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G W Sharp

Publications and source records attributed to G W Sharp.

At least 109 records · Page 6Linked to original sources
PubMed1981

Rapid changes in calcium content of rat pancreatic islets in response to glucose.

To study whether glucose affects total islet Ca content, 45Ca content was measured in isolated pancreatic islets, loaded with 45Ca++, during up to 48 h in tissue culture conditions. In islets maintained in 2.8 mM or 8.3 mM glucose the 45Ca content did not increase further after the 24 h, indicating that, at this time point, islets are in isotopic equilibrium. Islets maintained in 8.3 mM glucose had a significantly higher Ca content of 9 pmol/islet than the 5 pmol of islets maintained in 2.8 mM glucose. An increase in the glucose concentration from 2.8 mM to 8.3 mM after 24 h, in the continued presence of 45Ca++, caused a significant and rapid augmentation of the Ca content (+2.5 pmol/islet/5 min). Conversely, a decrease of the glucose concentration from 8.3 mM to 2.8 mM resulted in a rapid loss of cellular Ca. Thus, glucose rapidly affects the total Ca content of islets, the turnover of Ca amounting to approximately 10% of the total content per minute. Islets maintained in 8.3 mM glucose for 24 h exhibited a greater insulin release in response to 16.7 mM glucose than islets maintained for 24 h in 2.8 mM glucose. The increased response, however, was not correlative with the difference in Ca content.

Animals↗
PubMed1981

Insulin release. Demonstration of a priming effect of 3-isobutyl-1-methyl-xanthine (IBMX) on islets of Langerhans.

Exposure of isolated rat pancreatic islets to 1 mM 3-isobutyl-1-methyl-xanthine (IBMX) resulted in a 5.7-fold increase in the rate of insulin release. After a 30-min rest period under basal conditions, reexposure of the islets to 1 mM IBMX resulted in an enhanced rate of release when compared with either a timed control -islets simultaneously exposed to IBMX for the first time-or with the first priming response to IBMX. The possibility of a continuous priming effect was suggested by the gradually rising rate of insulin release during exposure to IBMX. It is concluded that islets have a memory for IBMX exposure that results in increased responsiveness to subsequent stimulation by IBMX.

1-Methyl-3-isobutylxanthine↗
PubMed1981

Defective Ca++ handling and insulin release in islets of diabetic hamsters and mice.

In islets from normal Chinese hamsters preloaded with 45Ca, glucose-induced biphasic insulin release was accompanied by increased 45Ca++ efflux. Islets from diabetic hamsters showed decreased insulin release and no rise in 45Ca++ efflux. No rise in 45Ca++ efflux was seen, even when insulin release was potentiated by 3-isobutyl-1-methylxanthine (IBMX). Since glucose-induced 45Ca++ uptake was normal in diabetic hamsters a defect in cellular Ca++ handling may be involved in the defective insulin release. A similar decrease of glucose-induced insulin release in islets from diabetic C57BL/KsJ-db/db mice was, on the other hand, associated with a small increase in 45Ca++ efflux. In these islets, IBMX potentiated both insulin release and 45Ca++ efflux suggesting that the release defect may be restricted to glucose.

1-Methyl-3-isobutylxanthine↗
PubMed1980

Role of Ca2+ in impaired insulin release from islets of diabetic (C57BL/KsJ-db/db) mice.

The involvement of Ca2+ in the impaired insulin release of diabetic C57BL/KsJ-db/db mice was studied. Twenty-week-old severely hyperglycemic mice were compared to nondiabetic C57BL/KsJ mice as controls. Collagenase-isolated islets were maintained for 46 h in tissue culture allowing for equilibration at the same glucose concentration (8.3) mM). The insulin content of both types of islets was similar. In control islets preloaded during culture with 45Ca2+ glucose-induced insulin release was associated with increased 45Ca2+ effux. Islets from diabetic mice showed markedly reduced insulin response to glucose and a smaller increase in 45Ca2+ efflux. Because insulin release was strikingly potentiated by 3-isobutyl-1-methylxanthine (IBMX), even more than in control islets, there was no generalized release defect. In both types of islets, IBMX potentiation was accompanied by a further enhanced 45Ca2+ efflux, possibly suggesting that cAMP effects are associated with increased cytosol Ca2+% concentrations. As Ca2+ uptake was stimulated by glucose in both types of islets, a defect may lie in the mechanism by which glucose uses cellulr calcium to raise cytosol Ca2+ in the beta-cell of these diabetic mice.

