Carbamylcholine and ouabain effects on Ca2+ handling and insulin release in islets from rats depleted in long-chain polyunsaturated omega 3 fatty acids

A number of metabolic, ionic and secretory variables were recently found to be affected in pancreatic islets obtained from second generation rats depleted in long-chain polyunsaturated ω3 fatty acids (ω3 rats). The present study further documents three sets of anomalies in such islets. First, after 90 min exposure to d-glucose (8.3 mM), the release of insulin from perifused islets, prelabelled with ⁴⁵Ca, is lower in ω3 rats than in control animals, despite comparable ⁴⁵Ca fractional outflow rate. Second, over 15 min exposure to carbamylcholine (0.1 mM), in the presence of d-glucose, the cytosolic concentration of Ca²⁺ is increased to a greater relative extent in dispersed islet cells from ω3 rats, as compared to control animals. This coincides with a greater relative increase in insulin output from perifused islets during the second phase of the secretory response to the cholinergic agent. Last, the increase provoked by ouabain (1.0 mM) in cytosolic Ca²⁺ concentration, ⁴⁵Ca fractional outflow rate and insulin release are all delayed in the ω3 rats. Taking into account the decreased activity of Na⁺, K⁺-ATPase in the islets of ω3 rats, these findings are interpreted as reflecting an impaired priming of insulin-producing cells when first exposed for 105 min to a physiological postprandial concentration of d-glucose.     

Effect ofD-mannoheptulose upon the cationic and secretory responses to α-D-glucose pentaacetate in perifused rat pancreatic islets

The metabolism of α-D-glucose pentaacetate and its positive insulinotropic action in isolated rat pancreatic islets are both unexpectedly resistant toD-mannoheptulose, as judged from experiments conducted over 90–120 min incubation. In the present study, the possible effects of the heptose upon the immediate cationic and secretory response to the ester were investigated in perifused islets prelabeled with either⁸⁶Rb or⁴⁵Ca. At a 10 mM concentration, sufficient to abolish the inhibitory action of unesterifiedD-glucose upon⁸⁶Rb outflow,d-mannoheptulose failed to suppress the decrease in⁸⁶Rb outflow and increase in⁴⁵Ca efflux caused by α-D-glucose pentaacetate at normal extracellular Ca²⁺ concentration and also failed to prevent the decrease in both⁴⁵Ca and insulin release provoked by the ester in the absence of extracellular Ca²⁺. The sole obvious effect of the heptose was to change the early peak-shaped positive secretory response to α-D-glucose pentaacetate to a transient inhibition of insulin release. This change was observed in islets either deprived of any other exogenous nutrient or exposed toL-leucine throughout the experiments. These findings support the view that the islet functional response to α-D-glucose pentaacetate is largely resistant toD-mannoheptulose. They also reinforce the concept that the insulinotropic action of this and other monosaccharide esters involves a dual modality of B-cell activation, linked to both the catabolism of their carbohydrate moieties and a direct effect of the esters themselves upon a specific receptor system.     

Influence of lactation upon pancreatic islet function

The activity of adenylate cyclase, its responsiveness to NaF and forskolin, the activity of the protein kinases A and C, and the oxidation of exogenous D-glucose, L-leucine, and L-glutamine were all higher in pancreatic islets removed from lactating, as distinct from nonlactating, rats. Yet, the release of insulin evoked by D-glucose or the association of L-leucine and L-glutamine was lower in the islets obtained from lactating animals. The lactation-induced decrease in secretory activity was not attributable to a change in the insulin content of the islets, was not corrected by exposure of the islets to theophylline or forskolin, and was also observed in response to stimulation by Ba2+. The rapidly exchangeable islet Ca pool, as estimated from the basal value for 45Ca net uptake, was severely decreased in lactation. Moreover, a hypoglycemic sulfonylurea, which stimulated islet 45Ca net uptake much more markedly in lactating than nonlactating animals, provoked, in association with 12-O-tetradecanoylphorbol-13-acetate, a greater insulin output in lactating than nonlactating rats. It is speculated that the decreased secretory activity in islets removed from lactating rats may be accounted for, in part at least, by a decreased Ca content of the islet cells.     

