Regulation of calcium fluxes in rat pancreatic islets. Quinine mimics the dual effect of glucose on calcium movements

Venom Peptides, Polyphenols and Alkaloids: Are They the Next Antidiabetics That Will Preserve β-Cell Mass and Function in Type 2 Diabetes?

Improvement of insulin secretion by pancreatic β-cells and preservation of their mass are the current challenges that future antidiabetic drugs should meet for achieving efficient and long-term glycemic control in patients with type 2 diabetes (T2D). The successful development of glucagon-like peptide 1 (GLP-1) analogues, derived from the saliva of a lizard from the Helodermatidae family, has provided the proof of concept that antidiabetic drugs directly targeting pancreatic β-cells can emerge from venomous animals. The literature reporting on the antidiabetic effects of medicinal plants suggests that they contain some promising active substances such as polyphenols and alkaloids, which could be active as insulin secretagogues and β-cell protectors. In this review, we discuss the potential of several polyphenols, alkaloids and venom peptides from snake, frogs, scorpions and cone snails. These molecules could contribute to the development of new efficient antidiabetic medicines targeting β-cells, which would tackle the progression of the disease.

Desensitization of insulin secretory response to imidazolines, tolbutamide, and quinine. I. Secretory and morphological studies

The desensitization of pancreatic B-cells against stimulation by insulin secretagogues that inhibit ATP-dependent K(+) channels (K(ATP) channels) was investigated by measuring insulin secretion of perifused pancreatic islets. Additionally, the islet insulin content and the number of secretory granules per B-cell were determined. Prior to the measurement of secretion, islets were cultured for 18 h in the presence or absence of the test agents in a cell-culture medium containing 5 mM glucose. The effects of three imidazolines, phentolamine, alinidine, and idazoxan (100 microM each) were compared with those of the well-characterized sulfonylurea, tolbutamide (500 microM), and those of the ion channel-blocking alkaloid, quinine (100 microM). Insulin secretion was strongly reduced upon re-exposure to phentolamine, alinidine, tolbutamide, and quinine, whereas idazoxan, which stimulated secretion only weakly, had no significant effect. The imidazoline secretagogues phentolamine and alinidine induced a cross-desensitization against the stimulatory effect of tolbutamide and quinine. A long-term depolarization with 40 mM KCl was also able to induce a significant reduction of the secretory response to all of the above secretagogues. The insulin content of cultured islets was moderately, but significantly reduced by alinidine, whereas the reduction by phentolamine, tolbutamide, and quinine was not significant. In contrast to these observations, the ultrastructural examination revealed that tolbutamide-treated B-cells had a high degree of degranulation, whereas the other test agents and 40 mM KCl produced only a partial degranulation, except for phentolamine, which produced no significant degranulation at all. These results suggest that the desensitization of insulin secretion is a common property of all agents that stimulate insulin secretion by depolarisation of the plasma membrane. Depending on the specific secretagogue, additional mechanisms, proximal and distal to Ca(2+) influx, appear to contribute to the desensitization (see Rustenbeck et al., pages 1695-1703, this issue).

Quinine Overdose: Hypoglycemia Potentiated by Age and Alcohol

Objective:

To discuss a case of quinine toxicity and the impact that age and alcohol can have on its clinical presentation of life-threatening hypoglycemia.

Case Summary:

A 55-year-old white woman with a history of ethanol abuse developed quinine toxicity after accidentally consuming approximately 4 g of quinine. During her hospital stay, the patient developed life-threatening hypoglycemia, signs of cinchonism (tinnitus, lethargy), hypotension, and hypokalemia. She was treated successfully with intravenous fluids, dextrose, and potassium.

Discussion:

In reviewing the literature involving quinine toxicity, few cases have discussed the role quinine may play in the development of hypoglycemia. Quinine has been implicated as the cause of hypoglycemia in patients with malaria, but has less commonly been considered the cause in healthy patients. In this case report, the patient developed life-threatening hypoglycemia after the consumption of 4.6 g of quinine. Toxicity associated with quinine is usually detected at doses >4 g, but lethal doses have been reported to range between 2 and 8 g. The cause of this patient's sudden drop in glucose concentration was determined to be multifactorial and attributed to a combination of quinine, ethanol, and potential biologic aging of metabolic organs.

