W. Schmeer

Cyclic variations of glucose-induced electrical activity in pancreatic B cells

Microelectrodes were used to record the effects of glucose on the membrane potential of single mouse B cells. In most cells, the slow waves of depolarization and the intervals of repolarization produced by a constant concentration of glucose displayed a great regularity. However, cyclic variations in the duration of these slow waves and/or intervals were observed in a certain number of B cells. These oscillations were more clearly visible and more frequent (47%) in the presence of 15 mM glucose, than in the presence of 10 mM glucose (19%). They sometimes disappeared with time, but sometimes persisted for over 90 min and were not affected by atropine, propanolol and phentolamine. Their mean period was 203 s at 10 mM glucose and 235 s at 15 mM glucose. The membrane potential and the degree of electrical activity were not different in B cells exhibiting these cyclic variations or not. These oscillations in the duration of slow waves and intervals induced by glucose could be due to fluctuations in metabolic events and in cytoplasmic K+ activity in B cells.

Mechanism of the stimulation of insulin release in vitro by HB 699, a benzoic acid derivative similar to the non-sulphonylurea moiety of glibenclamide

SummaryHB 699 is a benzoic acid derivative similar to the non-sulphonylurea moiety of glibenclamide. The mechanisms whereby it affects B-cell function have been studied in vitro with mouse islets. In the presence of 3 mmol/l glucose, HB 699 decreased 86Rb+ efflux and accelerated 45Ca2+ efflux from islet cells, depolarized the B-cell membrane and induced an electrical activity similar to that triggered by stimulatory concentrations of glucose, and increased insulin release. The changes in 45Ca2+ efflux and insulin release, but not the inhibition of 86Rb+ efflux, were abolished in the absence of Ca2+. In the presence of 10 mmol/l glucose, HB 699 increased 86Rb+ and 45Ca2+ efflux from the islets, caused a persistent depolarization of the B-cell membrane with continuous electrical activity, and markedly potentiated insulin release. All these changes were suppressed by omission of extracellular Ca2+. In the presence of 15 mmol/l glucose, diazoxide increased 86Rb+ efflux, hyperpolarized the B-cell membrane, suppressed electrical activity and inhibited insulin release. HB 699 reversed these effects of diazoxide. It is suggested that HB 699 decreases K+ permeability of the B-cell membrane, thereby causing a depolarization which leads to activation of voltage-dependent Ca channnels and Ca2+ influx, and eventually increases insulin release. A sulphonylurea group is thus not a prerequisite to trigger the sequence of events that is also thought to underlie the releasing effects of tolbutamide and glibenclamide.

The permissive effect of glucose, tolbutamide and high K+ on arginine stimulation of insulin release in isolated mouse islets.

SummaryMouse islets were used to study how glucose modulates arginine stimulation of insulin release. At 3 mmol/l glucose, arginine (20 mmol/l) decreased the resting membrane potential of B cells by about 10 mV, but did not evoke electrical activity. This depolarisation was accompanied by a slight but rapid acceleration of 86Rb+ efflux and 45Ca2+ influx. However, 45Ca2+ efflux and insulin release increased only weakly and belatedly. When the membrane was depolarised by threshold (7 mmol/l) or stimulatory (10–15 mmol/l) concentrations of glucose, arginine rapidly induced or augmented electrical activity, markedly accelerated 86Rb+ efflux, 45Ca2+ influx and efflux, and triggered a strong and fast increase in insulin release. When glucose-induced depolarisation of the B-cell membrane was prevented by diazoxide, arginine lost all effects but those produced at low glucose. However, the delayed increase in release still exhibited some glucose-dependency. In contrast, depolarisation by tolbut amide, at low glucose, largely mimicked the permissive effect of high glucose. Depolarisation by high K+ also amplified arginine stimulation of insulin release, but did not accelerate it as did glucose or tolbutamide. Omission of extracellular Ca2+ abolished the releasing effect of arginine under all conditions. The results thus show that the permissive action of glucose mainly results from its ability to depolarise the B-cell membrane. It enables the small depolarisation by arginine itself to activate Ca channels more rapidly and efficiently. Changes in the metabolic state of B cells may also contribute to this permissive action by increasing the efficacy of the initiating signal triggered by arginine.

