Calcium distribution in islets of Langerhans: a study of calcium concentrations and of calcium accumulation in B cell organelles

1975 ◽  
Vol 19 (2) ◽  
pp. 395-409
Author(s):  
S.L. Howell ◽  
W. Montague ◽  
M. Tyhurst
Keyword(s):  
Endoplasmic Reticulum ◽  
Cyclic Amp ◽  
B Cell ◽  
Islets Of Langerhans ◽  
Cyclic Gmp ◽  
Cell Organelles ◽  
Calcium Accumulation ◽  
Labile Pool ◽  
Storage Granules ◽  
Rat Islets Of Langerhans

Calcium concentrations of various pancreatic B cell organelles have been determined by X-ray microanalysis of areas of frozen sections of unfixed rat islets of Langerhans. Highest concentrations were detected in storage granules and in mitochondria, although calcium was also present in nuclei, in areas of endoplasmic reticulum and of cytoplasm. Accumulation of 45Ca by isolated organelles has been studied in homogenates and isolated subcellular fractions of rat islets of Langerhans. In the presence of a permeant anion (oxalate or phosphate), accumulation of 45Ca into mitochondria and microsomes was strongly stimulated by ATP. This net uptake was diminished during incubation of homogenates or of a mitochondria plus storage granule-rich fraction in the presence of cyclic AMP, dibutyryl cyclic GMP; 2:4-dinitrophenol or of ruthenium red. Investigations of the characteristics of 45Ca accumulation by homogenates prepared from storage granule-depleted islets showed no differences from those of normal islets, suggesting that the granules do not represent an important labile pool of calcium. With the exception of cyclic AMP and cyclic GMP none of the insulin secretagogues tested (glucose, leucine, arginine, adrenalin, noradrenalin, theophylline, glibenclamide) altered calcium accumulation by islet homogenates. On the basis of absolute calcium levels and of 45Ca uptake studies it is concluded that islet B cells contain a readily exchangeable mitochondrial calcium pool, and an endoplasmic reticulum pool containing a lower concentration of calcium which is also readily exchangeable. The storage granules, despite their high calcium content, do not appear to constitute a labile pool. It seems likely that the labile mitochondria and endoplasmic reticulum pools play a predominant role in the regulation of cytoplasmic free calcium levels, which may in turn be important in the regulation of rates of insulin secretion.

scholarly journals Interaction between insulin-storage granules and F-actin in vitro

1979 ◽  
Vol 178 (2) ◽  
pp. 367-371 ◽  
Author(s):  
S L Howell ◽  
M Tyhurst
Keyword(s):  
Insulin Secretion ◽  
Cyclic Amp ◽  
Phospholipase C ◽  
Islets Of Langerhans ◽  
Actin Filaments ◽  
Sedimentation Rates ◽  
Storage Granules ◽  
Rat Islets Of Langerhans ◽  
Insulin Storage

Possible interactions between polymerized (F-) actin and insulin-storage granules from rat islets of Langerhans were examined in vitro by comparing the sedimentation of the granules in the presence of various actin concentrations. Actin in the concentration range 0.1–0.5 mg/ml produced a retardation in granule-sedimentation rates consistent with binding of the granules to the actin filaments. The interaction was increased by addition of ATP (2mM), but was decreased by CaCl2 (0.1 mM). Binding of granules to actin was unaffected by cyclic AMP or by preincubation of the granules with phospholipase C. Specificity of the interaction was confirmed by the use of depolymerized (G-) actin and of myosin to provide a solution of comparable viscosity; neither of these caused any alteration of granule sedimentation. Possible implications of this interaction of insulin-storage granules with actin for the mechanism of insulin secretion are briefly discussed.

scholarly journals Cyclic nucleotide phosphodiesterase of rat pancreatic islets. Effects of Ca2+, calmodulin and trifluoperazine

