The pancreatic -cell recognition of insulin secretagogues. IV. Islet uptake of sulfonylureas

Antihyperglycemic activity and antidiabetic effect of methyl caffeate isolated from Solanum torvum Swartz. fruit in streptozotocin induced diabetic rats

Natural remedies from medicinal plants are considered to be effective and safe alternatives to treat diabetes mellitus. Solanum torvum Swartz. fruit is widely used in the traditional system of medicine to treat diabetes. In the present study methyl caffeate, isolated from S. torvum fruit, was screened for its efficacy in controlling diabetes in animal models. Antihyperglycemic effect of methyl caffeate was studied in normal glucose-fed rats. The effects of oral administration of methyl caffeate (10, 20 and 40 mg/kg) for 28 days on body weight, fasting blood glucose, plasma insulin, hemoglobin, glycated hemoglobin, total protein, hepatic glycogen and carbohydrate metabolism enzymes in streptozotocin induced diabetic rats were investigated. Histological observations in the pancreas and GLUT4 expression in skeletal muscles were also studied. Methyl caffeate at 40 mg/kg significantly prevented the increase in blood glucose level after glucose administration at 60 min in comparison to the hyperglycemic control group. In streptozotocin induced diabetic rats, methyl caffeate produced significant reduction in blood glucose and increased body weight. The levels and/or activities of other biochemical parameters were near normal due to treatment with methyl caffeate. Methyl caffeate treated diabetic rats showed upregulation of GLUT4 and regeneration of β-cells in the pancreas. These results substantiated that methyl caffeate possessed hypoglycemic effect, and it could be developed into a potent oral antidiabetic drug.

An experimental evaluation of the antidiabetic and antilipidemic properties of a standardized Momordica charantia fruit extract

BACKGROUND

The MCE, Momordica charantia fruit extract Linn. (Cucurbitaceae) have been documented to elicit hypoglycemic activity on various occasions. However, due to lack of standardization of these extracts, their efficacy remains questionable. The present study was undertaken by selecting a well standardised MCE. This study reports hypoglycemic and antilipidemic activities of MCE employing relevant animal models and in vitro methods.

METHODS

Diabetes was induced in Wistar rats by a s.c., subcutaneous injection of alloxan monohydrate (100 mg/kg) in acetate buffer (pH 4.5). MCE and glibenclamide were administered orally to alloxan diabetic rats at doses of 150 mg/kg, 300 mg/kg & 600 mg/kg, and 4 mg/kg respectively for 30 days, blood was withdrawn for glucose determination on 0, 7, 14, 21 and 30th days. On the 31st day, overnight fasted rats were sacrificed and blood was collected for various biochemical estimations including glycosylated haemoglobin, mean blood glucose, serum insulin, cholesterol, triglcerides, protein and glycogen content of liver. The hemidiaphragms and livers were also isolated, carefully excised and placed immediately in ice cooled perfusion solution and processed to study the glucose uptake/transfer processes. Hypolipidemic activity in old obese rats was evaluated by treating two groups with MCE (150 mg/kg & 300 mg/kg) orally for 30 days and determining total cholesterol, triglyceride and HDL-CH, LDL-CH and VLDL-CH levels from serum samples.

RESULTS

Subchronic study of MCE in alloxan induced diabetic rats showed significant antihyperglycemic activity by lowering blood glucose and GHb%, percent glycosylated haemoglobin. Pattern of glucose tolerance curve was also altered significantly. MCE treatment enhanced uptake of glucose by hemidiaphragm and inhibited glycogenolysis in liver slices in vitro. A significant reduction in the serum cholesterol and glyceride levels of obese rats following MCE treatment was also observed.

CONCLUSION

Our experimental findings with respect to the mechanism of action of MCE in alloxan diabetic rats suggest that it enhances insulin secretion by the islets of Langerhans, reduces glycogenesis in liver tissue, enhances peripheral glucose utilisation and increases serum protein levels. Furthermore, MCE treatment restores the altered histological architecture of the islets of Langerhans. Hence, the biochemical, pharmacological and histopathological profiles of MCE clearly indicate its potential antidiabetic activity and other beneficial effects in amelioration of diabetes associated complications. Further, an evaluation of its antilipidemic activity in old obese rats demonstrated significant lowering of cholesterol and triglyceride levels while elevating HDL-cholesterol levels. Also, the extract lowered serum lipids in alloxan diabetic rats, suggesting its usefulness in controlling metabolic alterations associated with diabetes.

