Cardiac Glycogen Research Articles

Exercise training attenuated the PKB and GSK-3 dephosphorylation in the myocardium of ZDF rats.

Cardiac dysfunction is a severe secondary effect of Type 2 diabetes. Recruitment of the protein kinase B/glycogen synthase kinase-3 pathway represents an integral event in glucose homeostasis, albeit its regulation in the diabetic heart remains undefined. Thus the following study tested the hypothesis that the regulation of protein kinase B/glycogen synthase kinase-3 was altered in the myocardium of the Zucker diabetic fatty rat. Second, exercise has been shown to improve glucose homeostasis, and, in this regard, the effect of swimming training on the regulation of protein kinase B/glycogen synthase kinase-3 in the diabetic rat heart was examined. In the sedentary Zucker diabetic fatty rats, glucose levels were elevated, and cardiac glycogen content increased, compared with wild type. A 13-wk swimming regimen significantly reduced plasma glucose levels and cardiac glycogen content and partially normalized protein kinase B-serine473, protein kinase B-threonine308, and glycogen synthase kinase-3alpha phosphorylation in Zucker diabetic fatty rats. In conclusion, hyperglycemia and increased cardiac glycogen content in the Zucker diabetic fatty rats were associated with dysregulation of protein kinase B/glycogen synthase kinase-3 phosphorylation. These anomalies in the Zucker diabetic fatty rat were partially normalized with swimming. These data support the premise that exercise training may protect the heart against the deleterious consequences of diabetes.

Fetal bradycardia at 28 weeks of gestation associated with cardiac glycogen phosphorylase b kinase deficiency

Fetal bradycardia at 28 weeks of gestation associated with cardiac glycogen phosphorylase b kinase deficiency

Impairment of glucose metabolism in hearts from rats treated with endotoxin.

In patients and animals with sepsis or critical illness, the mechanical function of the heart is often impaired. Although these conditions are accompanied by dramatic metabolic and hormonal changes, little is known about alterations of cardiac metabolism. In this study, we assessed the impact of an endotoxin-induced inflammation on cardiac glucose utilization. Bacterial endotoxin (1 mg/kg lipopolysaccharide from Salmonella typhimurium, LPS) was injected intravenously to rats. Six hours after LPS application, hearts were isolated and perfused in the Langendorff mode. Left ventricular pressure was reduced by 50% in hearts from LPS-treated rats, compared to those from saline-injected control animals. With glucose as the sole fuel, there was no difference in glycolysis between the groups. However, on addition of beta-hydroxybutyrate (an alternative fuel which inhibits phosphofructokinase via an increased citrate level), the glycolytic rate in the LPS group was 44 and 48% lower (in basal, and insulin-stimulated conditions, respectively; P<0.01) than in control hearts. At the end of perfusions with beta-hydroxybutyrate and insulin, the cardiac citrate content was 40% higher in LPS vs. controls (P<0.001). In addition to the reduced glycolysis, the insulin-dependent increase of cardiac glycogen was 77% smaller in LPS hearts. The difference between LPS and control glycolysis was abolished if the hearts were perfused with the ceramidase inhibitor N-oleyl-ethanolamine (5 microM), and also with the cyclooxygenase-2 inhibitor NS-398 (10 microM), or the thromboxane A2 receptor antagonist SQ-29548 (1 microM). The inflammatory reaction caused by endotoxin impairs cardiac glucose metabolism (and in particular, the action of insulin) in at least two ways: through the exacerbation of the counterregulatory effect of alternative fuels on glycolysis, and through a reduction in net glycogen synthesis. Impairment of glycolysis may be mediated by a sphingomyelin derivative, and COX-2-derived thromboxane A2.

