Involvement of nitric oxide in iodine deficiency-induced microvascular remodeling in the thyroid gland: role of nitric oxide synthase 3 and ryanodine receptors

Iodine deficiency (ID) induces microvascular changes in the thyroid gland via a TSH-independent reactive oxygen species-hypoxia inducible factor (HIF)-1α-vascular endothelial growth factor (VEGF) pathway. The involvement of nitric oxide (NO) in this pathway and the role of calcium (Ca(2+)) and of ryanodine receptors (RYRs) in NO synthase 3 (NOS3) activation were investigated in a murine model of goitrogenesis and in 3 in vitro models of ID, including primary cultures of human thyrocytes. ID activated NOS3 and the production of NO in thyrocytes in vitro and increased the thyroid blood flow in vivo. Using bevacizumab (a blocking antibody against VEGF-A) in mice, it appeared that NOS3 is activated upstream of VEGF-A. L-nitroarginine methyl ester (a NOS inhibitor) blocked the ID-induced increase in thyroid blood flow in vivo and NO production in vitro, as well as ID-induced VEGF-A mRNA and HIF-1α expression in vitro, whereas S-nitroso-acetyl-penicillamine (a NO donor) did the opposite. Ca(2+) is involved in this pathway as intracellular Ca(2+) flux increased after ID, and thapsigargin activated NOS3 and increased VEGF-A mRNA expression. Two of the 3 known mammalian RYR isoforms (RYR1 and RYR2) were shown to be expressed in thyrocytes. RYR inhibition using ryanodine at 10μM decreased ID-induced NOS3 activation, HIF-1α, and VEGF-A expression, whereas RYR activation with ryanodine at 1nM increased NOS3 activation and VEGF-A mRNA expression. In conclusion, during the early phase of TSH-independent ID-induced microvascular activation, ID sequentially activates RYRs and NOS3, thereby supporting ID-induced activation of the NO/HIF-1α/VEGF-A pathway in thyrocytes.

Placental lactogens induce serotonin biosynthesis in a subset of mouse beta cells during pregnancy

AIMS/HYPOTHESIS

Upregulation of the functional beta cell mass is required to match the physiological demands of mother and fetus during pregnancy. This increase is dependent on placental lactogens (PLs) and prolactin receptors, but the mechanisms underlying these events are only partially understood. We studied the mRNA expression profile of mouse islets during pregnancy to gain a better insight into these changes.

METHODS

RNA expression was measured ex vivo via microarrays and quantitative RT-PCR. In vivo observations were extended by in vitro models in which ovine PL was added to cultured mouse islets and MIN6 cells.

RESULTS

mRNA encoding both isoforms of the rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase (TPH), i.e. Tph1 and Tph2, were strongly induced (fold change 25- to 200-fold) during pregnancy. This induction was mimicked by exposing islets or MIN6 cells to ovine PLs for 24 h and was dependent on janus kinase 2 and signal transducer and activator of transcription 5. Parallel to Tph1 mRNA and protein induction, islet serotonin content increased to a peak level that was 200-fold higher than basal. Interestingly, only a subpopulation of the beta cells was serotonin-positive in vitro and in vivo. The stored serotonin pool in pregnant islets and PL-treated MIN6 cells was rapidly released (turnover once every 2 h).

CONCLUSIONS/INTERPRETATION

A very strong lactogen-dependent upregulation of serotonin biosynthesis occurs in a subpopulation of mouse islet beta cells during pregnancy. Since the newly formed serotonin is rapidly released, this lactogen-induced beta cell function may serve local or endocrine tasks, the nature of which remains to be identified.

Subplasmalemmal Ca(2+) measurements in mouse pancreatic beta cells support the existence of an amplifying effect of glucose on insulin secretion

AIMS/HYPOTHESIS

Glucose-induced insulin secretion is attributed to a rise of beta cell cytosolic free [Ca(2+)] ([Ca(2+)](c)) (triggering pathway) and amplification of the action of Ca(2+). This concept of amplification rests on observations that glucose can increase Ca(2+)-induced insulin secretion without further elevating an imposed already high [Ca(2+)](c). However, it remains possible that this amplification results from an increase in [Ca(2+)] just under the plasma membrane ([Ca(2+)](SM)), which escaped detection by previous measurements of global [Ca(2+)](c). This was the hypothesis that we tested here by measuring [Ca(2+)](SM).

METHODS

The genetically encoded Ca(2+) indicators D3-cpv (untargeted) and LynD3-cpv (targeted to plasma membrane) were expressed in clusters of mouse beta cells. LynD3-cpv was also expressed in beta cells within intact islets. [Ca(2+)](SM) changes were monitored using total internal reflection fluorescence microscopy. Insulin secretion was measured in parallel.

RESULTS

Beta cells expressing D3cpv or LynD3cpv displayed normal [Ca(2+)] changes and insulin secretion in response to glucose. Distinct [Ca(2+)](SM) fluctuations were detected during repetitive variations of KCl between 30 and 32-35 mmol/l, attesting to the adequate sensitivity of our system. When the amplifying pathway was evaluated (high KCl + diazoxide), increasing glucose from 3 to 15 mmol/l consistently lowered [Ca(2+)](SM) while stimulating insulin secretion approximately two fold. Blocking Ca(2+) uptake by the endoplasmic reticulum largely attenuated the [Ca(2+)](SM) decrease produced by high glucose but did not unmask localised [Ca(2+)](SM) increases.

CONCLUSIONS/INTERPRETATION

Glucose can increase Ca(2+)-induced insulin secretion without causing further elevation of beta cell [Ca(2+)](SM). The phenomenon is therefore a true amplification of the triggering action of Ca(2+).

Effects of fructosamine-3-kinase deficiency on function and survival of mouse pancreatic islets after prolonged culture in high glucose or ribose concentrations

Due to their high glucose permeability, insulin-secreting pancreatic beta-cells likely undergo strong intracellular protein glycation at high glucose concentrations. They may, however, be partly protected from the glucotoxic alterations of their survival and function by fructosamine-3-kinase (FN3K), a ubiquitous enzyme that initiates deglycation of intracellular proteins. To test that hypothesis, we cultured pancreatic islets from Fn3k-knockout (Fn3k(-/-)) mice and their wild-type (WT) littermates for 1-3 wk in the presence of 10 or 30 mmol/l glucose (G10 or G30, respectively) and measured protein glycation, apoptosis, preproinsulin gene expression, and Ca(2+) and insulin secretory responses to acute glucose stimulation. The more potent glycating agent d-ribose (25 mmol/l) was used as positive control for protein glycation. In WT islets, a 1-wk culture in G30 significantly increased the amount of soluble intracellular protein-bound fructose-epsilon-lysines and the glucose sensitivity of beta-cells for changes in Ca(2+) and insulin secretion, whereas it decreased the islet insulin content. After 3 wk, culture in G30 also strongly decreased beta-cell glucose responsiveness and preproinsulin mRNA levels, whereas it increased islet cell apoptosis. Although protein-bound fructose-epsilon-lysines were more abundant in Fn3k(-/-) vs. WT islets, islet cell survival and function and their glucotoxic alterations were almost identical in both types of islets, except for a lower level of apoptosis in Fn3k(-/-) islets cultured for 3 wk in G30. In comparison, d-ribose (1 wk) similarly decreased preproinsulin expression and beta-cell glucose responsiveness in both types of islets, whereas it increased apoptosis to a larger extent in Fn3k(-/-) vs. WT islets. We conclude that, despite its ability to reduce the glycation of intracellular islet proteins, FN3K is neither required for the maintenance of beta-cell survival and function under control conditions nor involved in protection against beta-cell glucotoxicity. The latter, therefore, occurs independently from the associated increase in the level of intracellular fructose-epsilon-lysines.

Increased glucose sensitivity of both triggering and amplifying pathways of insulin secretion in rat islets cultured for 1 wk in high glucose

Chronic hyperglycemia has been shown to induce either a lack of response or an increased sensitivity to glucose in pancreatic beta-cells. We reinvestigated this controversial issue in a single experimental model by culturing rat islets for 1 wk in 10 or 30 mmol/l glucose (G10, Controls; or G30, High-glucose islets) before testing the effect of stepwise glucose stimulation from G0.5 to G20 on key beta-cell stimulus-secretion coupling events. Compared with Controls, the glucose sensitivity of High-glucose islets was markedly increased, leading to maximal stimulation of oxidative metabolism and both triggering and amplifying pathways of insulin secretion in G6 rather than G20, hence to loss of glucose effect above G6. This enhanced glucose sensitivity occurred despite an approximately twofold increase in islet uncoupling protein 2 mRNA expression. Besides this increased glucose sensitivity, the maximal glucose stimulation of insulin secretion in High-glucose islets was reduced by approximately 50%, proportionally to the reduction of insulin content. In High-glucose islets, changes in (45)Ca(2+) influx induced by glucose and diazoxide were qualitatively similar but quantitatively smaller than in Control islets and, paradoxically, did not lead to detectable changes in the intracellular Ca(2+) concentration measured by microspectrofluorimetry (fura PE 3). In conclusion, after 1 wk of culture in G30, the loss of glucose stimulation of insulin secretion in the physiological range of glucose concentrations (G5-G10) results from the combination of an increased sensitivity to glucose of both triggering and amplifying pathways of insulin secretion and an approximately 50% reduction in the maximal glucose stimulation of insulin secretion.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Inhibition of protein synthesis sequentially impairs distinct steps of stimulus-secretion coupling in pancreatic beta cells