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.

Evidence for a saturable, energy-dependent and carrier-mediated uptake of oral antidiabetics into rat hepatocytes

The hepatic uptake of two sulfonylureas, glisoxepide and glibenclamide, was investigated in isolated rat hepatocytes. Two transport processes were defined: passive physical diffusion and saturable carrier transport. For diffusion at pH 7.4 the permeability coefficients were 3.3 x 10(-6) cm/s for glisoxepide and 10.6 x 10(-6) cm/s for glibenclamide. Saturable uptake differed among the sulfonylureas. Glibenclamide uptake was neither energy- nor sodium-dependent and temperature dependence was linear. The apparent activation energy for saturable glibenclamide uptake was 15.2 kJ/mol and Q10 values for uptake between 7 and 37 degrees C were 1.17 +/- 0.12. Saturable glibenclamide uptake exhibited a Km = 3.1 microM and a Vmax = 416 pmol/mg cell protein per min. Thus glibenclamide uptake was defined kinetically as a facilitated diffusion process. Glisoxepide uptake revealed two Km values: Km1 = 2-3 microM and Vmax1 = 200 pmol/mg protein per min, and Km2 = 110 microM and Vmax2 = 1600 pmol/mg protein per min. Uptake at low and high substrate concentration was energy-dependent, sodium-dependent and was inhibited by ouabain. Temperature dependence increased markedly beyond 22 degrees C and the apparent activation energy was 59.7 kJ/mol at low Km1 glisoxepide concentrations and 60.3 kJ/mol at high Km2 concentrations. Whereas glisoxepide was slowly taken up into AS-30D hepatoma cells, glibenclamide was not. The hepatic uptake of glibenclamide was not inhibited by glisoxepide but glibenclamide inhibited glisoxepide uptake. The inhibition by glibenclamide was noncompetitive. Isolated hepatocytes accumulated the sulfonylureas markedly and metabolized both. The metabolized radioligands were slowly released into the incubation buffer. The results indicate that the hepatic uptake of the two sulfonylureas is by carrier-mediated transport. The uptake processes are, however, strikingly different, indicating heterogeneity of sulfonylurea transporters.

Evidence for more than one binding site for sulfonylureas in insulin-secreting cells

Specific binding of both [3H]glibenclamide and [3H]gliquidone has been observed in a particulate fraction of insulin-secreting rat tumour (RIN m5F) cells. The binding of both the labels was time-dependent, of high affinity (including a low affinity binding site), saturable and reversible. The rank order of inhibition of [3H]glibenclamide binding was glibenclamide greater than gliquidone greater than AG-EE 388 = AG-EE 86 = AG-EE 319 greater than AG-EE 436 (AG coded drugs are benzoic acid derivatives which lack the sulfonylurea moiety of sulfonylureas). The Kds of high affinity binding for glibenclamide and gliquidone were 0.08 and 1.3 nM, respectively. When [3H]gliquidone was used as the labelled compound this rank order of binding and the affinities of drugs were different, e.g. glibenclamide was less potent than gliquidone. The Kd values of high affinity binding to the [3H]gliquidone binding site were 810 and 79 nM with respect to glibenclamide and gliquidone. The binding site labelled by [3H]gliquidone, in contrast to that labelled by [3H]glibenclamide, was not able to discriminate between the two enantiomers AG-EE 319 and AG-EE 436. The data indicate that there are different binding sites for glibenclamide and gliquidone in RIN m5F cells. In extension to data of other groups it is speculated that there exists more than one specific binding site for sulfonylureas and other related compounds, e.g. benzoic acid derivatives and that sulfonylureas behave differently not only in quantitative but in qualitative terms as well.

A comparison of cellular actions between gliclazide and a hypoglycaemic peptide fragment of human growth hormone (hGH 6-13)

The mechanisms of hypoglycaemic action of a 'second-generation' sulphonylurea, gliclazide, and a synthetic human growth hormone fragment, hGH 6-13 (Leu-Ser-Arg-Leu-Phe-Asp-Asn-Ala), were compared at the cellular level in rats. Both compounds were shown to be hypoglycaemic in vivo although their molecular structures were totally different. Gliclazide was markedly insulinotropic, as are all hypoglycaemic sulphonylureas, whereas hGH 6-13 had no visible effect on basal levels of plasma insulin. However, in vitro studies with isolated pancreatic islets revealed that hGH 6-13 significantly augmented insulin secretion in the presence of exogenous glucose. One other major difference was that gliclazide had no direct effect on insulin receptor function while the synthetic hGH 6-13 increased the binding of insulin to specific receptors on isolated cells. Results suggested that the human growth hormone fragment hGH 6-13 could be a potential anti-diabetes drug with the ability to potentiate circulating insulin action and to achieve blood glucose normalisation.