Enzyme replacement therapy in the mouse model of Pompe disease

Deficiency of acid alpha-glucosidase (GAA) results in widespread cellular deposition of lysosomal glycogen manifesting as myopathy and cardiomyopathy. When GAA−/− mice were treated with rhGAA (20 mg/kg/week for up to 5 months), skeletal muscle cells took up little enzyme compared to liver and heart. Glycogen reduction was less than 50%, and some fibers showed little or no glycogen clearance. A dose of 100 mg/kg/week resulted in ∼75% glycogen clearance in skeletal muscle. The enzyme reduced cardiac glycogen to undetectable levels at either dose. Skeletal muscle fibers with residual glycogen showed immunoreactivity for LAMP-1/LAMP-2, indicating that undigested glycogen remained in proliferating lysosomes. Glycogen clearance was more pronounced in type 1 fibers, and histochemical analysis suggested an increased mannose-6-phosphate receptor immunoreactivity in these fibers. Differential transport of enzyme into lysosomes may explain the strikingly uneven pattern of glycogen removal. Autophagic vacuoles, a feature of both the mouse model and the human disease, persisted despite glycogen clearance. In some groups a modest glycogen reduction was accompanied by improved muscle strength. These studies suggest that enzyme replacement therapy, although at much higher doses than in other lysosomal diseases, has the potential to reverse cardiac pathology and to reduce the glycogen level in skeletal muscle.

Transgenic mice overexpressing mutant PRKAG2 define the cause of Wolff-Parkinson-White syndrome in glycogen storage cardiomyopathy.

Mutations in the gamma2 subunit (PRKAG2) of AMP-activated protein kinase produce an unusual human cardiomyopathy characterized by ventricular hypertrophy and electrophysiological abnormalities: Wolff-Parkinson-White syndrome (WPW) and progressive degenerative conduction system disease. Pathological examinations of affected human hearts reveal vacuoles containing amylopectin, a glycogen-related substance. To elucidate the mechanism by which PRKAG2 mutations produce hypertrophy with electrophysiological abnormalities, we constructed transgenic mice overexpressing the PRKAG2 cDNA with or without a missense N488I human mutation. Transgenic mutant mice showed elevated AMP-activated protein kinase activity, accumulated large amounts of cardiac glycogen (30-fold above normal), developed dramatic left ventricular hypertrophy, and exhibited ventricular preexcitation and sinus node dysfunction. Electrophysiological testing demonstrated alternative atrioventricular conduction pathways consistent with WPW. Cardiac histopathology revealed that the annulus fibrosis, which normally insulates the ventricles from inappropriate excitation by the atria, was disrupted by glycogen-filled myocytes. These anomalous microscopic atrioventricular connections, rather than morphologically distinct bypass tracts, appeared to provide the anatomic substrate for ventricular preexcitation. Our data establish PRKAG2 mutations as a glycogen storage cardiomyopathy, provide an anatomic explanation for electrophysiological findings, and implicate disruption of the annulus fibrosis by glycogen-engorged myocytes as the cause of preexcitation in Pompe, Danon, and other glycogen storage diseases.

Physiological Factors Associated with Weak Neonatal Poults (Meleagris gallopavo)

Management related deaths in turkey hatchlings remain a problem for turkey growers. The etiologies of such deaths also remain obscure. The hypothesis proposed by the current study was that weak poults identified by their characteristic behavior would differ physiologically from normal poults in a way that interferes with development of critical systems such as the cardiovascular and nervous system. Eight groups of 500 poults (from a 20-wk laying cycle) of the same breeder flock were examined. Each placement was in the same brooder house operated at identical conditions. Observations of flip-over poults were made at 6-h intervals for the initial wk of age. Identified poults (n = 12) were sacrificed and sampled immediately. Flip- over poults differed from controls only in depressed heart weight and plasma thyroid hormone concentrations. The identified poults did not differ from controls in body weight, liver weight, yolk weight, blood glucose or organ glycogen concentrations. In a second trial, reduced heart weights were noted along with reduced capability to maintain cardiac glycogen concentrations. The reduced heart weight or function may result in failure to provide nutrients for proper brain function. The data suggest that management to aid weak neonatal turkey poult deaths may need to focus on conditions fostering heart growth and function.

Elevated hexokinase increases cardiac glycolysis in transgenic mice.

