Search PubMed⌕ Search

Biomedical subjects

I C Green

Publications and source records attributed to I C Green.

62 records · Page 4Linked to original sources
PubMed1976

Insulin release and the microtubular system of the islets of Langerhans: effects of insulin secretagogues on microtubule subunit pool size.

An assay system was devised to estimate the pool size of microtubule subunits in islet cells, and to study the importance of the equilibrium between polymerised microtubules and their subunits in the regulation of insulin release. The assay was based on the observation that colchicine binds specifically and quantitatively to microtubule protein subunits, but not to intact microtubules. Vinblastine and cold treatment, which have been shown to cause disaggregation of microtubules into subunits and to inhibit insulin release from islets, increased the number of colchicine binding subunits. D2O, which promotoes stability of microtubules and increases their number in islet cells, caused a decrease in subunit concentration. These results suggest that changes in the equilibrium between polymerised microtubules and their subunits could be studied by measuring the size of the subunit pool. When insulin release was stimulated, by incubating islets in high glucose concentrations or by increasing the intracellular concentration of cyclic AMP there was a reduction in the content of subunit protein. Conversely, when insulin release was inhibited by removal of calcium from the incubation medium there was a shift in the microtubule subunit equilibrium to give an increase in the number of subunits assayed. These results indicate that changes in the equilibrium between subunits and microtubules may play an important role in regulating rates of insulin secretion.

Animals↗
PubMed1975

Insulin release and the microtubular system of the islets of Langerhans. Identification and characterization of tubulin-like protein.

1. Incubation of islets of Langerhans in vitro in the presence of colchicine produced a progressive inhibition of the insulin-secretory response to glucose, which was dependent on the time of incubation. 2. The uptake of [3-H]colchicine by islet cells was a rapid process, equilibrium being reached in less than 30 min. Part of the colchicine taken up was bound to protein material, which was recovered largely in a post-microsomal supernatant fraction prepared from the islets. In contrast with this rapid uptake, the binding of colchicine by islet-cell proteins in intact islets or in islet homogenates was a slow process, and equilibrium was not reached for 60-90 min. After an initial 30 min delay, the time-course of the binding of [3-H]colchicine to islet-cell proteins paralleled that for the inhibitory effect of colchicine on insulin release. 3. Some purification of the colchicine-binding material present in islet homogenates could be achieved by precipitation of the protein with 2mM-CaCl2 (2.8-fold). However, ion-exchange chromatography on DEAE-Sephadex produced a further 27-fold purification on elution with 0.6M-NaCl. 4. Colchicine-binding protein prepared from islets by ion-exchange chromatography showed an intrinsic association constant for colchicine of 1.4muM and an apparent molecular weight on gel filtration of 110000. 5. These results suggest that colchicine-binding protein in islet cells closely resembles tubulin extracted from the other tissues. The delayed effectiveness of colchicine in inhibiting insulin secretion is not due to poor penetration of colchicine into the cells but rather to slow binding of the alkaloid to islet-cell tubulin. It seems likely that, as in other tissues, this binding prevents polymerization of the tubulin into microtubules, and thus interferes with the release process.

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↗
PubMedDate not supplied

Dual action of beta-endorphin on insulin release in genetically obese and lean mice.

Intraperitoneal administration of beta-endorphin (1 mg/kg) to ob/ob mice doubled fasting plasma insulin concentrations within 30 min, while plasma glucose concentrations were unaltered. In lean mice, beta-endorphin failed to alter plasma insulin or glucose responses. In glucose-loaded ob/ob mice, beta-endorphin (1 mg/kg) reduced insulin levels at 40 min, and delayed glucose disposal. A lower dose of beta-endorphin (0.1 mg/kg) decreased plasma insulin at 90 min, with no effect on plasma glucose disposal. In lean mice, only the higher dose of beta-endorphin suppressed the glucose-stimulated rise in plasma insulin concentrations, without affecting plasma glucose. Beta-endorphin's actions were blocked by naltrexone and could not be mimicked by N-acetyl-beta-endorphin. Beta-endorphin (10(-8)M) enhanced insulin release from isolated ob/ob and lean mouse islets incubated in medium containing 6 mM glucose, but inhibited release when 20 mM glucose was present. These effects were naloxone reversible. The results indicate that 1) ob/ob mice display a greater magnitude of response in vivo to beta-endorphin's actions on insulin release compared with lean mice, 2) high concentrations of beta-endorphin exacerbate glucose disposal in ob/ob mice. 3) the prevailing glucose concentration is an important determinant of whether beta-endorphin's effects on insulin release will be stimulatory or inhibitory and 4) these actions are mediated via opiate receptors.

Animals↗
PubMedDate not supplied

Pharmacological interference with phospholipase A2 activity reveals mechanistic differences between glucose and glyceraldehyde induced insulin release: implication for coupling of glucose metabolism to phospholipase A2 activity.

Prostaglandin E2 levels in isolated rat islets were increased from 64 +/- 11 pg/30 islets when incubated in medium containing 2 mM glucose to 115 +/- 9 pg/30 islets in medium containing 20 mM glucose. In contrast, glyceraldehyde (10 mM) reduced prostaglandin E2 levels to 29 +/- 6 pg/30 islets. Inhibition of glucose metabolism by mannoheptulose (10 mM) abolished the stimulatory effect of glucose on prostaglandin E2 levels and inhibited glucose-induced insulin release. The cyclooxygenase inhibitor, flurbiprofen (20 microM), did not affect insulin release caused by glucose or glyceraldehyde. In the presence of 1 mg/ml bovine serum albumin, insulin secretion induced by 20 mM glucose (6.9 +/- 1.1% of islet insulin content) was reduced by the lipoxygenase inhibitor BW755 C (20 microM) to 3.1 +/- 0.6%, and by the phospholipase A2 inhibitor, p-bromophenacyl bromide (10 microM), to 2.1 +/- 0.8%. In the absence of bovine serum albumin the inhibitory action of BW755 C and p-bromophenacyl bromide on glucose-induced insulin release was significantly more pronounced. These drugs whether in the presence or absence of bovine serum albumin, did not affect glyceraldehyde-stimulated insulin secretion. Glyceraldehyde (10 mM), potentiated glucose-induced insulin release in the presence of 2-8 mM glucose, but not for 10-20 mM glucose. Although the phospholipase A2 activator, melittin, initiated insulin release in the presence of 2 mM glucose and enhanced 10 mM glyceraldehyde-stimulated insulin secretion it had no effect on 20 mM glucose-induced insulin release. These two stimulatory effects of melittin on insulin release were totally abolished by p-bromophenacyl bromide.(ABSTRACT TRUNCATED AT 250 WORDS)

4,5-Dihydro-1-(3-(trifluoromethyl)phenyl)-1H-pyraz↗