Regulation of insulin secretion from islets of Langerhans rendered permeable by electric discharge

Bioaccumulation of the Selected Metal Ions in Saccharomyces cerevisiae Cells Under Treatment of the Culture with Pulsed Electric Field (PEF)

The obtained results demonstrated an influence of PEF on increase in accumulation of various ions in S. cerevisiae cells. Optimization of particular PEF parameters and ions concentrations in the medium caused twofold increase in accumulation of magnesium and zinc ions and 3.5-fold higher accumulation of calcium ions in the cells. In the case of ion couple, accumulation of magnesium and zinc was, respectively, 1.5-fold and twofold higher in comparison to the control cultures. Yeast cells biomass enriched with Mg(2+), Zn(2+), Ca(2+) as well as Mg(2+) and Zn(2+) (simultaneously) may be an alternative for pharmacological supplementation applied in deficiency of these cations.

Calcium dependencies of regulated exocytosis in different endocrine cells

Exocytotic machinery in neuronal and endocrine tissues is sensitive to changes in intracellular Ca(2+) concentration. Endocrine cell models, that are most frequently used to study the mechanisms of regulated exocytosis, are pancreatic beta cells, adrenal chromaffin cells and pituitary cells. To reliably study the Ca(2+) sensitivity in endocrine cells, accurate and fast determination of Ca(2+) dependence in each tested cell is required. With slow photo-release it is possible to induce ramp-like increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) that leads to a robust exocytotic activity. Slow increases in the [Ca(2+)](i) revealed exocytotic phases with different Ca(2+) sensitivities that have been largely masked in step-like flash photo-release experiments. Strikingly, in the cells of the three described model endocrine tissues (beta, chromaffin and melanotroph cells), distinct Ca(2+) sensitivity 'classes' of secretory vesicles have been observed: a highly Ca(2+)-sensitive, a medium Ca(2+)-sensitive and a low Ca(2+)-sensitive kinetic phase of secretory vesicle exocytosis. We discuss that a physiological modulation of a cellular activity, e.g. by activating cAMP/PKA transduction pathway, can switch the secretory vesicles between Ca(2+) sensitivity classes. This significantly alters late steps in the secretory release of hormones even without utilization of an additional Ca(2+) sensor protein.

Metabolic amplifying pathway increases both phases of insulin secretion independently of β-cell actin microfilaments

Two pathways control glucose-induced insulin secretion (IS) by β-cells. The triggering pathway involves ATP-sensitive potassium (KATP) channel-dependent depolarization, Ca2+ influx, and a rise in the cytosolic Ca2+ concentration ([Ca2+]c), which triggers exocytosis of insulin granules. The metabolic amplifying pathway augments IS without further increasing [Ca2+]c. The underlying mechanisms are unknown. Here, we tested the hypothesis that amplification implicates actin microfilaments. Mouse islets were treated with latrunculin B and cytochalasin B to depolymerize actin or jasplakinolide to polymerize actin. They were then perifused to measure [Ca2+]c and IS. Metabolic amplification was studied during imposed steady elevation of [Ca2+]c by tolbutamide or KCl or by comparing the magnitude of [Ca2+]c and IS changes produced by glucose and tolbutamide. Both actin polymerization and depolymerization augmented IS triggered by all stimuli without increasing (sometimes decreasing) [Ca2+]c, which indicates a predominantly inhibitory function of microfilaments in exocytosis at a step distal to [Ca2+]c increase. When [Ca2+]c was elevated and controlled by KCl or tolbutamide, the amplifying action of glucose was facilitated by actin depolymerization and unaffected by polymerization. Both phases of IS were larger in response to high-glucose than to tolbutamide in low-glucose, although triggering [Ca2+]c was lower. This difference in IS, due to amplification, persisted when the IS rate was doubled by actin depolymerization or polymerization. In conclusion, metabolic amplification is rapid and influences the first as well as the second phase of IS. It is a late step of stimulus-secretion coupling, which does not require functional actin microfilaments and could correspond to acceleration of the priming process conferring release competence to insulin granules.

Cross-linking of cellular proteins by tissue transglutaminase during necrotic cell death: a mechanism for maintaining tissue integrity

Tissue transglutaminase (tTG) is a Ca(2+)-dependent enzyme which cross-links proteins via epsilon(gamma-glutamyl)lysine bridges. There is increasing evidence that tTG is involved in wound repair and tissue stabilization, as well as in physiological mechanisms leading to cell death. To investigate the role of this enzyme in tissue wounding leading to loss of Ca(2+) homoeostasis, we initially used a model involving electroporation to reproduce cell wounding under controlled conditions. Two cell models were used whereby tTG expression is regulated either by antisense silencing in ECV 304 cells or by using transfected Swiss 3T3 cells in which tTG expression is under the control of the tet regulatory system. Using these cells, loss of Ca(2+) homoeostasis following electroporation led to a tTG-dependent formation of highly cross-linked proteinaceous shells from intracellular proteins. Formation of these structures is dependent on elevated intracellular Ca(2+), but it is independent of intracellular proteases and is near maximal after only 20 min post-wounding. Using labelled primary amines as an indicator of tTG activity within these 'wounded cells', we demonstrate that tTG modifies a wide range of proteins that are present in both the perinuclear and intranuclear spaces. The demonstration of entrapped DNA within these shell structures, which showed limited fragmentation, provides evidence that the high degree of transglutaminase cross-linking results in the prevention of DNA release, which may serve to dampen any subsequent inflammatory response. Comparable observations were shown when monolayers of cells were mechanically wounded by scratching. In this second model of cell wounding, redistribution of tTG activity to the extracellular matrix was also demonstrated, an effect which may serve to stabilize tissues post-trauma, and thus contribute to the maintenance of tissue integrity.