Proteins with a short half-life are potential sites of pancreatic ss cell dysfunction under pathophysiological conditions. In this study, mouse islets were used to establish which step in the regulation of insulin secretion is most sensitive to inhibition of protein synthesis by 10 microM cycloheximide (CHX). Although islet protein synthesis was inhibited approximately 95% after 1 h, the inhibition of insulin secretion was delayed and progressive. After long (18-20 h) CHX-treatment, the strong (80%) inhibition of glucose-, tolbutamide-, and K(+)-induced insulin secretion was not due to lower insulin stores, to any marked impairment of glucose metabolism or to altered function of K(+)-ATP channels (total K(+)-ATP currents were however decreased). It was partly caused by a decreased Ca(2+) influx (whole-cell Ca(2+) current) resulting in a smaller rise in cytosolic Ca(2+) ([Ca(2+)](i)). The situation was very different after short (2-5 h) CHX-treatment. Insulin secretion was 50-60% inhibited although islet glucose metabolism was unaffected and stimulus-induced [Ca(2+)](i) rise was not (2 h) or only marginally (5 h) decreased. The efficiency of Ca(2+) on secretion was thus impaired. The inhibition of insulin secretion by 15 h of CHX treatment was more slowly reversible (>4 h) than that of protein synthesis. This reversibility of secretion was largely attributable to recovery of a normal Ca(2+) efficiency. In conclusion, inhibition of protein synthesis in islets inhibits insulin secretion in two stages: a rapid decrease in the efficiency of Ca(2+) on exocytosis, followed by a decrease in the Ca(2+) signal mediated by a slower loss of functional Ca(2+) channels. Glucose metabolism and the regulation of K(+)-ATP channels are more resistant. Proteins with a short half-life appear to be important to ensure optimal Ca(2+) effects on exocytosis, and are the potential Achille's heel of stimulus-secretion coupling.

Oscillations of insulin secretion can be triggered by imposed oscillations of cytoplasmic Ca2+ or metabolism in normal mouse islets

Glucose-induced insulin secretion depends on an acceleration of glucose metabolism, requires a rise in the cytoplasmic free Ca2+ concentration ([Ca2+]i), and is modulated by activation of protein kinases in beta-cells. Normal mouse islets were used to determine whether oscillations of these three signals are able and necessary to trigger oscillations of insulin secretion. The approach was to minimize or abolish spontaneous oscillations and to compare the impact of forced oscillations of each signal on insulin secretion. In a control medium, repetitive increases in the glucose concentration triggered oscillations in metabolism [NAD(P)H fluorescence], [Ca2+]i (fura-PE3 method), and insulin secretion. In the presence of diazoxide, metabolic oscillations persisted, but [Ca2+]i and insulin oscillations were abolished. When the islets were depolarized with high K+ with or without diazoxide, [Ca2+]i was elevated, and insulin secretion was stimulated. Forced metabolic oscillations transiently decreased or did not affect [Ca2+]i and potentiated insulin secretion with oscillations of small amplitude. These oscillations of secretion followed metabolic oscillations only when [Ca2+]i did not change. When [Ca2+]i fluctuated, these changes prevailed over those of metabolism for timing secretion. Repetitive depolarizations with high K+ in the presence of stable glucose (10 mmol/l) induced synchronous pulses of [Ca2+]i and insulin secretion with only small oscillations of metabolism. Continuous stimulation of protein kinase A (PKA) and protein kinase C (PKC) did not dissociate the [Ca2+]i and insulin pulses from the high K+ pulses. However, the amplitude of the insulin pulses was consistently increased, whereas that of the [Ca2+]i pulses was either increased (PKA) or decreased (PKC). In conclusion, metabolic oscillations can induce oscillations of insulin secretion independently of but with a lesser effectiveness than [Ca2+]i oscillations. Although oscillations in metabolism may cyclically influence secretion through an ATP-sensitive K+ channel (K+-ATP channel)-independent pathway, their regulatory effects are characterized by a hysteresis that makes them unlikely drivers of fast oscillations, unless they also involve [Ca2+]i changes through the K+-ATP channel-dependent pathway.

Multiple sites of purinergic control of insulin secretion in mouse pancreatic beta-cells

In mouse pancreatic beta-cells, extracellular ATP (0.1 mmol/l) effectively reduced glucose-induced insulin secretion. This inhibitory action resulted from a direct interference with the secretory machinery, and ATP suppressed depolarization-induced exocytosis by 60% as revealed by high-resolution capacitance measurements. Suppression of Ca2+-dependent exocytosis was mediated via binding to P2Y1 purinoceptors but was not associated with inhibition of the voltage-dependent Ca2+ currents or adenylate cyclase activity. Inhibition of exocytosis by ATP resulted from G-protein-dependent activation of the serine/threonine protein phosphatase calcineurin and was abolished by cyclosporin A and deltamethrin. In contrast to the direct inhibitory action on exocytosis, ATP reduced the whole-cell ATP-sensitive K+ (K(ATP)) current by 30% (via activation of cytosolic phospholipase A2), leading to membrane depolarization and stimulation of electrical activity. The stimulatory effect of ATP also involved mobilization of Ca2+ from thapsigargin-sensitive intracellular stores. We propose that the inhibitory action of ATP, by interacting with the secretory machinery at a level downstream to an elevation in [Ca2+]i, is important for autocrine regulation of insulin secretion in mouse beta-cells.

Influence of cell number on the characteristics and synchrony of Ca2+ oscillations in clusters of mouse pancreatic islet cells

1. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in single cells and cell clusters of different sizes prepared from mouse pancreatic islets. 2. During stimulation with 15 mM glucose, 20 % of isolated cells were inert, whereas 80 % showed [Ca2+]i oscillations of variable amplitude, duration and frequency. Spectral analysis identified a major frequency of 0.14 min-1 and a less prominent one of 0.27 min-1. 3. In contrast, practically all clusters (2-50 cells) responded to glucose, and no inert cells were identified within the clusters. As compared to single cells, mean [Ca2+]i was more elevated, [Ca2+]i oscillations were more regular and their major frequency was slightly higher (but reached a plateau at approximately 0.25 min-1). In some cells and clusters, faster oscillations occurred on top of the slow ones, between them or randomly. 4. Image analysis revealed that the regular [Ca2+]i oscillations were well synchronized between all cells of the clusters. Even when the Ca2+ response was irregular, slow and fast [Ca2+]i oscillations induced by glucose were also synchronous in all cells. 5. In contrast, [Ca2+]i oscillations resulting from mobilization of intracellular Ca2+ by acetylcholine were restricted to certain cells only and were not synchronized. 6. Heptanol and 18alpha-glycyrrhetinic acid, two agents widely used to block gap junctions, altered glucose-induced Ca2+ oscillations, but control experiments showed that they also exerted effects other than a selective uncoupling of the cells. 7. The results support theoretical models predicting an increased regularity of glucose-dependent oscillatory events in clusters as compared to isolated islet cells, but contradict the proposal that the frequency of the oscillations increases with the number of coupled cells. Islet cell clusters function better as electrical than biochemical syncytia. This may explain the co-ordination of [Ca2+]i oscillations driven by depolarization-dependent Ca2+ influx during glucose stimulation.

Altered cAMP and Ca2+ signaling in mouse pancreatic islets with glucagon-like peptide-1 receptor null phenotype

1-Cells from rodents and humans express different receptors recognizing hormones of the secretin-glucagon family, which--when activated--synergize with glucose in the control of insulin release. We have recently reported that isolated islets from mice homozygous for a GLP-1 receptor null mutation (GLP-1R(-/-)) exhibit a well-preserved insulin-secretory response to glucose. This observation can be interpreted in two different ways: 1) the presence of GLP-1R is not essential for the secretory response of isolated islets to glucose alone; 2) beta-cells in GLP-1R(-/-) pancreases underwent compensatory changes in response to the null mutation. To explore these possibilities, we studied islets from control GLP-IR(+/+) mice in the absence or presence of 1 pmol/l exendin (9-39)amide, a specific and potent GLP-1R antagonist. Exendin (9-39)amide (15-min exposure) reduced glucose-induced insulin secretion from both perifused and statically incubated GLP-1R(+/+) islets by 50% (P < 0.05), and reduced islet cAMP production in parallel (P < 0.001). Furthermore, GLP-1R(-/-) islets exhibited: 1) reduced cAMP accumulation in the presence of 20 mmol/l glucose (knockout islets versus control islets, 12 +/- 1 vs. 27 +/- 3 fmol x islet(-1) x 15 min(-1); P < 0.001) and exaggerated acceleration of cAMP production by 10 nmol/l glucose-dependent insulinotropic peptide (GIP) (increase over 20 mmol/l glucose by GIP in knockout islets versus control islets: 66 +/- 5 vs. 14 +/- 3 fmol x islet(-1) x 15 min(-1); P < 0.001); 2) increased mean cytosolic [Ca2+] ([Ca2+]c) at 7, 10, and 15 mmol/l glucose in knockout islets versus control islets; and 3) signs of asynchrony of [Ca2+]c oscillations between different islet subregions. In conclusion, disruption of GLP-1R signaling is associated with reduced basal but enhanced GIP-stimulated cAMP production and abnormalities in basal and glucose-stimulated [Ca2+]c. These abnormalities suggest that GLP-1R signaling is an essential upstream component of multiple beta-cell signaling pathways.

