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PubMed1982

Value of the glucagon test in screening for hepatic glycogen storage disease.

The fasting glucagon test of 40 patients with hepatic glycogen storage disease (type I, 13 patients; type Ib, 5 patients; type III, 12 patients; type IX (phosphorylase kinase deficiency), 10 patients) has been reviewed. In all patients with types Ib and III the blood glucose level rose less than 1 mmol/1 but in types 1 and IX there was wide variation in response. Blood lactate levels were greater than 2.4 mmol/1 in all patients with type I and Ib 120 minutes after the administration of glucagon but were lower than 2.4 mmol/1 in types III and IX. The value of this test for screening was analysed using several criteria. From the data a simplified scheme for the investigation of patients with suspected hepatic glycogen storage disease is proposed.

Blood Glucose↗
PubMed1985

A fast-twitch oxidative-glycolytic muscle with a robust inward calcium current.

This report describes the histochemical and physiological properties of a rat skeletal muscle with a robust activity-dependent slow inward Ca2+ current. The muscle, the flexor digitorum brevis (FDB), is a small plantar flexor from the hindfoot. It is a homogeneous muscle consisting of approximately 90% fast-twitch oxidative-glycolytic (type IIA) fibers. Stimulation of the FDB with repetitive stimulus trains (30 or 50 Hz for 330 ms, 1 train/s for 2-5 min) produced a slow increase in the base-line or resting tension of the muscle between trains. This progressive increase in resting tension appears to be due to the activation of voltage-dependent slow Ca2+ channels, since it could be eliminated (i) by stimulating the muscle in a medium containing 2 mM EGTA and without Ca2+, and (ii) by the addition of either Co2+ or verapamil. The presence of a slow current may be associated with an increase in K+ efflux as stimulation continues, and with a prolongation of relaxation time. We also propose that the slow Ca2+ current may contribute to the allosteric activation of phosphorylase kinase during muscle activity. The FDB provides an excellent preparation to investigate the regulation of muscle metabolism by intra- and extra-cellular Ca2+ during exercise.

Action Potentials↗
PubMed1976

Calcium-binding proteins.

Calcium binding, which appears to be either specific or physiologically significant, has been reported for 70 norminally "different" proteins. First, I catalog these proteins. Only a few recent or general references can be cited. Those proteins that serve critical physiological functions or that may be regarded as chemical prototypes are discussed in more detail. Second, I present several generalizations. Inevitably the logic here is cyclic in that the generalizations dictate which data from all those available are actually presented. In order to focus our attention in the catalog, I outline these generalizations: 1. An examination of the five calcium-binding proteins of known structure does not reveal a correlation between ligand type and/or geometry and Ca2+ affinity or selectivity. 2. For many of the enzymes supposedly activated or stabilized by Ca2+ the available data dot not allow one to judge the physiological significance of the calcium binding or its contribution of the enzymic mechanism. 3. Of the enzymes requiring Ca2 and concanavalin A, only the nuclease of Staphylococcus and possibly phospholipase appear to bind Ca2 at the active site. 4. The calcium affinities of most of the extracellular enzymes are low, pKd = 3 to 4. This is consistent with the fact that the Ca2+ concentration of the extracellular environment is about 10(-3) M. 5. The cytosol concentration of free Ca2+ in most if not all eukaryotic cells is from 10(-6) to 10(-8) M. Following a stimulus to the cell after which Ca2+ functions as a second messenger, the free Ca2+ may rise to 10(-6) to 10(-5) M. The reported affinities of most enzymes of the cytosol are too low to be physiologically significant. 6. Several intracellular enzymes or enzyme activators have pKd values between 5 and 8. These enzymes therefore may be turned on and off or "modulated" in response to extracellular stimuli. 7. There is a distinct conceptual difference between extracellular enzymes that are "activated" by Ca2+ and those intracellular enzymes that are modulated by Ca2+. The activated proteins bind Ca2+ upon secretion or upon incorporation into a secretory vesicle and retain it throughout their functional lifetimes. The modulated proteins may bind and release Ca2+ many times in response to varying concentrations of this second messenger. 8. Several of the calcium-modulated proteins contain a characteristic conformation, consisting of a helix, calcium-binding loop, and second helix, referred to as the "EF hand". These proteins are homologous, that is, evolutionarily related.

Adenosine Triphosphatases↗
PubMed1986

Relationship between liver glycogen metabolism and glucose homeostasis in the fetal rat: studies with the gsd/gsd rat.