Localized calcium influx in pancreatic beta-cells: its significance for Ca2+-dependent insulin secretion from the islets of Langerhans

Ca2+ influx through voltage-dependent Ca2+ channels plays a crucial role in stimulus-secretion coupling in pancreatic islet beta-cells. Molecular and physiologic studies have identified multiple Ca2+ channel subtypes in rodent islets and insulin-secreting cell lines. The differential targeting of Ca2+ channel subtypes to the vicinity of the insulin secretory apparatus is likely to account for their selective coupling to glucose-dependent insulin secretion. In this article, I review these studies. In addition, I discuss temporal and spatial aspects of Ca2+ signaling in beta-cells, the former involving the oscillatory activation of Ca2+ channels during glucose-induced electrical bursting, and the latter involving [Ca2+]i elevation in restricted microscopic "domains," as well as direct interactions between Ca2+ channels and secretory SNARE proteins. Finally, I review the evidence supporting a possible role for Ca2+ release from the endoplasmic reticulum in glucose-dependent insulin secretion, and evidence to support the existence of novel Ca2+ entry pathways. I also show that the beta-cell has an elaborate and complex set of [Ca2+]i signaling mechanisms that are capable of generating diverse and extremely precise [Ca2+]i patterns. These signals, in turn, are exquisitely coupled in space and time to the beta-cell secretory machinery to produce the precise minute-to-minute control of insulin secretion necessary for body energy homeostasis.

Glycolytic metabolites and intracellular signaling in the pancreatic beta cell

In the pathways modulating the secretion of insulin and other physiologically important molecules, the critical role played by calcium in the moment-to-moment regulation of secretory processes may be modulated by additional factors, and these factors may include the glycolytic metabolites. We studied these early glucose breakdown products for effects on calcium release and inositol 1,4, 5-trisphosphate (IP3) binding to the IP3 receptor in a pancreatic beta cell preparation. The physiological significance of the response was also examined in terms of the insulinotropic effects of these metabolites. In studies of calcium release from the pancreatic beta cell, the metabolite 2,3-bisphosphoglycerate (DPG) exerted a statistically significant stimulatory effect on calcium release. A lesser but nonetheless significant effect also occurred in the presence of 3-phosphoglycerate and glucose-6-phosphate. The DPG-induced effect was concentration dependent. It is likely that the effects of DPG and other glycolytic metabolites on pancreatic beta cell signaling are physiologically significant inasmuch as we were also able to demonstrate that DPG and other glycolytic metabolites promoted the release of insulin from the pancreatic beta cell.

A practical assessment of transdermal drug delivery by skin electroporation

Transdermal drug delivery has many potential advantages, but the skin's poorly-permeable stratum corneum blocks delivery of most drugs at therapeutic levels. Short high-voltage pulses have been used to electroporate the skin's lipid bilayer barriers and thereby deliver compounds at rates increased by as much as four orders of magnitude. Evidence that the observed flux enhancement is due to physical alteration of the skin by electroporation, as opposed to only providing an iontophoretic driving force, is supported by a number of different transport, electrical and microscopy studies. Practical applications of electroporation's unique effects on skin are motivated by large flux increases for many different compounds, rapidly responsive delivery profiles, and efficient use of skin area and electrical charge. Greater enhancement can be achieved by combining skin electroporation with iontophoresis, ultrasound, and macromolecules. Sensation due to electroporation can be avoided by using appropriate electrical protocols and electrode design. To develop skin electroporation as a successful transdermal drug delivery technology, the strong set of existing in vitro mechanistic studies must be supplemented with studies addressing in vivo/clinical issues and device design.

Tubocurarine blocks a calcium-dependent potassium current in rat tumoral pituitary cells

We investigated the effects of potassium channel inhibitors on electrical activity, membrane ionic currents, intracellular calcium concentration ([Ca2+]i) and hormone release in GH3/B6 cells (a line of pituitary origin). Patch-clamp recordings show a two-component after hyperpolarization (AHP) following each action potential (current clamp) or a two-component tail current (voltage-clamp). Both components can be blocked by inhibiting Ca2+ influx. Application of D-tubocurarine (dTc) (20-500 microM) reversibly suppressed the slowly decaying Ca2+-activated K+ tail current (I AHPs) in a concentration-dependent manner. On the other hand, low doses of tetraethylammonium ions (TEA+) only blocked the rapidly decaying voltage- and Ca2+-activated K+ tail current (I AHPf). Therefore, GH3/B6 cells exhibit at least two quite distinct Ca2+-dependent K+ currents, which differ in size, voltage- and Ca2+-sensitivity, kinetics and pharmacology. These two currents also play quite separate roles in shaping the action potential. d-tubocurarine increased spontaneous Ca2+ action potential firing, whereas TEA increased action potential duration. Thus, both agents stimulated Ca2+ entry. I AHPs is activated by a transient increase in [Ca2+]i such as a thyrotrophin releasing hormone-induced Ca2+ mobilization. All the K+ channel inhibitors we tested: TEA, apamin, dTC and charybdotoxin, stimulated prolactin and growth hormone release in GH3/B6 cells. Our results show that I AHPs is a good sensor for subplasmalemmal Ca2+ and that dTc is a good pharmacological tool for studying this current.

Ca(2+)-induced loss of Ca2+/calmodulin-dependent protein kinase II activity in pancreatic beta-cells

Elevations in intracellular Ca2+ in electrically permeabilized islets of Langerhans produced rapid insulin secretory responses from beta-cells, but the Ca(2+)-induced secretion was not maintained and was irrespective of the pattern of administration of elevated Ca2+. Ca(2+)-insensitive beta-cells responded normally to activators of protein kinase C or cAMP-dependent kinase with increased insulin secretion. The loss of secretory responsiveness to Ca2+ was paralleled by a reduction in Ca(2+)-induced protein phosphorylation. This was caused by a reduction in Ca2+/calmodulin-dependent protein kinase II (CaMK II) activity in the desensitized cells, as assessed by measuring the phosphorylation of a CaMK II-specific exogenous substrate, autocamtide-2. The Ca(2+)-induced reductions in kinase activity and protein phosphorylation were not dependent on the activation of Ca(2+)-dependent protein kinases and were not caused by the activation of phosphoprotein phosphatases or of Ca(2+)-activated proteases. The concomitant reductions in CaMK II activity and Ca(2+)-induced insulin secretion suggest that the activation of CaMK II is required for normal insulin secretory responses to increased intracellular Ca2+ concentrations.

Pulsatile insulin release from pancreatic islets with nonoscillatory elevation of cytoplasmic Ca2+

The relationship between insulin release and cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in isolated pancreatic islets from ob/ob mice. Although [Ca2+]i was low and stable in the presence of 3 mM glucose, basal insulin release exhibited low amplitude pulsatility, with a frequency of 0.32 +/- 0.04 min-1. Depolarization by raising K+ from 5.9 to 30.9 mM or by the addition of 1 mM tolbutamide caused a pronounced initial insulin pulse followed by declining pulses, but there was no change in frequency. This decline in amplitude of the insulin pulses was prevented in similar experiments performed in the presence of 11 mM glucose. Corresponding measurements of [Ca2+]i in islets exposed to tolbutamide or the high K+ concentration revealed stable elevations without oscillations. Although the [Ca2+]i level is an important determinant for the rate of secretion, the results indicate that pulsatile insulin release does not always depend on [Ca2+]i oscillations. It is suggested that cyclic generation of ATP may fuel pulsatile release under conditions when [Ca2+]i remains stable.