Uptake and release of Ca2+ by the endoplasmic reticulum contribute to the oscillations of the cytosolic Ca2+ concentration triggered by Ca2+ influx in the electrically excitable pancreatic B-cell

The role of intracellular Ca2+ pools in oscillations of the cytosolic Ca2+ concentration ([Ca2+]c) triggered by Ca2+ influx was investigated in mouse pancreatic B-cells. [Ca2+]c oscillations occurring spontaneously during glucose stimulation or repetitively induced by pulses of high K+ (in the presence of diazoxide) were characterized by a descending phase in two components. A rapid decrease in [Ca2+]c coincided with closure of voltage-dependent Ca2+ channels and was followed by a slower phase independent of Ca2+ influx. Blocking the SERCA pump with thapsigargin or cyclopiazonic acid accelerated the rising phase of [Ca2+]c oscillations and increased their amplitude, which suggests that the endoplasmic reticulum (ER) rapidly takes up Ca2+. It also suppressed the slow [Ca2+]c recovery phase, which indicates that this phase corresponds to the slow release of Ca2+ that was taken up by the ER during the upstroke of the [Ca2+]c transient. Glucose promoted the buffering capacity of the ER and amplified the slow [Ca2+]c recovery phase. The slow phase induced by high K+ pulses was not affected by modulators of Ca2+- or inositol 1,4,5-trisphosphate-induced Ca2+ release, did not involve a depolarization-induced Ca2+ release, and was also observed at the end of a rapid rise in [Ca2+]c triggered from caged Ca2+. It is attributed to passive leakage of Ca2+ from the ER. We suggest that the ER displays oscillations of the Ca2+ concentration ([Ca2+]ER) concomitant and parallel to [Ca2+]c. The observation that thapsigargin depolarizes the membrane of B-cells supports the proposal that the degree of Ca2+ filling of the ER modulates the membrane potential. Therefore, [Ca2+]ER oscillations occurring during glucose stimulation are likely to influence the bursting behavior of B-cells and eventually [Ca2+]c oscillations.

Temporal and quantitative correlations between insulin secretion and stably elevated or oscillatory cytoplasmic Ca2+ in mouse pancreatic beta-cells

An increase in cytoplasmic Ca2+ in beta-cells is a key step in glucose-induced insulin secretion. However, whether changes in cytoplasmic free Ca2+ ([Ca2+]i) directly regulate secretion remains disputed. This question was addressed by investigating the temporal and quantitative relationships between [Ca2+]i and insulin secretion. Both events were measured simultaneously in single mouse islets loaded with fura-PE3 and perifused with a medium containing diazoxide (to prevent any effect of glucose on the membrane potential) and either 4.8 or 30 mmol/l K+. Continuous depolarization with 30 mmol/l K+ in the presence of 15 mmol/l glucose induced a sustained rise in [Ca2+]i and insulin release. No oscillations of secretion were detected even after mathematical analysis of the data (pulse, spectral and sample distribution analysis). In contrast, alternating between 30 and 4.8 mmol/l K+ (1 min/2 min or 2.5 min/5 min) triggered synchronous [Ca2+]i and insulin oscillations of regular amplitude in each islet. A good correlation was found between [Ca2+]i and insulin secretion, and it was independent of the presence or absence of oscillations. This quantitative correlation between [Ca2+]i and insulin secretion was confirmed by experiments in which extracellular Ca2+ was increased or decreased (0.1-2.5 mmol/l) stepwise in the presence of 30 mmol/l K+. This resulted in parallel stepwise increases or decreases in [Ca2+]i and insulin secretion. However, while the successive [Ca2+]i levels were unaffected by glucose, each plateau of secretion was much higher in 20 than in 3 mmol/l glucose. In conclusion, in our preparation of normal mouse islets, insulin secretion oscillates only when [Ca2+]i oscillates in beta-cells. This close temporal relationship between insulin secretion and [Ca2+]i changes attests of the regulatory role of Ca2+. There also exists a quantitative relationship that is markedly influenced by the concentration of glucose.

Interplay between cytoplasmic Ca2+ and the ATP/ADP ratio: a feedback control mechanism in mouse pancreatic islets

In pancreatic beta cells, the increase in the ATP/ADP ratio that follows a stimulation by glucose is thought to play an important role in the Ca2+-dependent increase in insulin secretion. Here we have investigated the possible interactions between Ca2+ and adenine nucleotides in mouse islets. Measurements of both parameters in the same single islet showed that the rise in the ATP/ADP ratio precedes any rise in the cytoplasmic free-Ca2+ concentration ([Ca2+]i) and is already present during the initial transient lowering of [Ca2+]i produced by the sugar. Blockade of Ca2+ influx with nimodipine did not prevent the concentration-dependent increase in the ATP/ADP ratio produced by glucose and even augmented the ratio at all glucose concentrations which normally stimulate Ca2+ influx. In contrast, stimulation of Ca2+ influx by 30 mM K+ or 100 microM tolbutamide lowered the ATP/ADP ratio. This lowering was of rapid onset and reversibility, sustained and prevented by nimodipine or omission of extracellular Ca2+. It was, however, not attenuated after blockade of secretion by activation of alpha2-adrenoceptors. The difference in islet ATP/ADP ratio during blockade and stimulation of Ca2+ influx was similar to that observed between threshold and submaximal glucose concentrations. The results suggest that the following feedback loop could control the oscillations of membrane potential and [Ca2+]i in beta cells. Glucose metabolism increases the ATP/ADP ratio in a Ca2+-independent manner, which leads to closure of ATP-sensitive K+ channels, depolarization and stimulation of Ca2+ influx. The resulting increase in [Ca2+]i causes a larger consumption than production of ATP, which induces reopening of ATP-sensitive K+ channels and arrest of Ca2+ influx. Upon lowering of [Ca2+]i the ATP/ADP ratio increases again and a new cycle may start.

Tolbutamide and diazoxide influence insulin secretion by changing the concentration but not the action of cytoplasmic Ca2+ in beta-cells

Sulfonylureas stimulate insulin secretion by blocking ATP-sensitive K+ channels (K+-ATP channels) of the beta-cell membrane, thereby causing depolarization, Ca2+ influx, and rise in cytoplasmic Ca2+ concentration ([Ca2+]i), whereas diazoxide inhibits insulin secretion by opening K+-ATP channels. It has been suggested recently that these drugs also respectively increase and decrease the efficacy of Ca2+ on exocytosis. This hypothesis was tested here with intact islets or single beta-cells from normal mice. Depolarizing islet cells by raising extracellular K+ from 4.8 to 15, 30, and 60 mmol/l progressively raised [Ca2+]i and stimulated insulin secretion. The magnitude of the [Ca2+]i rise produced by a subsequent addition of 100 micromol/l tolbutamide decreased as the concentration of K+ was increased. The effect on insulin secretion paralleled that on [Ca2+]i. Similarly, the magnitudes of the [Ca2+]i drop and of the inhibition of insulin secretion produced by 250 micromol/l diazoxide were inversely related to the concentration of K+. Either drug was effective on secretion only when it increased or decreased [Ca2+]i. Exocytosis of insulin granules from single, voltage-clamped beta-cells was also studied by measuring cell capacitance changes. In the perforated patch configuration, exocytosis was evoked by depolarizing pulses. Addition of tolbutamide to the extracellular medium did not affect the Ca2+ current and the resulting change in cell capacitance. In the whole-cell configuration, cell capacitance increased with the concentration of free Ca2+ in the solution diffusing from the pipette into the cell. It was markedly potentiated by cAMP, was inhibited by activation of alpha2-adrenoceptors with clonidine, and was strongly augmented by acetylcholine. In contrast, tolbutamide was ineffective whether applied intra- or extracellularly, at low or high free Ca2+, and with or without cAMP. Diazoxide also failed to interfere directly with exocytosis. These results indicate that tolbutamide and diazoxide affect insulin secretion by changing the concentration, not the action, of Ca2+ in beta-cells.

Functional significance of Ca2+ oscillations in pancreatic beta cells

Several aspects of pancreatic beta cell function display marked oscillations even during continuous stimulation with a stable glucose concentration. This review article focuses on the characteristics, mechanisms and potential roles of the oscillations of cytoplasmic Ca2+ concentration [(Ca2+]i) in beta cells. These oscillations result from an intermittent influx of Ca2+ through voltage-dependent Ca2+ channels activated by periodic depolarizations of the plasma membrane. In each islet, [Ca2+]i oscillations are synchronous in all beta cells and trigger similar oscillations of insulin secretion. Changes in [Ca2+]i are thought to play a minute-to-minute regulatory role in secretion, but the effectiveness of Ca2+ on the secretory process is markedly influenced by various amplification mechanisms. It is still unclear whether the oscillations of [Ca2+]i reflect functional advantages for the beta cell itself or are simply necessary to ensure oscillations of plasma insulin levels through pulsatile secretion of the hormone.

