Effects of vitamin D on insulin secretion and glucose transporter GLUT2 under static magnetic field in rat.

The present study investigated the effects of vitamin D supplementation on insulin secretion and glucose transporter following static magnetic field (SMF) exposure in rat. Wistar male rats were divided into the following groups: control, SMF-exposed rat (128 mT; 1 h/day for 5 days), vitamin D-treated rats (1600 IU/100 g, received by gavage for five consecutive days), and co-exposed rats (the last day and after exposure rats received a single dose of vitamin D per os). Our results showed that exposure to SMF induced an increase in plasma glucose level and a decrease in plasma insulin concentration. Moreover, ß cell insulin content and islet area were lower in SMF-exposed group compared to control. Likewise, we reported the absence of GLUT2 expression in extracellular membrane of pancreatic islet in SMF-exposed group. Interestingly, supplementation with single dose of vitamin D per os corrected insulinemia and glycemia disturbances caused by SMF. By contrast, the same treatment failed to correct pancreatic area. This study provides evidence that vitamin D supplementation has curative effect on pancreas insulin content and on GLUT2 disruption caused by SMF exposure.

Glycerol metabolism alteration in adipocytes from n3-PUFA-depleted rats, an animal model for metabolic syndrome.

Aquaglyceroporin 7 (AQP7) is a glycerol transporter expressed in adipocytes. Its expression has been shown to be modulated in obesity. Metabolic syndrome is characterized by abdominal obesity, insulin resistance, dyslipidemia, and hypertension. An animal model displaying several features of metabolic syndrome was used to study the AQP7 expression at both mRNA and protein level and glycerol flux in adipocytes. Second generation n3-PUFA depleted female rats is a good animal model for metabolic syndrome as it displays characteristic features such as liver steatosis, visceral obesity, and insulin resistance. Our data show a reduced expression of AQP7 at the protein level in adipose tissue from n3-PUFA-depleted rats, without any changes at the mRNA levels. [U-(14)C]-Glycerol uptake was not modified in adipocytes from n3-PUFA-depleted animals.

Does NAD(P)H oxidase-derived H2O2 participate in hypotonicity-induced insulin release by activating VRAC in ß-cells?

NAD(P)H oxidase (NOX)-derived H(2)O(2) was recently proposed to act, in several cells, as the signal mediating the activation of volume-regulated anion channels (VRAC) under a variety of physiological conditions. The present study aims at investigating whether a similar situation prevails in insulin-secreting BRIN-BD11 and rat ß-cells. Exogenous H(2)O(2) (100 to 200 µM) at basal glucose concentration (1.1 to 2.8 mM) stimulated insulin secretion. The inhibitor of VRAC, 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) inhibited the secretory response to exogenous H(2)O(2). In patch clamp experiments, exogenous H(2)O(2) was observed to stimulate NPPB-sensitive anion channel activity, which induced cell membrane depolarization. Exposure of the BRIN-BD11 cells to a hypotonic medium caused a detectable increase in intracellular level of reactive oxygen species (ROS) that was abolished by diphenyleneiodonium chloride (DPI), a universal NOX inhibitor. NOX inhibitors such as DPI and plumbagin nearly totally inhibited insulin release provoked by exposure of the BRIN-BD11 cells to a hypotonic medium. Preincubation with two other drugs also abolished hypotonicity-induced insulin release and reduced basal insulin output: 1) N-acetyl-L-cysteine (NAC), a glutathione precursor that serves as general antioxidant and 2) betulinic acid a compound that almost totally abolished NOX4 expression. As NPPB, each of these inhibitors (DPI, plumbagin, preincubation with NAC or betulinic acid) strongly reduced the volume regulatory decrease observed following a hypotonic shock, providing an independent proof that VRAC activation is mediated by H(2)O(2). Taken together, these data suggest that NOX-derived H(2)O(2) plays a key role in the insulin secretory response of BRIN-BD11 and native ß-cells to extracellular hypotonicity.

Functional role of aquaglyceroporin 7 expression in the pancreatic beta-cell line BRIN-BD11.

