Glycogen phosphorylase in fish muscle: demonstration of three interconvertible forms

Differential expression for tissue-specific isozymes in three species of Hypostomus Lacépède, 1803 (Teleostei: Loricariidae)

The expression patterns of 14 enzymatic systems in skeletal muscle, liver and heart tissues of three species of Hypostomus from the Iguaçu River basin (Brazil) were investigated. Although the patterns were similar for the three species, different tissues showed differential expressions, and the data showed that differential tissue expressions of isoperoxidases may be due to preferential combination or association of polypeptide subunits. The detected patterns for SOD isozymes showed that the quaternary structures of these enzymes were in disagreement with the subunit number reported for the majority of other vertebrate groups. Tissue-specific restriction on heterotetramer formation also were described in LDH and MDHP isozymes. Thus, these tissue-specific gene expression character in the species of Hypostomus have the greatest potential to be recognized and applied in systematic studies among species of Hypostomus.

Pharmacological and molecular characterization of muscular cholecystokinin receptors in the human lower oesophageal sphincter

In vitro cholecystokinin (CCK) contracts the human lower oesophageal sphincter by stimulating muscular receptors. The aim of this study was to characterize the muscular CCK receptor subtypes in the human lower oesophageal sphincter. Twenty-five circular strips from six patients were studied. RNA was extracted, reverse transcribed, and cDNAs were amplified with primers for human CCK-A and B receptors. The potency of the contraction induced by CCK-8, desulphated CCK-8, and gastrin-I, and the effect of the CCK-A (loxiglumide and SR 27897) and the CCK-B (YM022 and L-365 260) specific receptor antagonists were compared. Both CCK-A and CCK-B receptor mRNAs were found in functional lower oesophageal sphincter strips. The potency of the CCK-8 concentration-dependent contraction was two and three orders of magnitude higher than that of desulphated CCK-8 and gastrin-I, respectively. The CCK-8-induced contraction was blocked by the CCK-A receptor antagonists loxiglumide (IC50 11 micromol L-1) and SR 27897 (IC50 74 nmol L-1) but not by CCK-B receptor antagonists (1 micromol L-1). Our data suggest that, although the human lower oesophageal sphincter expresses both CCK-A and CCK-B receptors, the contractile effect of CCK-8 on the circular muscle is mainly due to the activation of CCK-A receptors.

The Pasteur effect in facultative anaerobic metazoa

The existence and the regulatory mechanisms of the Pasteur effect in facultative anaerobic metazoa are discussed. There are three reasons for the controversy surrounding this phenomenon. 1) The different definitions of the Pasteur effect, 2) the antagonistic effect of metabolic depression and its species specific response to hypoxia, as well as 3) the laboratory-specific differences in the experimental procedures for analyzing the Pasteur effect and its regulation. This review aims to clarify the confusion about the existence of the Pasteur effect in facultative anaerobic metazoa and to offer possible molecular mechanisms.

Control of adenine nucleotide metabolism and glycolysis in vertebrate skeletal muscle during exercise

The turnover of adenosine triphosphate (ATP) in vertebrate skeletal muscle can increase more than a hundredfold during high-intensity exercise, while the content of ATP in muscle may remain virtually unchanged. This requires that the rates of ATP hydrolysis and ATP synthesis are exactly balanced despite large fluctuations in reaction rates. ATP is regenerated initially at the expense of phosphocreatine (PCr) and then mainly through glycolysis from muscle glycogen. The increased ATP turnover in contracting muscle will cause an increase in the contents of adenosine diphosphate (ADP), adenosine monophosphate (AMP) and inorganic phosphate (P(i)), metabolites that are substrates and activators of regulatory enzymes such as glycogen phosphorylase and phosphofructokinase. An intracellular metabolic feedback mechanism is thus activated by muscle contraction. How muscle metabolism is integrated in the intact body under physiological conditions is not fully understood. Common frogs are suitable experimental animals for the study of this problem because they can readily be induced to change from rest to high-intensity exercise, in the form of swimming. The changes in metabolites and effectors in gastrocnemius muscle were followed during exercise, post-exercise recovery and repeated exercise. The results suggest that glycolytic flux in muscle is modulated by signals from outside the muscle and that fructose 2,6-bisphosphate is a key signal in this process.