Okadaic acid-induced decrease in the magnitude and efficacy of the Ca2+ signal in pancreatic beta cells and inhibition of insulin secretion

1. Phosphorylation by kinases and dephosphorylation by phosphatases markedly affect the biological activity of proteins involved in stimulus-response coupling. In this study, we have characterized the effects of okadaic acid, an inhibitor of protein phosphatases 1 and 2A, on insulin secretion. Mouse pancreatic islets were preincubated for 60 min in the presence of okadaic acid before their function was studied. 2. Okadaic acid dose-dependently (IC50 approximately 200 nM) inhibited insulin secretion induced by 15 mM glucose. At 0.5 microM, okadaic acid also inhibited insulin secretion induced by tolbutamide, ketoisocaproate and high K+, and its effects were not reversed by activation of protein kinases A or C. 3. The inhibition of insulin secretion did not result from an alteration of glucose metabolism (estimated by the fluorescence of endogenous pyridine nucleotides) or a lowering of the ATP/ADP ratio in the islets. 4. Okadaic acid treatment slightly inhibited voltage-dependent Ca2+ currents in beta cells (perforated patch technique), which diminished the rise in cytoplasmic Ca2+ (fura-2 method) that glucose and high K+ produce in islets. However, this decrease (25%), was insufficient to explain the corresponding inhibition of insulin secretion (90%). Moreover, mobilization of intracellular Ca2+ by acetylcholine was barely affected by okadaic acid, whereas the concomitant insulin response was decreased by 85%. 5. Calyculin A, another inhibitor of protein phosphatases 1 and 2A largely mimicked the effects of okadaic acid, whereas 1-norokadaone, an inactive analogue of okadaic acid on phosphatases, did not alter beta cell function. 6. In conclusion, okadaic acid inhibits insulin secretion by decreasing the magnitude of the Ca2+ signal in beta cells and its efficacy on exocytosis. The results suggest that, contrary to current concepts, both phosphorylation and dephosphorylation of certain beta cell proteins may be involved in the regulation of insulin secretion.

Emptying of intracellular Ca2+ stores stimulates Ca2+ entry in mouse pancreatic beta-cells by both direct and indirect mechanisms

1. In non-excitable cells, the depletion of intracellular Ca2+ stores triggers Ca2+ influx by a process called capacitative Ca2+ entry. In the present study, we have investigated how the emptying of these stores by thapsigargin (1 microM) influences Ca2+ influx in electrically excitable pancreatic beta-cells. The cytoplasmic Ca2+ concentration ([Ca2+]i) was monitored in clusters of mouse beta-cells or in whole islets loaded with fura-2. 2. The membrane was first held hyperpolarized by diazoxide, an opener of ATP-sensitive K+ (KATP) channels, in the presence of 4.8 mM K+. Alternating between Ca(2+)-free medium and medium containing 2.5 mM Ca2+ caused a minor rise in [Ca2+]i (approximately 14 nM) in clusters of beta-cells. A larger rise (approximately 65 nM), resistant to the blockade of voltage-dependent Ca2+ channels by D600, occurred when extracellular Ca2+ was readmitted after emptying intracellular Ca2+ stores with thapsigargin or acetylcholine. Thus there exists a small capacitative Ca2+ entry in beta-cells. 3. When the membrane potential was clamped at depolarized levels with 10, 20 or 45 mM K+ in the presence of diazoxide, [Ca2+]i increased to different plateau levels ranging between 100 and 900 nM. Thapsigargin consistently caused a further transient rise in [Ca2+]i, but had little (at 10 mM K+) or no effect on the plateau level. This confirms that the capacitative Ca2+ entry is small. 4. In clusters of cells whose membrane potential was not clamped with diazoxide, 15 mM glucose (in 4.8 mM K+) induced [Ca2+]i oscillations by promoting Ca2+ influx through voltage-dependent Ca2+ channels. The application of thapsigargin accelerated these oscillations and increased their amplitude, sometimes causing a sustained elevation of [Ca2+]i. Similar results were obtained from whole islets perifused with a medium containing > or = 6 mM glucose. The effect of thapsigargin was always much larger than expected from the capacitative Ca2+ entry, probably because of a potentiation of Ca2+ influx through voltage-dependent Ca2+ channels. 5. This potentiating effect of thapsigargin did not result from an acceleration of cell metabolism since the drug did not affect glucose-induced changes in NAD(P)H fluorescence. It is also unlikely to involve the inhibition of KATP channels because thapsigargin steadily elevated [Ca2+]i in cells in which [Ca2+]i oscillations persisted in the presence of a maximally effective concentration of tolbutamide. 6. In conclusion, the emptying of intracellular Ca2+ stores in beta-cells induces a small capacitative Ca2+ entry and activates a depolarizing current which potentiates glucose-induced Ca2+ influx through voltage-dependent Ca2+ channels.

G protein-dependent inhibition of L-type Ca2+ currents by acetylcholine in mouse pancreatic B-cells

1. The effect of acetylcholine (ACh) on voltage-dependent Ca2+ currents in mouse pancreatic B-cells was studied using the whole-cell configuration of the patch-clamp technique. 2. ACh (0.25-250 microM) reversibly and dose-dependently inhibited the Ca2+ current elicited by depolarizations from -80 mV to +10 mV. Maximal inhibition was observed at concentrations > 25 microM where it amounted to approximately 35%. The effect was voltage independent and prevented by atropine (10 microM) suggesting that it was mediated by muscarinic receptors. 3. The inhibitory action of ACh on the Ca2+ current was abolished when the cytoplasmic solution contained GDP beta S (2 mM) and became irreversible when the non-hydrolysable GTP analogue GTP gamma S (10 microM) was included in the pipette. This indicates the participation of G proteins in the inhibitory effect of ACh but pretreatment of the cells with either pertussis or cholera toxin failed to prevent the effect of ACh on the Ca2+ current. 4. ACh remained equally effective as an inhibitor of the whole-cell Ca2+ current in the presence of the L-type Ca2+ channel agonist (-)-Bay K 8644 and after partial inhibition of the current by nifedipine. Addition of omega-agatoxin IVA, omega-conotoxin GVIA or omega-conotoxin MVIIC neither affected the peak Ca2+ current amplitude nor the extent of inhibition produced by ACh. These pharmacological properties indicate that ACh acts by inhibiting L-type Ca2+ channels. 5. The inhibitory action of ACh on the B-cell Ca2+ current was not secondary to elevation of [Ca2+]i and ACh remained equally effective as an inhibitor when Ba2+ was used as the charge carrier, when [Ca2+]i was buffered to low concentrations using EGTA and under experimental conditions preventing the mobilization of Ca2+ from intracellular stores. 6. These results suggest that ACh reduces the whole-cell Ca2+ current in the B-cell through a G protein-regulated, voltage- and Ca(2+)-independent inhibition of L-type Ca2+ channels.

Direct glucocorticoid inhibition of insulin secretion. An in vitro study of dexamethasone effects in mouse islets

The direct effects of glucocorticoids on pancreatic beta cell function were studied with normal mouse islets. Dexamethasone inhibited insulin secretion from cultured islets in a concentration-dependent manner: maximum of approximately 75% at 250 nM and IC50 at approximately 20 nM dexamethasone. This inhibition was of slow onset (0, 20, and 40% after 1, 2, and 3 h) and only slowly reversible. It was prevented by a blocker of nuclear glucocorticoid receptors, by pertussis toxin, by a phorbol ester, and by dibutyryl cAMP, but was unaffected by an increase in the fuel content of the culture medium. Dexamethasone treatment did not affect islet cAMP levels but slightly reduced inositol phosphate formation. After 18 h of culture with or without 1 microM dexamethasone, the islets were perifused and stimulated by a rise in the glucose concentration from 3 to 15 mM. Both phases of insulin secretion were similarly decreased in dexamethasone-treated islets as compared with control islets. This inhibition could not be ascribed to a lowering of insulin stores (higher in dexamethasone-treated islets), to an alteration of glucose metabolism (glucose oxidation and NAD(P)H changes were unaffected), or to a lesser rise of cytoplasmic Ca2+ in beta cells (only the frequency of the oscillations was modified). Dexamethasone also inhibited insulin secretion induced by arginine, tolbutamide, or high K+. In this case also the inhibition was observed despite a normal rise of cytoplasmic Ca2+. In conclusion, dexamethasone inhibits insulin secretion through a genomic action in beta cells that leads to a decrease in the efficacy of cytoplasmic Ca2+ on the exocytotic process.

No evidence for a role of reverse Na(+)-Ca2+ exchange in insulin release from mouse pancreatic islets

We studied whether reverse Na(+)-Ca2+ exchange can increase cytoplasmic Ca2+ ([Ca2+]i) in mouse islets and contribute to insulin release. The exchange was stimulated by replacing Na+ with choline, sucrose, or lithium in a medium containing 15 mM glucose. Na+ omission increased electrical activity in B cells, [Ca2+]i, and insulin release. When voltage-dependent Ca2+ channels were blocked by nimodipine or closed by holding the membrane polarized with diazoxide, Na+ omission caused a slight hyperpolarization, a small rise in [Ca2+]i, and a marginal increase in insulin release (the latter only with choline). This small rise in [Ca2+]i was dependent on extracellular Ca2+ but was hardly augmented when intracellular Na+ was raised with alanine. When B cells were depolarized by 30 mM K+, Na+ omission did not affect the membrane potential but increased [Ca2+]i and insulin release. If Ca2+ channels were blocked by nimodipine, only marginal increases in Ca2+ and insulin release persisted, which were not different from those observed when the cells were not depolarized. This indicates that Ca2+ influx through voltage-dependent Ca2+ channels rather than via reverse Na(+)-Ca2+ exchange underlies the rise in [Ca2+]i and in insulin release produced by Na+ removal. No decisive support for Ca2+ influx by reverse Na(+)-Ca2+ exchange could be found.