Both mouse and rat pancreatic islet beta-cells were recently found to express aquaglyceroporin 7 (AQP7). In the present study, the expression and role of AQP7 in the function of BRIN-BD11 cells were investigated. AQP7 mRNA and protein were detected by RT-PCR and Western blot analysis, respectively. In an isoosmolar medium, the net uptake of [2-(3)H]glycerol displayed an exponential time course reaching an equilibrium plateau value close to its extracellular concentration. Within 2 min of incubation in a hypotonic medium (caused by a 50 mM decrease in NaCl concentration), the [2-(3)H]glycerol uptake averaged 143.2 +/- 3.8% (n = 24; P < 0.001) of its control value in isotonic medium, declining thereafter consistently with previously demonstrated volume regulatory decrease. When isoosmolarity was restored by the addition of 100 mM urea to the hypotonic medium, [2-(3)H]glycerol uptake remained higher (112.1 +/- 2.8%, n = 24; P < 0.001) than its matched control under isotonic conditions, indicating rapid entry of urea and water. Insulin release by BRIN-BD11 cells was 3 times higher in hypotonic than in isotonic medium. When glycerol (100 mM) or urea (100 mM) were incorporated in the hypotonic medium, the insulin release remained significantly higher than that found in the control isotonic medium, averaging respectively 120.2 +/- 4.2 and 107.0 +/- 3.8% of the paired value recorded in the hypotonic medium. These findings document the rapid entry of glycerol and urea in BRIN-BD11 cells, likely mediated by AQP7.

Pancreatic islet function in omega-3 fatty acid-depleted rats: alteration of calcium fluxes and calcium-dependent insulin release.

Considering the insufficient supply of long-chain polyunsaturated omega-3 fatty acids often prevailing in Western populations, this report deals mainly with alterations of Ca(2+) fluxes and Ca(2+)-dependent insulin secretory events in isolated pancreatic islets from omega-3-depleted rats. In terms of (45)Ca(2+) handling, the islets from omega-3-depleted rats, compared with those from normal animals, displayed an unaltered responsiveness to an increase in extracellular K(+) concentration, a lower inflow rate and lower fractional outflow rate of the divalent cation, and higher (45)Ca(2+)-labeled cellular pool(s) at isotopic equilibrium. The latter anomaly was corrected 120 min after intravenous injection of a novel medium-chain triglyceride-fish oil (MCT:FO) emulsion, distinct from a control omega-3-poor MCT-olive oil (MCT:OO) emulsion. At 8.3 mM D-glucose, insulin release was higher in islets from omega-3-depleted rats vs. control animals, coinciding with a higher cytosolic Ca(2+) concentration. The relative magnitude of the increase in insulin output attributable to a rise in D-glucose as well as extracellular Ca(2+) or K(+) concentration, to the absence vs. presence of verapamil and to the presence vs. absence of extracellular Ca(2+), theophylline, phorbol 12-myristate 13-acetate, or Ba(2+), was always more pronounced in islets from omega-3-depleted rats injected with the MCT:OO compared with the MCT:FO emulsion. A comparable situation prevailed when comparing islets from noninjected omega-3-depleted and normal rats. In light of these and previous findings, we propose that an impairment of Na(+),K(+)-ATPase activity plays a major, although not an exclusive, role in the perturbation of Ca(2+) fluxes and Ca(2+)-dependent secretory events in the islets from omega-3-depleted rats.

Labeling of pancreatic glycogen by D-[U-14c]glucose in hyperglycemic rats.

Under conditions of sustained hyperglycemia, glycogen accumulates in pancreatic islets, but not so in acinar pancreatic cells. We investigated whether advantage could be taken of such a situation in the perspective of the noninvasive imaging of the endocrine pancreas. Control rats or animals injected with streptozotocin (STZ) were infused with solutions of D-glucose mixed with a tracer amount of D-[U-14C]glucose, and the radioactive glycogen content of both liver and pancreas was then measured. After 48 h of infusion, the radioactive glycogen content of the pancreas was 30 times lower in STZ rats than in control animals, coinciding with a 50 times lower insulin content. In the control rats, a sizable labeling of pancreatic glycogen was also recorded when D-[U-14C]glucose was infused for only the last 4 h of unlabeled D-glucose infusion; such a labeling was not decreased when the animals were further infused for 1 h with only the unlabeled hexose. Moreover, a pronounced difference in the pancreatic gland and blood radioactive content of control rats was still observed when the hyperglycemic animals were killed only 40 min after the i.v. injection of D-[U-14C]glucose. In STZ rats transplanted with islets and later infused with D-[U-14C]glucose, the total radioactive content and radioactive glycogen content were both much higher in the transplanted islets than in the pancreatic gland. These results allow one to define the conditions under which the administration of either 2-deoxy-2-[18F]fluoro-D-glucose or 11C-labeled D-glucose could conceivably be used to favor the selective labeling of the endocrine, as distinct from exocrine, pancreas.