Introducing specific antibodies into electropermeabilized cells is a valuable tool for eliminating specific cell functions

A technique is established for the role of intracellular proteins to be eliminated and thereby gives information about their specific role in signal transduction within cells. Rat pancreatic islets as well as INS-1 cells (an insulin secreting cell line) were electrically permeabilized in order to introduce high molecular weight compounds. Optimized conditions were five exposures with 15-s intervals, tau = 200 ms, an electric field of 1.36 kV per 0.4 cm in a specific permeabilization buffer at a calculated Ca++ concentration of 5 x 10(-8) M. In electroporation control experiments the spectrophotometrically measured uptake of the cell membrane-impermeable propidium iodide, FITC-labelled dextran (MW approximately 4000) and FITC-labelled antibodies (MW approximately 150,000) was established as being 81.5 +/- 5.0, 82.7 +/- 3.0 and 81.0 +/- 1.0 per cent of maximum, respectively. These data were corroborated qualitatively by visualizing microscopically the fluorescence of the FITC-labelled compounds in islets as well as in INS-1 cells. The cells appear to reseal since control experiments indicated a short-lived outflow of lactate dehydrogenase (MW of 140,000 which is similar to that of antibodies) and of insulin for the first 15-20 min. After electroporation the cells were functionally intact, i.e. responded to the stimulus carbachol (CCh). Only 18.0 +/- 10.1 per cent of cells had not resealed after 2 h (propidium iodide uptake measured at various time intervals after electroporation). As was shown recently the effect of specific compounds such as CCh and CCK8 on insulin release was eliminated selectively by antibodies against specific G proteins thus proving this method to be a valuable tool. In conclusion, adding antibodies to electrically permeabilized cells is a valuable tool for eliminating a specific cell function in order to elucidate the specific role of intracellular compounds. This method can probably be used for testing the specific role of other proteins in cell functions.

Stimulation of insulin release by D-glucose in depolarized pancreatic islets

This study reevaluates the relevance of plasma membrane depolarization in the pancreatic islet B-cell to the stimulation of insulin release by D-glucose. In rat pancreatic islets exposed to increasing concentrations of D-glucose, a sigmoidal relationship is observed between the output of insulin and concentration of the hexose. The release of insulin recorded in the absence of D-glucose or at a non-stimulatory concentration (2.5 mM) of the hexose is significantly enhanced when the islets are incubated in the presence of the meglitinide analogue A-4166 (20 microM) or at a high concentration (30 microM) of extracellular K+. Nevertheless, even in the presence of A-4166 or at the high K+ concentration, increasing concentrations of D-glucose, up to 20 microM, still enhance, in a sigmoidal manner, the secretion of insulin, with a 15-fold or greater increment in insulin output relative to the value found in the absence of the hexose. These findings indicate that D-glucose provides a concentration-related signal for insulin release, that cannot be attributed solely to either the closing of ATP-sensitive K+ channels, the depolarization of the plasma membrane, or the role of D-glucose as a nutrient to cover the energy expenditure in the islet cells.

Measurement of tissue transglutaminase activity in a permeabilized cell system: its regulation by Ca2+ and nucleotides

Electropermeabilized human endothelial cells (ECV-304) were used to study the regulation of tissue transglutaminase (tTGase) activity in the intracellular environment. An ELSA (enzyme-linked sorbent assay) plate assay was developed for intracellular tTGase activity, using the incorporation of a biotinylated primary amine, 5-{[(N-biotinoylamino)hexanoyl]amino}pentylamine(biotin-x-cadaveri ne; BTC), into endogenous protein substrates of tTGase. This incorporation process was inhibited by competitive inhibitors of tTGase, cystamine and monodansylcadaverine, in a dose-dependent manner. Over a 30 min period tTGase and its protein substrates did not leak out of the cell, and no incorporation of BTC occurred in unpermeabilized cells, indicating the reaction to be intracellular. In the presence of 10 nM or 10 muM CA2+, when nucleotides ATP and GTP were added at concentrations mimicking cytosolic levels, tTGase activity was decreased virtually to zero. Only at 100 muM Ca2+, when nucleotides were low or absent was tTGase activity observed. Under these conditions a variety of proteins was labelled by the enzyme, with the major labelling found in a protein of molecular mass around 51 kDa when analysed by SDS/PAGE/Western blotting.

Hyperresponse in calcium-induced insulin release from electrically permeabilized pancreatic islets of diabetics GK rats and its defective augmentation by glucose

In spontaneously diabetic GK rats, insulin secretion from pancreatic beta cells in response to glucose is selectively impaired, probably due to deficient intracellular metabolism of glucose and impaired closure of KATP channels during glucose stimulation. By using electrically permeabilized islets of GK rats, we explored the functional modulations in exocytotic steps distal to the rise in [Ca2+]i in the diabetic condition. At 30 nmol/l Ca2+ (basal conditions) insulin release was similar between GK and non-diabetic control Wistar rats. In response to 3.0 mumol/l Ca2+ (maximum stimulatory conditions), insulin release was significantly augmented in permeabilized GK islets (p < 0.01). Raising glucose concentrations from 2.8 to 16.7 mmol/l further augmented insulin release induced by 3.0 mumol/l Ca2+ from permeabilized control islets (p < 0.001), but had no effect on that from permeabilized GK islets. The stimulatory effect of glucose on insulin release from permeabilized control islets was partly inhibited by 2,4-dinitrophenol, an inhibitor of mitochondrial oxidative phosphorylation (p < 0.01). The hyperresponse to Ca2+ in GK islets may play a physiologically compensatory role on the putative functional impairment both in [Ca2+]i rise and energy state in response to glucose in diabetic beta cells, and may explain the relative preservation of insulin release induced by non-glucose depolarizing stimuli, such as arginine, from pancreatic islets in non-insulin-dependent diabetes mellitus.

