oxidation. Both decreased efficiency and defective mitochondrial oxidation result in an increased activation of glycogen phosphorylase, and may be improved by exercise training.

Adenosine Diphosphate↗
PubMed1996

Effect of parathyroid hormone on rat skeletal muscle in vivo.

The secondary hyperparathyroidism of chronic renal failure has been implicated in the pathogenesis of metabolic abnormalities in skeletal muscle. We studied the muscle metabolism in a model of hyperparathyroidism (Wistar rats injected with parathyroid hormone or saline for 4 days). 31P magnetic resonance spectroscopy allowed measurements of the concentration of cytosolic metabolically active inorganic phosphate [Pi] at rest and the rates of oxidative and anaerobic adenosine triphosphate turnover during exercise and recovery. Parathyroid hormone caused significant reductions in plasma [Pi] and intracellular [Pi], but had no effect upon oxidative or glycogenolytic adenosine triphosphate turnover.

Adenosine Triphosphate↗
PubMed1996

Is a high glycogen content beneficial or detrimental to the ischemic rat heart? A controversy resolved.

A high glycogen level may be beneficial to the ischemic heart by providing glycolytic ATP or detrimental by increasing intracellular lactate and protons. To determine the effect of high glycogen on the ischemic myocardium, the glycogen content of Langendorff-perfused rat hearts was either depleted or elevated before 32 minutes of low-flow (0.5 mL/min) ischemia with Krebs-Henseleit buffer with or without 11 mmol/L glucose, followed by 32 minutes of reperfusion with buffer containing 11 mmol/L glucose. 31P nuclear magnetic resonance spectra were acquired sequentially throughout. Further experiments involved early reperfusion or the addition of HOE 694, a Na+-H+ exchange inhibitor, during reperfusion. When glucose was supplied throughout ischemia, no ischemic contracture occurred, and postischemic recovery of contractile function was highest, at 88% of preischemic function. In the absence of glucose, normal-glycogen hearts underwent ischemic contracture at 5 minutes, had an end-ischemic pH of 6.87, and recovered to 54%, whereas in high-glycogen hearts, contracture was delayed to 13 minutes, the end-ischemic pH was 6.61, and functional recovery decreased to 13%. Contracture onset coincided with the decrease in glycolysis, which occurred as glycogen became fully depleted. Functional recovery in the high-glycogen hearts increased to 89% when reperfused before contracture and to 56% when reperfused in the presence of HOE 694. Thus, during brief ischemia in the high-glycogen hearts, ischemic glycogen depletion and contracture were avoided, and the hearts were protected from injury. In contrast, during prolonged ischemia in the high-glycogen hearts, glycogen became fully depleted, and myocardial injury occurred; the injury was exacerbated by the lower ischemia pH in these hearts, leading to increased Na+-H+ exchange during reperfusion. The contradictory findings of past studies concerning the effect of high glycogen on the ischemic myocardium may thus be due to differences in the extent of glycogen depletion during ischemia.

Adenosine Triphosphate↗
PubMed1996

Changes in energy metabolism of calf muscle in patients with intermittent claudication assessed by 31P magnetic resonance spectroscopy: a phase II open study.

Energy status and metabolism in skeletal muscle of nine patients with peripheral arterial disease and suffering from intermittent claudication were evaluated using 31phosphorus magnetic resonance spectroscopy (MRS) before and after treatment for 3 months with propionyl-L-carnitine (PLC; 2 g/day p.o.). Maximum walking distance (MWD) was assessed on a standard treadmill (4 km/h, zero incline). For the group as a whole 31P MRS results did not change significantly with PLC. Although MWD increased by a mean of 36%, this change did not reach significance. However, when these variables were assessed with respect to the change in MWD, there were significant differences between those who increased MWD by > 30% (responders, R; n = 5) and those who did not (nonresponders, NR; n = 4). Compared with pretreatment values, during exercise the decrease in muscle pH in R relative to the decrease in phosphocreatine was less after PLC (p = 0.04). After exercise there was a significant inverse correlation between the changes in recovery half-time (t1/2) for phosphocreatine and in MWD (r = -0.91, p = 0.01). With PLC, Vmax increased in R (p = 0.04), but not in NR. For the patient group as a whole, the changes in Vmax and MWD correlated positively (r = 0.90, p = 0.01). This study helps to identify the changes in muscle metabolism that correlate with changes in exercise performance, and may accompany treatment with PLC.