Uptake of D-mannoheptulose by normal and tumoral pancreatic islet cells.

D-mannoheptulose was recently proposed as a possible tool to label preferentially insulin-producing cells in the pancreatic gland. In the present study, D-[3H]-mannoheptulose uptake by rat pancreatic islets or dispersed islet cells was found to represent a time-related and temperature-sensitive process inhibited by cytochalasin B. This mould metabolite also inhibited the efflux of D-[3H]-mannoheptulose from prelabelled islets. After 60 min incubation at 37 degrees C, the apparent intracellular distribution space of the tritiated heptose was close to or somewhat higher than that of D-[5-3H]glucose and close to 50% of the intracellular 3HOH space. It was further enhanced by D-glucose and a high concentration of 10 mM of D-mannoheptulose. The uptake of D-[3H]mannoheptulose was much lower however than that of D-[3H]mannoheptulose hexaacetate. As judged from the fate of D-mannoheptulose hexa[2-14C]acetate, the latter ester was efficiently hydrolyzed in the islet cells. The internalization of D-[3H]mannoheptulose (or its ester) coincided with the generation of tritiated acidic metabolites, reflecting phosphorylation of the heptose. The situation found in normal islet cells sharply differed from that found in tumoral islet cells of either the RINm5F or INS-1 line, in which the apparent distribution space of D-[3H]mannoheptulose represented only about 3 and 9%, respectively, of the intracellular 3HOH space. These results indicate that the entry of D-mannoheptulose into islet cells represents a carrier-mediated process, possibly mediated at the intervention of GLUT2 and, hence, provide further support to the possible use of a suitable D-mannoheptulose analog as a tool for the preferential labelling of insulin-producing cells in the pancreatic gland.

Uptake of 1-deoxy-1-[125I]iodo-D-mannoheptulose by different cell types: in vitro and in vivo experiments.

D-mannoheptulose was recently proposed as a tool to label preferentially insulin-producing cells in the pancreatic gland in the perspective of the non-invasive imaging of the endocrine pancreas. In such a perspective, we have now synthesized 1-deoxy-1-[125I]iodo-D-mannoheptulose ([125I]MH) and examined its uptake by different rat cell types. No phosphorylation of [125I]MH by bovine heart hexokinase could be detected. The apparent distribution space of [125I]MH largely exceeded that of [U-14C]sucrose, considered as an extracellular marker, in erythrocytes, parotid cells, hepatocytes, pancreatic pieces and isolated pancreatic islets. Relative to the mean intracellular distribution space of 3HOH, that of [125I]MH was not significantly different in pancreatic pieces from either normal rats or streptozotocin-induced diabetic animals (STZ rats). In pancreatic islets, the uptake of [125I]MH was decreased at low temperature, but failed to be significantly affected by cytochalasin B. Sixty min after the intravenous injection of [125I]MH, the radioactive content of selected organs displayed the following hierarchy: muscle

Possible role of diadenosine polyphosphates in glucose-stimulated insulin release: effects of NaF upon metabolic and functional variables in rat isolated islets.

Diadenosine polyphosphates, such as diadenosine triphosphate (A2P3) and diadenosine tetraphosphate (A2P4), were recently proposed to participate in the stimulus-secretion coupling for nutrient-stimulated insulin release. Since NaF, an inhibitor of inorganic pyrophosphatase, was reported to lower A2P3 and A2P4 content in glucose-stimulated pancreatic islets, its effects upon metabolic, cationic, biosynthetic and secretory variables in rat pancreatic islets were investigated in the present study. Up to a concentration close to 0.1 mM, NaF failed to affect most of these variables, except for a decrease in 45Ca net uptake. Much higher concentrations of NaF (e.g. 5.0 mM) were required to cause inhibition of the metabolic, ionic, biosynthetic and secretory responses of the islets to nutrient secretagogues. Yet, even at this high concentration, NaF failed to lower the islet content in tritiated A2P3 and A2P4 in islets prelabelled with [2,8-3H]adenosine and failed to prevent the glucose-induced increase in such a content. It is concluded, therefore, that NaF may not represent a suitable tool to assess the participation of diadenosine polyphosphates in the process of nutrient-induced insulin secretion.

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.

Effect of the meglitinide analog S21403 on cationic fluxes and insulin release in perifused rat pancreatic islets exposed to a high concentration of D-glucose.