Litters containing both heterozygous and homozygous gsd/gsd rats have been studied to determine whether fetal liver glycogen contributes to the fetal blood glucose pool. It is shown that homozygous gsd/gsd fetuses (unable to mobilise liver glycogen) have consistently lower blood glucose concentrations than their heterozygous (normal) littermates at all ages studied. This difference is independent of maternal type and maternal blood glucose concentration. There is a marked activation of liver phosphorylase in heterozygous fetuses at 21 days of gestation, associated with a significant rise in the fetal/maternal blood glucose ratio. In contrast, there is no significant activation of phosphorylase or rise in the ratio for homozygous littermates. In homozygous fetuses, the blood glucose concentrations remained low and liver glycogen high. The possibility of fetal liver glycogen being derived from gluconeogenic precursors is discussed.

Animals↗
PubMed2005

Transcriptome and proteome analysis of soleus muscle of hormone-sensitive lipase-null mice.

Hormone-sensitive lipase (HSL), a key enzyme in fatty acid mobilization in adipocytes, has been demonstrated also in skeletal muscle. To gain further insight into the role and importance of HSL in skeletal muscle, a transcriptome analysis of soleus muscle of HSL-null mice was performed. A total of 161 transcripts were found to be differentially expressed. Increased mRNA levels of fructose-1,6-bisphosphatase, fructose-2,6-bisphosphatase, and phosphorylase kinase gamma1A suggest a higher glycogen flux in soleus muscle of HSL-null mice. An observed increase in the utilization of glycogen stores supports this finding. Moreover, an increased amount of intramyocellular lipid droplets, observed by transmission electron microscopy, suggests decreased mobilization of lipid stores in HSL-null mice. To complement the transcriptome data, protein expression analysis was performed. Five spots were found to be differentially expressed: pyruvate dehydrogenase E1alpha, creatine kinase (CK), ankyrin-repeat domain 2, glyceraldehyde-3-phosphate dehydrogenase, and one protein yet to be identified. The increased protein level of CK indicates creatine phosphate degradation to be of increased importance in HSL-null mice. The results of this study suggest that in the absence of HSL, a metabolic switch from reliance on lipid to carbohydrate energy substrates takes place, supporting an important role of HSL in soleus muscle lipid metabolism.

Animals↗
PubMed2001

Protecting the myocardium from ischemic injury: a critical role for alpha(1)-adrenoreceptors?

Ischemic preconditioning (IPC) refers to the ability of short periods of ischemia to make the myocardium more resistant to a subsequent ischemic insult. It is the most powerful form of endogenous protection against myocardial infarction and has been demonstrated in all species evaluated to date. However, the cellular mechanisms that drive IPC remain poorly understood. This hypothesis describes an important role for alpha(1)-adrenoreceptors in mediating IPC and discusses the underlying mechanisms by which this is likely achieved. alpha(1)-Adrenoreceptors are present in the myocardium of all mammalian species, and several lines of evidence suggest that they play an important role in mediating IPC. During periods of myocardial hypoxia/ischemia, cardiomyocytes have to rely solely on anaerobic glycolysis for energy production; for this, the cells have to depend on increased glucose entry inside the cell as well as increased glycolysis. Stimulation of alpha(1)-adrenoreceptors increases glucose transport inside the cardiomyocytes by translocating glucose transporter (GLUT)-1 and GLUT-4 from the cytoplasm to the plasma membrane, enhances glycogenolysis by activating phosphorylase kinase, increases the rate of glycolysis by activating the enzyme phosphofructokinase, reduces intracellular acidity produced during excessive glycolysis by activating the Na(+)/H(+) exchanger, and inhibits apoptosis by increasing the levels of the antiapoptotic protein Bcl-2. Myocardial ischemia produces an increase in the expression of alpha(1)-adrenoreceptors in cardiomyocytes, as well as increases the levels of its agonist norepinephrine by several fold. During ischemic states, upregulation of alpha(1)-adrenoreceptors and increase in norepinephrine release could be a powerful adaptive mechanism that drives IPC. An understanding into the role of alpha(1)-adrenoreceptors in mediating IPC could not only point to newer treatments for limiting myocardial damage during myocardial infarction or heart surgery, but could also help in avoiding the use of alpha(1)-antagonists in patients with ischemic heart disease.

Animals↗
PubMed1987

ATP-dependent proteolysis and the role of ubiquitin in rabbit cardiac muscle.