Abnormal calcium handling by perifused pancreatic islets from neonatal streptozotocin diabetic model rats

To elucidate the mechanism of impaired insulin release in case of non-insulin-dependent diabetes (NIDDM), we investigated insulin release and 45Ca++ efflux from perifused islets obtained from neonatal streptozotocin diabetic model rats. The model rats were prepared by the intraperitoneal administration of 65 mg/kg streptozotocin (STZ) to neonatal males. Rats treated with STZ did not differ from controls in body weight from 1 week to 16 weeks. The model rats had significant hyperglycemia both in the fasting state and after intraperitoneal administration of 2 g/kg glucose. Although the diameter of the islets from the model rats was not significantly different from that of controls, immunoreactivity to anti-insulin was slightly diminished, and degranulation was slightly observed in B-cells. Insulin content was reduced to 45.6% of the control. Insulin release from the perifused islets of STZ-treated rats responded little to 16.7 mmol/L glucose, but normally to 20 mmol/L arginine in the presence of 5.5 mmol/L glucose. In experiments to test the 45Ca++ efflux from the perifused islets prelabeled with 45Ca++, a rise of 45Ca++ efflux concomitant with the second phase of insulin release from the islets of the model rats was inhibited although a sharp increase of 45Ca++ efflux concomitant with the first phase of insulin release was maintained. 45Ca++ uptake for 30 minutes was reduced in the islets from the model rats in the basal and stimulated state of insulin secretion although the incremental 45Ca++ uptake was similar. It is possible that the abnormal calcium handling in pancreatic B-cells may be one of the causes of defect in insulin release in our model rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Possible participation of an islet B-cell calcium-sensing receptor in insulin release

The calcium-sensing receptor gene was recently shown to be expressed in rat pancreatic islets and purified islet B-cells. In this study, we investigated the possible role of this receptor in the regulation of insulin release from isolated rat pancreatic islets. Poly-l-arginine (0.2–0.3 μM) and poly-l-lysine (0.03–0.1 μM) increased insulin output evoked by d-glucose (8.3 mM). This positive effect faded out at higher concentrations of the basic peptides. Likewise, the release of insulin evoked by 8.3 mM d-glucose was significantly lower at high (1.0 mM) than low (0.05–0.1 mM) concentrations of neomycin. The insulinotropic action of Ba²⁺ in Ca²⁺-deprived islets was potentiated in rats pretreated with pertussis toxin. However, Gd³⁺ inhibited insulin release evoked by dd-glucose in islets prepared from normal rats or animals pretreated with pertussis toxin and incubated in the absence or presence of either theophylline or forskolin. Gd³⁺ (0.3 mM) failed to affect effluent radioactivity from islets prelabeled with myo-[2-³H]inositol and cyclic AMP net production in islets incubated in the absence or presence of forskolin. Gd³⁺ decreased, however, ⁴⁵Ca efflux from prelabeled islets perifused in the absence or presence of extracellular Ca²⁺. It is speculated that a negative insulinotropic action mediated by the calcium-sensing receptor, and possibly attributable to a fall in cytosolic Ca²⁺ concentration, may prevent excessive insulin secretion in pathological situations of hypercalcemia.     