Cardiac glucose metabolism is critical to normal and pathological function. The significance of the first committed metabolic step, glucose phosphorylation, has not been established. In this study a new transgenic model was developed in order to investigate the importance of this enzymatic step in cardiac glycolysis. Transgenic mice were produced that overexpress yeast hexokinase B under the control of a cardiac specific promoter. Yeast hexokinase B is a high affinity enzyme that is not inhibited by glucose-6-phosphate. Hexokinase enzyme activity was measured by a modified radiometric procedure. Cardiac glucose metabolism and contractility were measured in the Langendorff mode. Cardiac glycogen content and glucose-6-phosphate independent glycogen synthase activity were also determined. In transgenic hearts hexokinase activity was significantly elevated and increased glucose metabolism, particularly in the presence of insulin and during cardiac reperfusion. However during ischemic perfusion the effect of the transgene on glycolysis was minimal. Under all conditions tested there was no effect of hexokinase on contractility. Glycogen content of transgenic hearts was elevated 2-fold and glucose-6-phosphate independent glycogen synthase was also increased. These results demonstrate that glucose phosphorylation is a key step in determining cardiac glucose metabolism under oxidative conditions.

Akt Like a Woman

The oncogenic murine AKT8 retrovirus was identified almost 25 years ago, and the presence of the causative oncogenic agent v- akt was subsequently demonstrated in transformed cells or tumors from both mice and humans.1 2 The cellular homolog of v- akt was cloned in the early 1990s and was termed c-Akt (or simply Akt). Largely on the basis of its sequence similarity to both protein kinases A and C, it was concluded that Akt (alternatively known as protein kinase B [PKB]) represented a Ser-/Thr- protein kinase. Three mammalian genes have been identified, with transcripts of akt1 and akt2 being highly expressed in heart. The explosion of interest in Akt was the direct consequence of the realization that it is an effector of the lipid signaling molecule phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] (Figure⇓). Figure 1. Possible actions of Akt in the cardiovascular system. Arrows indicate activation; bars indicate inhibition. Phosphorylation of Bad inhibits its proapoptic activity by reducing its inhibition of the antiapoptotic actions of Bcl-2 and Bcl-XL. The overall consequences of activation of Akt are shown in boxes. For additional explanation, see the text. PDK indicates PtdIns(3,4,5)P3-dependent kinase; eNOS, endothelial NO synthase. By binding to their transmembrane receptor protein tyrosine kinases, a variety of growth factors, including insulin and insulin-like growth factor 1 (IGF1), activate the lipid kinase phosphatidylinositol 3-kinase (PI3K) to phosphorylate the membrane phospholipid PtdIns(4,5)P2, producing PtdIns(3,4,5)P3. PtdIns(3,4,5)P3 remains in the plane of the membrane and may serve to recruit Akt to this compartment from its normal cytoplasmic location by binding to the Akt pleckstrin-homology domain.1 2 In addition, PtdIns(3,4,5)P3 activates PtdIns(3,4,5)P3-dependent protein kinase, which phosphorylates Akt on a Thr- residue (Thr308 in Akt1/PKBα, Thr309 in Akt2/PKBβ, and Thr305 in PKBγ). …

Human Tissue Kallikrein Gene Delivery Attenuates Cardiac Glycogen Accumulation in Streptozotocin-Induced Diabetic Rats