Adenosine Diphosphate↗
PubMed1996

Effects of treatment of spontaneously hypertensive rats with the angiotensin-converting enzyme inhibitor trandolapril and the calcium antagonist verapamil on the sensitivity of glucose metabolism to insulin in rat soleus muscle in vitro.

We measured the sensitivity of glucose metabolism to insulin in soleus muscle preparations isolated from spontaneously hypertensive (SH) rats and normotensive age-matched Wistar-Kyoto (WKY) rats. SH rats were treated with the angiotensin-converting enzyme (ACE) inhibitor trandolapril (1 mg/kg) and/or a second antihypertensive drug, the calcium antagonist verapamil, alone (100 mg/kg) or as combination therapy (50 mg/kg). Treatment of SH rats with trandolapril or trandolapril in combination with verapamil for 6 weeks normalized the blood pressure. The estimated concentration of insulin required for half-maximal stimulation of glycogen synthesis (i.e., EC50 values) was approximately 500 microU/ml for muscles from both WKY and SH rats. This value is five times higher than the value obtained from soleus muscle preparations isolated from insulin-sensitive Wistar rats. This indicates that glycogen synthesis is insensitive to insulin in SH and WKY rat soleus muscle. Treatment of SH rats with trandolapril with or without verapamil improved the sensitivity of glycogen synthesis to insulin in soleus muscle. Further experiments investigated whether acute exposure (1 h) of insulin-sensitive skeletal muscle with either trandolaprilat (the active metabolite of trandolapril) or bradykinin (levels of which may be raised by ACE inhibition) could affect the insulin-stimulated rate of glucose metabolism. These results show that both trandolaprilat and bradykinin caused a small but significant increase in the rates of glucose metabolism. In conclusion, 1) SH and WKY rat skeletal muscle was insulin resistant, 2) chronic treatment of SH rats with trandolapril with or without verapamil normalized blood pressure and improved the response of glycogen metabolism to insulin, and 3) bradykinin and trandolaprilat acutely caused a small but significant increase in the rate of glycogen synthesis to a submaximal physiological concentration of insulin.

Angiotensin-Converting Enzyme Inhibitors↗
PubMed1996

Control of energy metabolism during muscle contraction.

Skeletal muscle energetics can be studied noninvasively at rest, during exercise, and in recovery using phosphorus nuclear magnetic resonance (31P-NMR). In resting muscle, inorganic phosphate (P(i)) and total cellular phosphate concentration are regulated by Na(+)-dependent P(i) transport. Insulin was shown to stimulate P(i) uptake in G-8 muscle cells, in isolated rat soleus muscle, and in human muscle in vivo under conditions of hyperinsulinemic-euglycemic clamp. The relationship between plasma P(i) and intracellular muscle P(i) was examined in a group of patients with elevated plasma P(i) resulting from renal failure. The total creatine content of muscle cells is controlled by an active creatine uptake in which beta 2-receptor stimulation and the activity of the Na(+)-K(+)-ATPase play a significant role. Recovery after exercise is entirely oxidative; the rate of ATP synthesis is largely controlled by ADP, the concentration of which is determined by the creatine kinase equilibrium that includes the concentration of H+. At the onset of aerobic dynamic exercise, ATP is maintained largely by glycolysis, producing lactic acid, and by phosphocreatine breakdown. After vasodilation, ATP synthesis becomes predominantly oxidative. The above processes can be quantitatively evaluated by 31P-NMR.

Animals↗