Muscarinic stimulation increases Na+ entry in pancreatic B-cells by a mechanism other than the emptying of intracellular Ca2+ pools

Stimulation of muscarinic (M3) receptors depolarizes pancreatic B-cells by increasing Na+ influx. Here, we measured [Na+]i and [Ca2+]i in B-cell clusters to investigate whether depletion of intracellular Ca2+ pools triggers this unusual transduction pathway for muscarinic receptors. Acetylcholine emptied Ca2+ pools less completely than did the SERCA pump inhibitors, thapsigargin, and cyclopiazonic acid. However, the rise in [Na+]i produced by acetylcholine was not mimicked by thapsigargin or cyclopiazonic acid and was not prevented by previous depletion of Ca2+ pools. Depolarization of B-cells by acetylcholine stimulates Ca2+ influx and steadily increases [Ca2+]i. In the presence of glucose and extracellular Ca2+, B-cells treated with thapsigargin or cyclopiazonic acid displayed large [Ca2+]i oscillations. Subsequent application of acetylcholine was followed by a sustained rise in [Ca2+]i as in untreated cells. In conclusion, intracellular Ca2+ pool depletion does not mediate acetylcholine stimulation of Na+ entry and of subsequent events. We propose that the muscarinic receptors are coupled to Na+ channels in B-cells.

Sulphonylureas do not increase insulin secretion by a mechanism other than a rise in cytoplasmic Ca2+ in pancreatic B-cells

The following sequence of events is thought to underlie the stimulation of insulin release by hypoglycaemic sulphonylureas. Interaction of the drugs with a high-affinity binding site (sulphonylurea receptor) in the B-cell membrane leads to closure of ATP-sensitive K+ channels, depolarization, opening of voltage-dependent Ca2+ channels, Ca2+ influx and rise in cytoplasmic [Ca2+]i. Recent experiments using permeabilized islet cells or measuring changes in B-cell membrane capacitance have suggested that sulphonylureas can increase insulin release by a mechanism independent of a change in [Ca2+]i. This provocative hypothesis was tested here with intact mouse islets. When B-cells were strongly depolarized by 60 mM K+, [Ca2+]i was increased and insulin secretion stimulated. Under these conditions, tolbutamide did not further increase [Ca2+]i or insulin release, whether it was applied before or after high K+, and whether the concentration of glucose was 3 or 15 mM. This contrasts with the ability of forskolin and phorbol 12-myristate 13-acetate (PMA) to increase release in the presence of high K+. Tolbutamide also failed to increase insulin release from islets depolarized with barium (substituted for extracellular Ca2+) or with arginine in the presence of high glucose. Glibenclamide and its non-sulphonylurea moiety meglitinide were also without effect on insulin release from already depolarized B-cells. In the absence of extracellular Ca2+, acetylcholine induced monophasic peaks of [Ca2+]i and insulin secretion which were both unaffected by tolbutamide. Insulin release from permeabilized islet cells was stimulated by raising free Ca2+ (between 0.1 and 23 microM). This effect was not affected by tolbutamide and inconsistently increased by glibenclamide. In conclusion, the present study does not support the proposal that hypoglycaemic sulphonylureas can increase insulin release even when they do not also raise [Ca2+]i in B-cells.

Ketoisocaproic acid and leucine increase cytoplasmic pH in mouse pancreatic B cells: role of cytoplasmic Ca2+ and pH-regulating exchangers

The effects of nonglucose nutrient insulin secretagogues on cytoplasmic pH (pHi) in pancreatic B cells are unclear. These were studied with intact mouse islets loaded with BCECF and stimulated with ketoisocaproic acid (KIC) or leucine, which, unlike glucose, are exclusively metabolized in mitochondria. The changes in pHi were compared to those in cytoplasmic Ca2+ ([Ca2+]i; islets loaded with fura-2), metabolism [NAD(P)H fluorescence], and insulin release. In HCO3- buffer containing 3 mM glucose, 10 mM KIC produced a rapid and sustained increase in metabolism, [Ca2+]i, pHi, and insulin release. In HEPES buffer, the increase in metabolism, [Ca2+]i, and release were also rapid but not as sustained, whereas the alkalinization was delayed. The changes in release thus follow a time course more similar to that of [Ca2+]i and metabolism than to that of pHi. The role of [Ca2+]i in pHi changes was next examined. A similar rapid rise in pHi was produced by KIC in both HCO3- and HEPES buffers when its effects on [Ca2+]i were prevented, whether [Ca2+]i was kept low (4.8 mM KCl plus diazoxide) or high (30 mM KCl plus diazoxide). When the Na(+):H+ exchanger was blocked by dimethylamiloride, the alkalinizing effect of KIC was unaffected in HCO3- buffer, indicating that it does not result from an activation of this exchanger. In HEPES buffer, however, KIC strongly decreased pHi unless the rise in [Ca2+]i was prevented, in which case KIC increased pHi. When the HCO3-/Cl-exchanger was blocked by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), the effect of KIC in HCO3- buffer became similar to that in HEPES buffer without or with DIDS. The effects of leucine on pHi were similar to those of KIC. In conclusion, the effect of KIC and leucine on islet cell pHi, like that of glucose, is the complex result of an alkalinizing action of their metabolism and an acidifying action of the [Ca2+]i rise that they also produce. Compensation of this acidification is achieved by operation of the pHi-regulating exchangers, of which the HCO3-/Cl- exchanger plays a predominant role.

Distinct effects of glucose on the synchronous oscillations of insulin release and cytoplasmic Ca2+ concentration measured simultaneously in single mouse islets

There exists a good temporal correlation between the oscillations of cytoplasmic Ca2+ ([Ca2+]i) in pancreatic B cells and insulin release. Here, single mouse islets loaded with fura-2 were used to investigate whether there also exists a quantitative correlation between both events during stimulation by different glucose concentrations. The frequency of [Ca2+]i oscillations was decreased by raising the CaCl2 concentration to 10 mM in the perifusion medium (to ensure adequate resolution of insulin oscillations by the immunoassay), and the glucose concentration was increased from 10 to 20 or from 15 to 30 mM. Raising the glucose level was followed by changes in [Ca2+]i oscillations. Their duration increased slightly, and that of the intervals decreased, whereas the nadir between the oscillations became less deep. On the other hand, the peak of [Ca2+]i oscillations did not change. As a result, the rhythm was accelerated, and the average [Ca2+]i was increased. The concomitant increase in insulin release resulted from similar changes, with, in addition, a marked increase in the peak of insulin oscillations. In two thirds of the islets, variations in the amplitude of successive [Ca2+]i oscillations occurred during stimulation with 10 mM glucose and disappeared at higher glucose levels. This was due to temporal variations in the responsiveness of all regions of the islet, rather than to the existence of nonresponsive regions that would be recruited into an active state by high glucose. In conclusion, there exists a good temporal correlation between insulin and [Ca2+]i oscillations in islets stimulated by various glucose concentrations. The quantitative correlation is not as close, indicating that the relationship between the two phenomena is nonlinear and supporting previous evidence that glucose also increases the efficacy of Ca2+ on secretion. This mechanism, rather than the development of a Ca2+ rise in nonresponsive cells, might underlie B cell recruitment in intact islets.

Role of cyclic GMP in the control of capacitative Ca2+ entry in rat pancreatic acinar cells

We have investigated the possible roles of cyclic GMP (cGMP) in initiating or regulating capacitiative Ca2+ entry in rat pancreatic acinar cells. In medium containing 1.8 mM external Ca2+, thapsigargin activated Ca2+ entry and slightly but significantly increased intracellular cGMP concentration. This rise in cGMP levels was prevented by pretreating the cells with the guanylate cyclase inhibitor, LY-83583, or by omitting Ca2+ during stimulation by thapsigargin or methacholine. LY-83583 and NG-nitro-L-arginine (L-NA, an inhibitor of NO synthase) both had a small inhibitory effect on Ca2+ entry when they were added after thapsigargin in Ca2(+)-containing medium, and they reduced by 32 and 48% respectively the thapsigargin-induced capacitative Ca2+ entry when added to the cells during a 20 min preincubation period. However, neither dibutyryl cGMP (Bt2cGMP) nor sodium nitroprusside, an NO mimic, affected either basal intracellular Ca2+ concentration [Ca2+]i or thapsigargin-induced capacitative Ca2+ entry. Further, the inhibitory effects observed after preincubation with LY-83583 or L-NA could not be prevented by preincubation with Bt2cGMP, nor could they be reversed by adding Bt2cGMP, 8-bromo-cGMP or sodium nitroprusside acutely after activation of capacitative Ca2+ entry by thapsigargin. Finally, pretreatment of cells with LY-83583 or L-NA did not affect Ca2+ signalling in response to 1 microM methacholine, including the pattern of [Ca2+]i oscillations. In conclusion, in pancreatic acinar cells, the rise in cellular cGMP levels appears to depend on, rather than cause, the increase in [Ca2+]i with agonist stimulation.