P58 The tissue kallikrein-kinin system is locally present in the heart and is reduced under diabetic conditions. In the present study, we explored the effects of kallikrein gene delivery in the heart under diabetic conditions. Intravenous injection of streptozotocin (STZ) (60mg/kg) into 8-week-old male Sprague-Dawley rats produced hyperglycemia (460 ± 10 mg/dl, n=30). Three weeks after STZ treatment, adenovirus containing the human tissue kallikrein gene (Ad.CMV-cHK) or green fluorescence protein gene (Ad.CMV-GFP) under the control of CMV promoter/enhancer were injected via the tail vein into STZ-diabetic rats. Adenovirus-mediated kallikrein gene delivery caused a significant reduction of blood pressure as compared to control rats receiving control virus (170.8 ± 3.5 vs. 187.4 ± 4.2 mmHg, n=8, P&lt;0.01). Rats were sacrificed and tissues were collected for analysis at 16 days after gene delivery. Histological examination of heart sections stained with periodic acid-Schiff (PAS) showed marked accumulation of glycogen in cardiomyocytes of diabetic rats. Kallikrein gene delivery significantly reduced cardiac glycogen accumulation as compared to rats receiving control adenovirus. Quantitative analysis confirmed the morphological observation that kallikrein gene delivery significantly reduced cardiac glycogen levels as compared to control diabetic rats injected withAd.CMV-GFP (0.5 ± 0.07 vs. 2.15 ± 0.83 mg glycogen/mg protein, n=6, P&lt;0.05). Cardiac cAMP levels were significantly increased after kallikrein gene delivery. This is the first study to demonstrate that kallikrein gene delivery exerts a protective effect in the reduction of glycogen accumulation in cardiomyocytes of STZ-induced diabetic rats.

Response to ischaemia and susceptibility to ventricular arrhythmias in the diabetic rat heart

Diabetic patients are more prone to congestive heart failure and rhythm disorders, associated with an enhanced sensitivity to ischaemia. However, experimental studies are inconsistent on this issue. Our objectives were to compare susceptibility to ventricular arrhythmias in rats with chronic diabetes induced by streptozotocin injection (45 mg/kg, i.v.), utilizing two different models. After 8 weeks, heart dysfunction developed, and either anaestbetised open-chest rats in vivo or isolated Langendorff-perfused hearts were subjected to 30 min regional zero-flow ischaemia induced by occlusion of LAD coronary artery. Cardiac glycogen and lactate estimation, as well as electrophoretic protein separation followed by silver staining, in conjunction with Western blot analysis of mitogen-activated protein kinases (MAPKs) were performed before and after ischaemia. In open-chest rats, 90% of the controls exhibited ventricular tachycardia (VT) that represented 55.4% of total arrhythmias, whereby only 19.9% of arrhythmias occurred as VT in 44% of the diabetic rats (P < 0.05 vs controls). Duration of VT and ventricular fibrillation (VF) was reduced from 42.9 l 14.4 s in the controls to 4.8 f 2.5 s in the diabetics (P < 0.05). In the isolated hearts, high incidence of VF was decreased in the diabetic hearts, and although VT occurred in almost all of the diabetic hearts, the total duration of VT and VF was substantially shorter (46.5 f 12.5 vs 657 f 25 s in the controls; P < 0.05). Baseline glycogen and lactate were similar, while postischaemic accumulation of lactate was lower in the diabetic myocardium (20.4 f. 1.9 vs 29.5 f 2.9 @I/g w.wt. in the controls; P < 0.05). Significant differences in the amounts of some cytosolic and particulate proteins between the diabetic and control hearts and no differences in MAPKs were observed under baseline conditions. Ischaemia resulted in different pattern of subcellular distribution of p38/MAPK and SAPK/JNKs in the diabetic myocardium. These results suggest that rat heart with chronic diabetes is less susceptible to ventricular arrhythmias disregarding the model. It is thus conceivable to suggest that some endogenous mechanisms of cardioprotection are operating in the diabetic myocardium to counteract metabolic disorders leading to cardiac failure and dysrhythmias. In the protocol of zero-flow ischaemia, altered carbohydrate metabolism resulting in reduced accumulation of acid products, as well as MAPKs-mediated cell signaling might be involved in modulation of the response to ischaemia in the diabetic myocardium. Supported by the grants VEGA SR 2/6094/99 and GACR 306/98/@470.