Differentiation between the short and long term effects of glucose on the intracellular calcium content of the pancreatic beta-cell

The problem of how glucose affects the intracellular (La3+-nondisplaceable) calcium content of pancreatic beta-cells was approached by combining measurements of 45Ca in ob/ob-mouse islets loaded to isotopic equilibrium with determinations of calcium using electrothermal atomic absorption spectroscopy. Whereas short term changes of the glucose concentration induced marked alterations of insulin release, the islet content of intracellular 45Ca was remarkably stable. The chronic actions of glucose differed from the acute ones in being readily demonstrable and sometimes resulting even in a suppression of the calcium content. Thus, after 7 days of culture in 20 mM glucose, the amount of intracellular calcium was actually lower than when the islets were cultured at 5.5 mM glucose. The long term effect of glucose in suppressing the islet content of intracellular calcium was associated with degranulation and loss of immunoreactive insulin, indicated both from staining of the beta-cells and measurements of the extracted hormone by RIA. The previously unknown ability of glucose to suppress the islet content of intracellular calcium may consequently result from mobilization of the secretory granules.     

The effect of glucose stimulation on 45calcium uptake of rat pancreatic islets and their total calcium content as measured by a fluorometric micro-method

Summary Glucose-stimulated ⁴⁵calcium uptake and total calcium content of rat pancreatic islets has been studied, using a new fluorometric micro-method to estimate total calcium. Extracellular calcium was separated from incubated tissue by a rapid micro-filtration procedure. Islets incubated up to 60 min with calcium chloride 2.5 mmol/l and glucose 2.5 mmol/l maintained the same calcium content (670±7.5 pmol/g DNA). When the glucose concentration was raised to 15 mmol/l no change in the total calcium content could be detected. On incubation with glucose 2.5 mmol/l in the absence of calcium, the calcium content decreased to 488±27 pmol/g DNA. On incubation with ⁴⁵calcium chloride 2.5 mmol/l for 5 or 30 min at 2.5 mmol/l glucose, islets exchanged 21 ±2 and 28±1% of their total calcium content and, at 15 mmol/l glucose, 30±3 and 45±2%, respectively. Thus, islet calcium has a high turn-over rate. Glucose stimulation results in an increase of the calcium uptake without enhancing the total calcium content and hence must increase the calcium-exchangeable pool.     

Cationic determinants of D-fructose insulinotropic action

In the absence of any other exogenous nutrient, D-fructose stimulates insulin release from rat pancreatic islets provided that it is tested at high concentrations in excess of a threshold value close to 80 mM, an optimal secretory response being recorded at 240 mM. In the present study, the cationic determinants of the insulinotropic action of D-fructose, used at the latter high concentration, were explored in perifused rat islets that had been prelabelled with either ⁸⁶Rb or ⁴⁵Ca. The changes in ⁸⁶Rb outflow and ⁴⁵Ca efflux evoked by 240 mM D-fructose were comparable to those caused by 11.1 mM D-glucose in that both hexoses inhibited ⁸⁶Rb and ⁴⁵Ca outflow and, at normal Ca²⁺ concentration, caused a secondary rise in ⁴⁵Ca efflux. These cationic changes coincided wit stimulation of insulin release. The major differences between the two series of experiments consisted, in the islets exposed to D-fructose, in the occurrence of an early and transient increase in ⁴⁵Ca efflux at normal extracellular Ca²⁺ concentration, a secondary reascension in ⁸⁶Rb outflow and a dramatic off-response in both ⁸⁶Rb and ⁴⁵Ca outflow as well as insulin release. These phenomena were also observed in islets exposed to 240 mM 3-O-methyl-D-glucose, suggesting that they may be linked to the massive influx (or efflux) of monosaccharides, as possibly accompanied by Na⁺ inward co-transport, mobilization of Ca²⁺ from intracellular stores and activation of voltage- and/or Ca²⁺-sensitive K⁺ channels. This interpretation was supported by the finding that, at high concentrations (80.0 mM) of D-glucose or D-mannose, the aldohexoses, also provoked a reascension in ⁸⁶Rb outflow and off-response in insulin release. The cationic determinants of the insulinotropic action of D-fructose, in high concentration (240 mM), thus appear similar, if not identical, to those currently incriminated in the stimulation of insulin release by D-glucose. Received: 31 May 1999 / Accepted in revised form: 22 December 1999     

Effect of age onAgaricus bisporus PHA-B stimulated insulin release and45Ca2+ uptakein vitro by islets of langerhans

