Cardiac glycogen in long-evans rats: diurnal pattern and response to exercise

Effect of nonworking heterotopic transplantation on rat heart glycogen metabolism

To determine whether the contractile work history of cardiac muscle influences its responsiveness to insulin, we examined the effect of insulin infusion on glycogen metabolism in the rat heart 1 wk after transplantation into a nonworking heterotopic infrarenal position. Nonworking heterografts had higher basal glycogen concentrations than did in situ working hearts of the same animals (29.9 +/- 2.7 vs. 23.3 +/- 0.8 mumol/g; P < 0.05), and a smaller fraction of their glycogen synthase enzyme activity was in the physiologically active glycogen synthase I form (8 +/- 2 vs. 22 +/- 3%; P < 0.02). During a 25-min infusion of insulin (1 U/min) and glucose (30 mg.kg-1.min-1), the fractional glycogen synthase I activity of heterografts remained lower than that of in situ hearts (29 +/- 5 vs. 56 +/- 7%; P < 0.02) and heterografts synthesized glycogen more slowly (0.126 +/- 0.07 vs. 0.352 +/- 0.06 mumol.g-1.min-1; P < 0.02). These effects could be duplicated by a 24-h fast, which similarly increased myocardial glycogen concentration (to 32.9 +/- 5.6 mumol/g). These observations suggest that the performance of repetitive contractile work is necessary to maintain the myocardium maximally responsive to insulin. Mechanical unloading increases myocardial glycogen concentration, thereby reducing the magnitude of insulin's stimulation of glycogen synthase and consequently the rate of incorporation of circulating glucose into glycogen.

Fasting and lactate unmask insulin responsiveness in the isolated working rat heart

We have previously reported that the nutritional state in vivo results in differential insulin responses by the perfused heart in vitro. To further assess the effects of insulin on glucose uptake at physiological work loads, hearts from fed and fasted (16-20 h) rats were perfused with buffer containing 2-[18F]fluoro-2-deoxy-D-glucose (2-FDG) and glucose (10 mM) alone or plus lactate (10 mM) as a competing substrate, with insulin (10 mU/ml) added after a control period. When glucose was the only substrate, the addition of insulin decreased the fractional rate of 2-FDG uptake in hearts from either fed or fasted rats. The effect of insulin on increasing myocardial 2-FDG uptake was immediate and sustained only in hearts from fasted rats in the presence of lactate, despite no change in cardiac work. At the same time, the increase in 2-FDG uptake and phosphorylation was associated with an increase in the tissue content of glycogen in hearts from fasted rats. We conclude that lactate unmasks insulin sensitivity in heart muscle at physiological work loads but that this unmasking of insulin-mediated glucose uptake is dependent on the nutritional state of the animal. The glucose up as a result of insulin stimulation is preferentially utilized for glycogen repletion and does not enter the glycolytic pathway. This observation also suggests that myocardial glycogen synthesis in vitro is affected by the nutritional state in vivo and that lactate provides a substrate for oxidative phosphorylation while glucose is preferentially utilized for glycogen synthesis.

Regulation of glycogen metabolism in canine myocardium: effects of insulin and epinephrine in vivo

Myocardial glycogen synthesis and glucose, lactate, and oxygen extraction were measured in the hearts of anesthetized dogs during infusions of insulin and epinephrine. Glycogen was monitored in vivo using 13C-nuclear magnetic resonance during an infusion of [1-13C]glucose into the left anterior descending artery. Glycogen synthesis was observed during a venous infusion of insulin (1.8 microU.min-1.kg-1), and this newly synthesized glycogen was neither broken down nor was more glycogen synthesized during a subsequent epinephrine infusion (0.5 micrograms.min-1.kg-1). During recovery from epinephrine, glycogen synthesis occurred at 2.1 times the rate seen in the control period. Glycogen synthesis was not stimulated in the absence of epinephrine by control infusions of saline. Glucose uptake was increased by insulin during the control period (from 0.09 to 0.39 mumol.min-1.g-1), so that the combined extraction of glucose and lactate exceeded the requirement for oxidizable substrate calculated from oxygen consumption. The "excess" glucose (0.15 mumol.min-1.g wet wt-1) is presumably available for glycogen synthesis. During recovery from epinephrine, lactate uptake was increased over threefold. Because this additional lactate supplies most of the fuel required for oxidation, the excess glucose available for glycogen synthesis during this period was two times that seen before epinephrine (an average of 0.32 mumol.min-1.g wet wt-1 between 20 and 40 min postepinephrine). These data are consistent with the notion that glycogen synthesis can be activated in the heart without an accompanying increase in glucose uptake by providing an alternative substrate (i.e., lactate) for oxidation.

Adaptability of the pulmonary system to changing metabolic requirements

The conventional view of the healthy pulmonary system during exercise is of a very precise and mechanically efficient homeostatic regulator of ventilation and gas exchange occurring within the reserves of a near ideal architecture of the lung and chest wall. These regulatory and architectural limits may be exceeded in the healthy pulmonary system when extremely high levels of metabolic demand are needed. For example, arterial hypoxemia will often occur at exercise intensities demanding greater than 25 liters/min cardiac output. This may be due to inadequate red cell transit time in the pulmonary capillary bed whose blood volume has been maximally recruited, thereby resulting in alveolar-end-capillary oxygen disequilibrium. At these extreme levels of exercise the hyperventilatory response may be minimal (and clearly inadequate in terms of alveolar oxygenation) despite substantial and progressive metabolic acidosis or hypoxemia or both. This evidence of compromised ventilatory response and inadequate gas exchange in the highly fit human suggests that the pulmonary system may not be reasonably designed or adaptable (with long-term physical training) to the extreme demands imposed on gas transport by a truly adapted cardiovascular system.