EFFECTS OF OROTOC ACID ON ISCHAEMIC\\REPERFUSED MYOCARDIAL FUNCTION ANDGLYCOGEN CONTENT IN ISOLATED WORKING RAT HEARTS

The present study was carried out to investigate the effect of orotic acid on ischaemic\\reperfused myocardial function and cardiac glycogen content in isolated working rat hearts. In a preliminary series of studies, hearts isolated from male Wistar rats (300–350 g) were perfused with oxygenated Krebs–Henseleit buffer containing cumulative concentrations of orotic acid from 0.01 to 10.00 mg l−1. In the concentration range of 0.01–0.10 mg l−1, orotic acid significantly improved left ventricular function. Therefore, in the second series of studies, rats were treated intravenously with 0.01 mg kg−1orotic acid for 4 days. Hearts were then isolated and subjected to 30 min of global no-flow ischaemia followed by 10 min of reperfusion. Orotic acid treatment significantly improved post-ischaemic myocardial function and increased pre-ischaemic and post-ischaemic glycogen content of the heart. We conclude that orotic acid improves ischaemic\\reperfused cardiac performance and this effect may be based on the elevation of myocardial glycogen content.

Rapid decrease of glycogen concentration in the hearts of senescence-accelerated mice during aging

Age-related changes in glycogen concentration in the heart of SAMP8, a substrain of senescence-accelerated prone mouse, were compared with that in SAMR1, a substrain of senescence-accelerated resistant mouse. The decrease by about 50% in the glycogen concentration in the hearts of 4-8 weeks old SAMP8 was observed in comparison with that in age-matched SAMR1, whereas there was no difference in adult hearts between the two substrains. Additionally, glycogen phosphorylase activity was significantly increased (1.3-fold) in the heart of 4–8 weeks old SAMP8 without the change in glycogen synthase activity between the two substrains. At the same age, the lipid peroxide concentration was also increased (1.4-fold) in the hearts of SAMP8, suggesting that oxidative stress was involved in the early damage to the hearts of SAMP8. Furthermore, the present findings that the cardiac glycogen concentration in SAMP8 decreases more quickly than that in SAMR1 may indicate the possibility that an accelerated aging of SAMP8 is related to the early changes in the energy sources in the heart.

Intermittent Ischemia: Energy Metabolism, Cellular Volume Regulation, Adenosine and Insights into Preconditioning

Interruption of ischemia by brief reperfusions (I/R) is better tolerated by the heart than continuous ischemia. The present study aims to determine the metabolic profiles of isolated rat hearts during intermittent ischemia, the possible cardioprotective role of adenosine and the influence of I/R on intracellular volumes, using multinuclear NMR spectroscopy. After five I/R (5/5 min) episodes, hearts paced at 5 Hz developed pressures comparable to those of hearts continuously perfused for 50 min at 37°C (CP). Following the first 5 min episode of no-flow ischemia, [ADP] dropped from 72±9 to 43±5μm(P<0.001) and remained stable at the end of the following reperfusions, despite a 2.5–4-fold increase during each episode of 5 min ischemia. Intracellular volumes were stable during CP at a value of 2.50±0.06 ml/g dry weight, and decreased by 4, 8, and 12% after 1, 3, and 5 I/R episodes. The phosphorylation potentials decreased from 54±8 to 4 mm−1during each period of 5 min ischemia and were 40±6 and 28±6 mm−1after CP and I/R5, respectively. Cardiac glycogen had decreased during 50 min of CP from 103±13 to 81±9μmol/g dry weight and lactate production was 116±15μmol/heart. Five I/R episodes decreased glycogen to 46±7μmol/g dry weight (P<0.005vCP) and increased lactate efflux to 262±31μmol/heart (P<0.005vCP). These findings suggest that a brief ischemia/reperfusion episode increases anaerobic metabolism of exogenous glucose, reduces [ADP] and induces cellular shrinkage. Administration of the adenosine receptor blocker 8-phenyl theophylline (8PT) during intermittent perfusion depressed the developed pressure to 78±7%, accentuated the decrease in phosphorylation potential (14±4 mm−1), abolished cellular shrinkage, reduced lactate efflux and blunted the decrease in ADP following the first I/R episode. In variance, no detectable changes were observed during intermittent ischemia when the ATP-sensitive potassium channel blocker glibenclamide was administered. These data demonstrate: (a) a brief episode of ischemia/reperfusion stimulates anaerobic metabolism of exogenous glucose and lowers intracellular ADP concentration; (b) adenosine receptors are partially responsible for the glycolytic stimulation during intermittent ischemia; (c) cellular shrinkage is related to the rate of glycolysis during intermittent ischemia/reperfusion.