Effect of cAMP-dependent protein kinase on catecholamine secretion from bovine adrenal chromaffin cells

We examined the role of cAMP-dependent protein kinase in Ca(2+)-elicited catecholamine secretion from bovine adrenal chromaffin cells. When the digitonin-treated cells were incubated with the catalytic subunit of cAMP-dependent protein kinase, the secretion of catecholamines from the cells occurred in the absence of Ca2+. The effect of the catalytic subunit was dependent on its activity (50-100 units/ml) and the presence of ATP-Mg2+ in the incubation medium. However, incubation of the cells with the regulatory subunit of cAMP-dependent protein kinase did not affect the secretion. Ca2+ (43 nM-10 microM) also increased the secretion, which was ATP-Mg(2+)-dependent. The catalytic subunit (25-200 units/ml) enhanced the Ca(2+)-evoked secretion at the suboptimal but not optimal Ca2+ concentration, which induced maximal secretion. A potent synthetic peptide inhibitor of cAMP-dependent protein kinase abolished the catalytic subunit-induced secretion, but not the Ca(2+)-evoked secretion. On the other hand, K-252a, a potent inhibitor of protein kinases, inhibited both the catalytic subunit-induced and the Ca(2+)-evoked secretion, but not KT5823, a much less potent inhibitor of protein kinases. These results strongly suggest that the catalytic subunit of cAMP-dependent protein kinase produces the secretion of catecholamines via protein phosphorylation. The results further suggest that the cAMP-dependent protein kinase does not participate in an intrinsic process of Ca(2+)-elicited secretion but it may act as a modulator.

Regulation of insulin secretion: the role of second messengers

This review summarises briefly studies performed in the last 5-6 years concerning the role of second messengers in the regulation of insulin secretion, using intact and electrically permeabilized rat islets of Langerhans. It is concluded that cyclic AMP (through protein kinase A), calcium (through calcium-calmodulin dependent protein kinases) and diacylglycerol (through protein kinase C) may be important second messengers in modulating the effects of specific secretagogues on insulin release. However, recent studies strongly suggest that neither protein kinase A nor protein kinase C are directly involved in the regulation of insulin secretion by glucose. The possible involvement of other second messengers, nitric oxide and arachidonic acid, in the regulation of secretion is also briefly reviewed.

Thapsigargin inhibits the glucose-induced decrease of intracellular Ca2+ in mouse islets of Langerhans

Stimulation of pancreatic islets of Langerhans with glucose results in changes in intracellular Ca2+ concentration ([Ca2+]i). With the use of mouse islets loaded with fura 2, the earliest glucose-induced alteration of [Ca2+]i was a pronounced decline in [Ca2+]i. This effect (phase 0) was evident 1 min after increasing extracellular glucose from 2 to 12 mM and was sustained for 3-5 min. Phase 0 was also observed when glucose was increased from 5 to 12 mM, indicating that it was not an experimental artifact resulting from substrate depletion. The [Ca2+]i-lowering effect of glucose was mimicked by D-glyceraldehyde but not by 2-deoxyglucose, pyruvate, glyburide, or 30 mM extracellular KCl. Mannoheptulose inhibited phase 0, whereas diazoxide, sodium azide, calmidazolium, or increasing extracellular [Ca2+] to 10 mM were all without effect. After the elevation of islet [Ca2+]i with 5 microM glyburide, 12 mM glucose caused a considerable transient decrease in [Ca2+]i. Under similar conditions, 5 mM caffeine attenuated phase 0, whereas 1 microM thapsigargin, a specific inhibitor of the sarcoplasmic and endoplasmic reticulum family of Ca(2+)-adenosinetriphosphatases (SERCA), almost completely inhibited any glucose-induced reduction of [Ca2+]i. These observations suggest that glucose causes an elevation of beta-cell SERCA activity triggered by factors generated during the cytosolic stages of glycolysis.

Electropermeabilization of cells in tissues assessed by the qualitative and quantitative electroloading of bleomycin

Using cells in suspension, electropermeabilization is a technique extensively used to transfect living cells or to introduce a variety of compounds inside the cells. Here we demonstrate the reality of the tissue electropermeabilization using qualitative and quantitative determinations of the electroloading of bleomycin considered as an nonpermeant molecule that serves as an indicator of the permeabilization. In tissues, cell electropermeabilization is achieved for electric field intensities lower than those necessary to permeabilize the same cells in suspension. We also emphasize the importance of the geometry of the electric field lines defined by the electrodes for permeabilizing a whole tissue, for example a tumor.

The inhibition of human duodenal adenylate cyclase activity by Ca2+ and the effects of EGTA

This study demonstrates that the inhibition of adenylate cyclase activity by Ca2+ is enhanced in the presence of increasing [EGTA] (0, 0.3, 1, 2.5 mM) by 2 orders of magnitude. It has been established that this effect is not because of poor Ca2+ buffering by low [EGTA] or high Ca2+ binding by the membrane preparation. It is present irrespective of stimulus. We suggest the enhanced sensitivity of adenylate cyclase to Ca2+ induced by EGTA is caused by the Ca-EGTA complex being a more inhibitory species than Ca2+. Thus consideration of the effects of the Ca-EGTA complex should be made when interpreting the results from experiments involving Ca2+ and EGTA.

Cell electropermeabilization: a new tool for biochemical and pharmacological studies

Cell electropermeabilization is the transient permeabilization of the plasma membrane by means of short and intense electric pulses. Under optimized conditions, electropermeabilization is compatible with cell survival. It provides a direct access into the cytosol to ions, small molecules, exogenous drugs and macromolecules. As cells remain functional, a large variety of cell biology questions can be addressed. Such 'in situ biochemistry' opens new possibilities beside the more classical studies dealing with unpermeabilized cells or subcellular extracts. Electropermeabilization also allows pharmacological studies with cells, cultured monolayers and in vivo tissues as well as the design of drug controlled-release systems.

Ca2(+)-induced insulin secretion from electrically permeabilized islets. Loss of the Ca2(+)-induced secretory response is accompanied by loss of Ca2(+)-induced protein phosphorylation

Increasing the cytosolic Ca2+ concentration of electrically permeabilized rat islets of Langerhans caused rapid increases in insulin secretion and in 32P incorporation into islet proteins. However, the secretory responsiveness of permeabilized islets was relatively transient, with insulin secretion approaching basal levels within 20-30 min despite the continued presence of stimulatory concentrations of Ca2+. The loss of Ca2(+)-induced insulin secretion was accompanied by a marked reduction in Ca2(+)-dependent protein phosphorylation, but not in cyclic AMP-dependent protein phosphorylation. Similarly, permeabilized islets which were no longer responsive to Ca2+ were able to mount appropriate secretory responses to cyclic AMP and to a protein kinase C-activating phorbol ester. These results suggest that prolonged exposure to elevated cytosolic Ca2+ concentrations results in a specific desensitization of the secretory mechanism to Ca2+, perhaps as a result of a decrease in Ca2(+)-dependent kinase activity. Furthermore, these studies suggest that secretory responses of B-cells to cyclic AMP and activators of protein kinase C are not dependent upon the responsiveness of the cells to changes in cytosolic Ca2+.

