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Biomedical subjects

W Montague

Publications and source records attributed to W Montague.

At least 55 records · Page 3Linked to original sources
PubMed1974

Regulation of guanylate cyclase in guinea-pig islets of Langerhans.

1. Guanylate cyclase activity was determined in homogenates of guinea-pig islets of Langerhans by measurement of the conversion of [alpha-(32)P]GTP into cyclic [(32)P]GMP, the reaction products being separated on columns of neutral alumina. 2. The pH optimum of the enzyme was 7.3; it showed a requirement for bivalent cations, the effectiveness of the cations tested being Mn(2+)>>Ca(2+)>Mg(2+). 3. About 70% of enzyme activity was sedimented by centrifugation at 105000g for 60min; activity was increased 2.3-fold by treatment of homogenates with 0.1% Triton X-100. 4. Guanylate cyclase activity of homogenates was increased by acetylcholine, secretin or pancreozymin, but was inhibited by adrenaline, noradrenaline or ATP. Insulin, glucagon, prostaglandins E(1) or E(2), glucose, F(-), diazoxide or glibenclamide were ineffective. 5. Determination of cyclic GMP amounts in islets by radioimmunoassay showed a basal concentration of 2.0pmol/mg of protein, which was increased by incubation of the islets in the presence of acetylcholine or the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, but was unaffected by glucose. 6. Dibutyryl cyclic GMP had significant stimulatory effects on rates of insulin biosynthesis in isolated rat islets of Langerhans. 7. These results suggest a possible role for cyclic GMP in the regulation of insulin biosynthesis and secretion.

Acetylcholine↗
PubMed1973

The mode of action of adenosine 3':5'-cyclic monophosphate in mammalian islets of Langerhans. Effects of insulin secretagogues on islet-cell protein kinase activity.

1. Protein kinase activity was measured in islets of Langerhans that had been incubated in the presence of agents known to affect insulin release. 2. Glucagon, theophylline, caffeine and 3-isobutyl-1-methylxanthine, agents that raise cyclic AMP concentrations in islet cells and stimulate insulin release, increased protein kinase activity. Adrenaline and diazoxide, agents that decrease cyclic AMP concentrations and inhibit insulin secretion, decreased the activity. 3. The increase in protein kinase activity produced by different concentrations of 3-isobutyl-1-methylxanthine was apparently related to the increase in intracellular concentrations of cyclic AMP. 4. The sulphonylureas, tolbutamide and glibenclamide, agents that increase insulin release, also increased the protein kinase activity; however, leucine, arginine and xylitol, which also stimulate insulin release, were without effect on the kinase activity. 5. Increasing the glucose concentration of the incubation medium from 2 to 20mm had no effect on protein kinase activity. Further, the ability of 3-isobutyl-1-methylxanthine to increase the protein kinase activity was not affected by the glucose concentration of the incubation medium. 6. These results suggest that agents which affect insulin secretion by altering cyclic AMP concentrations may exert their effects on hormone release by altering the activity of a cyclic AMP-dependent protein kinase in islet cells.

Animals↗
PubMed1973

Regulation of insulin release from isolated islets of Langerhans of the rat in pregnancy. The role of adenosine 3':5'-cyclic monophosphate.

1. The concentrations of cyclic AMP were compared in islets of Langerhans isolated from the pancreases of normal female and pregnant rats and were higher in islets in pregnancy. 2. There was also a significant increase in adenylate cyclase activity in homogenates of islets from pregnant rats compared with those from normal rats. 3. Increased cyclic AMP concentration in islets from pregnant rats was reflected in increased protein kinase activity. When the cyclic AMP-dependent protein kinase activity was increased by 3-isobutyl-1-methylxanthine this stimulated activity was significantly greater in pregnancy. 4. Insulin-secretion studies with islets from normal and pregnant rats showed that theophylline or 3-isobutyl-1-methylxanthine, which raise intracellular cyclic AMP concentrations, caused a significantly greater insulin secretion in pregnancy. 5. It was also found that in the presence of a glucose concentration too low to stimulate insulin secretion, the latter could be induced if the cyclic AMP concentrations were raised sufficiently with 3-isobutyl-1-methylxanthine. 6. It is suggested that the higher cyclic AMP concentrations observed in islets in pregnancy mediate the greater insulin-secretory capacity, as well as the greater sensitivity of these islets to low glucose concentrations.

