The digitonin-permeabilized pancreatic islet model. Effect of myo-inositol 1,4,5-trisphosphate on Ca2+ mobilization

Control of insulin secretion by cytochrome C and calcium signaling in islets with impaired metabolism

The aim of the study was to assess the relative control of insulin secretion rate (ISR) by calcium influx and signaling from cytochrome c in islets where, as in diabetes, the metabolic pathways are impaired. This was achieved either by culturing isolated islets at low (3 mm) glucose or by fasting rats prior to the isolation of the islets. Culture in low glucose greatly reduced the glucose response of cytochrome c reduction and translocation and ISR, but did not affect the response to the mitochondrial fuel α-ketoisocaproate. Unexpectedly, glucose-stimulated calcium influx was only slightly reduced in low glucose-cultured islets and was not responsible for the impairment in glucose-stimulated ISR. A glucokinase activator acutely restored cytochrome c reduction and translocation and ISR, independent of effects on calcium influx. Islets from fasted rats had reduced ISR and cytochrome c reduction in response to both glucose and α-ketoisocaproate despite normal responses of calcium. Our data are consistent with the scenario where cytochrome c reduction and translocation are essential signals in the stimulation of ISR, the loss of which can result in impaired ISR even when calcium response is normal.

Intracellular Ca(2+) modulation of ATP-sensitive K(+) channel activity in acetylcholine-induced activation of rat pancreatic beta-cells

We investigated the mechanism by which acetylcholine (ACh) regulates insulin secretion from rat pancreatic beta-cells. In an extracellular solution with 5.5 mM glucose, ACh increased the rate of insulin secretion from rat islets. In islets treated with bisindolylmaleimide (BIM), a PKC inhibitor, ACh still increased insulin secretion, but the increment was lower than that without BIM. In the presence of nifedipine, an L-type Ca(2+) channel blocker, on the other hand, ACh did not increase insulin secretion. In isolated rat pancreatic beta-cells, ACh caused depolarization followed by action potentials. This ACh effect was observed even in cells treated with BIM. In the presence of nifedipine, ACh caused only depolarization. These ACh effects were prevented by atropine. In the perforated whole-cell configuration, ramp pulses from -90 to -50 mV induced membrane currents mostly through ATP-sensitive K(+) channels (K(ATP)). These currents were reduced in size by ACh in cells either treated or untreated with BIM; whereas the loading of cells with U-73122 (a phospholipase C inhibitor) or BAPTA/AM (a Ca(2+) chelator) abolished the ACh effect. In the standard whole-cell configuration, ACh reduced the currents through K(ATP) with 0.5 mM EGTA, but not with 10 mM EGTA, in the pipette solution. Intracellular application of GDPbetaS or heparin also inhibited the ACh effect. In the inside-out single-channel recordings, elevation of the Ca(2+) concentration inside the membrane from 10 nM-10 microM decreased K(ATP) activity only in the presence of ATP. The affinity of ATP to K(ATP) became 4.5 times higher with the higher concentration of Ca(2+). These results suggest that Ca(2+) from ACh receptor signaling modulates the sensitivity of K(ATP) to ATP. A positive-feedback mechanism of intracellular Ca(2+)-dependent Ca(2+) influx was also demonstrated.

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

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

Synaptotagmin III/VII isoforms mediate Ca2+-induced insulin secretion in pancreatic islet beta -cells

Synaptotagmins (Syt) play important roles in Ca(2+)-induced neuroexocytosis. Insulin secretion of the pancreatic beta-cell is dependent on an increase in intracellular Ca(2+); however, Syt involvement in insulin exocytosis is poorly understood. Reverse transcriptase-polymerase chain reaction studies showed the presence of Syt isoforms III, IV, V, and VII in rat pancreatic islets, whereas Syt isoforms I, II, III, IV, V, VII, and VIII were present in insulin-secreting betaTC3 cell. Syt III and VII proteins were identified in rat islets and betaTC3 and RINm5F beta-cells by immunoblotting. Confocal microscopy showed that Syt III and VII co-localized with insulin-containing secretory granules. Two-fold overexpression of Syt III in RINm5F beta-cell (Syt III cell) was achieved by stable transfection, which conferred greater Ca(2+) sensitivity for exocytosis, and resulted in increased insulin secretion. Glyceraldehyde + carbachol-induced insulin secretion in Syt III cells was 2.5-fold higher than control empty vector cells, whereas potassium-induced secretion was 6-fold higher. In permeabilized Syt III cells, Ca(2+)-induced and mastoparan-induced insulin secretion was also increased. In Syt VII-overexpressing RINm5F beta-cells, there was amplification of carbachol-induced insulin secretion in intact cells and of Ca(2+)-induced and mastoparan-induced insulin secretion in permeabilized cells. In conclusion, Syt III/VII are located in insulin-containing secretory granules, and we suggest that Syt III/VII may be the Ca(2+) sensor or one of the Ca(2+) sensors for insulin exocytosis of the beta-cell.