Phorbol esters modulate phospholipid metabolism in a human cholinergic cell line, LA-N-2: a possible role for the base exchange enzymes

It was shown that in LA-N-2 cells prelabeled with [3H-methyl]choline for 24 hr (Singh et al.: Mol Chem Neuropathol 14:53-66, 1991) the major intracellular and extracellular hydrophilic compound was phosphorylcholine. LA-N-2 cells were labeled with [14C-methyl]choline for 24 hr, harvested, and incubated in Hepes/BSA/saline buffer for varying periods of time. The radioactive compound present in the cytosol and released into Hepes/BSA/saline buffer medium in the presence or absence of TPA was phosphorylcholine. There was a gradual increase in the appearance of radioactivity in the medium and this corresponded to a gradual decline in the radioactivity present in the cytosolic compartment with a statistically significant P value of less than .005. Identical results were obtained with prelabeled cells subsequently incubated with TPA. There was no significant change in the amount of radioactivity associated with lipid suggesting that the phosphorylcholine may be released directly from the cytosolic compartment into the medium rather than originating through a phospholipase-C catalyzed hydrolysis of phosphatidylcholine. This possibility received support from experiments in which cells were electropermeabilized in the presence of radioactive phosphorylcholine. It was found that the introduced [14C]phosphorylcholine was released intact into the incubation medium from the cytosolic compartment. The incorporation of [14C]choline, [14C]ethanolamine, and [14C]serine by LA-N-2 cells into their corresponding phospholipids was investigated in the presence or absence of TPA. The presence of TPA increased the amount of radioactivity incorporated into the phospholipids with a corresponding decrease in the amount of radioactivity in the cytosolic compartment compared to control cultures. There were no detectable differences between TPA exposed and control cells in the distribution of radioactivity in free choline, phosphorylcholine, or CDP-choline of [14C] choline labeled cells. This indicates that the increased lipid labeling was not accompanied by enhanced labeling of the intermediates of the de novo pathway. This effect of TPA in altering the distribution of labeling of the cytosolic and lipid components was not demonstrable with cells grown in the presence of 10(-5) M retinoic acid.

Calmodulin independence of human duodenal adenylate cyclase

The calmodulin and calcium dependence of human adenylate cyclase from the second part of the duodenum was assessed in washed particulate preparations of biopsy specimens by investigating (a) the concentration dependent effects of free [Ca2+] on enzyme activity, (b) the effects of exogenous calmodulin on enzyme activity in ethylene glycol bis (b-aminoethyl ether)N,N'-tetra-acetic acid (EGTA) washed particulate preparations, and (c) the effects of calmodulin antagonists on enzyme activity. Both basal (IC50 = 193.75 (57.5) nmol/l (mean (SEM)) and NaF stimulated (IC50 = 188.0 (44.0) nmol/l) adenylate cyclase activity was strongly inhibited by free [Ca2+] greater than 90 nmol/l. Free [Ca2+] less than 90 nmol/l had no effect on adenylate cyclase activity. NaF stimulated adenylate cyclase activity was inhibited by 50% at 2.5 mmol/l EGTA. This inhibition could not be reversed by free Ca2+. The addition of exogenous calmodulin to EGTA (5 mmol/l) washed particulate preparations failed to stimulate adenylate cyclase activity. Trifluoperazine and N-(8-aminohexyl)-5-IODO-1-naphthalene-sulphonamide (IODO 8) did not significantly inhibit basal and NaF stimulated adenylate cyclase activity when measured at concentrations of up to 100 mumol/l. These results suggest that human duodenal adenylate cyclase activity is calmodulin independent but is affected by changes in free [Ca2+].

Late events in regulated exocytosis

To understand the intracellular mechanisms that control exocytosis it is necessary to have access to the cell interior. This is achieved by plasma membrane permeabilisation or by application of patch-pipettes. These conditions permit control over the cytosol composition and also allow leakage of soluble factors that may have roles in the exocytotic mechanism. Different permeabilisation methods allow different extents of leakage and therefore provide complementary data. The exocytotic machinery itself remains intact and can be activated by providing Ca2+ and/or a guanine nucleotide. In some cells there is evidence for the participation of two guanine nucleotide-binding proteins (GP and GE), as well as a Ca(2+)-binding protein. In others Ca2+ is the only requirement. In a number of cell types, ATP is not required for the late steps in the secretory pathway.

Simultaneous monitoring of cytosolic free calcium and exocytosis at the single cell level

Abstract Quinacrine, a fluorescent basic molecule, accumulates in secretory granules of pituitary cells, as was revealed by its colocalization with immunoreactive prolactin. Thus quinacrine fluorescence may be used to monitor secretory activity at the single cell level. Rat pituitary cells in primary culture were loaded with quinacrine and stimulated with physiological secretagogues, such as thyrotrophin-releasing hormone or bradykinin, which induced a multiphasic lowering of fluorescence, corresponding to the loss of quinacrine contained in exocytosed granules. Quinacrine was further used in combination with the fluorescent calcium probe fura-2, in order to monitor simultaneously exocytosis and variations in the cytosolic free calcium concentration, [Ca(2+)](i). With an appropriate selection of the excitation wavelengths, in dual excitation microfluorimetry experiments, it was possible to distinguish between fluorescence changes due to altered [Ca(2+)](i) versus quinacrine exocytosis. Transient elevations of [Ca(2+)](i) were provoked in individual pituitary cells by enhancing calcium influx through voltage gated channels. In part of the cells an initial increase in [Ca(2+)](i) coincided with stimulated quinacrine release. The approach was also applied to cells of the neuroblastoma line NCB20, where stimulation with bradykinin caused a transient rise in [Ca(2+)](i), concomitantly with enhanced exocytosis. No increase in exocytosis was ever detected without an elevation of [Ca(2+)](i), suggesting that in both cellular systems, an increase in [Ca(2+)](i), is absolutely necessary, but not sufficient to induce secretion.