The effect of the meglitinide analog S21403 (10 microm) upon(86)Rb and(45)Ca outflow and insulin release was investigated in perifused rat islets exposed to a high concentration of D-glucose (16.7 mm) in order to simulate the situation found in diabetic patients. Under these conditions, S21403 provoked a rapid, sustained and rapidly reversible increase in(86)Rb outflow, (45)Ca efflux and insulin release. These effects were suppressed or reversed when the experiments were conducted in the absence of extracellular Ca2+. They support the view that S21043 could be used as a novel insulinotropic tool in the treatment of non-insulin-dependent diabetes mellitus, the cationic and secretory responses to the drug displaying a favourable time course for prompt and not unduly prolonged activation of islet B-cells.

Inhibition of glucose-induced insulin release by 3-O-methyl-D-glucose: enzymatic, metabolic and cationic determinants.

The analog of D-glucose, 3-O-methyl-D-glucose, is thought to delay the equilibration of D-glucose concentration across the plasma membrane of pancreatic islet B-cells, but not to exert any marked inhibitory action upon the late phase of glucose-stimulated insulin release. In this study, however, 3-O-methyl-D-glucose, when tested in high concentrations (30-80 mM) was found to cause a rapid, sustained and not rapidly reversible inhibition of glucose-induced insulin release in rat pancreatic islets. In relative terms, the inhibitory action of 3-O-methyl-D-glucose was more marked at low than high concentrations of D-glucose. It could not be attributed to hyperosmolarity and appeared specific for the insulinotropic action of D-glucose, as distinct from non-glucidic nutrient secretagogues. Although 3-O-methyl-D-glucose and D-glucose failed to exert any reciprocal effect upon the steady-state value for the net uptake of these monosaccharides by the islets, the glucose analog inhibited D-[5-3H]glucose utilization and D-[U-14C]glucose oxidation. This coincided with increased 86Rb outflow and decreased 45Ca outflow from prelabelled islets, as well as decreased 45Ca net uptake. A preferential effect of 3-O-methyl-D-glucose upon the first phase of glucose-stimulated insulin release was judged compatible with an altered initial rate of D-glucose entry into islet B-cells. The long-term inhibitory action of the glucose analog upon the metabolic and secretory response to D-glucose, however, may be due, in part at least, to an impaired rate of D-glucose phosphorylation. The phosphorylation of the hexose by beef heart hexokinase and human B-cell glucokinase, as well as by parotid and islet homogenates, was indeed inhibited by 3-O-methyl-D-glucose. The relationship between insulin release and D-glucose utilization or oxidation in the presence of 3-O-methyl-D-glucose was not different from that otherwise observed at increasing concentrations of either D-glucose or D-mannoheptulose. It is concluded, therefore, that 3-O-methyl-D-glucose adversely affects the metabolism and insulinotropic action of D-glucose by a mechanism largely unrelated to changes in the intracellular concentration of the latter hexose.

Effects of alpha-D- and beta-L-glucose pentaacetate on effluent radioactivity from pancreatic islets labelled with [(32)P]phosphate and myo-[2-(3)H]inositol.

The anomers of both D-glucose pentaacetate and L-glucose pentaacetate were recently found to display insulinotropic potential. In order to progress in understanding the mode of action of these esters in islet cells, we have now investigated whether they mimic the effect of nutrient secretagogues to cause a phosphate flush and activation of phospholipase C in isolated islets. For this purpose, rat pancreatic islets were prelabelled with either [(32)P]orthophosphate or myo-[2-(3)H]inositol and placed in a perifusion chamber. In the absence of any other exogenous nutrient, the administration of alpha-D-glucose pentaacetate (1.7 mM) from 46 to 70 min of perifusion increased, after an initial transient fall, both 32P and 3H fractional outflow rates and stimulated insulin release from the perifused islets. No secondary rise in either (32)P or (3)H outflow and no sizeable stimulation of insulin release was observed, however, in response to Beta-L-glucose pentaacetate (also 1.7 mM). These findings are consistent with the view that the insulinotropic action of alpha-D-glucose pentaacetate entails a nutrient-like component leading to the occurrence of both a phosphate flush and hydrolysis of phosphoinositides. This is not the case, however, for Beta-L-glucose pentaacetate. The latter ester might act directly on a yet unidentified receptor, the early secretory response to alpha-D-glucose pentaacetate also apparently involving such a direct effect of the ester itself.

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 microM) and poly-L-lysine (0.03-0.1 microM) 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 D-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.