Journal Article↗
PubMed1973

A possible role of adenylate cyclase in the long-tern dietary regulation of insulin secretion from rat islets of Langerhans.

1. Adenylate cyclase activity and patterns of insulin release in response to various concentrations of glucose were determined in islets of Langerhans isolated from starving, fed, or glucose-loaded rats. 2. Basal and glucagon-stimulated activities of adenylate cyclase were lower in islets from starved than from fed rats. The minimum glucose concentration required for stimulation of insulin secretion was higher, whereas the maximum secretory response to glucose was lower, in islets from starved than from fed rats. 3. Adenylate cyclase activity in islets of Langerhans obtained from fed rats loaded with glucose by intermittent intravenous or intraperitoneal injections over 5h was significantly higher than that seen in islets from normal fed rats. Islets obtained from glucose-loaded rats required a lower glucose concentration for stimulation of insulin secretion and attained a higher maximal response to glucose stimulation than those derived from fed rats. 4. Incubation in vitro of islets isolated from normal fed rats, for periods of 1 to 24h in the presence of high concentrations of glucose resulted in an activation of adenylate cyclase that occurred progressively from 2 to 7h and which was maintained during 24h of incubation. The increase of adenylate cyclase activity in isolated islets incubated for 4h in the presence of glucose was not prevented by addition of cycloheximide or actinomycin D. Galactose or 2-deoxyglucose was ineffective in increasing adenylate cyclase activity, and pyruvate (20mm) was less effective than glucose. 5. It is suggested that glucose or a glucose metabolite may exert long-term effects on islet cell adenylate cyclase.

Adenylyl Cyclases↗
PubMed1972

The mode of action of adenosine 3':5'-cyclic monophosphate in mammalian islets of Langerhans. Preparation and properties of islet-cell protein phosphokinase.

1. A protein was demonstrated in mammalian islets of Langerhans that after purification appeared as a single component possessing both cyclic-AMP (adenosine 3':5'-cyclic monophosphate)-binding and cyclic-AMP-dependent protein phosphokinase activities. 2. The protein had an intrinsic association constant for cyclic AMP of 1.15x10(-8)m, which was similar to the K(m) for cyclic AMP (1.11x10(-8)m) of the protein phosphokinase activity. 3. Incubation of the protein in the presence of cyclic AMP resulted in its dissociation into cyclic-AMP-independent protein phosphokinase (catalytic) and cyclic-AMP-binding (receptor) subunits, which could be separated on Sephadex G-200. 4. The cyclic-AMP-dependent protein phosphokinase was capable of phosphorylating a variety of proteins, the most readily phosphorylated being histone, casein and protein components of sub-cellular fractions prepared from islets of Langerhans. 5. The cyclic-AMP-dependent phosphorylation of histone had a K(m) for ATP of 1.1x10(-5)m. 6. The endogenous protein phosphokinase activity in rat islets incubated with agents that are known to alter the intracellular concentration of cyclic AMP was investigated. Theophylline and 3-isobutyl-1-methylxanthine, agents that raise cyclic AMP concentrations in islets, increased the activity of the protein phosphokinase, whereas adrenaline, which lowers islet cyclic AMP concentrations, decreased its activity. 7. It is suggested that cyclic AMP may exert its effects on insulin release by increasing the activity of a protein phosphokinase and may thereby promote the phosphorylation and activity of a rate-determining component of the secretory mechanism.

Animals↗
PubMed1972

The role of adenosine 3':5'-cyclic monophosphate in the regulation of insulin release. Properties of islet-cell adenosine 3':5'-cyclic monophosphate phosphodiesterase.