Summary Thein vitro basal insulin release and⁴⁵Ca²⁺ uptake by rat islets of Langerhans remain unaffected by the animal’s age. However, whenAgaricus bisporus lectin PHA-B is added to the medium, there is a significant stimulation of basal insulin secretion and⁴⁵Ca²⁺ uptake. The islets isolated from younger rats are more sensitive toA. bisporus PHA-B stimulation and register 1.5-fold and 3-fold more increases of insulin release and⁴⁵Ca²⁺ uptake respectively as compared to islets isolated from 12-month-old rats. Age-related conformational changes in the B-cell membrane are suggested as a possible explanation for the above observation. CDRI Communication No. 3428     

Altered K+ fluxes and insulin release in pancreatic islets from omega3 fatty acid-depleted rats

A low intake of long-chain polyunsaturated ω3 fatty acid often prevails in Western populations. Its consequences in terms of the control of fuel homeostasis led us to explore functional events in pancreatic islets isolated from either normal or ω3-depleted rats (second generation). In the latter rats, the inflow of K⁺ by both ouabain-sensitive and ouabain-resistant modalities was decreased, this coinciding with an impaired insulin secretory response to ouabain. The intravenous injection of a medium-chain triglyceride: fish oil emulsion to ω3-depleted rats 2 h before sacrifice restored a normal value for the inflow of K⁺ by the ouabain-sensitive modality, i.e., that linked to the activity of the Na,K-ATPase, but failed to correct the entry of K⁺ by the ouabain-resistant modality and the defect of the insulin secretory response to ouabain. In conclusion, an impaired activity of the Na,K-ATPase in insulin-producing cells apparently represents a key determinant of altered islet function in ω3-depleted rats.     

Effects of glucose, leucine and adenosine on insulin release, 45Ca2+ net uptake, NADH/NAD ratios and oxygen consumption of islets isolated from fed and starved mice

In order to elucidate further the effects of starvation on islet metabolism and insulin release, pancreatic islets of mice were isolated and incubated in the presence of various nutrient secretagogues. Starvation for 60 h completely blocked the insulin release in response to either 16.7 mM glucose or 10 mM leucine. The further addition of 20 mM adenosine partly restored the insulin response. Glucose, adenosine, glucose + adenosine, glucose + leucine or leucine + adenosine all increased the NADH/NAD ratios over basal values in islets from both fed and starved mice. No effects of starvation were observed on islet NADH/NAD ratios in any of the above media, but when islets of starved animals were incubated in the absence of any metabolic substrates the NADH/NAD ratios were decreased. In the absence of exogenous substrates the respiratory rate was also lower in islets from starved animals. Respiratory stimulation evoked by either 16.7 mM glucose or 10 mM leucine + 10 mM glutamine was lower after starvation, whereas glucose + adenosine, glucose + leucine and adenosine all induced normal respiratory responses. No differences between the 45Ca2+ uptake of islets from either starved or fed mice were observed under any conditions. It is concluded that, in starvation, a dissociation between islet insulin release and metabolism (measured as NADH/NAD ratios, oxygen consumption and 45Ca2+ uptake) may exist in the presence of certain nutrient secretagogues.     

Tricyclic antidepressant imipramine reduces the insulin secretory rate in islet cells of Wistar albino rats through a calcium antagonistic action