Insulinotropic action of beta-L-glucose pentaacetate.

The metabolism of beta-L-glucose pentaacetate and its interference with the catabolism of L-[U-14C]glutamine, [U-14C]palmitate, D-[U-14C]glucose, and D-[5-3H]glucose were examined in rat pancreatic islets. Likewise, attention was paid to the effects of this ester on the biosynthesis of islet peptides, the release of insulin from incubated or perifused islets, the functional behavior of individual B cells examined in a reverse hemolytic plaque assay of insulin secretion, adenylate cyclase activity in a membrane-enriched islet subcellular fraction, cAMP production by intact islets, tritiated inositol phosphate production by islets preincubated with myo-[2-3H]inositol, islet cell intracellular pH, 86Rb and 45Ca efflux from prelabeled perifused islets, and electrical activity in single isolated B cells. The results of these experiments were interpreted to indicate that the insulinotropic action of beta-L-glucose pentaacetate is not attributable to any nutritional value of the ester but, instead, appears to result from a direct effect of the ester itself on a yet unidentified receptor system, resulting in a decrease in K+ conductance, plasma membrane depolarization, and induction of electrical activity.

Effects of the polyacetate esters of nutrient and nonnutrient monosaccharides on 45calcium efflux and insulin release from perifused rat pancreatic islets.

CONCLUSION: The polyacetate esters of selected nonnutrient monosaccharides represent potential tools for either stimulation of insulin release in noninsulin-dependent diabetes or inhibition of insulin secretion in hyperinsulinemic syndromes. BACKGROUND: The polyacetate esters of several monosaccharides were recently shown to display greater nutritional value or biological efficiency than the corresponding unesterified carbohydrates. METHODS: The effects of seven polyacetate esters of monosaccharides, all tested at a 1.7-mM concentration, on both 45Ca efflux and insulin release were investigated in prelabeled rat pancreatic islets perifused in the presence of 10.0 mM succinic acid dimethyl ester. RESULTS: Both alpha-D-glucose penta-acetate and, to a lesser extent, beta-L-glucose penta-acetate stimulated insulin release. Inversely, alpha-D-galactose penta-acetate, but not beta-D-galactose penta-acetate inhibited insulin secretion evoked by succinic acid dimethyl ester. Esters of carbohydrates which are inhibitors of D-glucose metabolism, such as D-mannoheptulose hexa-acetate and the two anomers of 2-deoxy-D-glucose tetra-acetate also enhanced insulin output, with a preference for the alpha-anomer of 2-deoxy-D-glucose tetra-acetate. Only those esters with positive insulinotropic action augmented 45Ca efflux from the prelabeled islets.

Secretory, biosynthetic and cationic responses to 2-(N-PHENYL-INDOYL)IMIDAZOLE in rat pancreatic islets.

The secretory, biosynthetic and cationic effects of a novel insulinotropic agent with an imidazoline structure, 2-(N-phenyl-indoyl)imidazole hydrochloride (RX 871024) was investigated in rat pancreatic islets. In the 1.0-10-microM range, this agent augmented, in a concentration-related manner, the release of insulin from islets incubated at intermediate concentrations of d-glucose (4.0-7.0 mm), this enhancing action fading out at both lower a nd higher d-glucose levels. When the concentration of RX 871024 was raised to 1.0 mm, severe inhibition of glucose-stimulated insulin output was observed. The imidazole derivative (10 microM) failed to enhance glucose-stimulated biosynthetic activity in islets exposed to l-[4-3H]phenylalanine; a modest inhibition of the islet peptide tritiation was even recorded at 4.0 mm d-glucose. The positive insulinotropic action of RX 871024 (10 microM) coincided with a decrease in 45Ca net uptake, unchanged outflow of 86Rb and stimulation of 45Ca efflux from prelabelled islets, the latter effect being only partially suppressed in the absence of extracellular Ca2+. These findings suggest a multifactorial mode of action of RX 871024 in islet cells, with emphasis on both an apparent stimulation of Ca2+ influx and, independently of this effect, an intracellular redistribution of the divalent cation. The imidazole compound is proposed, therefore, to display suitable attributes to bypass site-specific defects of d-glucose metabolism in the B-cell of non-insulin-dependent diabetic patients.

Effect of repaglinide on insulin secretion in islets from rats infused for two days with a hypertonic solution of D-glucose.

