RFX6 haploinsufficiency predisposes to diabetes through impaired beta cell function

AIMS/HYPOTHESIS

Regulatory factor X 6 (RFX6) is crucial for pancreatic endocrine development and differentiation. The RFX6 variant p.His293LeufsTer7 is significantly enriched in the Finnish population, with almost 1:250 individuals as a carrier. Importantly, the FinnGen study indicates a high predisposition for heterozygous carriers to develop type 2 and gestational diabetes. However, the precise mechanism of this predisposition remains unknown.

METHODS

To understand the role of this variant in beta cell development and function, we used CRISPR technology to generate allelic series of pluripotent stem cells. We created two isogenic stem cell models: a human embryonic stem cell model; and a patient-derived stem cell model. Both were differentiated into pancreatic islet lineages (stem-cell-derived islets, SC-islets), followed by implantation in immunocompromised NOD-SCID-Gamma mice.

RESULTS

Stem cell models of the homozygous variant RFX6-/- predictably failed to generate insulin-secreting pancreatic beta cells, mirroring the phenotype observed in Mitchell-Riley syndrome. Notably, at the pancreatic endocrine stage, there was an upregulation of precursor markers NEUROG3 and SOX9, accompanied by increased apoptosis. Intriguingly, heterozygous RFX6+/- SC-islets exhibited RFX6 haploinsufficiency (54.2% reduction in protein expression), associated with reduced beta cell maturation markers, altered calcium signalling and impaired insulin secretion (62% and 54% reduction in basal and high glucose conditions, respectively). However, RFX6 haploinsufficiency did not have an impact on beta cell number or insulin content. The reduced insulin secretion persisted after in vivo implantation in mice, aligning with the increased risk of variant carriers to develop diabetes.

CONCLUSIONS/INTERPRETATION

Our allelic series isogenic SC-islet models represent a powerful tool to elucidate specific aetiologies of diabetes in humans, enabling the sensitive detection of aberrations in both beta cell development and function. We highlight the critical role of RFX6 in augmenting and maintaining the pancreatic progenitor pool, with an endocrine roadblock and increased cell death upon its loss. We demonstrate that RFX6 haploinsufficiency does not affect beta cell number or insulin content but does impair function, predisposing heterozygous carriers of loss-of-function variants to diabetes.

DATA AVAILABILITY

Ultra-deep bulk RNA-seq data for pancreatic differentiation stages 3, 5 and 7 of H1 RFX6 genotypes are deposited in the Gene Expression Omnibus database with accession code GSE234289. Original western blot images are deposited at Mendeley ( https://data.mendeley.com/datasets/g75drr3mgw/2 ).

Functional, metabolic and transcriptional maturation of human pancreatic islets derived from stem cells

Transplantation of pancreatic islet cells derived from human pluripotent stem cells is a promising treatment for diabetes. Despite progress in the generation of stem-cell-derived islets (SC-islets), no detailed characterization of their functional properties has been conducted. Here, we generated functionally mature SC-islets using an optimized protocol and benchmarked them comprehensively against primary adult islets. Biphasic glucose-stimulated insulin secretion developed during in vitro maturation, associated with cytoarchitectural reorganization and the increasing presence of alpha cells. Electrophysiology, signaling and exocytosis of SC-islets were similar to those of adult islets. Glucose-responsive insulin secretion was achieved despite differences in glycolytic and mitochondrial glucose metabolism. Single-cell transcriptomics of SC-islets in vitro and throughout 6 months of engraftment in mice revealed a continuous maturation trajectory culminating in a transcriptional landscape closely resembling that of primary islets. Our thorough evaluation of SC-islet maturation highlights their advanced degree of functionality and supports their use in further efforts to understand and combat diabetes.

Pancreatic islets derived from stem cells are benchmarked against primary cells.

Paracrine control of α-cell glucagon exocytosis is compromised in human type-2 diabetes

Glucagon is released from pancreatic α-cells to activate pathways that raise blood glucose. Its secretion is regulated by α-cell-intrinsic glucose sensing and paracrine control through insulin and somatostatin. To understand the inadequately high glucagon levels that contribute to hyperglycemia in type-2 diabetes (T2D), we analyzed granule behavior, exocytosis and membrane excitability in α-cells of 68 non-diabetic and 21 T2D human donors. We report that exocytosis is moderately reduced in α-cells of T2D donors, without changes in voltage-dependent ion currents or granule trafficking. Dispersed α-cells have a non-physiological V-shaped dose response to glucose, with maximal exocytosis at hyperglycemia. Within intact islets, hyperglycemia instead inhibits α-cell exocytosis, but not in T2D or when paracrine inhibition by insulin or somatostatin is blocked. Surface expression of somatostatin-receptor-2 is reduced in T2D, suggesting a mechanism for the observed somatostatin resistance. Thus, elevated glucagon in human T2D may reflect α-cell insensitivity to paracrine inhibition at hyperglycemia.

Glucagon is elevated Type-2 diabetes, which contributes to poor glucose control in patients with the disease. Here the authors report that secretion of the hormone is controlled by paracrine inhibition, and that resistance of α-cells to somatostatin can explain hyperglucagonemia in type-2 diabetes.

1421Dyskalemias in hypertensive patients treated with combination of antihypertensive drugs: a nested case-control study using Danish national registries

Aims

Little is known about the occurrence of potassium disturbances in relation to combination therapy in hypertension. Using data from Danish electronic registries, we investigated the association between different combinations of antihypertensive therapy and potassium imbalances, in 22,060 individuals, between 1995–2012.

Methods

Using incidence density matching, two comparison patients without hypokalemia were matched to each corresponding patient with hypokalemia on age, gender, renal function, time from HTN date to date of potassium measurement. The same approach was applied to identify matches for patients with hyperkalemia. The ten most common antihypertensive drug combinations in our population were: (1) Beta-blockers + Angiotensin converting enzyme inhibitors, (2) Angiotensin converting enzyme inhibitors + Thiazides, (3) Angiotensin converting enzyme inhibitors + Thiazides + Potassium supplement, (4) Angiotensin receptor blockers + Other diuretics, (5) Beta-blockers + Angiotensin converting enzyme inhibitors + Potassium supplement (ATC: A12B), (6) Beta-blockers + Calcium channel blockers, (7) Beta-blockers + Thiazides + Potassium supplement, (8) Calcium channel blockers + Angiotensin converting enzyme inhibitors, (9) Calcium channel blockers + Thiazides + Potassium supplement, (10) Other antihypertensive drug combinations. We used conditional logistic regression analysis to examine the risk of developing hypo- and hyperkalemia in relation to different combinations of antihypertensive drugs within one year. The multivariable model was adjusted for serum sodium, malignancy, inflammatory bowel disease, diabetes, alcoholism and beta2-agonists.

