Identification of the calmodulin-regulated protein phosphatase, calcineurin, in rat pancreatic islets

The aim of this work was the identification of the calmodulin-stimulated protein phosphatase, calcineurin, in rat pancreatic islets. For this purpose, a high-affinity calcineurin antibody and the Western blotting technique were used to detect the presence of calcineurin in freshly collagenase-isolated islets. The calcineurin content detected by this method was about 0.30 ng islet (approx. 0.07% of the total islet protein). The subunit composition and Mr of islet calcineurin were similar to those of bovine brain calcineurin. Incubation of nitrocellulose membranes of the Western blotting, containing the islet protein fractions, with 125I-labeled calmodulin and 45Ca2+ demonstrated that the A subunit bound calmodulin, while the B subunit bound Ca2+. The presence of calcineurin in the islets of Langerhans would suggest its possible participation, as a counterpart of the kinases effect, in the regulatory mechanism of insulin secretion.

Decreased Ca2(+)-ATPase activity after glycosylation of erythrocyte membranes in vivo and in vitro

Erythrocyte membranes drawn from diabetic patients with poor metabolic control have increased protein glycosylation and decreased Ca2(+)-ATPase activity. A significant relationship was found between these two parameters. Similar results were obtained when protein glycosylation and Ca2(+)-ATPase activity were measured in membranes from normal erythrocytes preincubated with glucose. In this condition, both parameters showed a clear time and dose dependence. Incubation of erythrocyte membranes instead of intact erythrocytes with glucose and glucose-6-phosphate strongly suggests that only the glycosylation of the membrane inner-surface proteins can affect Ca2(+)-ATPase activity. The simultaneous presence of 10 mM glucose and 5 mM ATP in the incubation medium did not affect the degree of erythrocyte membrane protein glycosylation but significantly blocked the inhibitory effect of glucose on Ca2(+)-ATPase activity. However, 5 mM ATP only partially blocked the inhibitory effect of 100 mM glucose, suggesting a competitive mechanism of glucose and ATP for the enzyme active site. Our results show that glycosylation of erythrocyte membrane proteins significantly inhibits Ca2(+)-ATPase activity. This effect could contribute to the development of the capillary closure process observed in diabetic patients. Furthermore, it could represent an index of a general impairment of enzyme function arising in cells chronically exposed to high glucose levels.

Changes induced by glucose in the plasma membrane properties of pancreatic islets

Partially purified membranes obtained from rat pancreatic isolated islets preincubated for 3 min with 3.3 and 16.6 mM glucose were labelled with 1,6-diphenyl-1,3,5-hexatriene to study fluorescence polarization. Other islets, incubated for 5 min with the same glucose concentration, were extracted and phospholipids separated by thin-layer chromatography. The composition of phospholipids of fatty acids was then studied by gas-liquid chromatography. Arrhenius plots of the microviscosity in membranes obtained from islets exhibited two components, a steeper slope below 18 degrees C and a gentler slope above 18 degrees C, indicating greater flow activation energy at temperatures below the transition point. Exposure of islets to 16.6 mM glucose significantly increased the flow activation energy (delta E), below and above the transition point. Islets incubated for 5 min with 16.6 mM glucose showed an increase in the percentage composition of 12:0 and 18:2 together with a decrease in the 20:2 W6 and 22:3 W3 fatty acids esterified to phospholipids. Regardless of these changes, no significant alterations occurred in the proportion of saturated fatty acids or in the double bond index; these measurements therefore did not account for the effects of glucose concentration in flow activation energy. The thermotropic changes reported here might be the consequence of some degree of disorder induced by glucose upon the membrane structure. This order alteration could either favor the membrane fusion which occurs during the emiocytosis or only reflects the consequence of such a process.

Early changes in the rat pancreatic B cell size induced by glucose

The perimeter, cell area and volume density (Vvi) of B cells and exocytotic images present in these cells were measured in rat pancreas perfused with 3.3 or 16.6 mM glucose. Four minutes after the beginning of 16.6-mM glucose perfusion and coincident with the appearance at the apex of the first phase of insulin secretion, all these parameters underwent a significant increase. The changes observed in the perimeter, the cell area and the Vvi of B cells suggest an increase in their surface area. An imbalance in the rate of endocytosis:exocytosis processes with a relative predominance of the latter would increase the length of the plasma membrane and could be responsible, at least partly, for the changes in the B cell size.