Aims/hypothesis Treatments with antidepressants have been associated with modifications in glucose homeostasis. The aim of this study was to assess the effect of imipramine, a tricyclic antidepressant, on insulin-secreting cells.Methods Insulin radioimmunoassay, radioisotopic, fluorimetric and patch-clamp methods were used to characterise the effects of imipramine on ionic and secretory events in pancreatic islet cells from Wistar albino rats.Results Imipramine induced a dose-dependent decrease in glucose-stimulated insulin output (IC₅₀=5.2 µmol/l). It also provoked a concentration-dependent reduction in ⁴⁵Ca outflow from islets perifused in the presence of 16.7 mmol/l glucose. Moreover, imipramine inhibited the increase in ⁴⁵Ca outflow mediated by K⁺ depolarisation. Patch-clamp recordings further revealed that imipramine provoked a marked and reversible decrease of the inward Ca²⁺ current. In single islet cells, imipramine counteracted the rise in [Ca²⁺]_i evoked by glucose or high K⁺ concentrations.Conclusions/interpretation These data indicate that imipramine dose-dependently reduces the insulin secretory rate from rat pancreatic beta cells. Such an effect appears to be mediated by the inhibition of voltage-sensitive Ca²⁺ channels with subsequent reduction in Ca²⁺ entry. Thus, it is possible that some adverse effects of imipramine are related, at least in part, to its capacity to behave as a Ca²⁺ entry blocker.Abbreviations FOR fractional outflow rate     

Regulation of calcium fluxes in pancreatic islets: the role of membrane depolarization

The effect of K+-induced depolarization on calcium fluxes and insulin release from isolated islets were investigated in order to elucidate the mechanism by which glucose initially reduces and later increases 45Ca efflux from prelabeled and perifused rat pancreatic islets. Raising the extracellular K+ concentration from 5.0 to 20.0 mM produced a 2- to 3-fold increase in 45Ca net uptake and efflux from isolated islets. The latter effect was dependent on the presence of extracellular Ca2+, suggesting that it resulted from the entry of calcium into the islet cells. In the presence of 20 mM K+, 16.7 mM glucose failed to stimulate 45Ca efflux, while 20 mM K+ further enhanced 45Ca efflux from islets perifused in the presence of the high concentration of glucose. These findings suggest that the effect of glucose to stimulate 45Ca efflux from perifused islets depends mainly on the glucose-induced depolarization of the cell membrane. In the absence of extracellular calcium, 20 mM K+ failed to mimick the effect of glucose to reduce 45Ca efflux. Glucose (16.7 mM) decreased 45Ca efflux from islets perifused in the presence of 20 mM K+ and antagonized the effect of 20 mM K+ to stimulate 45Ca efflux from perifused islets. It is concluded that K+-induced plasma membrane depolarization reproduces the effect of glucose to stimulate but not to inhibit 45Ca efflux from perifused islets.     

Stimulus-secretion coupling of arginine-induced insulin release: comparison between the cationic amino acid and its methyl ester

The role currently ascribed to the accumulation of L-arginine in the pancreatic islet B-cell as a determinant of its insulinotropic action was reevaluated by comparing the uptake and the metabolic, ionic, electric, and secretory effects of the cationic amino acid with those of its more positively charged methyl ester in rat pancreatic islets. The response to L-arginine methyl ester differed from that evoked by the unesterified amino acid by a lower uptake and oxidation, lack of inhibitory action on D-glucose metabolism, more severe inhibition of the catabolism of endogenous L-glutamine, inhibition of 45Ca net uptake, decrease in both 86Rb outflow from prelabeled islets perifused at normal extracellular Ca2+ concentration and 45Ca efflux from prelabeled islets perifused in the absence of extracellular Ca2+, and delayed and lesser insulinotropic action. These findings reinforce the view that the carrier-mediated entry of L-arginine into the islet B-cells, with resulting depolarization of the plasma membrane, represents the essential mechanism for stimulation of insulin release by this cationic amino acid.     