Results

The multivariable analysis showed 10.5 times increased odds for developing hypokalemia if administered Calcium channel blockers + Thiazides + Potassium supplement (95% CI 4.97–22.06) compared to Angiotensin converting enzyme inhibitors + Beta blockers. Other drug combinations significantly associated with increased hypokalemia risk were: Angiotensin converting enzyme inhibitors + Thiazides (OR 5.01, 95% CI 2.32–10.79), Angiotensin converting enzyme inhibitors + Loop + Potassium supplement (A12B) (OR 4.03, 95% CI 1.69–9.62), Angiotensin converting enzyme inhibitors + Thiazides + Potassium supplement (OR 4.16, 95% CI 2.01–8.64) and Calcium channel blockers + Angiotensin converting enzyme inhibitors (OR 4.04, 95% CI 1.72–9.50). None of the ten groups were associated with increased odds for developing hyperkalemia in the multivariable analysis.

Cumulative incidence curves for hypokale

Conclusion

Thiazide diuretics in combination with angiotensin converting enzyme inhibitors or calcium channel blockers were strongly associated with hypokalemia risk within one year from treatment initiation.

Acknowledgement/Funding

None

Biaryl sulfonamide motifs up- or down-regulate ion channel activity by activating voltage sensors

Voltage-gated ion channels are key molecules for the generation of cellular electrical excitability. Many pharmaceutical drugs target these channels by blocking their ion-conducting pore, but in many cases, channel-opening compounds would be more beneficial. Here, to search for new channel-opening compounds, we screen 18,000 compounds with high-throughput patch-clamp technology and find several potassium-channel openers that share a distinct biaryl-sulfonamide motif. Our data suggest that the negatively charged variants of these compounds bind to the top of the voltage-sensor domain, between transmembrane segments 3 and 4, to open the channel. Although we show here that biaryl-sulfonamide compounds open a potassium channel, they have also been reported to block sodium and calcium channels. However, because they inactivate voltage-gated sodium channels by promoting activation of one voltage sensor, we suggest that, despite different effects on the channel gates, the biaryl-sulfonamide motif is a general ion-channel activator motif. Because these compounds block action potential-generating sodium and calcium channels and open an action potential-dampening potassium channel, they should have a high propensity to reduce excitability. This opens up the possibility to build new excitability-reducing pharmaceutical drugs from the biaryl-sulfonamide scaffold.

Ca2+ channel clustering with insulin-containing granules is disturbed in type 2 diabetes

Loss of first-phase insulin secretion is an early sign of developing type 2 diabetes (T2D). Ca²⁺ entry through voltage-gated L-type Ca²⁺ channels triggers exocytosis of insulin-containing granules in pancreatic β cells and is required for the postprandial spike in insulin secretion. Using high-resolution microscopy, we have identified a subset of docked insulin granules in human β cells and rat-derived clonal insulin 1 (INS1) cells for which localized Ca²⁺ influx triggers exocytosis with high probability and minimal latency. This immediately releasable pool (IRP) of granules, identified both structurally and functionally, was absent in β cells from human T2D donors and in INS1 cells cultured in fatty acids that mimic the diabetic state. Upon arrival at the plasma membrane, IRP granules slowly associated with 15 to 20 L-type channels. We determined that recruitment depended on a direct interaction with the synaptic protein Munc13, because expression of the II–III loop of the channel, the C2 domain of Munc13-1, or of Munc13-1 with a mutated C2 domain all disrupted L-type channel clustering at granules and ablated fast exocytosis. Thus, rapid insulin secretion requires Munc13-mediated recruitment of L-type Ca²⁺ channels in close proximity to insulin granules. Loss of this organization underlies disturbed insulin secretion kinetics in T2D.

Resin-acid derivatives as potent electrostatic openers of voltage-gated K channels and suppressors of neuronal excitability

Voltage-gated ion channels generate cellular excitability, cause diseases when mutated, and act as drug targets in hyperexcitability diseases, such as epilepsy, cardiac arrhythmia and pain. Unfortunately, many patients do not satisfactorily respond to the present-day drugs. We found that the naturally occurring resin acid dehydroabietic acid (DHAA) is a potent opener of a voltage-gated K channel and thereby a potential suppressor of cellular excitability. DHAA acts via a non-traditional mechanism, by electrostatically activating the voltage-sensor domain, rather than directly targeting the ion-conducting pore domain. By systematic iterative modifications of DHAA we synthesized 71 derivatives and found 32 compounds more potent than DHAA. The most potent compound, Compound 77, is 240 times more efficient than DHAA in opening a K channel. This and other potent compounds reduced excitability in dorsal root ganglion neurons, suggesting that resin-acid derivatives can become the first members of a new family of drugs with the potential for treatment of hyperexcitability diseases.

Identification of novel NaV1.7 antagonists using high throughput screening platforms

Congenital Insensitivity to Pain (CIP) is a loss of function mutation resulting in a truncated NaV1.7 protein, suggesting a pivotal role in pain signaling and rendering it an important pharmaceutical target for multiple pain conditions. The structural homology in the NaV-channel family makes it challenging to design effective analgesic compounds without inducing for example cardiotoxicity or seizure liabilities. An additional approach to structural isoform selectivity is to identify compounds with use- or state-dependent profiles, i.e. inhibition efficacy based on the gating of the ion channel. In general nerve cells in damaged or inflamed tissue are more depolarized and electrically active compared to healthy nerve cells in for instance the heart. This observation has led to the design of two types of screening protocols emulating the voltage condition of peripheral neurons or cardiac tissue. The two voltage protocols have been developed to identify both use- and state-dependent antagonists. In this paper we describe an attempt to merge the two different protocols into one to increase screening efficacy, while retaining relevant state- and use-dependent pharmacology. The new protocol is constructed of two stimulation pulses and a slow voltage ramp for simultaneous assessment of resting and state-dependent block. By comparing all protocols we show that the new protocol indeed filter compounds for state-dependence and increase the prediction power of selecting use-dependent compounds.

Potency optimization of Huwentoxin-IV on hNav1.7: a neurotoxin TTX-S sodium-channel antagonist from the venom of the Chinese bird-eating spider Selenocosmia huwena

The spider venom peptide Huwentoxin-IV (HwTx-IV) 1 is a potent antagonist of hNav1.7 (IC50 determined herein as 17 ± 2 nM). Nav1.7 is a voltage-gated sodium channel involved in the generation and conduction of neuropathic and nociceptive pain signals. We prepared a number of HwTx-IV analogs as part of a structure-function study into Nav1.7 antagonism. The inhibitory potency of these analogs was determined by automated electrophysiology and is reported herein. In particular, the native residues Glu(1), Glu(4), Phe(6) and Tyr(33) were revealed as important activity modulators and several peptides bearing mutations in these positions showed significantly increased potency on hNav1.7 while maintaining the original selectivity profile of the wild-type peptide 1 on hNav1.5. Peptide 47 (Gly(1), Gly(4), Trp(33)-HwTx) demonstrated the largest potency increase on hNav1.7 (IC50 0.4 ± 0.1 nM).