Loss of glycogen during preconditioning is not a prerequisite for protection of the rabbit heart.

Depletion of glycogen has been proposed as the mechanism of protection from ischemic preconditioning. The hypothesis was tested by seeing whether pharmacological manipulation of preconditioning causes parallel changes in cardiac glycogen content. Five groups of isolated rabbit hearts were studied. Group 1 experienced 30 min of ischemia only. Group 2 (PC) was preconditioned with 5 min of global ischemia followed by 10 min of reperfusion. Group 3 was preconditioned with 5 min exposure to 400 nM bradykinin followed by a 10 min washout period. Group 4 experienced exposure to 10 microM adenosine followed by a 10 min washout period, and the fifth group was also preconditioned with 5 min ischemia and 10 min reperfusion but 100 microM 8-(p-sulfophenyl)theophylline (SPT), which blocks adenosine receptors, was included in the buffer to block preconditioning's protection. Transmural biopsies were taken before treatment, just prior to the 30 min period of global ischemia, and after 30 min of global ischemia. Glycogen in the samples was digested with amyloglucosidase and the resulting glucose was assayed. Baseline glycogen averaged 17.3 +/- 0.6 mumol glucose/g wet weight. After preconditioning glycogen decreased to 13.3 +/- 1.3 mumol glucose/g wet weight (p < 0.005 vs. baseline). Glycogen was similarly depleted after pharmacological preconditioning with adenosine (14.0 +/- 1.0 mumol glucose/g wet weight, p < 0.05 vs. baseline) suggesting a correlation. However, when preconditioning was performed in the presence of SPT, which blocks protection, glycogen was also depleted by the same amount (13.3 +/- 0.7 mumol glucose/g wet weight, p = ns vs. PC). Bradykinin, which also mimics preconditioning, caused no depletion of glycogen (16.3 +/- 0.8 mumol glucose/g wet weight, p = ns vs. baseline). Because preconditioning with bradykinin did not deplete glycogen and because glycogen continued to be low when protection from preconditioning was blocked with SPT, we conclude that loss of glycogen per se does not cause the protection of preconditioning.

Physiological Differences in Late Embryos from Turkey Breeders at Different Ages

Physiological mechanisms were measured in embryos from turkey hens of different ages to determine associations with declines in hatchability as breeder hens age. As the hens aged from 32 to 54 wk of age, embryonic viability declined (P < 0.05). The greatest proportional increase (P < 0.01) in embryonic mortality of aging hens occurred at the plateau stage in oxygen consumption or immediately thereafter at pipping. Eggshell conductance constants increased (P < 0.01) as hens aged but did not change after mid-lay, suggesting an alteration in respiration for the embryos in eggs produced by older hens compared to eggs produced by the same hens at younger ages. The alteration may cause embryos in eggs from older hens to reach the plateau stage in oxygen consumption (approximately 25 to 26 d of incubation) earlier in development than embryos from young hens. Hepatic and cardiac glycogen concentrations were greatest (P < 0.001) in embryos from hens at the youngest age and then declined (P < 0.05) as the hens aged. Embryonic blood plasma glucose concentrations declined (P < 0.05) similarly. Plasma thyroxine (T4) and triiodothyronine (T3) concentrations were measured in embryos from the hens at different ages as well. Increased (P < 0.05) T4 was evidenced in embryos from the youngest hens, whereas increased (P < 0.05) T3 activity was evident in embryos from hens of older ages. It was concluded that the decline in hatchability seen as turkey breeder hens age may have a basis in the differences seen in the physiology of hatching in embryos. Specifically, thyroid influences on growth and carbohydrate metabolism may be involved in decreased embryonic viability.

Role of high glycogen in underperfused diabetic rat hearts with added norepinephrine.

