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PubMed1983

Response of liver glycogen synthase and phosphorylase to in vivo glucose and glucose analogues.

Glucose causes a rapid increase in the proportion (%) of glycogen synthase in the active (I) form and a rapid decrease in the proportion of phosphorylase in the active (a) form in both fed and fasted rats. The changes in synthase I and phosphorylase a are more rapid in fasted animals. With graded doses of glucose, the maximal decrease in phosphorylase a occurred at a dose that was considerably smaller than that required to maximally stimulate an increase in % synthase I. Thus, in the intact animal a dissociation between the effects of glucose on the synthase and phosphorylase systems was observed. Sorbitol, mannose, galactose, and arabinose all stimulated an increase in synthase I but did not significantly affect the proportion of phosphorylase in the a form. The % synthase I was not significantly affected by a number of other glucose homologues, pentoses, or three-carbon gluconeogenic substrates. The ketoses fructose and mannoheptulose both caused a striking increase in % phosphorylase a and a decrease in % synthase I, i.e., results opposite to those of glucose. The mechanism by which fructose induces these changes is not known, but the mannoheptulose effects may be accounted for by a rise in liver cAMP concentration.

Animals↗
PubMed1981

Age-specific incidence of neoplasms in untreated NZR/Gd rats: an inbred strain with cardiovascular tumours and liver glycogen storage disease.

NZR/Gd is a new inbred strain of albino rats subject to epithelial mesotheliomas of the heart and also to hepatic glycogen storage disease. The age-specific incidences of neoplasms in 650 untreated NZR rats are tabulated by sex and histopathologic type. In males the most common neoplasms were artio-caval mesotheliomas (12 per cent.) and lung adenomas (10 per cent.), while in females most were pituitary adenomas (23 per cent.), lung adenomas (10 per cent.) or tumours of breast (7 per cent.), heart (4 per cent.) or liver or uterine cervix (3 per cent.). Uncommon tumours (less than 2 per cent.) were pancreas islet cell adenomas, lymphoma, hemangioma, fibroma or tumours of kidney, ovary or bile ducts. The detailed histopathology of the artio-caval tumours reported earlier was confirmed, but it now appears there is a male preponderance (4 : 1), and larger tumours showed their malignancy by invasion of adjacent structures and occasionally by distant metastasis. In males the primary site was evenly divided between right atrium and inferior vena cava. Metastasis of tumours of all types was uncommon, but multiple primary tumours were frequently found, especially in the endocrine system of female rats. Almost one-third of the tumours were associated with at least one other neoplasm arising in a different tissue. Overall tumour statistics in NZR/Gd rats were quite similar to other inbred strains for which quantitative reports are available, and the virtual absence of other diseases and long life-span (up to 1250 days) indicated no serious effect of the hepatic glycogen storage disease on survival. The NZR rats seem to be unique in producing atrio-caval mesotheliomas, and they have a much higher incidence of alveolar lung adenomas (10 per cent.) tham other reported strains.

Age Factors↗
PubMed1986

The mechanism of caffeine-enhanced glucose stimulation of liver glycogen synthase phosphatase activity.

Our report that glucose within its physiological range stimulates glycogen synthase phosphatase activity, provided an appropriate second effector is present, has been expanded. The nature of the stimulatory process, particularly the roles of glucose, and of caffeine which represents the potential second effectors, has been studied. Glucose and caffeine stimulated synthase phosphatase activity in a synergistic manner. With 0.5 mM caffeine the A0.5 for glucose was 11 mM (from 27 mM), whereas in the presence of 30 mM glucose the A0.5 for caffeine was 0.06 mM (from 0.7 mM). At 10 mM glucose the A0.5 for caffeine was 0.1 mM. Glucose stimulation remained non-cooperative, unaffected by the presence of caffeine, whereas the cooperative stimulation of caffeine was unaffected by glucose. Some slight stimulation of synthase activity was observed with caffeine and with glucose over a wide concentration range. However, they did not act synergistically to influence the measurement of synthase activity. Glucose-6-phosphate, which also stimulates synthase phosphatase activity, acted independently, not synergistically with caffeine. All the methylxanthines were tested as potential second effectors in an effort to discover the essential structural elements of the agent. All dimethylxanthines, 3- and 7-methylxanthine and 1-methyl-3-isobutylxanthine enhanced glucose stimulation but none of them alone was stimulatory. Judged from the half-maximal concentrations, in the presence of 10 mM glucose, caffeine was the most potent second effector by a significant margin. The maximum velocity was also greatest with caffeine, whereas that with other methylxanthines was generally lower, and varied. 1-Methylxanthine with increased concentration was slightly inhibitory even in the presence of 10 mM glucose. Xanthine (0.5 mM), itself, strongly inhibited synthase phosphatase activity, an effect not influenced by glucose. Xanthine did not influence the measurement of synthase or phosphorylase phosphatase activity with or without glucose. In general, conditions of methylxanthine-enhanced, glucose stimulation of synthase phosphatase and phosphorylase phosphatase activities differed markedly, confirming that separate, distinct mechanisms are involved.

1-Methyl-3-isobutylxanthine↗
PubMed2000

Activation of the liver glycogen phosphorylase by Ca(2+)oscillations: a theoretical study.

Cytosolic calcium plays a crucial role as a second messenger in cellular signalling. Various cell types, including hepatocytes, display Ca(2+)oscillations when stimulated by an extracellular signal. However, the biological relevance of this temporal organization remains unclear. In this paper, we investigate theoretically the effect of Ca(2+)oscillations on a particular example of cell regulation: the phosphorylation-dephosphorylation cycle controlling the activation of glycogen phosphorylase in hepatocytes. By modelling periodic sinusoidal variations in the intracellular Ca(2+)concentration, we show that Ca(2+)oscillations reduce the threshold for the activation of the enzyme. Furthermore, as the activation of a given enzyme depends on the kinetics of its phosphorylation-dephosphorylation cycle, specificity can be encoded by the oscillation frequency. Finally, using a model for signal-induced Ca(2+)oscillations based on Ca(2+)-induced Ca(2+)release, we show that realistic Ca(2+)oscillations can potentiate the response to a hormonal stimulation. These results indicate that Ca(2+)oscillations in hepatocytes could contribute to increase the efficiency and specificity of cellular signalling, as shown experimentally for gene expression in lymphocytes (Dolmetsch et al., 1998).

Animals↗