Phenyl isoxazole voltage-gated sodium channel blockers: structure and activity relationship

Blocking of certain sodium channels is considered to be an attractive mechanism to treat chronic pain conditions. Phenyl isoxazole carbamate 1 was identified as a potent and selective Na(V)1.7 blocker. Structural analogues of 1, both carbamates, ureas and amides, were proven to be useful in establishing the structure-activity relationship and improving ADME related properties. Amide 24 showed a good overall in vitro profile, that translated well to rat in vivo PK.

Ion channel screening technology

Ion channels are at present the third biggest target class in drug discovery. Primary research is continually uncovering potential new ion channel targets in indications such as cancer, diabetes and respiratory diseases, as well as the more established fields of pain, cardiovascular disease, and neurological disorders. Despite the physiological significance and therapeutic relevance in a wide variety of biological systems, ion channels still remain under exploited as drug targets. This is to a large extent resulting from the historical lack of screening technologies to provide the throughput and quality of data required to support medicinal chemistry. Although technical challenges still lie ahead, this historic bottleneck in ion channel drug discovery is now being overcome by novel technologies that can be integrated into lead generation stages of ion channel drug discovery to allow the development of novel therapeutic agents. This review describes the variety of technologies available for ion channel screening and discusses the opportunities these technologies provide. The challenges that remain to be addressed are highlighted.

Cellular HTS assays for pharmacological characterization of Na(V)1.7 modulators

Ion channels are challenging targets in the early phases of the drug discovery process, especially because of the lack of technologies available to screen large numbers of compounds in functionally relevant assays. The electrophysiological patch-clamp technique, which is the gold standard for studying ion channels, has low throughput and is not amenable to screening large numbers of compounds. However, for random high-throughput screening (HTS) of compounds against ion channel targets, a number of functional cellular assays have become available during the last few years. Here we use the sodium channel NaV1.7 stably expressed in human embryonic kidney 293 cells and compare three HTS assays-a Li flux atomic absorption spectroscopy (AAS) assay, a fluorescent imaging plate reader (FLIP, Molecular Devices, Sunnyvale, CA) membrane potential assay, and a fluorescence resonance energy transfer (FRET)-based membrane potential assay-to an automated electrophysiological assay (the Ionworks HT [Molecular Devices] platform) and characterize 11 known NaV inhibitors. Our results show that all three HTS assays are suitable for identification of NaV1.7 inhibitors, but as an HTS assay the Li-AAS assay is more robust with higher Z' values than the FLIPR and FRET-based membrane potential assays. Furthermore, there was a better correlation between the Ionworks assay and the Li-AAS assay regarding the potency of the NaV inhibitors investigated. This paper describes the first comparison between all the HTS assays available today to study voltage-gated NaVs, and the results suggest that the Li-AAS assay is more suited as a first HTS assay when starting an NaV drug discovery campaign.

A stop codon mutation in SCN9A causes lack of pain sensation

The general lack of pain experience is a rare occurrence in humans, and the molecular causes for this phenotype are not well understood. Here we have studied a Canadian family from Newfoundland with members who exhibit a congenital inability to experience pain. We have mapped the locus to a 13.7 Mb region on chromosome 2q (2q24.3-2q31.1). Screening of candidate genes in this region identified a protein-truncating mutation in SCN9A, which encodes for the voltage-gated sodium channel Na(v)1.7. The mutation is a C-A transversion at nucleotide 984 transforming the codon for tyrosine 328 to a stop codon. The predicted product lacks all pore-forming regions of Na(v)1.7. Indeed, expression of this altered gene in a cell line did not produce functional responses, nor did it cause compensatory effects on endogenous voltage-gated sodium currents when expressed in ND7/23 cells. Because a homozygous knockout of Na(v)1.7 in mice has been shown to be lethal, we explored why a deficiency of Na(v)1.7 is non-lethal in humans. Expression studies in monkey, human, mouse and rat tissue indicated species-differences in the Na(v)1.7 expression profile. Whereas in rodents the channel was strongly expressed in hypothalamic nuclei, only weak mRNA levels were detected in this area in primates. Furthermore, primate pituitary and adrenal glands were devoid of signal, whereas these two glands were mRNA-positive in rodents. This species difference may explain the non-lethality of the observed mutation in humans. Our data further establish Na(v)1.7 as a critical element of peripheral nociception in humans.

Ryanodine receptor‐operated activation of TRP‐like channels can trigger critical Ca2+signaling events in pancreatic β‐cells

There is little information available concerning the link between the ryanodine (RY) receptors and the downstream Ca(2+) signaling events in beta-cells. In fura-2 loaded INS-1E cells, activation of RY receptors by 9-methyl 5,7-dibromoeudistomin D (MBED) caused a rapid rise of [Ca(2+)]i followed by a plateau and repetitive [Ca(2+)]i spikes on the plateau. The [Ca(2+)]i plateau was abolished by omission of extracellular Ca(2+) and by SKF 96365. In the presence of SKF 96365, MBED produced a transient increase of [Ca(2+)]i, which was abolished by thapsigargin. Activation of RY receptors caused Ca(2+) entry even when the ER Ca(2+) pool was depleted by thapsigargin. The [Ca(2+)]i plateau was not inhibited by nimodipine or ruthenium red, but was inhibited by membrane depolarization, La(3+), Gd(3+), niflumic acid, and 2-aminoethoxydiphenyl borate, agents that inhibit the transient receptor potential channels. The [Ca(2+)]i spikes were inhibited by nimodipine and ryanodine, indicating that they were due to Ca(2+) influx through the voltage-gated Ca(2+) channels and Ca(2+)-induced Ca(2+) release (CICR). Activation of RY receptors depolarized membrane potential as measured by patch clamp. Thus, activation of RY receptors leads to coherent changes in Ca(2+) signaling, which includes activation of TRP-like channels, membrane depolarization, activation of the voltage-gated Ca(2+) channels and CICR.

Intracellular calcium dependence of large dense-core vesicle exocytosis in the absence of synaptotagmin I

Synaptotagmin I is a synaptic vesicle-associated protein essential for synchronous neurotransmission. We investigated its impact on the intracellular Ca(2+)-dependence of large dense-core vesicle (LDCV) exocytosis by combining Ca(2+)-uncaging and membrane capacitance measurements in adrenal slices from mouse synaptotagmin I null mutants. Synaptotagmin I-deficient chromaffin cells displayed prolonged exocytic delays and slow, yet Ca(2+)-dependent fusion rates, resulting in strongly reduced LDCV release in response to short depolarizations. Vesicle recruitment, the shape of individual amperometric events, and endocytosis appeared unaffected. These findings demonstrate that synaptotagmin I is required for rapid, highly Ca(2+)-sensitive LDCV exocytosis and indicate that it regulates the equilibrium between a slowly releasable and a readily releasable state of the fusion machinery. Alternatively, synaptotagmin I could function as calcium sensor for the readily releasable pool, leading to the destabilization of the pool in its absence.