Absorption↗
PubMed1980

Dependency of cyclic AMP-induced insulin release on intra- and extracellular calcium in rat islets of Langerhans.

Calcium and cyclic AMP are important in the stimulation of insulin release. The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) raises islet cAMP levels and causes insulin release at nonstimulatory glucose concentrations. In isolated rat pancreatic islets maintained for 2 d in tissue culture, the effects of IBMX on insulin release and 45Ca++ fluxes were compared with those of glucose. During perifusion at 1 mM Ca++, 16.7 mM glucose elicited a biphasic insulin release, whereas 1 mM IBMX in the presence of 2.8 mM glucose caused a monophasic release. Decreasing extracellular Ca++ a monophasic release. Decreasing extracellular Ca++ to 0.1 mM during stimulation reduced the glucose effect by 80% but did not alter IBMX-induced release. Both glucose and IBMX stimulated 45Ca++ uptake (5 min). 45Ca++ efflux from islets loaded to isotopic equilibrium (46 h) was increased by both substances. IBMX stimulation of insulin release, of 45Ca++ uptake, and of efflux were not inhibited by blockade of Ca++ uptake with verapamil, whereas glucose-induced changes are known to be inhibited. Because IBMX-induced insulin release remained unaltered at 0.1 mM calcium, it appears that cAMP-stimulated insulin release is controlled by intracellular calcium. This is supported by perifusion experiments at 0 Ca++ when IBMX stimulated net Ca++ efflux. In addition, glucose-stimulated insulin release was potentiated by IBMX. These results suggest that cAMP induced insulin release is mediated by increases in cytosolic Ca++ and that cAMP causes dislocation of Ca++ from intracellular stores.

1-Methyl-3-isobutylxanthine↗
PubMed1980

Possible role for calmodulin in insulin release. Studies with trifluoperazine in rat pancreatic islets.

The role of calmodulin in insulin secretion from rat pancreatic islets has been examined by the use of trifluoperazine, an inhibitor of calmodulin-Ca++-directed functions. It was found that 30 microM trifluoperazine caused 50% inhibition, and 100 microM, up to 73% inhibition of 16.7 mM glucose-stimulated insulin release. 100 microM trifluoperazine caused a similar inhibition of 10 mM glyceraldehyde-stimulated release. Therefore, the site of action of trifluoperazine in glucose stimulus-secretion coupling appears to be after the trioses. As trifluoperazine had no effect upon insulin release stimulated by 1 mM 3-isobutyl-1-methylxanthine, the inhibitory effect of trifluoperazine appears to be rather specific. Further, the process of exocytosis per se is not affected. It was also found that although trifluoperazine inhibited the effect of glucose to stimulate insulin release, it did not affect the synergism between glucose and 3-isobutyl-1-methylxanthine to potentiate insulin release. It may be concluded that trifluoperazine selectively inhibits one part of the mechanism by which glucose stimulates insulin release. Calmodulin plays a role in the stimulation of insulin release by glucose at a site between metabolism of trioses and elevation of cytosol Ca++, but is not involved in the final process of exocytosis.

1-Methyl-3-isobutylxanthine↗
PubMed1980

Evidence for the involvement of Na/Ca exchange in glucose-induced insulin release from rat pancreatic islets.