Flying insects: model systems in exercise physiology

Insect flight is the most energy-demanding exercise known. It requires very effective coupling of adenosine triphosphate (ATP) hydrolysis and regeneration in the working flight muscles. 31P nuclear magnetic resonance (NMR) spectroscopy of locust flight muscle in vivo has shown that flight causes only a small decrease in the content of ATP, whereas the free concentrations of inorganic phosphate (Pi), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) were estimated to increase by about 3-, 5- and 27-fold, respectively. These metabolites are potent activators of glycogen phosphorylase and phosphofructokinase (PFK). Activation of glycolysis by AMP and Pi is reinforced synergistically by fructose 2,6-biphosphate (F2,6P2), a very potent activator of PFK. During prolonged flight locusts gradually change from using carbohydrate to lipids as their main fuel. This requires a decrease in glycolytic flux which is brought about, at least in part, by a marked decrease in the content of F2,6P2 in flight muscle (by 80% within 15 min of flight). The synthesis of F2,6P2 in flight muscle can be stimulated by the nervous system via the biogenic amine octopamine. Octopamine and F2,6P2 seem to be part of a mechanism to control the rate of carbohydrate oxidation in flight muscle and thus function in the metabolic integration of insect flight.

Intracellular acidosis in murine fibrosarcomas coincides with ATP depletion, hypoxia, and high levels of lactate and total Pi

Bioenergetic and metabolic status of murine FSaII tumours were evaluated using 31P MRS, acid extracts ('global' techniques) and quantitative bioluminescence ('microregional' assay). Data obtained from s.c. tumours of varying sizes (44-600 mm3) have been correlated with the oxygenation status evaluated using O2-sensitive needle electrodes. beta-NTP/Pi and phosphocreatine (PCr)/Pi ratios derived from 31P MRS were positively correlated to the median tissue pO2 values. pH declined during growth with intracellular acidosis being evident in tumours > 350 mm3. Whereas lactic acid formation greatly contributed to this decline in small- and medium-sized tumours, ATP hydrolysis and slowing down of the activities of pumps involved in pHi regulation seem to be major factors responsible for intracellular acidification in bulky tumours. PCr levels decreased at an early growth stage, whilst ATP concentrations dropped in bulky malignancies only, coinciding with a decrease in adenylate energy charge and a substantial rise in the levels of total Pi. MRS observable (mobile) Pi was consistently lower than [Pi] measured in acid extracts. On average, median pO2 values of ca 10 mmHg represent a critical threshold for energy metabolism. At higher median O2 tensions, levels of ATP, phosphomonoester and total Pi were relatively constant. This coincided with intracellular alkalosis or neutrality and stable adenylate ratios. On average, median pO2 values < 10 mmHg coincided with intracellular acidosis, ATP depletion, a drop in energy charge and rising Pi levels.

Identification of a putative collagen-binding protein from chicken skeletal muscle as glycogen phosphorylase

We have purified and generated antisera to a 95 kDa skeletal muscle protein that constitutes the largest mass fraction of gelatin-agarose binding proteins in skeletal muscle. Preliminary results indicated that this 95 kDa chicken skeletal muscle protein bound strongly to gelatin-agarose and type IV collagen-agarose, suggesting a possible function in muscle cell adhesion to collagen. However, N-terminal sequencing of proteolytic fragments of the 95 kDa protein indicates that it is the chicken skeletal muscle form of glycogen phosphorylase, the binding of which to gelatin-agarose is unlikely to be biologically relevant. Further characterization showed that the skeletal muscle form of glycogen phosphorylase is immunologically distinct from the liver and brain forms in the chicken, and suggests that, unlike mammalian skeletal muscle, chicken skeletal muscle may have two phosphorylase isoforms. Furthermore, immunolocalization data and solubility characteristics of glycogen phosphorylase in muscle extraction experiments suggest the enzyme may interact strongly with an unidentified component of the muscle cytoskeleton. Thus, this study yields a novel purification technique for skeletal muscle glycogen phosphorylase, provides new information on the distribution and isoforms of glycogen phosphorylase, and provides a caveat for using gelatin affinity chromatography as a primary step in purifying collagen-binding proteins from skeletal muscle.

Phosphorylation-induced conformational changes in the phosphorylase ab hybrid as revealed by resolution of pyridoxal 5'-phosphate with imidazole citrate and cysteine

The accessibility of pyridoxal 5'-phosphates of the phosphorylase ab hybrid to resolution by imidazole citrate and cysteine was studied and compared with that of the b and a forms. Promotion of resolution of phosphorylated forms by raising the temperature or in the presence of glycogen indicates that the resistance of phosphorylase a and ab to resolution at 0 degrees C is due rather to their tetrameric state than their phosphorylation-related active conformation. The pattern of resolution of the ab hybrid was similar to that of the a and differed from that of the b forms in that it occurred at 30 degrees C and 37 degrees C but not at 0 degrees C, moreover, it did not show first-order kinetics. On the other hand, inhibition of resolution by ligands binding to the nucleotide site of phosphorylase reflected an intermediate sensitivity of the ab form between that of the b and a forms. We conclude that partial phosphorylation of phosphorylase b elicits conformational change(s) in both subunits which influence the monomer-monomer interactions and resolution of pyridoxal 5'-phosphates. Resistance of ab hybrid to monomerizing agents as imidazole citrate, comparable to that of other forms, argues for its stability, ruling out its reshuffling into mixtures of phosphorylase b and a.