2-aminoethoxydiphenyl borate reveals heterogeneity in receptor-activated Ca(2+) discharge and store-operated Ca(2+) influx

We have investigated Ca(2+) release and receptor- and store-operated Ca(2+) influxes in Chinese hamster ovary-K1 (CHO) cells, SH-SY5Y human neuroblastoma cells and RBL-1 rat basophilic leukemia cells using Fura-2 and patch-clamp measurements. Ca(2+) release and subsequent Ni(2+)-sensitive, store-operated influx were induced by thapsigargin and stimulation of G protein-coupled receptors. The alleged noncompetitive IP3 receptor inhibitor,2-aminoethoxydiphenyl borate (2-APB) rapidly blocked a major part of the secondary influx response in CHO cells in a reversible manner. It also reduced Mn(2+) influx in response to thapsigargin. Inhibition of Ca(2+) release was also seen but this was less complete, slower in onset, less reversible, and required higher concentration of 2-APB. In RBL-1 cells, I(CRAC) activity was rapidly blocked by extracellular 2-APB whereas intracellular 2-APB was less effective. Store-operated Ca(2+) influxes were only partially blocked by 2-APB. In SH-SY5Y cells, Ca(2+) influxes were insensitive to 2-APB. Ca(2+) release in RBL-1 cells was partially sensitive but in SH-SY5Y cells the release was totally resistant to 2-APB. The results suggest, that 2-APB (1) may inhibit distinct subtypes of IP3 receptors with different sensitivity, and (2) that independently of this, it also inhibits some store-operated Ca(2+) channels via a direct, extracellular action.

Reconstitution of neurotransmission by determining communication between differentiated PC12 pheochromocytoma and HEL 92.1.7 erythroleukemia cells

Neurotransmitter release was monitored using fura-2-loaded HEL 92.1.7 cells dispersed among differentiated PC12 cells (loaded with another Ca2+ indicator fluo-3) and immobilised using transparent polycarbonate membrane filters with uniform pore size. Depolarisation with K+ caused a rapid rise in Ca2+ concentration in the PC12 cells, followed by a delayed secondary Ca2+ response in simultaneously monitored nearby HEL cells. There was a lag period of about 20 s between the responses of the two cell types. Voltage-gated Ca2+ channels in PC12 cells were inhibited by the P/Q-type (omega-conotoxin MVIIC, omega-agatoxin IVA), N-type (omega-conotoxin GVIA) and L-type channel blockers (nifedipine) as determined using fura-2 or whole-cell patch-clamp recordings. The communication between the cell types on the other hand was sensitive to P/Q- and N-type but not to L-type channel blockers. This suggests that, as in neurons, P/Q- and N-type Ca2+ channels mediate the release of neurotransmitters acting on HEL cells. Theoretically, the procedure employed should be sensitive enough to detect single exocytotic events. Our results demonstrate that a random distribution between effector and target cells is sufficient to allow communication between cells in a manner similar to extrasynaptic transmission.

The orexin OX1 receptor activates a novel Ca2+ influx pathway necessary for coupling to phospholipase C

Ca(2+) elevations in Chinese hamster ovary cells stably expressing OX(1) receptors were measured using fluorescent Ca(2+) indicators fura-2 and fluo-3. Stimulation with orexin-A led to pronounced Ca(2+) elevations with an EC(50) around 1 nm. When the extracellular [Ca(2+)] was reduced to a submicromolar concentration, the EC(50) was increased 100-fold. Similarly, the inositol 1,4,5-trisphosphate production in the presence of 1 mm external Ca(2+) was about 2 orders of magnitude more sensitive to orexin-A stimulation than in low extracellular Ca(2+). The shift in the potency was not caused by depletion of intracellular Ca(2+) but by a requirement of extracellular Ca(2+) for production of inositol 1,4,5-trisphosphate. Fura-2 experiments with the "Mn(2+)-quench technique" indicated a direct activation of a cation influx pathway by OX(1) receptor independent of Ca(2+) release or pool depletion. Furthermore, depolarization of the cells to +60 mV, which almost nullifies the driving force for Ca(2+) entry, abolished the Ca(2+) response to low concentrations of orexin-A. The results thus suggest that OX(1) receptor activation leads to two responses, (i) a Ca(2+) influx and (ii) a direct stimulation of phospholipase C, and that these two responses converge at the level of phospholipase C where the former markedly enhances the potency of the latter.

Oscillations in oxygen tension and insulin release of individual pancreatic ob/ob mouse islets

AIMS/HYPOTHESIS

The role of beta-cell metabolism for generation of oscillatory insulin release was investigated by simultaneous measurements of oxygen tension (pO2) and insulin release from individual islets of Langerhans.

METHODS

Individual islets isolated from the ob/ob-mice were perifused. Insulin in the perifusate was measured with a sensitive ELISA and PO2 with a modified Clark-type electrode inserted into the islets.

RESULTS

In the presence of 3 mmol/l D-glucose, PO2 was 102 +/- 9 mmHg and oscillatory (0.26 +/- 0.04 oscillations/min). Corresponding insulin measurements showed oscillatory release with similar periodicity (0.25 +/- 0.02 oscillations/min). When the D-glucose concentration was increased to 11 mmol/l, PO2 decreased by 30% to 72 +/- 10 mmHg with maintained frequency of the oscillations. Corresponding insulin secretory rate rose from 5 +/- 2 to 131 +/- 16 pmol x g(-1) x s(-1) leaving the frequency of the insulin pulses unaffected. The magnitude of glucose-induced change in pO2 varied between islets but was positively correlated to the amount of insulin released (r2 = 0.85). When 1 mmol/l tolbutamide was added to the perifusion medium containing 11 mmol/l glucose no change in average oscillatory pO2 was observed despite a doubling in the secretory rate. When 8 mmol/l 3-oxymethyl glucose was added to perifusion medium containing 3 mmol/l D-glucose, neither pO2 nor insulin release of the islets were changed. Temporal analysis of oscillations in pO2 and insulin release revealed that maximum respiration correlated to maximum or close to maximum insulin release.

CONCLUSION/INTERPRETATION

The temporal relation between oscillations in pO2 and insulin release supports a role for metabolic oscillations in the generation of pulsatile insulin release.

Comparison of the activation of the Ca2+ release-activated Ca2+ current ICRAC to InsP3 in Jurkat T-lymphocytes, pulmonary artery endothelia and RBL-1 cells