Conclusions:

Quinine can cause hypoglycemia; in this case it occurred at a toxic dose of 4.6 g. In older patients or patients with a significant history of alcohol abuse, this effect may be potentiated.

Delayed cardiotoxicity following quinine overdose: a case report

Quinine poisoning typically results in a constellation of non-life threatening symptoms which include tinnitus, deafness, nausea, vomiting, vision changes, headache, and hypotension. Cardiac conduction defects, dysrhythmias, and cardiovascular collapse have all been reported after overdose and generally occur within 8 hours of ingestion. We report a unique case of delayed cardiotoxicity following quinine ingestion. Toxicity included marked ventricular conduction abnormalities for which serum alkalinization appeared to be therapeutically beneficial, and torsades de pointes requiring overdrive pacing for termination.

Diazoxide unmasks glucose inhibition of insulin release by counteracting entry of Ca2+

The interaction of diazoxide with the effects of glucose on the insulin-releasing mechanism was analyzed in beta-cell-rich pancreatic islets isolated from ob/ob mice. When added at a concentration of 400 microM to a medium containing 1.28 mM Ca2+, diazoxide converted glucose stimulation of insulin release into inhibition. Further addition of 2 mM theophylline restored the insulin secretory response to glucose. The paradoxical glucose inhibition of insulin release was accounted for by a diazoxide interaction with the entry of Ca2+, unmasking a capacity of the sugar to lower cytoplasmic Ca2+ below its resting concentration.

Failure of Leiurus quinquestriatus venom to affect potassium movements in pancreatic islets

The venom from the Israeli scorpion Leiurus quinquestriatus failed to affect 86Rb and 45Ca outflow from rat pancreatic islets perifused in the presence of tetrodotoxin and stimulated by the Ca2+-ionophore A23187 or the hypoglycaemic sulfonylurea tolbutamide. In non-stimulated islets, the venom components whose effects are insensitive to tetrodotoxin did not affect 45Ca and 86Rb outflow. Last, the venom did not alter 86Rb inflow. These findings suggest that 86Rb, 45Ca fluxes and more specifically the Ca2+-activated K+ permeability in the pancreatic B-cell are insensitive to the venom.

High incidence of hypoglycaemia in African patients treated with intravenous quinine for severe malaria

Changes in plasma glucose and insulin concentrations were monitored over 24 hours in 28 African patients receiving quinine intravenously in an average dose of 8.5 mg base/kg over one hour eight hourly for severe malaria. The patients (nine children and 19 adults) were moderately undernourished; none was pregnant or had renal insufficiency. Plasma insulin concentrations rose during the infusion and then declined. Plasma glucose concentrations were decreased at two, three, and four hours after the start of the infusion. Insulin: glucose ratios were raised between half an hour and two hours after the start of the infusion. The three infusions of quinine increased plasma insulin concentrations in a similar way. In nine patients, including four children, plasma glucose concentrations fell below 2.8 mmol/l on one or two occasions. At the time of the hypoglycaemia plasma insulin concentrations were inappropriately high as shown by a consistent and often considerable increase in the insulin:glucose ratio. Hypoglycaemia that may pass unnoticed in comatose patients is thus a common complication of treating severe malaria with quinine, in particular in children. Its high incidence calls for attentive monitoring and preventive measures.

Glucose homeostasis in rats treated acutely and chronically with quinine

Glucose homeostasis in normal rats was studied after chronic or acute administration of quinine. Male rats received a daily dose of 10-30 mg/kg of quinine in the drinking water for 20 weeks. The high dose caused a slight decrease in food intake and weight gain. Though basal plasma insulin levels were increased in treated rats, their plasma glucose levels were only slightly and not consistently decreased. After oral or intravenous administration of glucose, the plasma insulin levels were higher and the disappearance rate of glucose was greater in rats receiving quinine than in the controls. The insulin content of the pancreas was not affected by quinine treatment. Intraperitoneal injection of a high dose of quinine (30 mg/kg) transiently increased plasma glucose and insulin levels. The insulin response was increased during a subsequent administration of glucose but the glucose levels were not modified. This study shows that chronic administration of quinine increases plasma insulin levels, accelerates disposal of oral or intravenous glucose but does not cause hypoglycaemia in normal rats.