Internalization of 3H-glibenclamide in pancreatic islet cells

As judged by autoradiographic criteria at the optical level, rat islet cells progressively accumulate 3H-glibenclamide over 1 to 30 min incubation at 24 degrees C. The labeled material associates with all endocrine cells, but is preferentially seen within B cells. The total uptake of 3H-glibenclamide (1.0 mumol/l) is little affected by the presence of unlabeled glibenclamide (0.2 mmol/l), but is significantly decreased at 4 degrees C (p less than 0.05). At the electron microscopic level, less than 15% of the autoradiographic grains are located at the B cell plasma membrane and 72-79% of the grains found over the cytoplasm are associated with insulin secretory granules. Such a pattern is observed both after short (1 min) or prolonged (30 min) incubation, and at 4 or 24 degrees C. It is proposed that the insulinotropic action of glibenclamide is not necessarily attributable to primary events located solely at the cell surface.

Effects of glibenclamide and tetracaine on 86Rb+ fluxes in mouse pancreatic beta-cells

Potassium transport was measured in beta-cell-rich islets from ob/ob-mice using the K+-analogue 86Rb+. Both tetracaine (0.1 mM) and glibenclamide (0.1 microM) reduced the ouabain-resistant 86Rb+ influx but did not significantly affect the ouabain-sensitive portion (Na+/K+ pump). Tetracaine (0.5 - 1 mM) or glibenclamide (0.2 mM) decreased the 86Rb+ equilibrium content and glibenclamide (1 microM) transiently reduced the 86Rb+ efflux rate but 0.1 mM tetracaine had only a slight effect on this flux rate. The results suggest that a change in ouabain-resistant (passive) K+ fluxes, but not the Na+/K+ pump, is involved in stimulation of insulin secretion by glibenclamide and tetracaine. Both drugs may exert similar effects on the beta-cell plasma membrane.

Different effects of glibenclamide and the structural analogue HB 699 on the 45Ca2+ uptake by ob/ob-mouse islets

The effects on 45Ca2+ uptake of HB 699, an acyl-amino-alkyl benzoic acid derivative, was compared to those of glibenclamide in incubations using the La3+ wash technique. HB 699 enhanced the 45Ca2+ net uptake in a concentration range (10-200 microM) where insulin release was also stimulated. Glibenclamide showed maximum stimulation of 45Ca2+ net uptake already at 1 microM. HB 699 did not clearly stimulate the short-term 45Ca2+ uptake whether or not the islets were preincubated with the drug. It is suggested that HB 699-induced insulin release is mediated, at least partly, by increased mobility of beta-cell Ca2+.

Binding of hypoglycaemic sulphonylureas to an artificial phospholipid bilayer

Hypoglycaemic sulphonylureas bind to multilamellar liposomes formed of egg yolk phosphatidylcholine. In this artificial model, both specific and non-specific components of the binding phenomenon can be characterized by the same criteria as those used in studies performed with natural membranes. The relative ability of distinct sulphonylureas to inhibit the binding of 3H-glibenclamide or 3H-gliquidone to the liposomes parallels their relative potency as insulin secretagogues. It is proposed that the insertion of hypoglycaemic sulphonylureas into the phospholipid domain of the B cell membrane could represent a primary event in the mechanism by which these agents stimulate insulin release.