Muscarinic stimulation exerts both stimulatory and inhibitory effects on the concentration of cytoplasmic Ca2+ in the electrically excitable pancreatic B-cell

Mouse pancreatic islets were used to investigate how muscarinic stimulation influences the cytoplasmic Ca2+ concentration ([Ca2+]i) in insulin-secreting B-cells. In the absence of extracellular Ca2+, acetylcholine (ACh) triggered a transient, concentration-dependent and thapsigargin-inhibited increase in [Ca2+]i. In the presence of extracellular Ca2+ and 15 mM glucose, ACh induced a biphasic rise in [Ca2+]i. The initial, transient phase increased with the concentration of ACh, whereas the second, sustained, phase was higher at low (0.1-1 microM) than at high (> or = 10 microM) concentrations of ACh. Thapsigargin attenuated (did not suppress) the first phase of the [Ca2+]i rise and did not affect the sustained response. This sustained rise was inhibited by omission of extracellular Na+ (which prevents the depolarizing action of ACh) and by D600 or diazoxide (which prevent activation of voltage-dependent Ca2+ channels). During steady-state stimulation, the Ca2+ action potentials in B-cells were stimulated by 1 microM ACh but inhibited by 100 microM ACh. When B-cells were depolarized by 45 mM K+, ACh induced a concentration-dependent, biphasic change in [Ca2+]i, consisting of a first peak rapidly followed by a decrease. Thapsigargin suppressed the peak without affecting the drop in [Ca2+]i. Measurements of 45Ca2+ efflux under similar conditions indicated that ACh decreases Ca2+ influx and slightly increases the efflux. All effects of ACh were blocked by atropine. In conclusion, three mechanisms at least are involved in the biphasic change in [Ca2+]i that muscarinic stimulation exerts in excitable pancreatic B-cells. A mobilization of Ca2+ from the endoplasmic reticulum contributes significantly to the first peak, but little to the steady-state rise in [Ca2+]i. This second phase results from an influx of Ca2+ through voltage-dependent Ca2+ channels activated by a Na(+)-dependent depolarization. However, when high concentrations of ACh are used, Ca2+ influx is attenuated.

The Ca(2+)-mobilizing actions of a Jurkat cell extract on mammalian cells and Xenopus laevis oocytes

Randriamampita and Tsien (Randriamampita, C., and Tsien, R. Y. (1993) Nature 364, 809-814) suggested that an acid-extracted fraction from a Jurkat cell line contains a messenger responsible for the coupling of calcium entry to the depletion of intracellular stores, i.e. capacitative calcium entry. We found that the extract, prepared as described by Randriamampita and Tsien, caused Ca2+ entry in 1321N1 astrocytoma cells which was not blocked by the D-myo-1,4,5-trisphosphate-receptor antagonist, heparin. In contrast to astrocytoma cells, when applied to mouse lacrimal acinar cells and rat hepatocytes the Jurkat extract always caused the release of intracellular Ca2+, followed by Ca2+ entry across the plasma membrane. This activity of the extract on lacrimal cells was blocked by either intracellular injection of heparin or extracellular atropine. Similarly prepared lacrimal cell extracts gave Ca2+ responses when applied to astrocytoma cells or lacrimal cells which were similar to those for Jurkat-derived extract. However, extracts from hepatocytes had no effect. In most Xenopus oocytes, the Jurkat extract had no effect, while in a few oocytes, the extract gave a [Ca2+]i response similar to that seen in lacrimal cells, that is, release of Ca2+ followed by Ca2+ entry. We conclude that the actions of the Jurkat cell extract are not consistent with its containing the long sought messenger for capacitative calcium entry. It is likely that this fraction contains a number of factors that mediate Ca2+ response in different cell types, possibly through receptor-mediated mechanisms.

Multiple effects and stimulation of insulin secretion by the tyrosine kinase inhibitor genistein in normal mouse islets

1. Islets from normal mice were used to test the acute effects of genistein, a potent tyrosine kinase inhibitor, on stimulus-secretion coupling in pancreatic beta-cells. 2. Genistein produced a concentration-dependent (10-100 microM), reversible, increase of insulin release. This effect was marginal on basal release or in the presence of non-metabolized secretagogues, and much larger in the presence of glucose or other nutrients. The increase in insulin release caused by 100 microM genistein was abolished by adrenaline or omission of extracellular Ca2+. It was not accompanied by any rise of cyclic AMP, inositol phosphate or adenine nucleotide levels. 3. Although genistein slightly inhibited ATP-sensitive K+ channels, as shown by 86Rb efflux and patch-clamp experiments, this effect could not explain the action of the drug on insulin release because the latter persisted when ATP-sensitive K+ channels were all blocked by maximally effective concentrations of glucose and tolbutamide. Genistein was also effective when ATP-sensitive K+ channels were opened by diazoxide and the beta-cell membrane depolarized by 30 mM K, but ineffective in the presence of diazoxide and normal extracellular K. 4. Genistein paradoxically decreased Ca2+ influx in beta-cells, as shown by the inhibition of glucose-induced electrical activity, by the inhibition of Ca2+ currents (perforated patches) and by the lowering of cytosolic [Ca2+]i (fura-2 technique). Genistein thus increases insulin release in spite of a lowering of [Ca2+]i in beta-cells. 5. Daidzein, an analogue of genistein reported not to affect tyrosine kinases, was slightly less potent than genistein on K+ and Ca2+ channels, but increased insulin secretion in a similar way. Three other tyrosine kinase inhibitors, tyrphostin A47, herbimycin A and an analogue of erbstatin variably affected insulin secretion.6. Genistein exerts a number of heretofore unrecognized effects. The unusual mechanisms, by which genistein increases insulin release in spite of a decrease in beta-cell [Ca2+]i and without activating known signalling pathways, do not seem to result from an inhibition of tyrosine kinases.

Culture duration and conditions affect the oscillations of cytoplasmic calcium concentration induced by glucose in mouse pancreatic islets

The pattern of the increase in cytoplasmic Cai2+ that glucose produces in beta cells has been reported to be highly variable. Here, we evaluated the influence of the culture duration (1-4 days) and conditions (5-10 mmol/l glucose) on Cai2+ in normal mouse islets stimulated by glucose. After 1 day of culture in 10 mmol/l glucose, a rise of the glucose concentration from 3 to 15 mmol/l induced a triphasic change of Cai2+ in the islets. A small initial decrease was followed by a large peak increase and then by regular fast oscillations (approximately 2.5/min). When the culture was prolonged to 2, 3 and 4 days, the initial decrease became inconsistent and the peak occurred earlier, whereas the oscillations decreased in frequency, increased in duration and eventually disappeared; on day 4 the Cai2+ rise was sustained. After culture in 5 mmol/l glucose, the pattern of Cai2+ changes induced by 15 mmol/l glucose was different. The initial decrease was very pronounced, the first peak was delayed and clearly separated from the subsequent oscillations. These were of a mixed type (fast Ca2+ transients on top of slow ones) after 1 day, and of a slow type only after 4 days. These alterations in the Cai2+ oscillations triggered by glucose could not be ascribed to desynchronization of the signal between different regions of the islets. In conclusion, culturing normal mouse islets in 5 or 10 mmol/l glucose for 1-4 days, markedly alters the characteristics of the changes in Cai2+ produced by glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of protein kinase-C in signal transduction through vasopressin and acetylcholine receptors in pancreatic B-cells from normal mouse

The role of protein kinase-C (PKC) in the potentiation of insulin release by arginine vasopressin (AVP) and acetylcholine (ACh) was investigated with normal mouse islets. The islets were submitted to a short term (30-min) or long term (22-h) treatment with phorbol 12-myristate 13-acetate (PMA) to stimulate acutely or down-regulate PKC before being stimulated by AVP or ACh. Control islets were treated with the inactive 4 alpha-phorbol 12,13-didecanoate. In the presence of 15 mM glucose and 2.5 mM Ca2+, AVP and ACh stimulated inositol phosphate (IP) formation, increased cytoplasmic Ca2+ (Cai2+), and potentiated insulin release. These effects were greater with ACh than with AVP, in particular on Cai2+, which was scarcely affected by AVP. In the absence of extracellular Ca2+, only ACh induced a short-lived increase in Cai2+ and insulin. Acute treatment with PMA in the presence of extracellular Ca2+ strongly increased insulin release in spite of a marked lowering of Cai2+. Under these conditions, the effects of AVP and ACh on IP production and Cai2+ were practically abolished, and only ACh transiently increased insulin release. In the absence of Ca2+, the small mobilization of Cai2+ by ACh triggered a peak of insulin, whereas a similar mobilization of Cai2+ by AVP was ineffective on insulin. After long term treatment of the islets with PMA, AVP normally increased IP formation, but did not affect insulin release. The effect of ACh on IP was still inhibited. However, ACh produced a marked transient increase in Cai2+, with a small transient release of insulin. The releasing effect of ACh was also inhibited in the absence of Ca2+. In conclusion, PKC plays a dual role in the B-cell responses to ACh and AVP. Its activation is necessary for the sustained potentiation of insulin release that both agents produce. This effect probably results from a sensitization of the secretory machinery to Cai2+, the triggering signal. PKC also exerts a negative feedback control on the signal transduction mechanisms involving phospholipase-C, but the ACh and AVP responses are not equally affected by this feedback.