Sparteine increases insulin release by decreasing the K+ permeability of the B-cell membrane

The effects of sparteine on the pancreatic B-cell function have been studied with mouse islets. In the presence of a non-stimulatory concentration of glucose (3 mM), sparteine (0.2-1 mM) decreased the rate of 86Rb+ efflux from islet cells, depolarized the B-cell membrane, induced a glucose-like electrical activity and stimulated insulin release. This increase in release was observed over a large range of glucose concentrations (3-20 mM), and was most marked in the presence of 10 mM glucose. At this concentration of glucose, the effect of sparteine was already detected with 0.02 mM and was maximal with 0.5 mM. Higher concentrations of sparteine only had a transient effect on insulin release. In the presence of 10 mM glucose, 0.2 mM sparteine decreased 86Rb+ efflux and increased 45Ca2+ efflux from islet cells. The effect on 86Rb+ efflux was only transient in the presence of extracellular calcium, whereas the effect on 45Ca2+ efflux required the presence of extracellular calcium. The electrical activity induced by glucose in B-cells was augmented by sparteine which, at a concentration of 0.5 mM, produced a persistent depolarization with continuous spike activity. The potentiation of insulin release by sparteine was not reversible, but was inhibited by adrenaline and completely blocked by omission of extracellular calcium. Sparteine reversed the increase in 86Rb+ efflux and the decrease in insulin release caused by diazoxide. These results show that sparteine increases insulin release by reducing the K+-permeability of the B-cell membrane.

Quinine inhibits Ca2+-independent K+ channels whereas tetraethylammonium inhibits Ca2+-activated K+ channels in insulin-secreting cells

The effects of quinine and tetraethylammonium (TEA) on single-channel K+ currents recorded from excised membrane patches of the insulin-secreting cell line RINm5F were investigated. When 100 microM quinine was applied to the external membrane surface K+ current flow through inward rectifier channels was abolished, while a separate voltage-activated high-conductance K+ channel was not significantly affected. On the other hand, 2 mM TEA abolished current flow through voltage-activated high-conductance K+ channels without influencing the inward rectifier K+ channel. Quinine is therefore not a specific inhibitor of Ca2+-activated K+ channels, but instead a good blocker of the Ca2+-independent K+ inward rectifier channel whereas TEA specifically inhibits the high-conductance voltage-activated K+ channel which is also Ca2+-activated.

Do hypoglycemic sulfonylureas inhibit Na+,K+-ATPase activity in pancreatic islets?

The view that the insulinotropic response to hypoglycemic sulfonylureas is somehow related to an inhibitory action of these drugs on either the Ca2+-activated K+ permeability or the Na+,K+-ATPase activity in pancreatic islet cells was investigated by measuring the effect of tolbutamide on 86Rb outflow and uptake, 45Ca outflow, and insulin release in rat pancreatic islets. Although tolbutamide inhibited 86Rb efflux from glucose-deprived islets, whether in the absence or presence of extracellular Ca2+ or ouabain, a primary action of the sulfonylurea on the Ca2+-responsive K+ channels appeared unlikely, because tolbutamide failed to suppress the increase in 86Rb outflow evoked by either the ionophore A23187 or ouabain. Glibenclamide also failed to suppress the increase in 86Rb outflow evoked by the ionophore. Moreover, tolbutamide itself stimulated quinine-sensitive 86Rb outflow from glucose-stimulated islets. Likewise, although tolbutamide inhibited an ouabain-resistant modality of 86Rb inflow into the islet cells, an inhibitory action of the sulfonylurea on the Na+,K+-ATPase appeared improbable, because tolbutamide failed to minimize and instead favored the cationic and secretory response to ouabain. It is concluded, therefore, that the capacity of tolbutamide to cause under suitable conditions continuous electrical activity in islet cells cannot be attributed to inhibition of either the Ca2+-sensitive K+ permeability or ouabain-sensitive Na+,K+-ATPase.