In many electrically non-excitable cells, Ca2+ entry is mediated predominantly by the store-operated Ca2+ influx pathway. The best-characterised store-operated Ca2+ current is the Ca2+ release-activated Ca2+ current (ICRAC). It is generally believed that high concentrations of intracellular Ca2+ buffer are required to measure ICRAC, due to Ca2+-dependent inactivation of the channels. Recently, we have recorded robust ICRAC in rat basophilic leukaemia (RBL-1) cells at physiological levels of Ca2+ buffering when stores were depleted by inhibition of the sarcoplasmic/ endoplasmic reticulum Ca2+-activated adenosine triphosphatase (SERCA) pumps. However, the second messenger inositol 1,4,5-trisphosphate (InsP3) was not able to evoke the current under such conditions, despite inducing substantial Ca2+ release. We have therefore suggested that a threshold exists within the Ca2+ stores which has to be overcome for macroscopic ICRAC to activate. To establish whether this is a specific feature of ICRAC in RBL-1 cells or whether it is a more general phenomenon, we investigated whether a threshold is also seen in other cell-types used to study store-operated Ca2+ entry. In Jurkat-T lymphocytes, ICRAC is activated weakly by InsP3 in the presence of low concentrations of Ca2+ buffer, whereas the current is large when SERCA pumps are blocked simultaneously, as in RBL-1 cells. Although the electrophysiological properties of ICRAC in the Jurkat cell are very similar to those of RBL-1 cells, the Na+ conductance in the absence of external divalent cations is quite different. Unexpectedly, we failed consistently to record any store-operated Ca2+ current in macrovascular pulmonary artery endothelia whereas robust ICRAC was seen under the same conditions in RBL-1 cells. Our results show that ICRAC has a similar profile of activation in the presence of physiological levels of Ca2+ buffering for Jurkat T-lymphocytes and RBL-1 cells, indicating that the threshold mechanism may be a general feature of ICRAC activation. Because ICRAC in pulmonary artery endothelia is, at best, very small, additional Ca2+ influx pathways may also contribute to agonist-induced Ca2+ entry.

Effects of acute and chronic angiotensin receptor blockade on myocardial vascular blood volume and perfusion in a pig model of coronary microembolization

Based on the reduction of ischemic cardiac events in clinical trials and experimental observations, inhibition of the effects of angiotensin II on coronary microcirculatory function may afford myocardial protection after injury. The immediate effects of intracoronary AT1 receptor blockade with irbesartan were examined in a pig model in the healthy myocardium and in acute ischemia induced by injection of 30-microm microspheres into the left anterior descending coronary artery (LAD). Electron-beam computed tomography was performed for in-vivo quantitative measurements of regional intramyocardial vascular blood volume (V(B)) and perfusion (F(M)), as well as left ventricular ejection fraction (LVEF) and muscle mass. Ratios of V(B) and F(M) in the anterior (LAD-supplied)/ inferior (control) myocardium were generated. At baseline, 0.2 mg/kg irbesartan injected into the LAD increased V(B) and F(M) ratios significantly by 27 +/- 8% and 51 +/- 13%, respectively. After anterior coronary microembolization, V(B) and F(M) ratios were 0.60 +/- 0.05 and 0.51 +/- 0.05, respectively, and were significantly increased by irbesartan (by 24 +/- 10% and by 36 +/- 11%, respectively). After 4 weeks of treatment with oral irbesartan (n = 7) or placebo (n = 7), an improved LVEF (56 +/- 4% v 44 +/- 4%, P = .046) was observed in irbesartan-treated animals, but no difference in LV end-diastolic volumes or muscle mass. Resting V(B) (0.95 +/- 0.06 v 0.76 +/- 0.06; P = .047) and F(M) (0.84 +/- 0.05 v 0.64 +/- 0.04; P = .016) ratios were significantly greater in irbesartan-treated animals. Using adenosine, there was a trend for higher V(B) and F(M) ratios in irbesartan- v placebo-treated animals. Therefore, in a pig model of acute myocardial ischemia, AT1 receptor blockade by irbesartan induced microvascular vasodilation and, ostensibly, conveyed myocardial protection. Long-term treatment with irbesartan resulted in moderate enhancements of resting V(B) and F(M) compared with placebo, suggesting a role for coronary microcirculatory effects of chronic AT1 receptor blockade in preserving LVEF.

Endogenous extracellular purine nucleotides redirect alpha2-adrenoceptor signaling

Many receptors coupled to inhibitory Go/Gi-type G proteins often also produce stimulatory signals like Ca2+ mobilisation. When expressed in CHO cells the alpha2-adrenoceptor subtypes alpha2A, alpha2B and alpkha2C mobilised Ca2+. These responses were strongly reduced by the P2Y-purinoceptor antagonist suramin. A large proportion of the total pool of purine nucleotides was found extracellularly. Removal of extracellular nucleotides with apyrase or by constant perfusion had a similar effect as suramin. These treatments did not affect the alpha2-adrenoceptor-mediated inhibition of cAMP production. This indicates that cells may be primed or their signaling pathways redirected towards Ca2+ mobilisation by 'autocrine' release of nucleotides.

Characterization of recombinant human P2X4 receptor reveals pharmacological differences to the rat homologue

We isolated a cDNA from human brain encoding a purinergic receptor that shows a high degree of homology to the rat P2X4 receptor (87% identity). By fluorescence in situ hybridization, the human P2X4 gene has been mapped to region q24.32 of chromosome 12. Tissue distribution analysis of human P2X4 transcripts demonstrates a broad expression pattern in that the mRNA was detected not only in brain but also in all tissues tested. Heterologous expression of the human P2X4 receptor in Xenopus laevis oocytes and human embryonic kidney 293 cells evoked an ATP-activated channel. Simultaneous whole-cell current and Fura-2 fluorescence measurements in human embronic kidney 293 cells transfected with human P2X4 cDNA allowed us to determine the fraction of the current carried by Ca2: this was approximately 8%, demonstrating a high Ca2+ permeability. Low extracellular Zn2+ concentrations (5-10 microM) increase the apparent gating efficiency of human P2X4 by ATP without affecting the maximal response. However, raising the concentration of the divalent cation (> 100 microM) inhibits the ATP-evoked current in a non-voltage-dependent manner. The human P2X4 receptor displays a very similar agonist potency profile to that of rat P2X4 (ATP > > 2-methylthio-ATP > or = CTP > alpha, beta-methylene-ATP > dATP) but has a notably higher sensitivity for the antagonists suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid, and bromphenol blue. Chimeric constructs between human and rat isoforms as well as single-point mutations were engineered to map the regions responsible for the different sensitivity to suramin and pyridoxal-phosphate-6-azophenyl-2'4'-disulfonic acid.

Coincidence of early glucose-induced depolarization with lowering of cytoplasmic Ca2+ in mouse pancreatic beta-cells

1. The temporal relationship between the early glucose-induced changes of membrane potential and cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in insulin-releasing pancreatic beta-cells. 2. The mean resting membrane potential and [Ca2+]i were about -70 mV and 60 nM, respectively, in 3 mM glucose. 3. Elevating the glucose concentration to 8-23 mM typically elicited a slow depolarization, which was paralleled by a lowering of [Ca2+]i. When the slow depolarization had reached a threshold of -55 to -40 mV, there was rapid further depolarization to a plateau with superimposed action potentials, and [Ca2+]i increased dramatically. 4. Imposing hyperpolarizations and depolarizations of 10 mV from a holding potential of -70 mV had no detectable effect on [Ca2+]i. Furthermore, glucose elevation elicited a decrease in [Ca2+]i even at a holding potential of -70 mV. 5. Step depolarizations induced [Ca2+]i transients, which decayed with time courses well fitted by double exponentials. The slower component became faster by a factor of about 4 upon elevation of glucose, suggesting involvement of ATP-dependent Ca2+ sequestration or extrusion of [Ca2+]i. 6. Glucose stimulation increased the size and accelerated the recovery of carbachol-triggered [Ca2+]i transients, and thapsigargin, an intracellular Ca(2+)-ATPase inhibitor, counteracted the glucose-induced lowering of [Ca2+]i, indicating that calcium transport into intracellular stores is involved in glucose-induced lowering of [Ca2+]i. 7. The results support the notion that in beta-cells, nutrient-induced elevation of ATP leads initially to ATP-dependent removal of Ca2+ from the cytoplasm, paralleled by a slow depolarization due to inhibition of ATP-sensitive K+ channels. Only after depolarization has reached a threshold do action potentials occur, inducing a sharp elevation in [Ca2+]i.