Effect of tetracaine and lidocaine on insulin release in isolated mouse pancreatic islets

The effect of tetracaine and lidocaine on insulin secretion and glucose oxidation by islets of ob/ob-mice was measured. Tetracaine, at a concentration of 1 microM to 0.1 mM, did not markedly influence the basal (3 mM glucose) insulin secretion, whereas 0.5-3.5 mM induced a marked increase. At 7 mM glucose, there was a dose-dependent increase with 0.1-2.5 mM tetracaine. Insulin release induced by 20 mM glucose was potentiated by 0.1 mM and 0.5 mM tetracaine, but this effect disappeared at 1 mM tetracaine. The stimulatory effect of 0.5-1 mM tetracaine on basal insulin release was blocked by the secretory inhibitors, adrenaline (1 microM), clonidine (1 microM) and by Ca2+-deficiency, but the stimulation by 3.5 mM tetracaine was not reduced by 1 microM clonidine or Ca2+ deficiency. Atropine (10 microM) did not affect the stimulation by 0.5 mM tetracaine at 3 mM glucose or by 0.25 mM tetracaine at 20 mM glucose. Tetracaine, at 0.1 mM, potentiated the secretory stimulation of 20 mM L-leucine, 20 mM D-mannose, or 1 microM glibenclamide. Mannoheptulose, 10 mM, abolished the combined effects of 0.1 mM tetracaine and 10 mM glucose. Lidocaine, 1-5 mM, stimulated basal insulin release, but 1 microM-1 mM of the drug did not affect glucose-induced (20 mM glucose) insulin release and 5 mM lidocaine inhibited glucose stimulation. The oxidation of 10 mM D-[U-14C]glucose was slightly enhanced by 0.1 and 1 mM tetracaine. The results indicate that tetracaine and lidocaine, at certain concentrations, can induce insulin release and that tetracaine potentiates secretion induced by other secretagogues. It is concluded that these effects may be associated with beta-cell functions related to the adrenergic receptors but probably not to cholinergic receptors.

Tolbutamide perifusion of rat islets. Sequential changes in calcium, phosphorus, sodium, potassium, and chlorine in single beta cells

Fluctuations of calcium, phosphorus, sodium, potassium, and chlorine in beta cells were followed during rat islet perifusion with tolbutamide and related to insulin secretion. In 24 paired experiments two chambers containing 100 islets were perifused with buffered medium containing 4.2 mM glucose alone or with added tolbutamide (200 micrograms/ml). Effluent was collected frequently for insulin determinations. At eight different time intervals from 0 to 20 min islets were acutely fixed, prepared for scanning electron microscopy and beta cells in islet tissue were identified. Element content in 480 single cells was measured by energy dispersive x-ray analysis. Tolbutamide elicited typical monophasic insulin release that exceeded control islet secretory rates from 2 to 6 min with a peak value at 3 min. This pattern was preceded by monophasic calcium accumulation in beta cells that abruptly rose 150% above control cells at 1 min and declined to base line by 4 min. The rapid ascent of calcium was associated with significant depressions of sodium and potassium content without alterations of cell phosphorus. Chlorine fell at 2 min and then rose greater than 50% above control cells at 4 min. After 6 min insulin secretion and element content remained near control levels. We conclude that monophasic calcium accumulation in beta cells is the earliest, most predictive event of islet insulin secretion after a tolbutamide stimulus. Oscillations of beta cell sodium and potassium reciprocally relate to calcium, and an elevation of chlorine content is a relatively late phenomenon in the stimulus-secretion coupling process.

On insulin secretion

Aspects of insulin secretory mechanisms and models of diabetogenic B cell damage are discussed. Measurements of fluxes of 3H-labelled triphenylmethylphosphonium ion, 86Rb+, 42K+, 22Na+, and 45Ca2+ in isolated islets indicate that the triggering of insulin release depends on alterations in the interaction of ions with the B cells. One difficulty in the detailed analysis of these alterations are uncertainties which arise when macroscopic concepts for homogenous phases are applied to microscopic and heterogenous compartments, as exemplified by the meaning of pH in insulin secretory granules and of membrane electric potential. Nonetheless, the importance of an apparent decreased K+ permeability in mediating the insulin-releasing action of glucose, and of an apparent increased Na+ permeability in mediating the potentiating action of acetylcholine is emphasized. Fluorescent probing of Ca2+ by chlorotetracycline revealed effects of glucose alone as well as glucose-dependent and atropine-sensitive effects of acetylcholine. Although acetylcholine, sulfonylureas, and certain thiol-blocking agents may stimulate insulin release by direct effects on the B cell plasma membrane, a high capacity for D-glucose transmembrane transport has probably evolved in order that the interior of the B cells can always sense the circulating glucose concentration. A signal to secretion is thought to be transmitted from glucose metabolism to altered ion fluxes by intervention of reduced pyridine nucleotides and hypothetical redox protein for which thioredoxin may be a model. The insulin secretory defect in hereditary diabetic C57BL/KsJ-db/db-mice is apparently linked to a decreased basal permeability for K+ and a failure of the B cells to decrease further this permeability in response to glucose. Functioning B cells are acutely damaged when exposed to heterologous serum or alloxan in vitro; cytotoxic activation of complement by the alternative pathway could perhaps occur during islet inflammation. Protection experiments with free-radical scavengers in vitro and in vivo support the theory that hydroxyl radicals are instrumental in the production of alloxan diabetes. Rapid reduction of alloxan by thioredoxin in the presence of molecular oxygen and NADPH leads to strong chemiluminescence from luminol indicative of an intense radical protection. The sensitivity of B cells to alloxan may be due to physiological specializations of their plasma membranes, involving the highly effective glucose carrier or the hypothetical oxidation/reduction systems or both.