The effects of cesium chloride on insulin release, ionic fluxes and membrane potential in pancreatic B-cells

Cs+ decreases K+ permeability in nerve and muscle cells. Its effects on the pancreatic B-cell function were studied with mouse islets. In the presence of 3 mM glucose, Cs+ substitution for K+ steadily inhibited 86Rb+ efflux and hyperpolarized the B-cell membrane. Addition of Cs+ to a K+-medium also inhibited 86Rb+ efflux, but depolarized the B-cell membrane. None of these changes altered insulin release. Substitution of Cs+ for K+ in a medium containing 10 mM glucose caused a Ca2+-dependent stimulation of insulin release and 45Ca2+ efflux, produced an initial fall and a secondary rise in 86Rb+ efflux and augmented the electrical activity in B-cells. Reintroduction of K+ to the medium was followed by a marked and transient inhibition of insulin release, that was blocked by ouabain and accompanied by an inhibition of 45Ca2+ and 86Rb+ efflux and by a hyperpolarization of the B-cell membrane. Addition of Cs+ to a K+ medium containing 10 mM glucose stimulated insulin release, 45Ca2+ efflux and 86Rb+ efflux. It also increased the electrical activity in B-cells. In the absence of Ca2+, however, Cs+ addition decreased the rate of 86Rb+ efflux. The effects of Cs+ on the B-cell function may be explained by its ability to decrease K+ permeability of the plasma membrane, by its inability to activate the sodium pump, and by a third unidentified effect likely brought about by the accumulation of intracellular Cs+.

Inhibition by mepacrine and p-bromophenacylbromide of phosphoinositide hydrolysis, glucose oxidation, calcium uptake and insulin release in rat pancreatic islets

Mepacrine and p-bromophenacylbromide were both found to impair 3H-inositol phosphate production in response to both nutrient and hormone-neurotransmitter stimuli in islets prelabelled with 3H-inositol. Both drugs also inhibited net 45Ca uptake in response to glucose or glibenclamide and considerably modified the patterns of 45Ca and 86Rb efflux from perifused islets under both basal and glucose-stimulated conditions. In addition, the oxidation of [U-14C] glucose in islets was impaired by either mepacrine or p-bromophenacylbromide. These inhibitory effects were found to be concentration-related for both mepacrine (0.01-1.0 mM) and p-bromophenacylbromide (0.03-0.3 mM) and were accompanied, in general, by a similar degree of inhibition of insulin secretion. These results suggest that both mepacrine and p-bromophenacylbromide can inhibit phospholipase C activity in intact islets, but also impair 45Ca and 86Rb fluxes and oxidation of nutrients. The diversity of these drugs' inhibitory actions makes them unsuitable tools for examining the role of specific cellular processes in the regulation of islet function.

The control of 86Rb efflux from rat isolated pancreatic islets by the sulphonylureas tolbutamide and glibenclamide

The efflux of 86Rb from rat isolated pancreatic islets preloaded with the isotope and perifused in vitro, has been used to monitor the effects of sulphonylureas on the potassium permeability, Pk, of pancreatic beta-cells. Tolbutamide (5 microM to 5 mM) had a dual effect, causing initially a decrease in 86Rb efflux (the 'on' response) which was rapidly superseded on drug removal by a large phasic increase in 86Rb efflux (the 'off' response). Each kinetic response had a different dose-dependency: the 'on' response was half-maximal at tolbutamide concentrations of 0.02 mM, maximal at 0.2 mM and decreased by concentrations greater than 0.2 mM whereas the 'off' response was half-maximal at 0.07 mM, maximal at 0.7 mM, with further increases in concentration (up to 5 mM) causing no further change in magnitude. Analysis of the time- and concentration-dependency of tolbutamide action, by presenting increasing concentrations (0 to 1.4 mM) of tolbutamide as a ramp or step function, established a critical dependence of the kinetics of 86Rb efflux during and after exposure to tolbutamide upon the initial rate of increase of the tolbutamide concentration rather than its final steady state. In the presence of quinine (10 microM), D600 (50 microM), or tetraethylammonium (20 mM), the secondary increase in 86Rb following tolbutamide (0.7 mM) removal was totally inhibited. Co2+ (2.56 mM) not only blocked the secondary 'off' response but also potentiated the initial 'on' response of tolbutamide. Glibenclamide produced a rapid decrease in 86Rb efflux but at a much lower concentration (10 microM) than tolbutamide and with no 'off' response apparent over a wide range of concentration (1 to 100 microM); moreover the decrease in 86Rb efflux was sustained and only slowly reversible. It is concluded that tolbutamide has two opposing actions on islet beta-cell 86Rb efflux, and therefore PK: (i) a tendency to increase a calcium-sensitive PK by stimulating calcium entry into the cell and (ii) a decrease in PK that may be due to a direct effect on the calcium-sensitive PK itself. The more sustained pharmacological action of glibenclamide is explained by the longer-lasting decrease in PK that it produces.