The relationship between cardiac dysfunction and glycogen level and/or duration of diabetes was examined during underperfusion (2 ml/min/g heart weight) with 10(-6)M norepinephrine (NE) in isolated 1- and 6-week streptozotocin-diabetic rat (diabetes mellitus, DM) hearts and non-DM hearts. Glycogen levels in non-DM and 1- and 6-week DM hearts were 85, 120, and 206 mumol/g dry weight, respectively, in the subendocardium. About 13 min after the start of underperfusion with NE, the diastolic tension in 1-week DM hearts began to increase when the glycogen level had decreased to half; in 6-week DM hearts, glycogen decreased more markedly without greater lactate accumulation, but these glycogen levels were still higher (104 mumol/g dry weight) than those in 1-week DM hearts and the diastolic tension did not increase. About 17 min after the onset of underperfusion, the glycogen decreased to the 13-min level of 1-week DM hearts (64 mumol/g dry weight) and the diastolic tension began to increase. Until 20 min after the onset of underperfusion, the injury was less in 6-week than in 1-week DM hearts. However, after 60-min underperfusion with NE, when the glycogen level was markedly low in both groups ( < 20 mumol/g dry weight), diastolic tension was increased twice as much in 6-week DM as in 1-week DM hearts and was related to the decreased subendocardial ATP level. The results indicate that the markedly high glycogen content in diabetic hearts probably helps delay the start of the increase in left ventricular (LV) stiffness during underperfusion with NE. Ultimately, however, the degree of the injury depends on the duration, i.e., the severity, of the diabetes.

Effects of vanadium treatment on the alterations of cardiac glycogen phosphorylase and phosphorylase kinase in streptozotocin-induced chronic diabetic rats.

Supersensitivity to isoproterenol (ISO) induced activation of cardiac phosphorylase in diabetic rat heart has been previously demonstrated and was also reproduced in this study. To explore further the nature of this supersensitivity, we examined the activity of phosphorylase kinase and the level of cyclic AMP (cAMP) in this tissue. We observed a significantly enhanced activation of phosphorylase kinase but no increase in cAMP levels in response to ISO stimulation in diabetic rat heart, suggesting that the supersensitivity of phosphorylase activation in diabetic heart may result from an enhanced activation of phosphorylase kinase that does not involve the cAMP pathway. On the other hand, perfusion of diabetic rat heart with verapamil (5 x 10(-8) M) prior to ISO stimulation abolished the enhanced cardiac phosphorylase activation, suggesting a role for calcium in the supersensitivity of phosphorylase activation. Furthermore, treatment of the diabetic rats with an insulin-like compound, vanadyl sulphate, completely abolished the enhanced cardiac phosphorylase activation and restored the increase in ISO-induced cAMP elevation in diabetic heart. The present study has provided further information on the changes of phosphorylase activation in the diabetic rat heart and demonstrated beneficial effects of vanadyl sulphate on the pathway leading to phosphorylase activation in diabetic rat heart.

Nonglucose substrates increase glycogen synthesis in vivo in dog heart

The effects of circulating nonglucose substrates on insulin-stimulated cardiac glycogen synthesis were studied in the dog heart in vivo using 13C-nuclear magnetic resonance (-NMR) and arteriovenous difference techniques. [1-13C]glycogen was monitored in hearts during an intravenous infusion of 20 mU/min insulin and glucose while [1-13C]glucose (10 mg/min) was infused into the left anterior descending coronary artery. When 1 mmol/min of lactate, pyruvate, or beta-hydroxybutyrate was added to the venous infusion, the measured rate of glycogen synthesis was increased, on average, sixfold. It was not increased further after a subsequent 10-min infusion of 5 micrograms/min epinephrine. Lactate extraction increased from 0.18 +/- 0.05 to 0.62 +/- 0.11 mumol.min-1.g wet wt-1 during lactate infusion, whereas glucose extraction did not change significantly (0.15 +/- 0.05 mumol.min-1.g wet wt-1 at 45 min of insulin and glucose infusion to 0.09 +/- 0.02 mumol.min-1.g wet wt-1 at 45 min of the lactate infusion). Therefore, the uptake and oxidation of circulating nonglucose substrates redirects the fate of extracted glucose from glycolysis to glycogen synthesis in the dog heart in vivo.