Glucose-induced inhibition of Ca(++) extrusion from the beta-cell may contribute to the rise in cytosol Ca(++) that leads to insulin release. To study whether interference with Na/Ca exchange is involved in this inhibition the effects of glucose were compared to those of ouabain. This substance inhibits Na/K ATPase, decreases the transmembrane Na(+) gradient in islets, and thus interferes with Na/Ca exchange. Collagenase isolated rat islets were maintained for 2 d in tissue culture with a trace amount of (45)Ca(++). Insulin release and (45)Ca(++) efflux were then measured during perifusion. In Ca(++)-deprived medium (to avoid changes in tissue specific radioactivity) 16.7 mM glucose inhibited (45)Ca(++) efflux. Initially 1 mM ouabain inhibited (45)Ca(++) efflux in a similar fashion, the onset being even faster than that of glucose. The effects of 16.7 mM glucose and ouabain were not additive, indicating that both substances may interfere with Na/Ca exchange. In the presence of Ca(++), 16.7 mM glucose induced biphasic insulin release. Ouabain alone caused a gradual increase of insulin release. Again, the effects of ouabain and 16.7 mM glucose were not additive. In contrast, at a submaximal glucose concentration (7 mM) ouabain enhanced both phases of release. An important role for Na/Ca exchange is suggested from experiments in which Ca(++) was removed at the time of glucose-stimulation (16.7 mM). The resulting marked inhibition of insulin release was completely overcome during first phase by ouabain added at the time of Ca(++) removal; second phase was restored to 60%. This could be due to the rapid inhibitory action of ouabain on Ca(++) efflux thereby preventing loss of cellular calcium critical for glucose to induce insulin release. It appears, therefore, that interference with Na/Ca exchange is an important event in the stimulation of insulin release by glucose.

Animals↗
PubMed1980

Dependency of acetylcholine-induced insulin release on Ca++ uptake by rat pancreatic islets.

The dependency of acetylcholine-induced insulin release on Ca++ uptake from extracellular fluid has been investigated in isolated rat pancreatic islets. Islets were maintained for 46 h in tissue culture, and 45Ca++ uptake and insulin release were measured over 5 min on the same batches of islets. Acetylcholine (10(-10)--10(-6) M) at 2.8 mM glucose stimulated insulin release and Ca++ uptake. Both effects were inhibited by atropine. When acetylcholine-stimulated Ca++ uptake was inhibited by verapamil, a blocker of the voltage-dependent Ca++ channel, acetylcholine-induced insulin release was abolished. In a previous report, verapamil did not inhibit glucose-stimulated insulin release over 5 min despite inhibition of the accompanying Ca++ uptake. Glucose (16.7 mM) stimulated insulin release to a similar extent as acetylcholine. Acetylcholine further enhanced glucose-stimulated insulin release without changing glucose-stimulated Ca++ uptake. Acetylcholine did not significantly alter cAMP levels in the islets. It is concluded that acetylcholine, in contrast to glucose, appears to stimulate the early phase of insulin release by enhancing Ca++ uptake from extracellular fluid. An additional effect of acetylcholine in the presence of high glucose may be operative.

Acetylcholine↗
PubMed1980

Stimulation of adenylate cyclase by Ca2+ and calmodulin in rat islets of langerhans: explanation for the glucose-induced increase in cyclic AMP levels.

The effect of Ca2+ and calmodulin has been studied on adenylate cyclase activity in homogenates of rat islets of Langerhans. EGTA had a stimulatory effect on the enzyme in accord with the known inhibitory effect of Ca2+. In contrast, the addition of Ca2+ together with calmodulin is stimulatory and demonstrates the existence of a Ca2+-dependent adenylate cyclase in islets of Langerhans. It is suggested that the glucose-induced increase in cyclic AMP concentrations in intact islets is a secondary consequence of the glucose-induced increase in cytosol free-Ca2+ concentrations which, with calmodulin, causes an increase in the activity of adenylate cyclase.

Adenylyl Cyclases↗
PubMed1979

Defective calcium handling and insulin release in islets from diabetic Chinese hamsters.

In pancreatic islets from normal Chinese hamsters preloaded with 45Ca2+, glucose-induced biphasic insulin release was associated with increased 45Ca2+ efflux; islets from diabetic hamsters showed decreased insulin release and no increase in 45Ca2+ efflux. The lack of stimulated 45Ca2+ efflux persisted even when glucose-induced insulin release was potentiated by 3-isobutyl-1-methylxanthine. Since glucose-stimulated 45Ca2+ uptake by diabetic islets was not impaired, a defect in intracellular Ca2+ handling may be involved in the defective insulin release of the diabetic Chinese hamster.

Animals↗