The stimulus -secretion coupling of glucose-induced insulin release. XIX. The insulinotropic effect of glyceraldehyde

Glyceraldehyde is known to stimulate insulin release. Its influence on various parameters of islet function was investigated in order to assess the possible significance of glycolysis in the insulinotropic action of glucose.In the absence of glucose, glyceraldehyde (5–20 mM), but neither dihydroxyacetone nor glycerol stimulated insulin release in rat isolated islets. The glucose-like effect of glyceraldehyde (10 mM) was characterized by a shift to the left of the curve relating insulin release to glucose concentration, without any significant increase in the maximal velocity of the secretory process. In the isolated perfused rat pancreas, glyceraldehyde provoked a biphasic secretory response. Glyceraldehyde-induced insulin release was inhibited in the absence of calcium or in the presence of epinephrine, unaffected by mannoheptulose or 3,3-tetramethyleneglutaric acid, and enhanced by theophylline and cytochalasin B. Glyceraldehyde also stimulated pro-insulin biosynthesis and ⁴⁵Ca net uptake by isolated islets, the latter effect being apparently due, in part at least, to inhibition of calcium outward transport across the cell membrane. At concentrations of nearly equivalent insulinotropic potency, glucose and glyceraldehyde were metabolized at rates yielding comparable output of both lactate and ¹⁴CO₂.These data indicate that glyceraldehyde mimics many effects of glucose on islet function, suggesting that the insulinotropic action of glucose may be related to its metabolism through the glycolytic pathway.     

Significance of the calcium content of mouse beta cells in the preservation of glucose-induced insulin release during culture

Pancreatic islets containing more than 90% beta cells from obese-hyperglycaemic (ob/ob) mice were cultured for 3 days in different concentrations of Ca2+ and glucose to evaluate the importance of intracellular Ca2+ sequestration in glucose-induced insulin release. The islet contents of calcium (total and exchangeable) and immunoreactive insulin were compared with the insulin secretory response to glucose after culture. The turnover of Ca2+ increased with increasing concentrations of glucose and Ca2+. Islets cultured in the presence of 5.5 mmol glucose/l contained more calcium and insulin than those cultured with 1 or 20 mmol glucose/l. During culture in 20 mmol glucose/l, a lowering of the Ca2+ concentration of the medium from 0.42 to 0.025 mmol/l resulted in a paradoxical increase in intracellular calcium, with improvement of the subsequent secretory response to the sugar. When the islets had been exposed to the calcium channel blocker D-600 during culture in a Ca2+-deficient medium, substantial insulin release was noted from islets containing relatively small amounts of calcium. The results suggest that the well-established role of glucose in maintaining insulin release is associated with an ability of the sugar to stimulate the retention of calcium in beta cells.     

Insulin secretion and the morphological and metabolic characteristics of pancreatic islets of hyperthyroid ob/ob mice.

Thyroxine treatment induced experimental hyperthyroidism in ob/ob mice, inhibited glucose-induced insulin secretion from the isolated perfused ob/ob mouse pancreas, and reduced total pancreas insulin content. In contrast, glucose-induced insulin release from incubated pancreatic islets and insulin content of pancreatic islets from ob/ob mice isolated by freehand microdissection were not reduced after thyroxine treatment when expressed per microgram dry islet. Histological examination of the ob/ob mouse pancreas revealed islets without degenerative lesions of islet cells. Granularity of beta cells was well preserved. The average number of pancreatic islets was unchanged. However, the beta cell area was significantly decreased in relation to the total pancreatic parenchyma after thyroxine treatment. This implies that insulin release and content per pancreatic islet was half of that of the controls. ATP content of islets was slightly reduced. Glucose oxidation and glucose utilization by islets from treated mice were slightly increased. Thyroxine treatment of the animals did not abolish the stimulation of 45Ca2+ uptake by glucose, but it did suppress the potentiating effect of fasting on the stimulatory effect of glucose on 45Ca2+ uptake. The metabolic characteristics of islets from experimentally hyperthyroid mice are those of all hyperthyroid tissues. The results provide no evidence for the view that the effects of thyroxine treatment may be due to disturbed metabolic function or energy deprivation of pancreatic islets. Inhibition of insulin secretion from the pancreas after thyroxine administration is apparently due to a reduction in pancreas insulin content and a diminished pancreatic islet volume. Reduced pancreatic islet volume represents most probably a reduction of individual islet cell volume.     