Long-term effects of glibenclamide on the insulin production, oxidative metabolism and quantitative ultrastructure of mouse pancreatic islets maintained in tissue culture at different glucose concentrations

In order to evaluate long-term effects of sulphonylureas on pancreatic islet structure and function, isolated mouse islets were maintained in tissue culture for one week at various glucose concentrations, and in the absence or presence of glibenclamide. When the islets were cultured at 3.3 or 5.5 mmol/1, but not at 16.7 mmol/1 glucose, it was found that the drug stimulated insulin secretion into the culture medium during the initial 3 days of culture. During the remainder of the culture period no such enhancement of secretion was demonstrated. Insulin release due to glibenclamide apparently resulted in rapid depletion of intracellular insulin stores. The finding of an enlarged B-cell Golgi apparatus in the drug-treated islets was probably associated with granule discharge. The failure of glibenclamide to promote insulin secretion during the whole culture period could reflect the adverse effects of the drug on islet insulin biosynthesis as indicated by short-term experiments performed after culture. Similar experiments showed that the impaired insulin biosynthesis could not be restored by withdrawal of the drug from the culture medium for 3 days. Furthermore, the capacity for insulin release in response to an acute glucose challenge at the end of the culture period, was abolished by culture in the presence of glibenclamide. The drug effects on insulin biosynthesis and intracellular insulin stores, which were most pronounced at 5.5 mmol/1 glucose, possibly resulted from changes in B-cell metabolism as suggested by the diminished islet glucose-oxidation rate. The spatial characteristics of islet mitochondria indicated that these changes might involve an adaptation to substrates other than glucose. In conclusion, our findings suggest that sulphonylureas have an insulinotropic effect, which is however transient. Indeed, it rather seems as if long-term exposure of islet B-cells to sulphonylureas in vitro were accompanied by functional deficiency.

Calcium and pancreatic beta-cell function. The mechanism of insulin secretion studied with the aid of lanthanum

La3+ was used to study the involvement of Ca2+ in insulin secretion in beta-cell-rich pancreatic islets micro-dissected from non-inbred ob/ob mice. Ultrastructural studies revealed that the localization of La3+ was entirely restricted to the exterior of the cells. Consistent with a membrane action, exposure to La3+ failed to affect glucose oxidation and either the sucrose space or the general ultrastructure of the islets. In contrast, La3+ had marked effects on insulin release and 45Ca fluxes. Exposure to La3+ resulted in pronounced inhibition of insulin release irrespective of the presence or absence of Ca2+, 3-isobutyl-1-methylxanthine or glucose. Perifusion experiments revealed that the inhibitory action was prompt, sustained and readily reversible. Removal of La3+ was associated with a subsequent prolonged stimulatory phase of insulin release even in medium deficient in Ca2+. This action could not be attributed to an increase in cyclic AMP, but was potentiated by 3-isobutyl-1-methylxanthine and abolished by L-adrenaline. La3+ displaced 45Ca from superficially located binding sites and inhibited the uptake and efflux of 45Ca. The stimulatory and inhibitory actions of glucose on 45Ca efflux were also abolished in the presence of 2 mM-La3+ Removal of La3+ was associated with the preferential mobilization of 45Ca incorporated in response to glucose. The results indicate that binding of La3+ to superficial sites in the plasma membrane leads to inhibition of insulin release by suppression of transmembrane Ca2+ fluxes. It is suggested that accumulation of Ca2+ in the cytoplasm accounts for the stimulation of insulin release seen after removal of La3+ from inhibitory binding sites in the beta-cell plasma membrane.