A soluble ATP/Mg2-dependent proteolytic system from rabbit cardiac muscle has been identified (m ca. 310 kDa) and purified ca. 9-fold. This enzyme which splits the substrate [3H]globin and 125I-bovine serum albumin (125I-BSA) has many similarities to the ATP-dependent proteolytic enzyme system from reticulocytes which utilizes ubiquitin: 1) The specific activities in reticulocyte lysates and cardiac muscle extracts are of the same magnitude (0.5-1 arb. unit/mg). 2) The binding and elution behavior on DEAE-cellulose is similar. 3) In both cases the pH optimum (substrate 125I-BSA) is pH 7.6. 4) Both enzymes are inhibited by hemin, NEM and iodoacetate but not e.g. by leupeptin, or inhibitors of serine proteases. 5) Neither enzyme system can utilize ATP-analogs such as AMP-CPP, AMP-PCP, AMP-PNP or ATP-gamma-S. There are however also significant differences: 1) The enzyme system from cardiac muscle is fully active in the absence of ubiquitin and cannot be activated by this peptide. 2) The enzyme from cardiac muscle can degrade methylated BSA. 3) The cardiac muscle enzyme can be further purified on Sepharose 4B; the enzyme from reticulocytes is inactivated by this procedure. 4) The cardiac enzyme cannot be inactivated by ribonuclease as the reticulocyte counterpart. Although ubiquitin does not appear to play a role in the isolated ATP/Mg2-dependent proteolytic system from cardiac muscle, it is demonstrated for the first time that 125I-ubiquitin can be conjugated to a wide variety of cardiac muscle proteins in vitro in an ATP-dependent manner. Apparent molecular masses of major conjugates were: 185 kDa, 140 kDa, 85 kDa, 65 kDa, 46 kDa, 38 kDa and 36 kDa as estimated by discontinuous SDS gel electrophoresis. Addition of purified phosphorylase kinase to cardiac muscle extract changed the ubiquitination pattern by the appearance of two novel protein bands. It is concluded that the ATP/Mg2-dependent proteolytic system of cardiac muscle must be differentiated from the proteolytic system of reticulocytes mainly because of its ubiquitin-independence. Nevertheless the conjugation of 125I-ubiquitin to many muscle proteins is a strong indication for a crucial role of this interesting peptide in striated muscle.

Adenosine Triphosphate↗
PubMed1989

Glycogenolysis during recovery from muscular work. The time course of phosphorylase activity is dependent on Pi concentration in the abdominal muscle of the shrimp Crangon crangon.

1) Glycogen is degraded in the abdominal muscle of the shrimp Crangon crangon (Decapoda, Crustacea) during the recovery period following work. The regulation of post-exercise glycogen breakdown and the properties of glycogen phosphorylase (EC 2.4.1.1) have been studied: 2) Glycogen phosphorylase exists as unphosphorylated b-form and phosphorylated a-form, the latter contains 1 molecule phosphate/subunit. Both forms of phosphorylase are dimers, isoenzymes have not been detected. 3) The purified b-form is inactive in absence of AMP and has very low affinities for AMP and Pi. For half-maximum activation 0.33 +/- 0.04 mM AMP is necessary, and the Km-value for Pi at 1 mM AMP is 48 +/- 5 mM. IMP does not affect the activity of the b-form. 4) The a-form is active without effectors, its Km-value for Pi is 5.3 +/- 1.5 mM. The proportion of phosphorylase a increases in vivo, from about 25% at rest, to approximately 90% upon work and remains at this high level during the first minutes of recovery. 5) It is concluded that the glycogenolytic flux in the abdominal muscle of the shrimp even during post-exercise periods depends on the level of the a-form the activity of which is restricted in time and extent by the cytoplasmic Pi concentration (Kamp, G. & Juretschke, H. P. (1987) Biochim. Biophys. Acta 929, 121-127).

Adenosine Monophosphate↗
PubMed1998

Novel aspects of the regulation of glycogen storage.

The storage polysaccharide glycogen is widely distributed in nature, from bacteria to mammals. Study of its regulated accumulation has resulted in the discovery or elaboration of several important biochemical principles. Many aspects of the control of glycogen storage still remain poorly understood and glycogen metabolism continues to provide interesting models of more general relevance.

Animals↗
PubMedDate not supplied

The molecular background of glycogen metabolism disorders.

The molecular pathology of classical glycogen storage disorders, glycogen synthase deficiency and Fanconi-Bickel syndrome is reviewed. The isolation of the respective cDNAs, the chromosomal localization of the genes and the elucidation of the genomic organization enabled mutation analysis in most disorders. The findings have shed light on the multi-protein structure of the glucose-6-phosphatase system, the phosphorylase kinase enzymatic complex and the molecular background of the differential tissue expression in debranching enzyme deficiency. The immediate practical benefit of these studies is our extending ability to predict the outcome of clinical variants and to offer genetic counseling to most families. The elucidation of the tertiary structure of these proteins and their structure-function relationship poses major challenges for the future.

Glycogen Storage Disease↗