Transglutaminase involvement in the secretion of insulin from electropermeabilised rat islets of Langerhans

Ca(2+)-Induced insulin release from electropermeabilised islets is inhibited by the transglutaminase inhibitors monodansylcadaverine, glycine methylester, methylamine and cystamine but not by the control compounds dimethyl monodansylcadaverine and sarcosine methylester which lack the primary amine group. Neither monodansylcadaverine nor glycine methylester inhibited insulin secretion induced by either cAMP or the phorbol ester PMA at basal levels (10 nM) of Ca2+. These data provide further evidence for the involvement of transglutaminase in Ca2+ induced insulin secretion, they also suggest that insulin secretion induced by either cAMP or PMA may act in part by a mechanism independent of that induced by Ca2+.

Signal transduction by membrane receptors in viable electropermeabilized cells: isoproterenol-stimulated cyclic AMP synthesis in C6 glioma cells

The activity of beta-adrenergic receptors at the plasma membrane level was investigated in viable, electropermeabilized C6 glioma cells. Electric field pulses were applied directly to the plated cells without any previous proteinase treatment. The affinity for isoproterenol and the density of the beta-adrenergic receptors, as judged from the number of [3H]CGP-12177 binding sites, were not affected by the electropermeabilization whereas the isoproterenol-stimulated cAMP accumulation was transiently impaired. This decrease in activity is due to an electropermeabilization-induced GTP leak. Normal activity could be obtained either by treating the cells by the electric field in a GTP-containing buffer, or by spontaneous recovery of the cells after the resealing of the plasma membrane, with a delay depending on the temperature. The activity of the receptors was not affected by the structural organization of the membrane associated to its electropermeabilization.

Phosphatidylcholine biosynthesis in cultured glioma cells: evidence for channeling of intermediates

The major pathway of choline (Cho) incorporation into phosphatidylcholine (PtdCho) in mammalian cells is sequential conversion of Cho to phosphocholine (PCho), cytidinediphosphate choline (CDP-Cho) and PtdCho. In intact cells, this sequence is usually demonstrated using radiolabeled Cho since PCho and CDP-Cho do not enter the cell intact. We have studied the incorporation of radiolabeled Cho, PCho and CDP-Cho into rat glioma (C6) cells following electropermeabilization. C6 cells were permeable as judged by [U-14C]sucrose and Erythrosin B uptake and more rapid incorporation of [1,2,3-3H]glycerol into cell lipids, and viable as assessed by uptake and incorporation of [methyl-3H]Cho, [1-14C]oleate and [1,2,3-3H]glycerol into complex lipids. Despite rapid incorporation of [methyl-3H]Cho into PtdCho in permeabilized cells, there was no incorporation of [methyl-14C]PCho or CDP-[methyl-14C]Cho into PtdCho. PCho (300 microM) and CDP-Cho (300 microM) failed to significantly reduce incorporation of 28 microM [methyl-3H]Cho into PtdCho. Radioactivity in PtdCho of cells prelabeled with [methyl-3H]Cho prior to permeabilization could be chased with 4 mM Cho but not with 4 mM PCho or 4 mM CDP-Cho. The water-soluble products of Cho metabolism--PCho, CDP-Cho and glycerophosphocholine--were retained at 37 degrees C in permeabilized cells compared with controls while there was uniform leakage from permeabilized cells at 4 degrees C. Hemicholinium-3, an inhibitor of high-affinity Cho transport, decreased [methyl-3H]Cho incorporation into PtdCho in permeabilized cells, as in controls, suggesting that even in permeabilized cells, Cho incorporation into PtdCho is linked to the transport system. We propose that individual steps of the cytidine pathway of PtdCho biosynthesis are functionally linked and that reaction intermediates are not freely diffusible within the cell but are channeled to PtdCho biosynthesis.

Time-course of Ca2+-induced insulin secretion from perifused, electrically permeabilised islets of Langerhans: effects of cAMP and a phorbol ester

The pattern of insulin secretion from electrically permeabilised islets was studied in a perifusion system. Increases in intracellular Ca2+ stimulated insulin secretion in a dose-related manner, but the secretory response to Ca2+ was only transient, with permeabilised islets becoming rapidly insensitive to a stimulatory concentration of Ca2+. Cyclic AMP and the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), both stimulated Ca2+-induced insulin secretion from perifused permeabilised islets by increasing the maximum secretory response to Ca2+, and by maintaining elevated rates of secretion when the permeabilised islets had become insensitive to the stimulatory effects of Ca2+. These results suggest that cAMP and PMA may act partly by modifying the magnitude of the secretory response to Ca2+, and also by Ca2+-independent effects on the secretory mechanism.

Glucose-induced buffering of cytoplasmic Ca2+ in the pancreatic beta-cell--an artifact or a physiological phenomenon?

The effects of stimulated metabolism on the cytoplasmic Ca2+ concentration (Ca2+i) of insulin-releasing pancreatic beta-cells were studied. When the glucose concentration was increased from 5 to 20 mM, some cell preparations responded with initial lowering of Ca2+i followed by a rise, whereas Ca2+i only increased in others. After prolonged exposure to 5 or 10 mM of the sugar, depolarization with high concentrations of sulfonylurea or K+ caused rapid increases of Ca2+i. However, when subsequently raising glucose to 20 mM there were pronounced temporary decreases of Ca2+i. Marked Ca2+i reducing effects were also obtained after prolonged exposure to 20 mM glucose, when metabolism was augmented further by exposure to leucine or beta-2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid. The results indicate that buffering of Ca2+i is not an artifact but may have physiological significance.

Effects of Bordetella pertussis toxin on catecholamine inhibition of insulin release from intact and electrically permeabilized rat islets

Noradrenaline- and clonidine-induced inhibition of insulin release from intact and electrically permeabilized rat islets was markedly relieved by prior exposure to 100 ng of Bordetella pertussis toxin/ml. The reversal of catecholamine inhibition of insulin secretion by this toxin was not associated with a decrease in specific binding of the alpha 2-adrenergic ligand [3H]yohimbine, and could not be fully explained by an increase in intracellular cyclic AMP. Exposure of intact islets to 1 microgram of pertussis toxin/ml for 2 h, followed by electrical permeabilization and incubation with 5 microCi of [alpha-32P]NAD+, resulted in the ADP-ribosylation in situ of a protein of molecular mass approx. 41 kDa. These results suggest that pertussis toxin alleviates catecholamine inhibition of beta-cell secretory responses by ADP-ribosylating at least one protein of molecular mass 41 kDa. In analogous systems the 41 kDa substrate of pertussis toxin has been shown to be the alpha subunit of Gi, but catecholamine-activated G proteins linked to effector systems other than adenylate cyclase might also be modified by this toxin in pancreatic beta-cells.