Effects of metabolic inhibitors on the efflux of 5-hydroxytryptamine from pancreatic Beta-cells

The influence of glucose and metabolic inhibitors on the efflux of tritiated 5-hydroxytryptamine (5-HT) was studied in perifused beta-cell-rich ob/ob mouse islets loaded with trace amounts of 3H-5-hydroxytryptophan. Glucose stimulated exocytotic release of 5-HT but the effect was rapidly inhibited by 2,4-dinitrophenol, antimycin A or N-ethylmalemide. This inhibition was followed by a marked stimulation of 5-HT efflux. The later phenomenon was reversible when 2,4-dinitrophenol was used by appeared irreversible with antimycin A or N-ethylmalemide. The results show that the maintenance of 5-HT within the beta-cell depends on energy. It is suggested that both inhibitory and stimulatory effects of 5-HT on insulin secretion depend on release of calcium from the secretory granules after short-circuiting a proton pump across the granule membrane.

Tolbutamide stimulation and inhibition of insulin release: studies of the underlying ionic mechanisms in isolated rat islets

The effects of tolbutamide on insulin release, 45Ca2+ uptake and 86Rb+ efflux were studied in isolated rat islets. At a low glucose concentration (75 mg/dl), tolbutamide (20-500 microgram/ml) produced a rapid, dose-dependent increase in insulin release from perifused islets. After 30-40 min however, the rate of secretion as well as the potentiating effect of theophylline were inversely related to the concentration of sulphonylurea. The monophasic release of insulin triggered by tolbutamide (100 microgram/ml) at low glucose could be evoked again by removing and reintroducing the drug, or by temporarily withdrawing calcium or adding cobalt to the medium. Tolbutamide (20 microgram/ml) accelerated and potentiated the biphasic insulin release in response to a secondary stimulation by glucose (150 mg/dl). By contrast, 100 microgram/ml tolbutamide reduced the releasing effect of glucose to a slow increase in secretion rates. Theophylline normalized the second phase of release, but did not restore the rapid phase. Tolbutamide stimulated 45Ca2+ influx (2 min-uptake) in islet cells; this effect was maximum immediately after addition of the drug and decreased later on, exhibiting a monophasic pattern. Glucose stimulation of Ca2+ uptake (5 min) was reduced in the presence of 100 microgram/ml tolbutamide. At a low glucose concentration, tolbutamide reversibly reduced 86Rb+ efflux (tracer of K+) from islet cells, without altering the further inhibition of this efflux by a later glucose increase. It is suggested that tolbutamide depolarizes B cells partially by reducing their K+ permeability. This depolarization leads to opening of voltage-dependent calcium channels and the resulting Ca2+ influx triggers insulin release. The important and maintained depolarization by high concentrations of tolbutamide may secondarily inactivate these channels and cause a decrease in Ca2+ influx. This could explain the monophasic release of insulin and the refractoriness of B cells to subsequent glucose stimulation.

Synergistic effects of hypoglycaemic sulphonylureas and antibiotic ionophores upon calcium translocation

1 Hypoglycaemic sulphonylureas, such as tolbutamide and gliclazide, provoke the translocation of calcium from an aqueous medium into or across a hydrophobic region. The combined effect of sulphonylureas and antibiotic ionophores upon such a process was investigated. 2 The magnitude of the sulphonylurea-induced translocation of calcium was more marked in the presence than in the absence of A23187. Gliclazide and tolbutamide also enhanced, although less markedly, X537A-mediated calcium translocation. The effect of the sulphonylureas was even less marked in the presence of both ionophores, which acted synergistically in causing calcium translocation. 3 A non-hypoglycaemic sulphonylurea and diazoxide failed to affect ionophore-mediated calcium translocation. Gliclazide failed to enhance X537A-mediated sodium translocation. 4 It is proposed that the primary site of action of hypoglycaemic sulphonylureas upon calcium-dependent physiological processes may correspond to a drug-induced facilitation of calcium transport across the plasma membrane, as mediated by native ionophores.

Extrapancreatic action of sulphonylureas: effect of gliquidone on insulin and glucagon binding to rat liver plasma membranes

Gliquidone (a second generation sulphonylurea) was administered orally to normal rats 1 h before killing. Gliquidone treatment led to a decrease in plasma glucose, an increase in insulin and a diminution in glucagon concentration. Insulin binding to liver plasma membranes was enhanced by 40% in comparison with controls, whereas glucagon binding was slightly diminished. These findings indicate a greater sensitivity of liver cells to insulin during sulphonylurea treatment and support the view that sulphonylureas potentiate insulin action on the liver.