Effect of a glycoprotein biosynthesis-blocker on insulin secretion

The effect of tunicamycin, a glycoprotein biosynthesis-blocker, on insulin secretion was studied using rat isolated pancreatic islets. Islets cultured with tunicamycin during 24 hr released significantly less insulin in response to 11 mM glucose than the corresponding controls. This inhibitory effect of tunicamycin was not significant in the presence of 3.3 mM glucose. The culture of these islets for a second 24 hr period in a medium devoid of tunicamycin did not remove the blocking effect of the drug on the glucose-induced insulin secretion, as demonstrated by measuring the concentration of insulin in the second culture medium or its release during a 60 min incubation period. Fresh isolated islets incubated for 60 min only with tunicamycin also released during this period less insulin in response to 16.6 mM glucose than the corresponding controls. These results might suggest that some islet glycoproteins participate in the mechanism of insulin secretion.

Effect of different stimulators and a blocker of insulin release on islet Na+, K+-ATPase activity

The effect of several insulin secretagogues and a blocker upon islet Na+, K+-ATPase activity was studied using rat islet homogenates. None of the agents tested modified the enzyme activity when added directly to the enzyme assay. Activity of Na+, K+-ATPase measured in islets preincubated during 3 min with glucose 3.3, 8 or 16.6 mM, as well as with 15 mM KIC or 1.2 microM somatostatin, did not significantly change. The presence of glucagon (1.4 microM) plus theophylline (10 mM) in the preincubation medium significantly enhanced activity while tolbutamide (1.48 mM) or gliclazide (76 microM) significantly decreased such activity. These results suggest that Na+, K+-ATPase activity would not be a main common step involved in the mechanism by which glucose, KIC, glucagon + theophylline and somatostatin exert their effect on insulin secretion. Conversely, the enzyme might contribute to the stimulatory effect of gliclazide and tolbutamide on insulin release. Such effect would be secondary to the release of some cellular mediator rather than a direct action of these compounds on the enzyme. Such effect would later favor a rise in the cytosolic concentration of calcium which might trigger the release of insulin.

Quantitative morphological changes in endocrine pancreas of rats with spontaneous diabetes mellitus

The present study describes the cytopathology of the pancreatic islets in 18-old male eSS rats with spontaneous diabetes mellitus as compared to aged-matched normal animals. Light-microscopic immunocytochemical and morphometric techniques were used to study islet-cell populations, while quantitative methods were employed specifically for the analysis of B-cell ultrastructure. The diabetic rats showed disruption of the islet structure and fibrosis in the stroma. The volume density (Vvi) of endocrine tissue and the Vvi and percentage of B cells were diminished, whereas the Vvi of exocrine tissue and the Vvi and percentage of D cells were increased. The number of medium and large islets as well as their mean volume (micron3) decreased in these animals. Pancreatic B cells from eSS rats showed an increase in the Vvi of endoplasmic reticulum, immature secretory granules and lysosomes. Conversely, the Vvi of total secretory granules and microtubules appeared diminished. The current observations contribute to our understanding of this useful animal model of diabetes mellitus, in the attempt to clarify the pathogenesis of the disease.

Lipid composition of normal male rat islets

Lipid composition was studied in fresh isolated islets from normal male rats. Extractable lipids represent 1856 micrograms per mg islet protein. In such extracts, phospholipids and neutral lipids represent 13.5% and 86.5%, respectively. Phosphatidylcholine (45.8%) and phosphatidylethanolamine (20.6%) were the major components of the phospholipid fraction, and phosphatidylinositol (8.9%) was the minor component. Esterified cholesterol (38.5%), cholesterol (25.5%) and free fatty acids (24.4%) were the major components of the neutral lipid fraction. Fatty acids esterified to phospholipids account for 619.7 pmol/islet, and 2710 pmol/islet were esterified to neutral lipids. In the phospholipid fraction, saturated and unsaturated fatty acids were in a similar proportion. Conversely, in the neutral lipids, two-thirds of the fatty acids were unsaturated. The omega 6 family was the main component of the phospholipid unsaturated fatty acids. In the omega 6 and omega 3 families, the long-chain fatty acids represent the main components. In the neutral lipid fraction, a different percentage of each family was found: omega 3 greater than omega 6 greater than omega 9. The long-chain polyunsaturated fatty acids were also predominant species in the omega 6 and omega 3 families. Further studies on the lipid composition of islets, obtained from rats with normal and altered islet functions, could provide new insights into the knowledge of the mechanism of insulin secretion.