Translocation of protein kinase C in rat islets of Langerhans. Effects of a phorbol ester, carbachol and glucose

In unstimulated rat islets (2 mM glucose), most of the ion-exchange purified protein kinase C (PKC) activity was associated with the cytosolic fraction. Both carbachol and phorbol myristate acetate caused a significant translocation of PKC activity from cytosolic to membrane fractions, but under the same conditions, glucose (20 mM) did not cause such a redistribution of PKC activity. PMA-induced translocation of PKC to the membrane fraction was also observed in electrically permeabilised islets, in which recovery of the enzyme activity was enhanced by buffering the intracellular Ca2+ concentration to 50 nM and supplying the permeabilised islets with protease inhibitors.

Intracellular calcium, insulin secretion, and action

Changes in cytosolic free calcium concentration [( Ca2+]i) constitute an important element of signal transduction in various cells. These changes either reflect alterations in calcium (Ca2+) fluxes or result from mobilization of intracellular Ca2+ stores. In pancreatic islet cells, an increase in [Ca2+]i is critical for secretagogue-induced insulin release. Thus, glucose evokes a rapid increase in [Ca2+]i, primarily by stimulating Ca2+ influx. Under physiologic conditions, glucose may also promote mobilization of intracellular Ca2+ stores by virtue of stimulating membrane phospholipid hydrolysis and formation of inositol triphosphate, a potent stimulus for Ca2+ mobilization. This action of glucose requires the presence of extracellular Ca2+. The magnitude of change in [Ca2+]i may not parallel the level of insulin release, suggesting that the role of [Ca2+]i in the process of insulin release must be considered in concert with other cellular mechanisms. The role of [Ca2+]i in promoting insulin action is a subject of continuous controversy. Recent observations that chelation of intracellular Ca2+ with quin-2 diminishes insulin action (and that of insulin mimetics) support the role of Ca2+ in mediating the insulin-generated signal. Insulin has also been demonstrated to increase [Ca2+]i in adipocytes in close association with its effect on 2-deoxyglucose uptake. Finally, in both pancreatic islet cells and adipocytes, high concentrations of either extracellular or intracellular Ca2+ inhibit cellular responsiveness. The optimal concentrations of cytosolic Ca2+ appear to be within the 140 to 350 nM range. When Ca2+ concentrations are too low or too high, the ability of pancreatic islets and insulin target cells to respond appropriately to physiologic stimuli is significantly diminished. Impaired cellular Ca2+ homeostasis (either primary or secondary to other cellular lesions) may represent a crucial and identical link in the pathogenesis of impaired insulin secretion and in the pathogenesis of impaired insulin action.

Protein phosphorylation in electrically permeabilized islets of Langerhans. Effects of Ca2+, cyclic AMP, a phorbol ester and noradrenaline

The incorporation of 32P from [gamma-32P]ATP into intracellular proteins was studied in electrically permeabilized rat islets of Langerhans. Ca2+ (10 microM), cyclic AMP (100 microM) and a protein kinase C-activating phorbol ester, phorbol 13-myristate 12-acetate (PMA; 100 nM) produced marked changes in the phosphorylation state of a number of proteins in permeabilized islets after incubation for 1 min at 37 degrees C. Ca2+ modified the effects of cyclic AMP and PMA on protein phosphorylation. Noradrenaline (10 microM) had no detectable effects on Ca2+-dependent protein phosphorylation, but significantly inhibited Ca2+-induced insulin secretion from electrically permeabilized islets. These results suggest that electrically permeabilized islets offer a useful model in which to study rapid events in protein phosphorylation as a mechanism of stimulus-secretion coupling. If the rapid Ca2+-induced effects on protein phosphorylation are involved in the control of insulin secretion, the results of this study also imply that part of the catecholamine inhibition of insulin secretion occurs at a stage in the secretory pathway beyond the activation of the regulated protein kinases.

Reversible plasma membrane ultrastructural changes correlated with electropermeabilization in Chinese hamster ovary cells

Chinese hamster ovary cells (CHO) grown in monolayers were permeabilized to molecules with molecular weight up to 1000 by high intensity 100 mus square wave electric field pulses. This permeability was transient and the cell viability was not affected. It was not possible for molecules with molecular weight larger than 1500 to penetrate inside the cytoplasm if lytic pulsing conditions were not used. In order to investigate the ultrastructural changes associated with this transient and limited permeabilization, cells were chemically fixed a few seconds after their pulsation and observed by electron microscopy. By scanning electron microscopy, numerous microvilli and blebs were observed almost immediately after application of the field. No other membrane changes were observed. Permeabilization of the membrane was visualized at the electron microscopic level by penetration of Ruthenium red. The appearance of osmotic pressure-dependent 'blebs' was indicative of local weakening of the plasma membrane. Most of these effects were fully reversible and disappeared within 30 min at 37 degrees C with the formation of huge polykaryons when cells were in contact before pulsing.

Characterization of the inositol 1,4,5-trisphosphate-induced Ca2+ release in pancreatic beta-cells

Pancreatic beta-cells isolated from obese-hyperglycaemic mice released intracellular Ca2+ in response to carbamoylcholine, an effect dependent on the presence of glucose. The effective Ca2+ concentration reached was sufficient to evoke a transient release of insulin. When the cells were deficient in Ca2+, the Ca2+ pool sensitive to carbamoylcholine stimulation was equivalent to that released by ionomycin. Unlike intact cells, cells permeabilized by high-voltage discharges failed to generate either inositol 1,4,5-triphosphate (InsP3) or to release Ca2+ after exposure to carbamoylcholine. However, the permeabilized cells released insulin sigmoidally in response to increasing concentrations of Ca2+. Also in the absence of functional mitochondria these cells exhibited a large ATP-dependent buffering of Ca2+, enabling the maintenance of an ambient Ca2+ concentration corresponding to about 150 nM even after several additional pulses of Ca2+. InsP3, maximally effective at 6 microM, promoted a rapid and pronounced release of Ca2+. The InsP3-sensitive Ca2+ pool was rapidly filled and lost its Ca2+ late after ATP depletion. The transient nature of the Ca2+ signal was not overcome by repetitive additions of InsP3. It was possible to restore the response to InsP3 after a delay of approx. 20 min, an effect which had less latency after the addition of Ca2+. These latter findings argue against degradation and/or desensitization as factors responsible for the transiency in InsP3 response. It is suggested that Ca2+ released by InsP3 is taken up by a part of the endoplasmic reticulum (ER) not sensitive to InsP3. On metabolism of InsP3, Ca2+ recycles to the InsP3-sensitive pool, implying that this pool indeed has a very high affinity for the ion. The presence of functional mitochondria did not interfere with the recycling process. The ER in pancreatic beta-cells is of major importance in buffering Ca2+, but InsP3 only modulates Ca2+ transport for a restricted period of time following immediately upon its formation. Thereafter the non-sensitive part of the ER takes over the continuous regulation of Ca2+ cycling.