Effect of methylene blue and thiol oxidants on pancreatic islet GSH/GSSG ratios and tolbutamide mediated insulin release in vitro

Methylene blue (an oxidant of NADPH), diamide (an oxidant of glutathion-SH [GSH]) and tertbutyl hydroperoxide (a substrate of glutathione peroxidase) significantly decreased the GSH content of pancreatic rat islets and decreased their GSH/GSSG ratio. They also significantly depressed the single peak insulin response to tolbutamide by the isolated perfused pancreas as well as its synergistic action with glucose in isolated pancreatic islets. These results suggest that the effect of tolbutamide alone and its synergistic action with glucose could depend on the islet NADPH and GSH. In addition it appears that augmentation of tolbutamide action by glucose in insulin release is mediated by the provision of additional NADPH and GSH through glucose metabolism.

Uptake of tolbutamide by islets of Langerhans and other tissues

3-H-tolbutamide was distributed in a volume exceeding the space occupied by 14-C-sucrose in islets as well as in liver, kidney, muscle, and fat. In contrast to previous reports, the findings suggest that tolbutamide is not restricted to the extracellular space of islets.

Dynamics of sulfonylurea-induced insulin release from the isolated perfused rat pancreas

In the isolated perfused rat pancreas various sulfonylurea drugs were tested with a basal glucose level of 1 mg/ml in the perfusion buffer and were found to cause a biphasic insulin response. NOVOCS 476, a new and potent sulfonylurea, and glibenclamide qualitatively differed from tolbutamide, glibornuride, glipizide, and glisoxepide, which were all alike in terms of the relationship between first and second phases of insulin release.

Effects of sulfonylureas on histochemical and ultracytochemical calcium distribution in B-cells of mice

The study examines the effects of sulfonylurea compounds on the histo- and ultracytochemical calcium distribution within the B-cells of mice using the glyoxal-bis-(2-hydroxyanil) (GBHA) and the pyroantimonate method combined with X-ray microanalysis. Treatment with tolbutamide (200 mg/kg), glibenclamide (2 mg/kg) and glisoxepide (2 mg/kg), causing moderate hypoglycemia and B-cell degranulation, was associated with an unchanged (30, 90 min) or slightly increased (180, 360 min, 4, 42d) GGHA staining intensity of the islet cells compared with controls. Ultra-cytochemically sulfonylureas provoked, compared with controls, a redistribution of calcium-rich, electron dense pyroantimonate precipitates (EDPP). Precipitation predominantly occurred along the inner surface of the plasma membranes and within the granule halos. In contrast, the cytoplasmic matrix, the Golgi complexes and the rough endoplasmic reticulum contained only few fine precipitates. The sulfonylureas investigated exerted identical effects on the histo- and ultracytochemical calcium distribution in B-cells. The results indicate that sulfonylurea-induced insulin secretion is associated with an accumulation and redistribution of calcium within the B-cells. This supports the hypothesis that an altered calcium handling by the B-cell mainly accounts for the insulinotropic effect of sulfonylureas.

Insulin secretion in the perinatal period of the rat in vivo and in vitro effects of glucose and gibenclamide (HB 419)

During the perinatal period of the rat the effect of glucose and glibenclamide (HB 419) on the secretion of insulin was studied in vivo and in vitro. In the in vitro experiments isolated islets of 21 day old fetal and 5 day old newborn rats were perifused wtih 16.7 mM glucose or 16.7 mM glucose plus 1 mug/ml glibenclamide, while in the in vivo ecperiments glucose, 0.5 g/kg of body weight, or glibenclamide, 0.5 mg/kg of body weight were tested. Glucose elicited a small first phase of insulin release in 21 days old fetal islets, while glucose plus glibenclamide evoked a biphasic pattern. The injection of glibenclamide to the mother lowered the blood sugar in the fetus and increased the fetal serum insulin concentration. In one day old newborn rats glibenclamide stimulated the secretion of insulin after an i.p. injection. Glucose was without effect. Both substances increaseded the serum insulin concentration in five day old newborn animals. Dynamic studies at that age revealed a monophasic response to glucose and a biphasic pattern to glucose plus glibenclamide.