Effect of different insulin secretagogues and blocking agents on islet cell Ca2+-ATPase activity

Plasma membrane Ca2+-ATPase activity was measured in rat islet homogenates. The enzyme was inhibited, in a dose-dependent manner, when the islets were preincubated for 5 min with different concentrations of glucose (2 to 16 mM). This inhibition disappeared almost entirely after 15 min incubation, regardless of the glucose concentration in the medium. Simultaneous measurement of insulin in the medium revealed a stimulatory effect of glucose upon insulin secretion. The Ca2+-ATPase activity was also inhibited when the islets were preincubated for 3 min with other stimulators of insulin secretion such as gliclazide (76 microM), tolbutamide (1.5 mM), glucagon (1.4 microM) + theophylline (10 mM) and ketoisocaproic acid (15 mM). Conversely, the activity of the enzyme was significantly enhanced when the islets were preincubated briefly with the insulin secretion blocker, somatostatin (1.4 microM). Neither glucose nor any of the other substances tested when added directly to the enzyme assay medium modified significantly the Ca2+-ATPase activity measured in the islet homogenates. These results would suggest that the activity of the islet plasma membrane is modulated by one or more of the intracellular metabolites produced when the islets are challenged by the insulin stimulator or blocking agents.

Osmotic dependence of the lysophosphatidylcholine lytic action on liposomes in the gel state

Multilamellar liposomes of dimyristoylphosphatidylcholine are susceptible to lytic action of lysophosphatidylcholine at the gel state, an effect which is not observed when liposomes are in the liquid crystalline state. The lytic action has been found to be enhanced when liposomes are dispersed in hypertonic solutions. On the contrary, hypotonic solutions decreased the effectiveness of the lysolipid. Shrunken liposomes present surface changes as detected by merocyanine 540 and 1-anilinonaphthalene-8-sulfonic acid which can be ascribed to the spontaneous curvature promoted by shrinkage.

Changes in insulin secretion and calcium distribution within B cells induced by gliclazide

Using the pyroantimonate technique, the ultracytochemical distribution of calcium within B cells was studied in isolated rat pancreatic islets incubated during 5, 15 and 30 min with 8.3 mM glucose alone or together with 76 microgram glicazide. Glucose alone produced a continuous increment in the total number of calcium pyroantimonate precipitates (CPP) throughout the incubation period studied. The CPP were mainly associated to the cytoplasmic matrix and the secretory granules at 5 and 15 min and almost evenly distributed between these structures and the plasma membrane at 30 min. Gliclazide plus glucose produced a significant increment, above the glucose values, of the total CPP at 5 min and a later decrease of such values at 15 and 30 min. At 5 min, the incremented total CPP was mainly associated to the secretory granules and the cytoplasmic matrix. The increment in CPP preceded the largest effect of gliclazide on insulin secretion. The latter diminution of CPP induced by gliclazide could contribute to the failure of this drug, as well as other oral hypoglycemic agents, to elicit a second phase of insulin secretion. Changes induced by gliclazide upon B-cell CPP content and distribution might suggest that beyond the effective role of cytosolic calcium in the control of insulin secretion, the cation might reach a threshold concentration in some cell structures to assure the normal development of the secretory process.

Thyroid hormone binding and deiodination by pancreatic islets: relationship with the in vitro effect upon insulin secretion

Thyroxine dehalogenation by rat pancreatic islets was studied incubating isolated islets with labelled T4. [125I]T4 added to the incubation medium was deiodinated by the islets with the consequent production of T3, rT3 and iodide. This deiodination process showed a clear glucose-dependence, being significantly increased in the presence of 16.6 mmol/l glucose. The existence of high and low affinity binding sites for T3 was also demonstrated incubating [125I]T3 with islets under different experimental conditions. The properties of these binding sites were greatly influenced by the extracellular concentration of glucose. Addition of T3 to the incubation medium, significantly modified the insulin release, but its effect varied according to the glucose concentration in the medium, i.e. it enhanced the insulin release at a glucose concentration between 2 to 8 mmol/l; it has no effect at 12 mmol/glucose, and significantly inhibited the secretion of insulin in the presence of 16.6 mmol/l glucose. Our results suggest that thyroid hormones might play a direct regulatory effect on insulin secretion, probably mediated by its deiodination and interaction with specific receptors in the islet cell.