Glucose-induced retention of intracellular 45Ca in pancreatic islets

beta-Cell-rich pancreatic islets from ob/ob mice were loaded with 45Ca in the presence of 20 mM glucose. When the islets were transferred to a nonradioactive incubation medium depleted of Ca2+, most of the 45Ca was lost during the first 10 min of incubation irrespective of whether or not glucose was present. Isolation of subcellular fractions under conditions minimizing 45Ca redistribution disclosed that the presence of 20 mM glucose during the nonradioactive incubation increased the retention of 45Ca in the mitochondrial fraction. This effect was not reproduced by K+ depolarization of the islets. Exposure of islets to 4 mM glucose failed to affect the total content of 45Ca in the islets but increased the incorporation of the isotope into cellular organelles. These results suggest that glucose, even at concentrations below the threshold for stimulation of insulin secretion, enhances the intracellular trapping of calcium.

Severe hypoglycemia and hyperinsulinemia in falciparum malaria

We studied the occurrence, clinical manifestations, and mechanism of hypoglycemia in patients with falciparum malaria in eastern Thailand. Hypoglycemia, which was often severe and recurrent, occurred in 17 patients, including 12 in a series of 151 patients with cerebral malaria. Thirty episodes were investigated. Plasma concentrations of insulin and C peptide were inappropriately high, and lactate and alanine concentrations were significantly higher than in patients with falciparum malaria who were normoglycemic (P less than 0.05). Sixteen patients had received quinine; plasma quinine and insulin concentrations were correlated at the time of hypoglycemia (P = 0.007). In seven healthy fasting volunteers intravenous quinine increased the mean plasma insulin concentration (+/- S.D.) from 8.9 +/- 3.1 to 17.1 +/- 8.4 mU per liter (P = 0.02) and reduced the mean plasma glucose concentration from 88 +/- 20 to 68 +/- 23 mg per deciliter (P = 0.002). Our observations indicate that in falciparum malaria quinine-induced insulin secretion may precipitate hypoglycemia, but other factors, including the large glucose requirements of the malaria parasites may also contribute. This important complication, associated with pregnancy and severe disease, must be excluded in all patients with falciparum malaria who have impaired or deteriorating consciousness.

The role of calcium in the initiation of superoxide release from alveolar macrophages

The role of calcium in the release of superoxide anion (O2-) was examined in alveolar macrophages after stimulation with the soluble stimuli: concanavalin A (Con A), N-formyl methionyl phenylalanine (FMP), and the calcium ionophore. A23187. The release of O2- by Con A was unaffected over a wide range of extracellular calcium concentrations (20 microM to 3 mM), whereas increasing the extracellular calcium above 2 mM inhibited FMP-stimulated O2- release. In contrast, A23187 did not stimulate O2- release in calcium-free medium (less than or equal to 30 microM). The addition of EGTA (50 microM) to calcium-free medium had no effect on Con A stimulation of O2- release or FMP-stimulated O2- release. These results suggest that, for the three soluble stimuli, there are different roles for Ca+2 in the activation and transmission of stimulatory signals across the cell membrane. Con A- or FMP-stimulated calcium efflux from calcium-loaded cells in either calcium-free medium or 0.5 mM calcium-containing medium. In calcium-free medium, FMP transiently retarded 45Ca+2 uptake, while in 0.5 mM calcium-containing medium, FMP transiently stimulated 45Ca+2 uptake. For either Con A or FMP, calcium efflux preceded O2- release by 30-45 sec. Quinine, an agent that blocks membrane hyperpolarization in macrophages, completely blocked O2- release by concanavalin A or FMP and inhibited 45CA+2 efflux by 50% or more for both agents. These results support the hypothesis that redistribution of cellular Ca+2 is one of the initial steps leading to the release of O2-.