Regulation of exocytosis in electrically permeabilized insulin-secreting cells. Evidence for Ca2+ dependent and independent secretion

The regulation of insulin secretion from RINm5F cells exposed to high voltage discharge has been investigated. Electron microscopy revealed that the overall structure of the cells was preserved after permeabilization. In this preparation insulin release was stimulated by Ca2+ (EC50 = 2.4 microM). The stable GTP analogue GTP gamma S enhanced secretion both at intermediate (nano- to micromolar) and vanishingly low (less than 10 pM) Ca2+ concentrations. At optimal Ca2+ (10 microM) the effect of GTP gamma S was greatly reduced. We investigated whether the secretory response to GTP analogues was mediated by any of three enzyme systems regulated by GTP-binding proteins, i.e. generation of cyclic AMP by adenylate cyclase, of diacylglycerol by phospholipase C and of arachidonic acid by phospholipase A2. The involvement of these messenger systems could be excluded as (i) cyclic AMP only had minor, Ca2+ dependent effects, (ii) phospholipase C was not activated in the absence of Ca2+ and insulin secretion due to the phorbol ester TPA displayed a different Ca2+ dependency, (iii) arachidonic acid did not elicit Ca2+ independent insulin secretion. These results, taken together with the finding that insulin secretion due to Ca2+ or TPA is attenuated by the inhibitory guanine nucleotide GDP beta S, suggest the existence of a regulatory site in exocytosis which is sensitive to guanine nucleotides.

The dual effector system for exocytosis in mast cells: obligatory requirement for both Ca2+ and GTP

The secretory process is a coordinated cellular response, initiated by occupation of surface receptors and comprising an ordered sequence of biochemical steps subject to multiple controls. Conceptually we can divide the sequence into two main sections comprising early, receptor-mediated events leading to generation of intracellular second messengers, and later events leading to membrane fusion and exocytosis. With the discovery that occupation of Ca2+ mobilising receptors leads to activation of polyphosphoinositide phosphodiesterase (PPI-pde) through the mediation of a G-protein (Gp), all the early events can be ascribed to the plasma membrane. Investigation of the exocytotic stage of secretion has been simplified by the use of permeabilised cells in which the composition of the cytosol can be precisely controlled. We have used streptolysin-O, a bacterial cytolysin which generates protein-sized pores in the plasma membrane, to investigate the exocytotic mechanism of rat mast cells. We find that in addition to the activation of PPI-dpe, GTP also acts in concert with Ca2+ at, or close to, the exocytotic site. Exocytosis can occur after substantial depletion of cytosol lactate dehydrogenase and 3-phosphoglycerate kinase indicating that soluble cytosol proteins are unlikely to play any role. There is no absolute requirement for ATP or phosphorylating nucleotide in exocytosis though when present the effective affinities of the two obligatory effectors (i.e. Ca2+ and GTP) are substantially enhanced.

Rat mast cells permeabilised with streptolysin O secrete histamine in response to Ca2+ at concentrations buffered in the micromolar range

Rat peritoneal mast cells have been permeabilised by treatment with streptolysin O which generates membrane lesions of macromolecular dimensions. In the presence of Ca2+ buffered at concentrations in the micromolar range, the permeabilised mast cells release histamine, beta-N-acetylglucosaminidase and lactate dehydrogenase. Release of the two secretory components (but not lactate dehydrogenase) has an obligatory requirement for a nucleoside triphosphate and micromolar concentrations of Ca2+. Inosine triphosphate (ITP) supports the release reaction better than ATP does. It is concluded that the secretory materials are released from the cells by an exocytotic mechanism, while lactate dehydrogenase leaks from the cells through the toxin-generated lesions. By initially withholding and then supplying Ca2+ to the permeabilised cells, it is shown that the exocytotic secretory reaction can persist even when the cytosol is depleted of the bulk of soluble proteins. The streptolysin O treated mast cell preparation represents a simplified system with which to study the mechanism of exocytosis.

Effect of pore formers on intracellular calcium

An increase in non-specific permeability across the plasma membrane of cells bathed in physiological media leads to an increase in intracellular Ca2+. The extent of the increase depends on the extent of the membrane lesion; the duration of the increase depends on the metabolic capability of cells to pump Ca2+ out. A slight, transient increase in cytosolic Ca2+ can be beneficial because it activates repair mechanisms; a large, sustained, increase is toxic because it impairs normal cell function. Extracellular Ca2+ prevents increases in non-specific permeability: only when the protective action of extracellular Ca2+ is overcome by pore-forming agents does intracellular Ca2+ rise to toxic levels. The beneficial actions of Ca2+, superimposed on its damaging effect, account for some of the paradoxical effects of Ca2+ that have been described in the literature.

Regulation of insulin secretion by cAMP in rat islets of Langerhans permeabilised by high-voltage discharge

Adenosine 3',5-cyclic monophosphate (cAMP) was shown to stimulate insulin secretion from electrically permeabilised islets of Langerhans incubated in Ca2+/EGTA buffers. cAMP-induced insulin secretion occurred in the presence of either sub-stimulatory (50 nM) or stimulatory (greater than 100 nM) concentrations of Ca2+. Similar effects on secretion were obtained in response to 8-bromo-cAMP (8-Br-cAMP) or the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine. Forskolin (0.2-20 microM) increased adenylate cyclase activity and enhanced insulin secretion from the permeabilised islets. These results suggest that, in electrically permeabilised islets, cAMP-induced insulin secretion is not dependent on changes in cytosolic Ca2+.