Prevalence of diabetes and obesity in an urban population of Argentina

The prevalence of diabetes mellitus and obesity was studied in the urban area of La Plata City, Argentina. In the chosen population the age ranged from 20 to 74 years. Overt diabetics were not included in the study. For each individual, sex, age, weight, height, body mass index, (BMI), education, type of job and clinical and diabetic background were registered. Plasma glucose values were measured by reading Dextrostix strips in the Dextrometer, in samples obtained by finger punction 2 h after a 50-g glucose load. The final size of the sample (809 people), with an age and sex distribution similar to that of the general population, was attained at random, considering a diabetes prevalence of 6% in the population and a precision and confidence limit in the estimation of 2% and 95%, respectively. The results were evaluated using the post-load glucose values recommended by the World Health Organization (WHO) (200 mg/100 ml). According to these values, the prevalence of diabetes in our population was 2.7%. When adding the 2.3% of known diabetics to these figures, a prevalence of 5% was obtained. The value of the ratio of number of normal newborns/number of pregnancies significantly diminished as a function of the increment in the plasma glucose values found in the probands. Taking into account the BMI values, 37% of our sample population were obese subjects. Even when the glucose values obtained in obese subjects were within the normal range, they were significantly higher than those obtained in the nonobese group.(ABSTRACT TRUNCATED AT 250 WORDS)

Appearance of immunoreactive endocrine cells during the development of the rat pancreas, with special reference to polypeptide-secreting cells

The chronological appearance of PP cells in fetal pancreatic islets was studied using specific anti-PP serum and the direct peroxidase method. The presence of A and B cells was also studied, using the same immunocytochemical technique, as a reference pattern related to data previously reported. Our data confirm that the A cell is the earliest endocrine cell type, appearing on the 12th day of gestation, followed by B cells (14th day) and later on by PP cells (19th day). Primitive islets were identified in the pancreas after the 15th day. However, the spatial cell disposition observed in the adult islet was only recognized at the 20th day of gestation. The data reported provide the necessary information to establish the complete chronology in the rat fetus. Consequently, the development of pancreatic islets in the rat fetus could be employed as a useful model to study the existence of factors that control the sequential appearance of endocrine cells and the possible changes occurring in the islets of animals with genetic diabetes during the fetal period.

Glucagon secretion and intracellular calcium distribution in pancreatic A cells

Quantitative changes in the distribution of intracellular calcium in A cells from perfused rat pancreas in relation to the secretory state of A cells were studied with the pyroantimonate technique for calcium precipitation. A cells stimulated with a 3.3 mM glucose concentration in the perfusate presented numerous calcium precipitates attached to cell membranes, nucleus, cytoplasm and secretion granules. Independently of the length of treatment, inhibition of glucagon release with 16.6 mM glucose decreased the calcium precipitates in every cell organelle studied. These results suggest that intracellular calcium rearrangement might be important in coupling stimulus to secretion in A cells, as it has been demonstrated for pancreatic B cells.

Role of phospholipase and calmodulin inhibitors on insulin, arachidonic acid and prostaglandin E2 release

Using several experimental approaches, we have studied simultaneously the effect of glucose upon insulin, arachidonic acid and prostaglandin E2 release by rat pancreatic islets. A 16.6 mmol/l glucose concentration stimulated the release of insulin, arachidonic acid and prostaglandins. All these effects were significantly reduced either by calmodulin and phospholipase A2 inhibitors, or by the omission of calcium in the incubation medium. Phospholipase A2 inhibitors do not modify the glucose-induced net 45Ca2+ uptake by isolated islets. Our results would suggest that activation of phospholipases, particularly A2, is involved in the mechanism by which glucose stimulates insulin release. This activation increases the intracellular concentration of arachidonic acid, prostaglandins and probably phospholipid degradation products, that could act as messengers for the stimulus-secretion coupling of insulin. The calcium-calmodulin complex would take part in this effect. Conversely, the glucose-induced net calcium uptake by the islets might either be preceded by phospholipase activation or not significantly affected by the blockade of its activity.