Pancreatic islet cells: effects of monosaccharides, glycolytic intermediates and metabolic inhibitors on membrane potential and electrical activity

1. The effects of monosaccharides, glycolytic intermediates, metabolic inhibitors and anxia, have been studied on the membrane electrical activity of mouse pancreatic islet cells in vitro using a single intracellular micro-electrode for both voltage recording and current injection. 2. In addition to D-glucose (28mM), D-mannose (16-6mM), and L-leucin (10mM), the substances D-glyceraldehyde (11mM), and acetoacetate (20 mM), induced action potentials in islet cells but other glucos analogues and metabolic intermediates including L-glucose dod not. 3. Mannoheptulose 20 mM), but not D-galactose or 2-deoxy-D-glucose, antagonized the electrical activity induced in islet cells by D-glucose, 28mM. Prior treatment of the cells with mannoheptulose caused them to hyperpolarize and completely prevented the appearance of electrical activity on subsequent exposure to D-glucose. 4. Electrical activity induced by D0glucose 28mM, was progressively inhibited by phloridzin, 10mM, if the cells were exposed to D-glucose and inhibitor simultaneously, and abolished on pretreatment with inhibitor for 30-60 min. Phloridzin also caused depolarization of the islet cells which was independent of extracellular glucose. 5. Anoxia completely blocked the electrical activity induced by glucose but not that evoked by D-glyceraldehyde, L-leucine, tolbutamide or glibenclamide. 6. Iodoacetic acid, 5 mM, rapidly blocked glucose-induced electrical activity whilst that elicited by tolbutamide was relatively resistant to inhibition. 7. The nature and possible location of the glucoreceptor in pancreatic islet cells is discussed in relation to the origin and functional significance of glucose-induced electrical activity and insulin secretion.

Effect of tolbutamide on aminophylline-, 3,5-AMP-dibutyrate- or glucagon-induced insulin release from pancreatic islets after impairment of pyridine nucleotide metabolism caused by 6-aminonicotinamide (6-AN)

The effect of tolbutamide on pyridine nucleotides and insulin secretion stimulated by aminophylline, 3,5-AMP-dibutyrate or glucagon was studied in pancreatic islets of rats previously treated with 6-aminonicotinamide (6-AN), an inhibitor of pyridine nucleotide synthesis. After being incubated for 60 min in a Krebs-Ringer-Bicarbonate-Buffer in the absence of glucose, pancreatic islets of rats i.p. injected with 35 mg/kg of 6-AN 6 hrs before pancreas removal contained about 30% less NADP and NADPH than did islets of control rats. No changes of NDA or NADH were observed in islets of 6-AN-treated animals. Addition of 16.5 mM glucose led to an increase of NADH, NADPH and a decrease of NADP in islets of both groups of animals; NAD levels remained unchanged. In vitro addition of tolbutamide to islets of control rats did not affect the levels of NADPH or NADP in the presence of 5.5 mM glucose. When 16.5 mM glucose were present, a decrease of NADPH and an increase of NADP was obvious. No effect of tolbutamide on insular NADPH or NADP was observed in islets of rats previously treated with 6-AN be it in the presence of 5.5 or 16.5 mM glucose. In islets of 6-AN-treated rats insulin release in response to aminophylline or 3,5-AMP-dibutyrate in the presence of 5.5 mM glucose was significantly depressed, when compared to islets of untreated controls. Addition of tolbutamide increased insulin release due to aminophylline, 3,5-AMP-dibutyrate or glucagon islets of controls. Tolbutamide alone was without effect. In islets of 6-AN-treated rats aminophylline, 3,5-AMP-dibutyrate or glucagon stimulated insulin release only when tolbutamide was present. Our data suggest that there is no direct interference of tolbutamide with pyridine nucleotides of pancreatic islets, and that tolbutamide increases the secretory response of the beta-cell to aminophylline, 3,5-AMP-dibutyrate or glucagon when insulin release due to these agents is inhibited during decrease of insular NADP and NADPH, caused by 6-AN.

Insulin release from the perfused rat pancreas. Mode of action of tolbutamide

Dextran-linked tolbutamide causes first-phase insulin release when perfused through the isolated rat pancreas. The linked sulphonylurea is also able to stimulate the membranal adenylate cyclase present in mouse islets. These facts make it highly likely that sulphonylureas exert their pharmacological action by interaction with the plasma membrane. Their action may be mediated via the adenylate cyclase enzyme.