Multiple stressor debriefing and the American Red Cross: the East Bay Hills fire experience

Workers who are mobilized to help with disaster relief are exposed to a multitude of stressors. Debriefing may prevent or minimize the negative consequences of stress reactions in relief personnel. The multiple stressor debriefing (MSD) intervention promotes the discussion of troubling aspects of the disaster work in a group format. This article discusses two models of debriefing and describes how the MSD model was used with American Red Cross personnel during the East Bay Hills firestorm in California. Specific recommendations are made for debriefing in large-scale, long-term disaster relief efforts.

Local changes of medium in studies of individual cells

A procedure is described for changing the medium surrounding individual cells attached to the bottom of a cell chamber. A small hole at the "apex" of a plastic U-tube allowed application and withdrawal of medium. The medium to be applied was perfused through the U-tube by pressure at one end and suction at the other. To prevent premature delivery of new medium from the U-tube, suction of the outlet dominated resulting in a net withdrawal of medium from the cell chamber. The flow of medium through the hole could be reversed rapidly by arresting the suction with an electromechanical valve. In this way it was possible to obtain 95% replacement of medium within 60 ms. A pressure transient arising from the closure of the valve was damped by the presence of a small air bubble in the system. To secure a precise deposition of medium and minimize the risk of mechanical disturbances to the cell it was essential to be able to inspect the medium changes visually. For this purpose the fluorescent indicator rhodamine B bound to dextran proved satisfactory. Free rhodamine B could not be used because it had biological effects, as was evident from studying ATP-regulated K+ channels in pancreatic beta-cells. When using a purpose-designed syringe pump for perfusing the U-tube, the technique allows well controlled exposure of individual cells to test substances added together with dextran-linked rhodamine B.

Variations in ATP-sensitive K+ channel activity provide evidence for inherent metabolic oscillations in pancreatic beta-cells

The cell-attached configuration of the patch clamp technique was used for studying slow variations in the activity of the ATP-sensitive K+ channels in pancreatic beta-cells isolated from mouse and man. In 0 or 3 mM glucose, the fraction of time the channels were open exhibited oscillations with frequencies in the 0.25-0.40/min range. This phenomenon is a strong argument for inherent fluctuations in the ATP production of the beta-cells. Variations in metabolism may thus be a major determinant for the characteristic large amplitude oscillations of cytoplasmic Ca2+ with equivalent frequency.

Glucose induces oscillatory Ca2+ signalling and insulin release in human pancreatic beta cells

Mechanisms of pulsatile insulin release in man were explored by studying the induction of oscillatory Ca2+ signals in individual beta cells and islets isolated from the human pancreas. Evidence was provided for a glucose-induced closure of ATP-regulated K+ channels, resulting in voltage-dependent entry of Ca2+. The observation of step-wise increases of capacitance in response to depolarizing pulses suggests that an enhanced influx of Ca2+ is an effective means of stimulating the secretory activity of the isolated human beta cell. Activation of muscarinic receptors (1-10 mumol/l carbachol) and of purinergic P2 receptors (0.01-1 mumol/l ATP) resulted in repetitive transients followed by sustained elevation of the cytoplasmic Ca2+ concentration ([Ca2+]i). Periodic mobilisation of intracellular calcium was seen also when injecting 100 mumol/l GTP-gamma-S into beta cells hyperpolarized to -70 mV. Individual beta cells responded to glucose and tolbutamide with increases of [Ca2+]i, manifested either as large amplitude oscillations (frequency 0.1-0.5/min) or as a sustained elevation. Glucose regulation was based on sudden transitions between the basal and the two alternative states of raised [Ca2+]i at threshold concentrations of the sugar characteristic for the individual beta cells. The oscillatory characteristics of coupled cells were determined collectively rather than by particular pacemaker cells. In intact pancreatic islets the glucose induction of well-synchronized [Ca2+]i oscillations had its counterpart in 2-5 min pulses of insulin. Each of these pulses could be resolved into regularly occurring short insulin transients. It is concluded that glucose stimulation of insulin release in man is determined by the number of beta cells entering into a state with Ca(2+)-induced secretory pulses.

Caffeine inhibits cytoplasmic Ca2+ oscillations induced by carbachol and guanosine 5'-O-(3-thiotriphosphate) in hyperpolarized pancreatic beta-cells

The effects of caffeine on cytoplasmic Ca2+ oscillations induced by carbachol and guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) were studied in individual mouse pancreatic beta-cells clamped at a hyperpolarized potential. Addition of 10 mM caffeine did not affect the cytoplasmic Ca2+ concentration ([Ca2+]i) in beta-cells exposed to 20 mM glucose and hyperpolarized with diazoxide. Under similar conditions 100 microM carbachol induced a typical response with a marked [Ca2+]i peak followed by a lower sustained elevation. Irrespective of whether 10 mM caffeine was present, there were [Ca2+]i transients with frequencies of 1-5/min superimposed on the sustained phase in 50-60% of the cells. In previously non-exposed cells the introduction of 10 mM caffeine caused temporary lowering of the sustained phase with disappearance of the transients. Subsequent omission of caffeine in the continued presence of carbachol caused a marked [Ca2+]i peak followed by reappearance of the [Ca2+]i transients. However, in cells oscillating in the presence of caffeine its omission caused disappearance of the transients. In this case reintroduction of caffeine restored the transients. In cells kept at -70 mV by a patch pipette containing 100 microM GTP-gamma-S and 3 mM Mg-ATP there were [Ca2+]i transients with frequencies of 0.5-2.5/min. These transients were sufficiently pronounced to activate repetitively a K+ current. Addition of 10 mM caffeine caused disappearance of the [Ca2+]i transients or reduction of their amplitudes and frequencies.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of Ca2+ in the release of pancreatic islet hormones

The role of Ca2+ in initiating exocytosis of granule-bound secretory products was evaluated with respect to pancreatic islet hormones. Apart from stimulating the transfer of the granules to the plasma membrane and their subsequent extrusion, a rise of the cytoplasmic Ca2+ concentration ([Ca2+]i) may under certain conditions also have depressive effects on insulin release. Glucose has a bidirectional action on [Ca2+]i by stimulating both the entry of the ion and its removal by organelle sequestration and outward transport. The recognition of glucose as a secretory stimulus is based on sudden transitions between oscillatory and steady-state [Ca2+]i at threshold concentrations of the sugar characteristic for the individual beta-cell. The intrinsic ability of each beta-cell to generate oscillations of [Ca2+]i and the subsequent synchronization of these signals result in a pulsatile release of insulin from isolated islets. Glucose regulation of this process is manifested as alterations of the amplitudes of the insulin pulses without effects on the frequency. It is suggested that electrical signalling from the beta-cells in combination with direct effects of glucose are important for regulating the release of glucagon and somatostatin.