Morphological and functional changes in several endocrine glands induced by hypothyroidism in the rat

The effect of hypothyroidism upon the morphology and the function of several endocrine glands was studied in radiothyroidectomized male rats. It was found that T3, T4, insulin, prolactin and corticosterone levels were significantly lower in hypothyroid rats. TSH levels were significantly higher in these animals while no changes were depicted in testosterone levels. The administration of T4 drew back to normal range the above-mentioned altered serum hormone levels. The studies performed with light microscopy revealed alterations only in the TSH secretory cells of the adenohypophysis. Conversely, when using the electron microscope to study the different endocrine glands, clear alterations were depicted in the TSH and prolactin secretory cells of the adenohypophysis, as well as in the pancreatic B cells and the cells of the zona fasciculata of the adrenal cortex. No abnormal changes were demonstrable at the level of the seminiferous tubules of the testis. All the above morphological changes were corrected by the administration of T4 to hypothyroid rats. These results suggest that the hypothyroid state is a complex hormonal dysfunction rather than a single hormonal defect. The secretory alterations are accompanied by fine cellular alterations in the corresponding glands.

Ultracytochemical nuclear calcium distribution in pancreatic B cells: its relation to glucose-stimulated insulin secretion

Calcium distribution in pancreatic B cells was studied, with the aid of the pyroantimonate technique, at different times of glucose-induced secretory activity in the perfused rat pancreas. Specificity of the pyroantimonate precipitates for calcium was assessed by EGTA cross-incubation. Quantitative studies for these calcium pyroantimonate precipitates were performed by a morphometric technique. Pyroantimonate precipitates within the B cell show both time and glucose dependence. At any time-point studied, in the nucleus as well as in other organelles, they were more numerous when glucose was increased in the medium from 3.3 to 16.6 mmol/l. The total number of nuclear calcium pyroantimonate precipitates rose sharply at the time corresponding to the refractory period, falling after that to almost the number found at the prestimulatory period. Otherwise, glucose significantly modifies the temporal distribution of precipitates bound to euchromatin, heterochromatin and perichromatin. These changes in nuclear calcium pyroantimonate precipitates during different periods of B cell secretory activity may indicate an active role of the cation in some nuclear regulatory function during glucose-induced release of insulin.

Chronobiological aspects of blood glucose regulation: a new scope for the study of diabetes mellitus

The levels of blood glucose undergo circadian variations in every species studied. Such rhythm is the consequence of the oscillation in the rate of provision and consumption of glucose. The liver plays an important role in this rhythm, retaining or releasing glucose from or to the circulation. The driving forces for this glucose flow come from outside (food intake) and inside (neural and hormonal control) the body. This blood glucose control might be referred as homeorheusis rather than homeostasis. The final regulatory adjustment of this precise and delicate mechanism is probably located around the suprachiasmatic and other hypothalamic nuclei. In diseases such as diabetes mellitus, where there is an alteration in the circadian rhythm of blood glucose, one of the main goals of treatment must be its restoration to normal levels.

Sequential determination of calcium distribution in B cells at the various phases of glucose-induced insulin secretion

Localization and quantification of calcium pyroantimonate precipitates within the B cells, and determination of insulin secretion were performed in rat pancreas perfused with 3.3 and 16.6 mmol/l glucose. Observations were carried out during the peak, the refractory period, and at 10 and 20 min in the second phase of glucose secretion after the start of a glucose challenge. Specific calcium pyroantimonate precipitates, assessed by EGTA cross-incubation, appeared attached to plasma membrane, Golgi complex, mitochondria, cytoplasmic matrix and secretory granules. The total number of cellular calcium pyroantimonate precipitates increased with perfusion time, being significantly higher at every time-point with the higher concentration of glucose (16.6 mmol/l) than with the 3.3 mmol/l glucose concentration. Calcium pyroantimonate precipitates showed a progressive increment both in plasma membranes and mitochondria. In the cytoplasmic matrix, B granules and Golgi complex, a sharp increase in the number of precipitates was detected at the refractory period, followed by a continuous decrease until the end of the experiment. These results show that the number of calcium pyroantimonate precipitates, localized in different organelles, changes according to the functional state of B cells. They stress the importance of intracellular readily exchangeable pools as regulators of calcium availability for insulin stimulus-secretion coupling.