Effects of a Calcium-channel Agonist On the Electrical, Ionic and Secretory Events in Mouse Pancreatic B-cells

The changes in pancreatic B-cell function produced by a Ca channel agonist, the dihydropyridine derivative CGP 28392, have been studied with mouse islets. CGP 28392 (5 microM) modified the electrical activity induced in B-cells by 10 mM glucose: the duration and the amplitude of the slow waves of membrane potential increased, but the overall spike activity decreased. Simultaneously, CGP 28392 markedly increased insulin release and 45Ca2+ efflux, and slightly accelerated 86Rb+ efflux from islet cells. These latter effects were abolished by omission of extracellular Ca2+. Qualitatively similar changes were observed at 15 mM glucose, whereas CGP 28392 was ineffective at 3 mM glucose. These results strongly suggest that an influx of Ca2+ contributes to the slow waves of membrane potential triggered by glucose, and underline the importance of this influx of Ca2+ for the control of insulin release by the sugar.

Effects of acute sodium omission on insulin release, ionic flux and membrane potential in mouse pancreatic B-cells

The effects of acute omission of extracellular Na+ on pancreatic B-cell function were studied in mouse islets, using choline and lithium salts as impermeant and permeant substitutes, respectively. In the absence of glucose, choline substitution for Na+ hyperpolarized the B-cell membrane, inhibited 86Rb+ and 45Ca2+ efflux, but did not affect insulin release. In contrast, Li+ substitution for Na+ depolarized the B-cell membrane and caused a Ca2+-independent, transient acceleration of 45Ca2+ efflux and insulin release. Na+ replacement by choline in the presence of 10 mM glucose and 2.5 mM Ca2+ again rapidly hyperpolarized the B-cell membrane. This hyperpolarization was then followed by a phase of depolarization with continuous spike activity, before long slow waves of the membrane potential resumed. Under these conditions, 86Rb+ efflux first decreased before accelerating, concomitantly with marked and parallel increases in 45Ca2+ efflux and insulin release. In the absence of Ca2+, 45Ca2+ and 86Rb+ efflux were inhibited and insulin release was unaffected by choline substitution for Na+. Na+ replacement by Li+ in the presence of 10 mM glucose rapidly depolarized the B-cell membrane, caused an intense continuous spike activity, and accelerated 45Ca2+ efflux, 86Rb+ efflux and insulin release. In the absence of extracellular Ca2+, Li+ still caused a rapid but transient increase in 45Ca2+ and 86Rb+ efflux and in insulin release. Although not indispensable for insulin release, Na+ plays an important regulatory role in stimulus-secretion coupling by modulating, among others, membrane potential and ionic fluxes in B-cells.

Forskolin Suppresses the Slow Cyclic Variations of Glucose-induced Electrical-activity in Pancreatic B-cells

The membrane potential of mouse pancreatic B cells was recorded with microelectrodes. In certain cells, both the slow waves of depolarization and the intervals of repolarization triggered by glucose (10 or 15 mM) displayed regular oscillations in their duration, though the concentration of the sugar remained constant. When forskolin (0.2 microM), an activator of adenylate cyclase, was added to the medium, the electrical activity rapidly became very regular, with slow waves and intervals of constant duration. This effect was unrelated to the overall increase in activity also brought about by forskolin. The oscillations resumed in 75% of the cells after withdrawal of the drug. Under similar conditions, forskolin rapidly and reversibly raised the cAMP concentration in the islets. The data suggest that cAMP is an important modulator of the electrical activity triggered by glucose in insulin-secreting cells.