1981 ◽  
Vol 197 (2) ◽  
pp. 459-464 ◽  
Author(s):  
M C Sugden ◽  
S J Ashcroft
Keyword(s):  
Cyclic Amp ◽  
Pancreatic Islets ◽  
Islets Of Langerhans ◽  
Cyclic Gmp ◽  
Cyclic Nucleotide ◽  
Specific Inhibitor ◽  
Cyclic Nucleotide Phosphodiesterase ◽  
Phosphodiesterase Activity ◽  
Rat Pancreatic Islets ◽  
Rat Islets Of Langerhans

Cyclic nucleotide phosphodiesterase activity towards cyclic AMP and cyclic GMP was studied in extracts of rat islets of Langerhans. Biphasic Eadie plots [Eadie (1942) J. Biol. Chem. 146, 85-93] were obtained with either substrate suggesting the presence of both ‘high’- and ‘low’-Km components. The apparent Km values were 6.2 +/- 0.5 (n = 8) microM and 103.4 +/- 13.5 (6) microM for cyclic AMP and 3.6 +/- 0.3 (12) microM and 61.4 +/- 7.5 (13) microM for cyclic GMP. With cyclic AMP as substrate, phosphodeisterase activity was increased by calmodulin and Ca2+ and decreased by trifluoperazine, a specific inhibitor of calmodulin. With cyclic GMP as substrate, phosphodiesterase activity was decreased by omission of Ca2+ or addition of trifluoperazine. Addition of exogenous calmodulin had no effect on activity. The data suggest that Ca2+ may influence the islet content of cyclic AMP and cyclic GMP via effects on calmodulin-dependent cyclic nucleotide phosphodiesterase(s).

scholarly journals Stimulation of Adenylate Cyclase by Ca2+ and Calmodulin in Rat Islets of Langerhans: Explanation for the Glucose-induced Increase in Cyclic AMP Levels

Diabetes ◽  
1980 ◽  
Vol 29 (1) ◽  
pp. 74-77 ◽  
Author(s):  
G. W. G. Sharp ◽  
D. E. Wiedenkeller ◽  
D. Kaelin ◽  
E. G. Siegel ◽  
C. B. Wollheim
Keyword(s):  
Cyclic Amp ◽  
Adenylate Cyclase ◽  
Islets Of Langerhans ◽  
Rat Islets ◽  
Rat Islets Of Langerhans ◽  
Stimulation Of

scholarly journals Effect of glucose on polyphosphoinositide metabolism in isolated rat islets of Langerhans

1985 ◽  
Vol 227 (2) ◽  
pp. 483-489 ◽  
Author(s):  
W Montague ◽  
N G Morgan ◽  
G M Rumford ◽  
C A Prince
Keyword(s):  
B Cell ◽  
Islets Of Langerhans ◽  
Sugar Metabolism ◽  
Receptor Occupancy ◽  
Early Event ◽  
Coupling Mechanism ◽  
Rat Islets ◽  
Plasma Membrane Receptor ◽  
Rat Islets Of Langerhans ◽  
Phosphatidylinositol 4

The metabolism of inositol-containing phospholipids during insulin secretion was studied in rat islets of Langerhans preincubated with [3H]inositol to label their phospholipids. Glucose (20 mM) caused a rapid breakdown of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate and an accumulation of inositol trisphosphate and inositol bisphosphate. This effect was maximal at 60s, did not require the presence of extracellular Ca2+, and was abolished by mannoheptulose (15 mM), but not by noradrenaline (1 microM). Mannose (20 mM) and DL-glyceraldehyde (10 mM) produced similar effects to those of glucose, but galactose (20 mM) and KCl (30 mM) were without effect. These results are compatible with the hypothesis that an early event in the stimulus-secretion coupling mechanism in the pancreatic B-cell is the rapid breakdown of polyphosphoinositides catalysed by phospholipase C. Moreover, they suggest that the breakdown of polyphosphoinositides is linked to sugar metabolism in the B-cell. This observation is important, since it demonstrates that events in a cell other than plasma-membrane receptor occupancy can promote polyphosphoinositide hydrolysis.

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