Dual effects of Na/K pump inhibition on cytoplasmic Ca2+ oscillations in pancreatic beta-cells

Inhibition of the Na/K pump by ouabain or removal of K+ resulted in gradual increase of intracellular sodium in beta-cell-rich pancreatic islets from ob/ob-mice exposed to 3 mM glucose. In individual beta-cells this action of ouabain was paralleled by closure of ATP-regulated K+ channels and a slow elevation of the cytoplasmic Ca2+ concentration ([Ca2+]i). In most beta-cells an increase of the glucose concentration to 11-20 mM induced large amplitude oscillations of [Ca2+]i with a frequency of 0.2-0.5/min. Ouabain had dual actions on these glucose-induced oscillations in promoting their appearance and at higher concentrations transforming them into a sustained increase of [Ca2+]i. At 100 microM ouabain reduced the frequency of the glucose-induced oscillations but nevertheless raised the time-average [Ca2+]i by increasing the amplitudes and half-widths of the Ca2+ peaks. When high concentrations of ouabain or removal of K+ transformed the oscillations into a sustained increase of [Ca2+]i, the level reached exceeded that obtained in response to rise of glucose alone. By favoring Ca2+ entry and counteracting removal of the cation from the cytoplasm, Na/K pump inhibition perturbs the balance between the processes determining glucose-induced oscillations of [Ca2+]i.

Cytoplasmic Ca2+ oscillations in pancreatic beta-cells

In the last 15 years it has been a growing interest in the cyclic variations of circulating insulin [46]. After the suggestion that this phenomenon may be due to oscillations of the beta-cell membrane potential [8,39], it was demonstrated that [Ca2+]i oscillates in the glucose-stimulated beta-cell with a similar frequency to that of pulsatile insulin release. The present review describes four types of [Ca2+]i oscillations in the pancreatic beta-cell. The slow sinusoidal oscillations, referred to as type-a, are those which most closely correspond to pulsatile insulin release. Although not affecting the properties of the type-a oscillations in individual beta-cells, the concentration of glucose is a determinant for their generation and further transformation into a sustained increase. Accordingly, cytoplasmic Ca2+ is regulated by sudden transitions between oscillatory and steady-state levels at threshold concentrations of glucose, which are characteristic for the individual beta-cell. This behaviour explains the observation of a gradual recruitment of previously non-secreting cells with increase of the extracellular glucose concentration [44]. However, it still remains to be elucidated how the sudden transitions between these three states translate into the co-ordinated slow oscillations of [Ca2+]i in the intact islet. Cyclic variations of circulating insulin require a synchronization of the [Ca2+]i cycles also among the islets in the pancreas. It is still an open question by which means the millions of islets communicate mutually to establish a pattern of pulsatile insulin release from the whole pancreas. The discovery that the beta-cell is not only the functional unit for insulin synthesis but also generates the [Ca2+]i oscillations required for pulsatile insulin release has both physiological and clinical implications. The fact that minor damage to the beta-cells prevents the type-a oscillations with maintenance of a glucose response in terms of raised [Ca2+]i reinforces previous arguments [54] that loss of insulin oscillations is an early indicator of type-2 diabetes. Further analyses of the [Ca2+]i oscillations in the beta-cells should include not only the mechanisms for their generation and subsequent propagation within or among the islets but also how modulation of their frequency affects the insulin sensitivity of various target cells. The latter approach may be important in the attempts to maintain normoglycemia under conditions minimizing the vascular effects of insulin supposed to precipitate hypertonia and atherosclerosis [70,71,77].

Stimulation of insulin release by isosmolar addition of permeant molecules

Pancreatic beta-cells are known to respond to hyposmolar stress by releasing insulin. It was evident from perifusion studies using islet cells from ob/ob-mice mixed with polyacrylamide beads that a similar type of secretory response can be obtained by isosmolar addition of 10-25 mM of the rapidly penetrating urea molecule. There was no effect with hyperosmolar addition of urea. The urea-induced insulin release differed from the ordinary stimulation of secretion in not disappearing but being more pronounced after previous heating to 45 degrees C or removal of extracellular Ca2+. Isosmolar urea was exceptional as an insulin secretagogue in being effective also in the presence of the alpha 2-adrenergic agonist clonidine or when lowering the temperature to 24 degrees C. Further support for the idea that isosmolar addition of rapidly penetrating molecules induces insulin release was obtained by testing non-metabolizable glucose analogues. Whereas 25 mM 3-O-methyl-D-glucose doubled the secretory rate within 4 min, the non-permeant L-glucose had only a slight initial action. When not compensating for the alterations of the medium osmolarity 3-O-methyl-D-glucose was without effect. Although expansion of beta-cells cannot explain the existence of a pronounced initial secretory response to D-glucose it may under certain conditions contribute to the stimulatory effects of the sugar.

Activation of G-proteins induces Ca2+ oscillations with hyperpolarizing K+ currents in pancreatic beta-cells

Activation of G-proteins by internal perfusion with GTP-gamma-S or external application of carbachol resulted in oscillations of cytoplasmic Ca2+ in isolated mouse pancreatic beta-cells. The Ca2+ transients were associated with the generation of K+ currents sufficiently pronounced to induce marked pulses of hyperpolarization. The oscillatory G-protein response remained largely unaffected when altering the membrane potential. The oscillations became less frequent in the presence of 1 mM neomycin and disappeared when the cells were internally perfused with 100 micrograms/ml heparin. The frequency of the oscillations was positively correlated with the basal level of cytoplasmic Ca2+. Addition of Ca2+ to the internal perfusion medium increased the oscillatory rate and buffering of the ion with Indo-1 or EGTA had the opposite effect. It is concluded that G-protein activation results in cyclic mobilisation of intracellular calcium mediated by inositol-1,4,5-triphosphate and that the basal concentration of cytoplasmic Ca2+ is an important determinant for the frequency of the oscillations.

Glucose has regulatory effects on insulin release also in the virtual absence of extracellular Ca2+

The glucose effect on insulin release in a Ca(2+)-deficient medium was analyzed in perifusion experiments with aggregates of cells prepared by dispersal of the beta-cell-rich pancreatic islets of ob/ob-mice. Hyperosmolar additions of 20 mM D-glucose or its poorly metabolized transport analogue 3-0-methyl-D-glucose resulted in 50% suppression of the secretory rate. However, after isosmolar additions of the sugars, replacing non-penetrating sucrose, there was a stimulation of insulin release. Whereas D-glucose was less effective than 3-O-methyl-D-glucose in stimulating insulin release after isosmolar addition, the opposite was found for the enhanced secretory response obtained when the sugars were excluded from the perifusion medium. The studies indicate that D-glucose has regulatory actions on insulin release also in the virtual absence of extracellular Ca2+. This effect is not only due to osmolar influences but involves also direct suppression of the secretory activity probably mediated by the metabolism of the sugar.