Multiple modulators of the glucose-induced net calcium uptake by isolated islets

Glucose-induced insulin secretion and net calcium uptake were simultaneously studied in isolated islets obtained from normal, adrenalectomized, ovariectomized and radiothyroidectomized rats, as well as from the corresponding hormone deprived rats following the administration of specific substitutive therapy. Both parameters were also studied in islets from normal rats incubated in the presence of Trifluoperazine (TFP). In all these unrelated experimental conditions simultaneous changes were obtained, observed in the release of insulin and the net calcium uptake elicited by glucose. Otherwise, the modifications of these two parameters obtained in the hormone deprived states were brought back to normal when the animals received the specific substitutive hormonal treatment. On the other hand, TFP also induces simultaneous diminution in both glucose-induced insulin release and net calcium uptake by isolated islets. On account of our results, we could suggest that the mechanism involved in the control of the glucose-induced net calcium uptake is actively modulated by adrenal and ovarian steroids and thyroid hormones as well as by calmodulin. Therefore, changes induced either in the level or activity of these modulators will modify the rate of influx and efflux of Ca2+ across the plasma membrane, with the consequent alteration in the mechanism of stimulus: secretion coupling of insulin.

Effect of the removal of different endocrine glands upon insulin secretion and B-cell ultrastructure

Glucose-induced insulin secretion and B-cell ultrastructure were studied in islets obtained from normal, adrenalectomized, radiothyroidectomized, ovariectomized and orchidectomized rats. Both parameters were also studied in the same experimental groups submitted to specific substitutive therapy. Insulin secretion in response to high glucose was significantly diminished in adrenalectomized, hypothyroid and male castrated rats. Conversely, this secretion was enhanced in ovariectomized rats. These abnormal insulin responses were restored to normal range by specific substitutive therapy. B-cell ultrastructure was markedly altered in hypothyroid and in female and male castrated rats. No significant changes were observed in the adrenalectomized rats. No conspicuous alterations were depicted in the other islet cell populations. The features of the morphological alterations were mainly related to changes in the B-granules and the rough endoplasmic reticulum. Modifications of the other B-cell organelles were less frequent. In the castrated rats, a distinctive feature was the appearance of a finely granulated colloid material. These B-cell alterations, consecutive to changes in the circulating levels of a given hormone, seemed to depend on the chemical structure of the hormone itself rather than on the changes induced in the B-cell secretory function. The ultrastructural changes described were reversed, as in the case of insulin release, by specific substitutive therapy. It is concluded that changes in the circulating levels of the hormones studied are followed by specific alterations in both B-cell secretion and ultrastructure.

Glucagon and insulin secretion during acid-base alterations

Previously, we reported that change from the normal pH of 7.4 surrounding the islet cells to 7.8 results in a decreased B-cell response to 16.6 mM glucose, 10 mM arginine or 400 micrograms/ml tolbutamide. In the present report we studied the effect of modifications in the extracellular pH on glucose- and arginine-induced glucagon and insulin secretion by the perfused rat pancreas. It was found that at pH 7.8, arginine-induced glucagon secretion was significantly greater than at pH 7.4. On the other hand, the switch from pH 7.4 to 7.8 in a pancreas already stimulated by either low glucose or arginine, produced fast and transient glucagon release. Sequential extracellular pH changes from 7.4 to 7.8 and back to 7.4 in the presence of 8.3 mM glucose and a 5 mM arginine stimulus demonstrated that A- and B-cells rapidly modify their secretion in response to extracellular alkalosis in opposite directions. While glucagon output was enhanced (mean secretory rates at pH 7.4, 0.692 +/- 0.042 ng/min and 0.948 +/- 0.57 at pH 7.8), insulin secretion was clearly reduced (72.6 +/- 6.2 ng/min and 35.7 +/- 2.8 ng/min at pH 7.4 and 7.8, respectively). The above observations, together with our previously reported data, indicate that extracellular pH plays an important role in the regulation of glucagon and insulin release. Particularly, extracellular alkalosis enhances A-cell response to 2.3 mM glucose and 5 mM arginine while partially inhibiting B-cell secretion in the perfused rat pancreas.