Mechanisms of the stimulation of insulin release by saturated fatty acids. A study of palmitate effects in mouse beta-cells

The mechanisms by which fatty acids increase insulin release are not known. In this study, mouse islets were used as a model and palmitate as a reference compound to study in vitro how saturated fatty acids influence pancreatic beta-cells. Palmitate (625 microM) was bound to albumin. It did not affect basal insulin release (3 mM glucose) but increased the release induced by 10-15 mM glucose. This effect was dependent on the concentration of free rather than total palmitate. It was reversible and abolished by epinephrine, diazoxide, nimodipine, or omission of extracellular Ca. Bromopalmitate and methyl palmoxirate, two inhibitors of fatty acid oxidation, were ineffective alone, and only bromopalmitate partially inhibited the effects of palmitate on insulin release. The increase in insulin release produced by palmitate could not be ascribed to a blockade of ATP-sensitive K(+)-channels because the fatty acid only barely decreased 86Rb efflux and did not depolarize beta-cells in 3 mM glucose. The small effect on 86Rb efflux might be attributed to a slight rise in the ATP/ADP ratio. No such rise occurred when palmitate was tested in 15 mM glucose, and the fatty acid consistently accelerated 86Rb efflux under these conditions. Measurements of beta-cell membrane potential (intracellular microelectrodes) and of free cytoplasmic calcium (Cai2+) in beta-cells (Fura 2 technique) showed that palmitate increases Ca2+ influx; it also caused a very small mobilization of intracellular Ca. The persistence of this stimulation of Ca2+ influx in the presence of diazoxide and high K+ suggests that palmitate might act on Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Multisite control of insulin release by glucose

Glucose controls insulin release by beta-cells at two sites at least. By controlling the membrane potential, it controls the influx of Ca2+ and the rise in cytoplasmic Ca2+ which triggers exocytosis. At this level, the principal targets of glucose are the K(+)-ATP channels whose activity may be modulated by changes in the ATP/ADP ratio. A second, newly identified, mechanism of regulation is independent of changes in beta-cell membrane potential and of changes in Cai2+. It is not sufficient to induce insulin release, but serves to increase the response. This appears to be achieved through an amplification of the effectiveness of Cai2+ on the secretory process and may also depend on the changes in energy state of beta-cells.

Two sites of glucose control of insulin release with distinct dependence on the energy state in pancreatic B-cells

The energy state of pancreatic B-cells may influence insulin release at several steps of stimulus-secretion coupling. By closing ATP-sensitive K+ channels (K(+)-ATP channels), a rise in the ATP/ADP ratio may regulate the membrane potential, and hence Ca2+ influx. It may also modulate the effectiveness of Ca2+ on its intracellular targets. To assess the existence of these two roles and determine their relative importance for insulin release, we tested the effects of azide, a mitochondrial poison, on mouse B-cell function under various conditions. During stimulation by glucose alone, when K(+)-ATP channels are controlled by cellular metabolism, azide caused parallel, concentration-dependent (0.5-5 mM), membrane repolarization, decrease in cytosolic Ca2+ concentration [Ca2+]i and inhibition of insulin release. When K(+)-ATP channels were closed pharmacologically (by tolbutamide in high glucose), azide did not repolarize the membrane or decrease [Ca2+]i, and was much less effective in inhibiting insulin release. A similar resistance to azide was observed when K(+)-ATP channels were opened by diazoxide, and high K+ was used to depolarize the membrane and raise [Ca2+]i. In contrast, azide similarly decreased ATP levels and increased ADP levels, thereby lowering the ATP/ADP ratio under all conditions. In conclusion, lowering the ATP/ADP ratio in B-cells can inhibit insulin release even when [Ca2+]i remains high. However, this distal step is much more resistant to a decrease in the energy state of B-cells than is the control of membrane potential by K(+)-ATP channels. Generation of the signal triggering insulin release, high [Ca2+]i, through metabolic control of membrane potential requires a higher global ATP/ADP ratio than does activation of the secretory process itself.

Oscillations of secretion driven by oscillations of cytoplasmic Ca2+ as evidences in single pancreatic islets

It is often assumed, but has not been demonstrated, that the oscillations of cytoplasmic calcium (Ca2+i) that occur in various secretory cells induce oscillations in secretion. Here, we have used single pancreatic islets from normal mice to monitor simultaneously insulin release and Ca2+i in beta-cells for periods up to 25 min. Ca2+i and insulin secretion were found to oscillate in synchrony during stimulation by glucose. This synchrony persisted when the frequency of both events changed spontaneously or upon addition of the hypoglycaemic sulfonylurea tolbutamide, or when their relative amplitudes varied. Repolarizing the beta-cell membrane with diazoxide abolished both Ca2+i and insulin oscillations. In contrast, epinephrine suppressed insulin oscillations in spite of the persistence of Ca2+i oscillations. This study is the first direct demonstration that, in an intact organ, sustained oscillations of Ca2+i induced by a physiological stimulus entrain synchronous oscillations of the functional response, and that both events can vary simultaneously or be dissociated depending on the experimental conditions.

Mechanisms by which glucose can control insulin release independently from its action on adenosine triphosphate-sensitive K+ channels in mouse B cells

Glucose stimulation of insulin release involves closure of ATP-sensitive K+ channels (K(+)-ATP channels), depolarization, and Ca2+ influx in B cells. However, by using diazoxide to open K(+)-ATP channels, and 30 mM K to depolarize the membrane, we could demonstrate that another mechanism exists, by which glucose can control insulin release independently from changes in K(+)-ATP channel activity and in membrane potential (Gembal et al. 1992. J. Clin. Invest. 89:1288-1295). A similar approach was followed here to investigate, with mouse islets, the nature of this newly identified mechanism. The membrane potential-independent increase in insulin release produced by glucose required metabolism of the sugar and was mimicked by other metabolized secretagogues. It also required elevated levels of cytoplasmic Cai2+, but was not due to further changes in Cai2+. It could not be ascribed to acceleration of phosphoinositide metabolism, or to activation of protein kinases A or C. Thus, glucose did not increase inositol phosphate levels and hardly affected cAMP levels. Moreover, increasing inositol phosphates by vasopressin or cAMP by forskolin, and activating protein kinase C by phorbol esters did not mimic the action of glucose on release, and down-regulation of protein kinase C did not prevent these effects. On the other hand, it correlated with an increase in the ATP/ADP ratio in islet cells. We suggest that the membrane potential-independent control of insulin release exerted by glucose involves changes in the energy state of B cells.

Activation of muscarinic receptors increases the concentration of free Na+ in mouse pancreatic B-cells

The fluorescent probe SBFI was used to monitor the influence of acetylcholine (ACh) on the cytosolic concentration of free Na+ (Na+i) in single mouse pancreatic B-cells. In the presence of 3 mM glucose and 135 mM extracellular Na+, Na+i averaged 16.6 mM. ACh (100 microM) increased Na+i by approximately 80%. This rise was prevented by atropine, a blocker of muscarinic receptors, and by omission of extracellular Na+, but still occurred if the sodium pump was blocked by ouabain. It was unaffected by tetrodotoxin, a blocker of voltage-sensitive Na+ channels, and was not mimicked by depolarization of the cells with high K+. It is concluded that activation of muscarinic receptors increases the membrane permeability to Na+ in pancreatic B-cells.

Influence of membrane potential changes on cytoplasmic Ca2+ concentration in an electrically excitable cell, the insulin-secreting pancreatic B-cell

Glucose stimulation of insulin release involves metabolism of the sugar and elevation of cytoplasmic calcium (Ca2+i) in pancreatic B-cells. We compared the dynamic changes of metabolism (fluorescence of endogenous reduced pyridine nucleotides, NAD(P)H), membrane potential (intracellular microelectrodes), and Ca2+i (fura-2 technique), in intact mouse islets. Glucose (15 mM) sequentially triggered an increase in NAD(P)H fluorescence, a depolarization with electrical activity, and a rise in Ca2+i. The change in NAD(P)H was monophasic and regular, whereas the changes in membrane potential and Ca2+i were multiphasic, with steady-state regular oscillations of similar average frequencies (about 2.2/min). Digital image analysis revealed that Ca2+i oscillations were synchronous in all regions of the islets. Omission of extracellular Ca2+ abolished the rise in Ca2+i but not the increase in NAD(P)H. Both electrical and Ca2+i oscillations disappeared in low external Ca2+ (1 mM), and became larger but slower in high Ca2+ (10 mM). Sustained depolarization (by tolbutamide, arginine, or high K+) and hyperpolarization (by diazoxide) of B-cells caused sustained increases and decreases of Ca2+i, respectively. In conclusion, the changes in membrane potential induced by various secretagogues trigger synchronous changes in Ca2+i in all B-cells of the islets. The oscillatory pattern of the electrical and Ca2+i responses induced by glucose is not accompanied by and thus probably not due to similar oscillations of metabolism.