This study investigates the insulin secretory responsiveness of pancreatic islets to repaglinide in an experimental model of B-cell glucose incompetence. Rats were infused for 2 d with a 1.67 M solution of D-glucose administered at a rate close to 2.8 mL/h. This resulted in a modest rise in glycemia, a severe increase in plasma insulin concentration, an increased sensitivity of B-cells to adrenergic stress, an abnormally high insulin output from isolated islets perifused in the presence of 16.7 mM D-glucose, and a paradoxical transient increase in insulin release from the islets in response to a fall in hexose concentration. The early increment in insulin output evoked by repaglinide, in the presence of 16.7 mM D-glucose, was not lower in the islets from glucose-infused rats than in those from control rats. Moreover, when the meglitinide analog was administered concomitantly with the removal of D-glucose from the perifusion medium, the early response to repaglinide was further increased. Even after 24 min of glucose deprivation, the output of insulin by the islets from glucose-infused rats was higher in the presence of repaglinide than in its absence. These findings indicate that, in this model of B-cell dysfunction, the secretory responsiveness to repaglinide, as distinct from that to glucose, is fully preserved. Therefore, when taken into consideration together with prior observations, these findings argue in support of the use of this insulinotropic agent in the treatment of noninsulin-dependent diabetes.

Insulinotropic action of the monoethyl ester of succinic acid.

1. Selected esters of succinic acid are currently under investigation as potential insulinotropic tools in the treatment of non-insulin-dependent diabetes. At variance with the methyl esters of succinic acid used in most of the work so far conducted from this perspective, the monoethyl ester of succinic acid (EMS) offers the advantage of avoiding the undesirable generation of methanol by intracellular hydrolysis. In the present study, the metabolism and functional effects of EMS were investigated, therefore, in rat pancreatic islets. 2. At a 10 mM concentration, EMS enhanced insulin release from islets stimulated by 7-17 mM D-glucose but failed to do so at lower concentrations of the hexose. EMS was efficiently metabolized, as judged from the generation of 14CO2 by islets exposed to the monoethyl ester of either [1,4-14C] or [2, 3-14C]succinic acid. D-Glucose (6 mM) failed to affect the metabolism of EMS (10 mM), which itself failed to affect the metabolism of D-[5-3H]glucose or D-[U-14C]glucose. EMS also stimulated biosynthetic activity in the islets. It inhibited 86Rb and 45Ca outflow from prelabeled islets perfused in the absence of D-glucose but enhanced the efflux of the two cationic tracers in the presence of the hexose (7 mM). 3. It is concluded that the insulinotropic action of EMS is attributable, to a large extent, to its capacity to act as a nutrient in islet cells.

Effect of 1,1-dimethyl-2-[2-morpholinophenyl]guanidine fumarate on pancreatic islet function.

The modality of the insulinotropic action of 1,1-dimethyl-2-[2-morpholinophenyl]guanidine fumarate (BTS 67 582), a new antidiabetic agent, was investigated in rat pancreatic islets. At a 0.1 mM concentration, which was sufficient to cause a close-to-maximal secretory response, BTS 67 582 failed to affect the utilization and oxidation of exogenous D-glucose, but slightly augmented 14CO2 production from islets prelabelled with either L-[U-14C]glutamine or [U-14C]palmitate. BTS 67 582 (0.1 mM) also failed to affect biosynthetic activity in islets incubated with L-[4-3H]phenylalanine. It augmented insulin release from islets incubated for 90 min in the absence or presence of D-glucose (2.8 to 16.7 mM), this coinciding with stimulation of 45Ca net uptake. In perifused islets deprived of extracellular D-glucose for 45 min, BTS 67 582 (0.1 mM) decreased 86Rb outflow from prelabelled islets, but failed to increase 45Ca efflux and insulin release. In the presence of D-glucose (7.0 mM), BTS 67 582, whilst failing to decrease 86Rb+ outflow, provoked rapid, sustained and rapidly reversible increases of both 45Ca2+ efflux and insulin output. The latter increases were attenuated, but not totally suppressed, in the absence of extracellular Ca2+. BTS 67 582 (0.1 mM) suppressed the inhibitory action of diazoxide (0.25 mM) upon glucose-stimulated insulin release, but nevertheless augmented insulin output from islets incubated in the presence of 90 mM K+. These findings support the view that the insulinotropic action of BTS 67 582 is mainly attributable to the inactivation of ATP-sensitive K+ channels. An intracellular redistribution of Ca2+ ions may also participate, however, to the islet functional response to BTS 67 582.