The influence of free fatty acids on glycogen recovery in rat heart after exercise

Glycogen supercompensation is the term used to denote the abnormally high levels of glycogen found in the heart shortly after an exercise-induced reduction of the substrate. Using rats, we tested whether this condition was linked to the use of plasma free fatty acids (FFA), which normally rise with exercise. Before a 1-h swim, animals received an injection of either saline (S) or nicotinic acid (NA). The nicotinic acid treatment dramatically suppressed the rise in plasma FFA observed in the S-group. Exercise caused a significant but similar reduction (35-38%) of the myocardial glycogen content in both groups. After 1 h of recovery in the S-group, myocardial glycogen reached a value of 30.3 +/- 1.7 mumol x g-1 or 113% of that measured before the exercise began. In contrast, the value for hearts from the NA-group with reduced FFA levels was 24.0 +/- 1.9 mumol x g-1 or only 91% of that measured before exercise. After 2 h the values were 33.8 +/- 1.4 and 29.0 +/- 1.9 mumol x g-1 respectively. These data indicate that glycogen repletion in rat heart after exercise is related to the amount of FFA present in the plasma. We suggest that carbohydrate metabolism is diverted towards synthesis and storage as a result of the glycolytic inhibition exerted by the increased use of fat as an energy source as previously observed in hearts from fasted or diabetic animals.

MgATP-induced inhibition of the adenosine triphosphatase activity of the chloroform-released mitochondrial adenosine triphosphatase

1. Preincubation of the ox heart chloroform-released mitochondrial ATPase with MgATP results in a time-dependent inhibition of ATPase activity. No re-activation occurs when MgATP remains in the preincubation medium. The enzyme activity returns when all the MgATP in the preincubation system has been hydrolysed. 2. The mechanism of the MgATP-induced inhibition was examined. Inhibition occurs on incubation with MgATP or other hydrolysable nucleotides. Incubation with MgADP or Pi does not cause any inhibition. Neither freshly bound adenine nucleotide nor Pi is associated with inhibited enzyme. The rate of MgATP-induced inhibition correlates with the rate of ATP hydrolysis in the preincubation medium. Changing the rate of ATP hydrolysis at a fixed concentration of ATP also changes the rate of MgATP-induced inhibition by the same proportion. The inhibition is thus related to the ATP-hydrolysis process itself. 3. We propose that intermediate enzyme species of the ATP-hydrolytic sequence can undergo a conformational change to form inhibited species. The kinetics of the inhibition suggest that a substrate-activation step is involved in ATP hydrolysis and MgATP-induced inhibition. 4. The effects of the nature of the preincubation medium on the process of MgATP-induced inhibition and its reversal were examined.

Exercise-induced increases in myocardial adenosine 3',5'-cyclic monophosphate and phosphodiesterase activity

Numerous cellular biochemical events caused by hormones are mediated through cyclic AMP. Although many changes occur in the cell during exercise that could be attributed to this nucleotide, little evidence is available implicating it as an important regulator of exercise metabolism. In this investigation it was found that a 60 min bout of treadmill exercise caused a 2.4-fold increase in myocardial cyclic AMP immediately following the work. Rather than the immediate nucleotide hydrolysis that was expected, it was found that the elevated cyclic AMP level remained for approx. 24 h before returning to control levels. Cardiac glycogen fell to 30% of control after work but supercompensated 60% above control within 1 h following exercise. Therefore, cardiac cyclic AMP was elevated at a time when glycogen was being synthesized. Study of the temporal relationship between the exercise-induced increase in cyclic AMP and cyclic nucleotide phosphodiesterase indicated that the work caused an increase in the hearts' capacity to hydrolyze cyclic AMP. Measurement of heart phosphodiesterase at substrate concentrations of 1.0 and 100 microM produced significant increases in enzyme activity immediately following exercise which remained elevated for 48 h and was back to control activity 96 h following work. These data present a potentially fascinating model for the study of the dissociation between cyclic AMP, glycogenesis and elevations in phosphodiesterase activity in the heart.

Effect of substrate supply and beta-adrenergic blockade on heart glycogen and triglyceride utilization during exercise in the rat

Exercise-induced heart glycogen and triglyceride mobilization was studied in control rats, in rats with reduced blood glycose supply (fasted rats), in rats with reduced plasma free fatty acids (FFA) supply (nicotinic acid-treated rats), and in rats with blockade of beta-adrenergic receptors (propranolol-treated rats). It was found in the fed control rats that both the heart glycogen and triglyceride levels were reduced at the beginning of the exercise and thereafter they returned to the control level despite the exercise being continued. The triglyceride level was reduced again during the exhaustive exercise. Reduced blood glucose supply increased the heart glycogen and triglyceride utilization during exercise. Partial prevention of the plasma FFA elevation during exercise increased the heart glycogen utilization and had no effect on utilization of the heart triglycerides. Blockade of the beta-adrenergic receptors fully prevented both the heart glycogen and triglyceride mobilization during exercise.

Effect of exercise on metabolism of glycogen and triglycerides in the respiratory muscles

It was shown that during muscular exertion the diaphragm muscle and the intercostal muscles utilize endogenous glycogen whereas only the diaphragm muscle utilizes endogenous triglycerides. The post-excercise glycogen repletion in the diaphragm muscle was much faster than in the intercostal muscles. In the diaphragm muscle, marked overshoot of the glycogen level occurred early after the exercise.