Intracellular ATP mimics GTP-gamma-S in generating Ca2+ oscillations in pancreatic beta-cells

Intracellular free calcium ([Ca2+]i) was measured in individual pancreatic beta-cells from mice using dual emission microfluorometry and the indicator Indo-1 applied by a patch clamp pipette. GTP-gamma-S (100 microM) injected together with 0.3 or 3 mM ATP evoked repetitive [Ca2+]i transients with a frequency of about 1 per min in beta-cells kept at a membrane potential of -70 mV. The oscillatory pattern was unaffected by the Ca2+ channel blocker verapamil (50 microM). When omitting GTP-gamma-S from the pipette medium it became evident that 3 mM ATP alone can induce oscillations. The results provide additional evidence for an important role of ATP in the ionic control of insulin release, indicating that such regulation may also involve activation of G-proteins.

Stimulation of pancreatic amylase release is associated with a parallel sustained increase of cytoplasmic calcium

The kinetics of the changes in the cytoplasmic Ca2+ concentration (Ca2+i) and amylase release were measured in fura-2-loaded pancreatic acinar cells and perifused pancreatic acini, respectively. Cholecystokinin octapeptide (CCK-8) and its amphibian analogue caerulein induced similar dose-related increases of Ca2+i and amylase secretion with threshold concentrations of 2-6 x 10(-12) M, and maximal effects at 2 x 10(-10) M. The action of CCK/caerulein on Ca2+i was complex and similar to that of carbachol and bombesin with a prompt several-fold increase within seconds followed by a gradual decline over more than 5 min to a new sustained suprabasal level. The kinetics of amylase release in response to CCK and carbachol correlated with the changes in Ca2+i. Additions of the antagonists N2,O2-dibutyrylguanosine 3':5'-cyclic monophosphate and atropine after 30 min of CCK-8 and carbachol stimulation, respectively, were associated with prompt lowerings of Ca2+i and inhibitions of amylase secretion. The patterns observed with substance P (SP) and eledoisin were different with high concentrations (10(-8)-10(-7) M) giving monophasic increases of Ca2+i and amylase release. An initial stimulation of cells with a high dose of CCK eliminated the Ca2+i response to further stimulation with CCK, carbachol, bombesin and SP, whereas cells subjected to initial stimulation with SP responded to subsequent exposure to CCK with prolonged elevation of Ca2+i. The data indicate that stimulation with CCK, carbachol and bombesin may be associated with intracellular mobilization of calcium from more than one pool, and that an increase of Ca2+i is involved even in threshold stimulation of amylase release.

The cytoplasmic Ca2+ response to glucose as an indicator of impairment of the pancreatic beta-cell function

The effects of glucose on the cytoplasmic Ca2+ concentration (Ca2+i) regulating insulin release were investigated using pancreatic beta-cells representative for the normal and diabetic situations. Increase of the glucose concentration resulted in a slight lowering of Ca2+i followed by a rise, often manifested as high amplitude oscillations. The Ca2+i-lowering component in the glucose action associated with suppression of insulin release became particularly prominent when the beta-cells were already depolarized by tolbutamide. Glucose-induced inhibition of insulin release was observed also in experiments with rats made diabetic with streptozotocin or alloxan. Other studies indicated lowering of plasma insulin after intravenous glucose administration in patients with insulin- and noninsulin-dependent diabetes mellitus. Brief exposure of beta-cells to 2.2 mmol l-1 streptozotocin resulted in impairment of the response to glucose, manifested as disappearance of the cyclic variation of Ca2+i. The results indicate that glucose-induced depolarisation is a vulnerable process, the disturbance of which may contribute to insulin secretory defects in diabetes mellitus.

Regulatory volume decrease of pancreatic beta-cells involving activation of tetraethylammonium-sensitive K+ conductance

Beta-cell-rich pancreatic islets from ob/ob-mice were used for evaluating the early effects of hypotonic stress. The beta-cells responded to an abrupt lowering of the osmotic pressure by 102 mOsm with both a transient stimulation of insulin release (peak value 25 times above basal) and a loss of potassium without major effects on sodium. The secretory response was obtained also in the presence of 100 microM quinine or 20 mM tetraethylammonium+. The loss of potassium was not affected by 20 mM glucose or 10 microM bumetanide, but became less apparent in the presence of 100 microM quinine and disappeared when the islets were exposed to 20 mM tetraethylammonium+. Amiloride and high concentrations of the hypoglycemic sulfonylureas tolbutamide and glibenclamide had only a slight suppressive action on potassium mobilization. Patch clamp analyses revealed an increased frequency of small channel openings after exposure to the hypotonic medium. It is concluded that the pancreatic beta-cells have the ability for a regulatory volume decrease involving activation of tetraethylammonium-sensitive K+ conductance. The stimulation of insulin release obtained by lowering the osmotic pressure seems to be related to the entry of water rather than to the ion movements responsible for the readjustment of the beta-cell volume.

Free and bound sodium in pancreatic beta-cells exposed to glucose and tolbutamide

The effects of glucose and tolbutamide on the sodium handling of the pancreatic beta-cells were evaluated by measuring the total sodium content in intact islets from ob/ob-mice by integrating flame photometry and the free ion in individual beta-cells by dual wavelength fluorometry. Whereas increasing the glucose concentration from 3 to 20 mM resulted in a lowering of sodium, the addition of 100 microM tolbutamide caused a rise. The above-mentioned effects were most marked (about 50%) for the physiologically significant free sodium. The data indicate a more important role for Na+ in the regulation of insulin release than so far acknowledged. Increase of Na+ may contribute to the secretory response to hypoglycemic sulfonylureas by providing an additional rise of cytoplasmic Ca2+.

Leucine induces initial lowering of cytoplasmic Ca2+ in pancreatic beta-cells without concomitant inhibition of insulin release

The early effects of glucose and leucine on cytoplasmic Ca2+ and insulin release were compared in suspensions of cells prepared by dispersal of the beta-cell-rich pancreatic islets of ob/ob-mice. Adequate temporal resolution was achieved by continuously recording the 340/380 nm fluorescence excitation ratio from cells loaded with the Ca2+ indicator fura-2 and measuring insulin in the perifusate from cells mixed with polyacrylamide beads. Raising the glucose concentration from 3 to 20 mM resulted in concomitant reductions of cytoplasmic Ca2+ and insulin release during the first minute. Whereas 10 mM leucine was as efficient as glucose in inducing temporary lowering of cytoplasmic Ca2+, this amino acid did not depress insulin release. It is concluded that the initial decrease of cytoplasmic Ca2+ is a phenomenon coupled to stimulation of the metabolism. The leucine-induced lowering of Ca2+ may essentially reflect changes in cytoplasmic pools other than in a peripheral one regulating insulin release.