Evidence that glucose can control insulin release independently from its action on ATP-sensitive K+ channels in mouse B cells

Glucose stimulation of insulin release involves closure of ATP-sensitive K+ channels, depolarization, and Ca2+ influx in B cells. Mouse islets were used to investigate whether glucose can still regulate insulin release when it cannot control ATP-sensitive K+ channels. Opening of these channels by diazoxide (100-250 mumol/liter) blocked the effects of glucose on B cell membrane potential (intracellular microelectrodes), free cytosolic Ca2+ (fura-2 method), and insulin release, but it did not prevent those of high K (30 mmol/liter). K-induced insulin release in the presence of diazoxide was, however, dose dependently increased by glucose, which was already effective at concentrations (2-6 mmol/liter) that are subthreshold under normal conditions (low K and no diazoxide). This effect was not accompanied by detectable changes in B cell membrane potential. Measurements of 45Ca fluxes and cytosolic Ca2+ indicated that glucose slightly increased Ca2+ influx during the first minutes of depolarization by K, but not in the steady state when its effect on insulin release was the largest. In conclusion, there exists a mechanism by which glucose can control insulin release independently from changes in K(+)-ATP channel activity, in membrane potential, and in cytosolic Ca2+. This mechanism may serve to amplify the secretory response to the triggering signal (closure of K(+)-ATP channels--depolarization--Ca2+ influx) induced by glucose.

Vanadate stimulation of insulin release in normal mouse islets

The effects of vanadate (Na3VO4) on pancreatic B-cell function were studied in normal mouse islets. Vanadate did not affect basal insulin release but potentiated the effect of 7-30 mM glucose at concentrations of 0.1-1 mM. This effect was progressive and slowly reversible. It was abolished by omission of extracellular Ca2+ but unaffected by blockers of adrenergic or muscarinic receptors. Comparison of the changes in membrane potential, 86Rb efflux and 45Ca efflux that vanadate and ouabain produced in B-cells made it possible to exclude the hypothesis that vanadate increases insulin release by blocking the sodium pump. Vanadate was also without effect on cAMP levels. On the other hand, it markedly changed the characteristics of the Ca(2+)-dependent electrical activity and of the oscillations of cytoplasmic Ca2+ recorded in B-cells stimulated by 15 mM glucose. In the steady state, Ca2+ influx was increased by vanadate, and this resulted in a rise in cytoplasmic Ca2+. The exact mechanisms underlying these changes could not be established but a blockade of K channels was excluded. In the presence of LiCl, vanadate markedly increased inositol phosphate levels in islet cells. This effect was attenuated but not suppressed by omission of Ca2+. A small increase in inositol bisphosphate was still produced by vanadate in the absence of LiCl. These results suggest that vanadate both stimulates phosphoinositide breakdown and inhibits inositol phosphate degradation. In conclusion, vanadate does not induce insulin release, but markedly potentiates the stimulation by glucose. This property is not due to an inhibition of the sodium pump or to a rise in cAMP concentration. It results from a complex interplay between changes in B-cell membrane potential, phosphoinositide metabolism and Ca2+ handling.

The influence of gamma-aminobutyric acid on hormone release by the mouse and rat endocrine pancreas

The present study was aimed at localizing gamma-aminobutyric acid (GABA) and its enzyme of synthesis, glutamic acid decarboxylase (GAD), in the mouse pancreas by immunocytochemical methods. The influence of GABA on hormone release was also studied with normal mouse and rat islets and the isolated perfused rat pancreas. Particular attention was paid to glucagon release to test a recent hypothesis suggesting that GABA mediates the still unexplained glucose-induced inhibition of glucagon release. GABA and GAD were identified only in islet cells and never in the exocrine tissue. Exogenous GABA, baclofen (agonist of GABAB receptors), muscimol (agonist of GABAA receptors), or bicuculline (antagonist of GABAA receptors) did not affect insulin and somatostatin release by isolated mouse or rat islets. GABA was also without effect on glucose-induced electrical activity in mouse B-cells. Glucagon secretion by mouse islets was only slightly inhibited (approximately 20%) by GABA. Since muscimol had a similar effect, and baclofen was ineffective, the inhibition by GABA probably involves GABAA receptor activation. Bicuculline, however, did not antagonize the inhibitory effects of GABA and muscimol, probably because the antagonist alone also decreased glucagon secretion. In contrast to GABA, low (3 mM) and high (20 mM) concentrations of glucose strongly inhibited (approximately 50-65%) glucagon release; this inhibition was not prevented by bicuculline. Similar results were obtained with the perfused rat pancreas; muscimol slightly inhibited glucagon release under various conditions, and bicuculline did not reverse the strong inhibition produced by 16.7 mM glucose. In conclusion, GABA does not affect insulin and somatostatin secretion, but inhibits A-cells, probably by acting on GABAA receptors. It is unlikely, however, that this small inhibitory effect can account for the inhibition of glucagon release produced by glucose.

Localization of GAD-like immunoreactivity in the pancreas and stomach of the rat and mouse

The aim of this study was to localize cells immunoreactive for glutamate decarboxylase (GAD), the enzyme of GABA synthesis, in pyloric and oxyntic regions of the rat stomach as well as in the rat and mouse pancreas. GAD immunocytochemistry was carried out on polyethylene glycol or cryostat sections of alkaline paraformaldehyde fixed tissue, with simultaneous immunolabelling of various gastro-pancreatic hormones for topographical comparison. In the rat stomach, nerve fibers displaying intense GAD-like immunoreactivity were seen in the myenteric plexus, the circular muscular layer, the submucosa and the lamina propria of the mucosa. But, they were absent from the submucous plexus. Colchicine treatment of the rats allowed to detect some labelled perikarya in the myenteric plexus suggesting that the GABAergic innervation is at least partly intrinsic to the stomach. In the oxyntic and pyloric mucosa, endocrine cells appeared immunostained for GAD. However, the nature of their hormones remained unknown since double immunodetections revealed that they were immunoreactive neither for gastrin nor for somatostatin. In the rat and mouse pancreas, GAD-like immunoreactivity was found in islet cells which corresponded only to insulin-secreting cells. Somatostatin-, glucagon- and pancreatic polypeptide-immunopositive cells were devoid of GAD immunolabelling. No GAD-like immunoreactivity was detected in the exocrine tissue and innervation. These results strenghten the hypothesis that GABA is not only a neurotransmitter in the stomach but that it could also be an endocrine or paracrine factor in the stomach and pancreas.

Immunocytochemical and autoradiographic studies of the endocrine cells interacting with GABA in the rat stomach

There are now increasing evidences suggesting that GABA is able of direct interaction with certain endocrine cells. In the present study, highly specific anti-GABA-glutaraldehyde antibodies and 3H-GABA uptake were used at the light and electron microscope levels to investigate the occurrence of cells containing endogenous GABA or taking up exogenous GABA in the mucosal antrum and corpus of the rat stomach. Only certain endocrine cell types of both regions were immunostained or grain-labelled. However, the morphology of their secretory granules did not allow to identify the nature of their hormone with certainty but suggested that somatostatin-like cells could interact with GABA. The combination of gastrin and somatostatin immunodetection with 3H-GABA uptake autoradiography at the light microscope level, revealed that a subpopulation of somatostatin-like cells and other still unidentified endocrine cells are able to take up GABA, while the gastrin-like cells are not. These results reinforce the hypothesis that certain endocrine cell types of the diffuse endocrine system of the digestive tract are able to directly interact with GABA.

High-affinity GABA uptake in a subpopulation of somatostatin cells in rat pancreas

The aim of this study was to localize the high-affinity uptake of [3H]-GABA in Langerhans islets of rats aged 2.5, 7.5, and 75 days. On high-resolution autoradiography, cells presenting characteristic somatostatin granules were labeled, whereas others containing similar granules appeared nearly devoid of silver grains. Immunogold detection with antisomatostatin antibodies and high-resolution autoradiography suggested that uptake of GABA is indeed performed by somatostatin cells. To test the heterogeneity of uptake frequency in somatostatin cells, a second approach, coupling immunohistochemistry with anti-somatostatin, anti-PP, anti-glucagon, anti-glicentin, and anti-CCK antibodies, and low-resolution autoradiography, was applied on paraffin sections. It demonstrated that the uptake ability is not characteristic of all the somatostatin cells but of only a subpopulation of them. A few cells not immunoreactive to the anti-somatostatin antiserum also appeared to be able to take up GABA. Moreover, except for a rare few, the PP-glucagon-, glicentin-, and CCK-39-immunoreactive cells were not labeled by autoradiography.

Localization of high-affinity GABA uptake and GABA content in the rat duodenum during development

The localization of high-affinity uptake sites for 3H gamma-aminobutyric acid (3H-GABA) was investigated in the rat duodenum during ontogenesis and also at the adult stage (from 15.5 days of fetal life up to 105 days post natum) by means of low- and high-resolution autoradiography. At all stages studied, specific endocrine cell types of the epithelium were labelled and an intense uptake was detected in the nervous tissue, especially in glial cells but also in scarce neurones. When the incubation medium was supplemented with beta-alanine (1 mM), a blocker of the glial uptake for GABA, the labelling persisted only in endocrine cells and in few neurones. The intensity and the frequency of the labelling decreased at later periods compared to the earlier developmental stages. The GABA content of the duodenum as measured by a new ion-exchange column chromatography-HPLC-coupled method was higher in the early postnatal period compared to later stages. These observations suggest that GABA, in addition to being a neurotransmitter, may play an important role during development of the duodenum.