Embryonic Viability and Metabolism in Turkey Lines Selected for Egg Production or Growth

Lines of turkeys selected genetically for rapid growth or egg production and their respective randombred control populations were examined for physiological factors influencing their hatchability. Hatchability of fertile eggs was improved (P ≤ .05) in the growth line but not in the egg production line when compared with randombred control populations. Embryonic mortality of both selected lines was lower during the 1st wk of incubation but higher during pipping than that observed for the mortality of the randombred control embryos. Eggshell conductance constants were lower (P ≤ .05) in genetically selected lines compared with constants for eggs of the randombred control strains, which suggests that embryos from selected lines have less access to oxygen. Thus, genetic selection regardless of whether for growth or egg production reduced oxygen consumption during external pipping and for hatched poults, so the reduction cannot be attributed specifically to growth or egg production. Metabolic rate as measured by oxygen consumption during external pipping and posthatch was reduced (P ≤ .05) in the embryos of the selected lines compared with controls. Thus, oxygen availability through eggshells prior to pipping may be vital in determining survival rates of late embryos. Lower oxygen consumption would cause embryos to rely more on stored glycogen for energy during pipping. Hepatic and cardiac glycogen stores and blood plasma glucose concentrations indicated the embryos of growth and egg production lines had less (P ≤ .05) stored glycogen than control embryos. The data suggest that there may be a relationship between carbohydrate accrual prior to pipping, its utilization during pipping, and subsequent embryonic survival during pipping and hatching among the lines and that these differences may be associated with the genetics of growth or egg production.

Loss of myocardial protection after preconditioning correlates with the time course of glycogen recovery within the preconditioned segment.

Although previous investigators have demonstrated that myocardial preconditioning reduces infarct size, the mechanisms of cardioprotection associated with preconditioning are not completely understood. To test the hypothesis that preconditioning (four 5-minute episodes of ischemia each followed by 5 minutes of reperfusion) reduces infarct size by depleting cardiac glycogen stores and attenuating the degree of intracellular acidosis during subsequent prolonged left coronary artery occlusion, preconditioned and control rats were subjected to 45 minutes of left coronary artery occlusion and 120 minutes of reflow immediately after preconditioning (groups 1P and 1C, respectively) or after 30 minutes (groups 2P+30m and 2C), 1 hour (groups 3P+60m and 3C), or 6 hours (groups 4P+360m and 4C) of nonischemic recovery after preconditioning but before prolonged ischemia. In each group, cardiectomy was performed in selected rats immediately before prolonged ischemia for cardiac glycogen assay. In selected animals, 31P magnetic resonance spectroscopy was performed to monitor intracellular pH and measure high-energy phosphate levels during ischemia and reperfusion. Group 1P rats demonstrated marked glycogen depletion after preconditioning compared with controls (0.72 +/- 0.39 [n = 9] versus 5.67 +/- 1.73 [n = 12] mg glucose/g wet wt; p < 0.001 versus group 1C) that was associated with attenuation of intracellular acidosis during ischemia, as measured by 31P magnetic resonance spectroscopy (6.8 +/- 0.3 [n = 11] versus 6.2 +/- 0.3 [n = 9] pH units; p < 0.01), and marked infarct size reduction (0.3 +/- 0.6% [n = 7] versus 38.1 +/- 11.3% [n = 7], infarct size divided by risk area; p < 0.0001). During ischemia, there were no differences in myocardial ATP or phosphocreatine levels or in any hemodynamic determinant of myocardial oxygen demand between groups 1P and 1C. In preconditioned rats that were allowed to recover before ischemia (groups 2P+30m, 3P+60m, and 4P+360m), the time course of glycogen repletion paralleled the loss of protection from ischemic injury. Glycogen depletion and the attenuation of intracellular acidosis during ischemia appear to be important factors in delaying irreversible injury and reducing infarct size in this animal model of myocardial preconditioning.