Properties of isolated human islets of langerhans: insulin secretion,glucose oxidation and protein phosphorylation

Summary In the present study, human islets were isolated by collagenase digestion from the pancreases of three kidney donors. Maintainance of the islets in tissue culture enabled insulin release, glucose oxidation and Ca²⁺-calmodulin-dependent protein phosphorylation to be determined using the same islets. Increasing glucose over a range 0–20 mmol/l resulted in a sigmoidal stimulation of insulin release (28.8±5.2 to 118.4±25.8 U-islet^–-h^–, n=10; threshold <4 mmol/l). There was a marked correlation between the insulin secretory response of the islets to glucose and their rate of glucose oxidation (5.9±0.3 at glucose 2 mmol/l up to 25.8±1.8 pmol-islet^–.h^– at 20 mmol/l, r = 0.98). N-acetylglucosamine (20 mmol/l) failed to elicit a secretory response from the islets. Stimulation of insulin secretion by glucose was dependent upon the presence of extracellular Ca²⁺. Extracts of the islets contained a Ca²⁺-calmodulin-dependent protein kinase which phosphorylated a 48-kdalton endogenous polypeptide. Myosin light-chain kinase activity was demonstrated in the presence of exogenous myosin light chains. This report demonstrates for the first time the sigmoidal nature of glucose-stimulated insulin release from isolated human islets, and its correlation with enhanced glucose oxidation. Furthermore, this is the first report of the presence of Ca²⁺-dependent protein kinases in human islets.     

Uptake of tritiated glibenclamide by endocrine and exocrine pancreas

Tritiated glibenclamide binds to specific receptors and is internalized in pancreatic insulin-producing B-cells. We investigated, therefore, whether tritiated glibenclamide could be used to preferentially label the endocrine, as distinct from exocrine, pancreas. In isolated rat pancreatic islets, the net uptake of ³H-glibenclamide reached within 30 min of incubation a near-equilibrium value, corresponding to an apparent distribution space close to three to four times the islet volume. In pieces of pancreas exposed up to 1 h to ³H-glibenclamide, however, its apparent distribution space progressively increased and, even at the min 60 of incubation, did not exceed a third of the wet weight of the pieces. Yet, no significant difference could be detected between the time course for ³H-glibenclamide uptake by pancreatic pieces from either control animals or rats injected with streptozotocin a few days before the experiments. Likewise, no significant difference in the paired ratio between the radioactive content of the pancreas and plasma could be found between the control and diabetic rats when examined 1, 5, or 24 h after the IV administration of ³H-glibenclamide. These findings indicate that the sulfonylurea does not represent a suitable tool for preferential labeling of the endocrine pancreas in the perspective of its imaging by a noninvasive procedure.     

Uptake of tritiated glibenclamide by endocrine and exocrine pancreas

Tritiated glibenclamide binds to specific receptors and is internalized in pancreatic insulin-producing B-cells. We investigated, therefore, whether tritiated glibenclamide could be used to preferentially label the endocrine, as distinct from exocrine, pancreas. In isolated rat pancreatic islets, the net uptake of ³H-glibenclamide reached within 30 min of incubation a near-equilibrium value, corresponding to an apparent distribution space close to three to four times the islet volume. In pieces of pancreas exposed up to 1 h to ³H-glibenclamide, however, its apparent distribution space progressively increased and, even at the min 60 of incubation, did not exceed a third of the wet weight of the pieces. Yet, no significant difference could be detected between the time course for ³H-glibenclamide uptake by pancreatic pieces from either control animals or rats injected with streptozotocin a few days before the experiments. Likewise, no significant difference in the paired ratio between the radioactive content of the pancreas and plasma could be found between the control and diabetic rats when examined 1, 5, or 24 h after the IV administration of ³H-glibenclamide. These findings indicate that the sulfonylurea does not represent a suitable tool for preferential labeling of the endocrine pancreas in the perspective of its imaging by a noninvasive procedure. An erratum to this article is available at .