1. An assay has been developed with sufficient sensitivity for determination of the adenosine 3':5'-cyclic monophosphate diesterase activity in islets of Langerhans, and has been used to investigate the response of the enzyme to various agents which are known to affect insulin release. 2. The subcellular distribution of the enzyme in islets of Langerhans prepared from guinea-pig pancreas was investigated and over 70% of the activity present in the original homogenate was recovered in the supernatant fraction. 3. Gel filtration of the activity present in the supernatant fraction on Sephadex G-200 gave a single peak of activity with an apparent molecular weight of 200000. The phosphodiesterase activity in the peak fraction showed two apparent K(m) values for adenosine 3':5'-cyclic monophosphate (cyclic AMP) of 3mum and 30mum, suggesting the presence of two activities. The pH optimum of the activity with the low K(m) value was 8.7. 4. Theophylline, caffeine, 3-isobutyl-1-methylxanthine (SC-2964), glibenclamide, tolbutamide, xylitol and leucine were inhibitors of the activity with the low K(m) value; imidazole and arginine stimulated the activity, and glucose and diazoxide were without significant effect. 5. It is suggested that the agents theophylline, caffeine, SC-2964, glibenclamide, tolbutamide, leucine and imidazole may alter the intracellular concentration of cyclic AMP in islets of Langerhans by affecting the cyclic AMP phosphodiesterase activity in islet cells and in this way may affect insulin release.

Animals↗
PubMed1971

The role of adenosine 3':5'-cyclic monophosphate in the regulation of insulin release by isolated rat islets of Langerhans.

1. Concentrations of cyclic AMP (adenosine 3':5'-cyclic monophosphate) and rates of insulin release were measured in islets of Langerhans isolated from rat pancreas and incubated for various times in the presence of glucose, 3-isobutyl-1-methylxanthine, caffeine, theophylline, adrenaline and diazoxide. 2. Caffeine and theophylline produced small but significant increases in both cyclic AMP and release of insulin when they were incubated in the presence of 10mm-glucose. 3. 3-Isobutyl-1-methylxanthine produced a marked increase in the intracellular concentration of cyclic AMP in the presence of 5mm- and 10mm-glucose. However, insulin release was stimulated only in the presence of 10mm-glucose. 4. In response to rising concentrations of extracellular glucose (5-20mm) there was no detectable increase in the intracellular concentration of cyclic AMP even though there was a marked increase in the rate of insulin release. 5. In response to 10mm-glucose insulin release occurred in two phases and 3-isobutyl-1-methylxanthine potentiated the effect of glucose on both phases. The intracellular concentration of cyclic AMP remained constant with glucose and rose within 10min to its maximum value with 3-isobutyl-1-methylxanthine. 6. Adrenaline and diazoxide inhibited insulin release and lowered the intracellular concentration of cyclic AMP when islets were incubated with glucose or 3-isobutyl-1-methylxanthine. 7. It is suggested that glucose does not stimulate insulin release by increasing the concentration of cyclic AMP in islet cells. However, the concentration of cyclic AMP in islet cells may modulate the effect of glucose on the release process.

Animals↗
PubMed1969

Islet-cell metabolism during insulin release. Effects of glucose, citrate, octanoate, tolbutamide, glucagon and theophylline.

1. Concentrations of glucose 6-phosphate and 6-phosphogluconate were studied in islets of Langerhans isolated from rat pancreas and incubated in the presence of various agents that induce insulin release. 2. In response to rising concentrations of extracellular glucose (2-10mm) there is a linear increase in the intracellular concentration of glucose 6-phosphate, though this is not the case for 6-phosphogluconate, the intracellular concentration of which only increases when the external glucose concentration exceeds 5mm. 3. Tolbutamide, octanoate and citrate, all of which promote insulin secretion from isolated islets, increase the intracellular concentrations of glucose 6-phosphate and 6-phosphogluconate. The results obtained in the presence of octanoate and citrate are compatible with an inhibitory effect of citrate on islet-cell phosphofructokinase. 4. Theophylline and glucagon when incubated with islets in vitro promote insulin release and cause a rise in 6-phosphogluconate concentration and not in that of glucose 6-phosphate. 5. It is suggested that the further metabolism of glucose 6-phosphate through a pathway other than glycolysis is essential for insulin release. One such pathway involves its oxidation to 6-phosphogluconate, which seems to be a necessary accompaniment of insulin secretion due to glucose. The possibility that agents other than glucose promote insulin release by enhancing the oxidation of glucose 6-phosphate through this pathway is discussed.

Animals↗