Regulation of the type III InsP(3) receptor by InsP(3) and ATP

Many hormones and neurotransmitters raise intracellular calcium (Ca(2+)) by generating InsP(3) and activating the inositol 1,4, 5-trisphosphate receptor (InsP(3)R). Multiple isoforms with distinct InsP(3) binding properties () have been identified (). The type III InsP(3)R lacks Ca(2+)-dependent inhibition, a property that makes it ideal for signal initiation (). Regulation of the type III InsP(3)R by InsP(3) and ATP was explored in detail using planar lipid bilayers. In comparison to the type I InsP(3)R, the type III InsP(3)R required a higher concentration of InsP(3) to reach maximal channel activity (EC(50) of 3.2 microM versus 0.5 microM for the types III and I InsP(3)R, respectively). However, the type III InsP(3)R did reach a 2.5-fold higher level of activity. Although activation by InsP(3) was isoform-specific, regulation by ATP was similar for both isoforms. In the presence of 2 microM InsP(3), low ATP concentrations (<6 mM) increased the open probability and mean open time. High ATP concentrations (>6 mM) decreased channel activity. These results illustrate the complex nature of type III InsP(3)R regulation. Enhanced channel activity in the presence of high InsP(3) may be important during periods of prolonged stimulation, whereas allosteric modulation by ATP may help to modulate intracellular Ca(2+) signaling.

Glucose regulation of free Ca(2+) in the endoplasmic reticulum of mouse pancreatic beta cells

Free Ca(2+) was measured in organelles of individual mouse pancreatic beta cells loaded with the low affinity indicator furaptra. After removal of cytoplasmic indicator by controlled digitonin permeabilization the organelle Ca(2+) was located essentially in the endoplasmic reticulum (ER), >90% being sensitive to inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPases. The Ca(2+) accumulation in the ER of intact beta cells depended in a hyperbolic fashion on the glucose concentration with half-maximal and maximal filling at 5.5 and >20 mM, respectively. Also elevation of cytoplasmic Ca(2+) by K(+) depolarization significantly enhanced the Ca(2+) accumulation. In permeabilized beta cells 1-3 mM ATP caused rapid Ca(2+) filling of the ER reaching almost 500 microM. At 50 nM, Ca(2+) ER became half-maximally filled at 45 microM ATP, whereas only 3.5 microM ATP was required at 200 nM Ca(2+). Inositol 1,4,5-trisphosphate induced a rapid release of about 65% of the ER Ca(2+), and its precursor phosphatidylinositol 4,5-bisphosphate was found to slowly mobilize 75% by another mechanism. It is concluded that glucose is an efficient stimulator of Ca(2+) uptake in the ER of pancreatic beta cells both by increasing ATP and cytoplasmic Ca(2+). Because physiological concentrations of cytoplasmic ATP are in the mM range, Ca(2+) sequestration can be anticipated to be modulated by factors reducing its ATP sensitivity.

Defects in inositol 1,4,5-trisphosphate receptor expression, Ca(2+) signaling, and insulin secretion in the anx7(+/-) knockout mouse

The mammalian anx7 gene codes for a Ca(2+)-activated GTPase, which supports Ca(2+)/GTP-dependent secretion events and Ca(2+) channel activities in vitro and in vivo. To test whether anx7 might be involved in Ca(2+) signaling in secreting pancreatic beta cells, we knocked out the anx7 gene in the mouse and tested the insulin-secretory properties of the beta cells. The nullizygous anx7 (-/-) phenotype is lethal at embryonic day 10 because of cerebral hemorrhage. However, the heterozygous anx7 (+/-) mouse, although expressing only low levels of ANX7 protein, is viable and fertile. The anx7 (+/-) phenotype is associated with a substantial defect in insulin secretion, although the insulin content of the islets, is 8- to 10-fold higher in the mutants than in the normal littermate control. We infer from electrophysiological studies that both glucose-stimulated secretion and voltage-dependent Ca(2+) channel functions are normal. However, electrooptical recordings indicate that the (+/-) mutation has caused a change in the ability of inositol 1,4,5-trisphosphate (IP(3))-generating agonists to release intracellular calcium. The principle molecular consequence of lower anx7 expression is a profound reduction in IP(3) receptor expression and function in pancreatic islets. The profound increase in islets, beta cell number, and size may be a means of compensating for less efficient insulin secretion by individual defective pancreatic beta cells. This is a direct demonstration of a connection between glucose-activated insulin secretion and Ca(2+) signaling through IP(3)-sensitive Ca(2+) stores.

Insulin receptor substrate 1-induced inhibition of endoplasmic reticulum Ca2+ uptake in beta-cells. Autocrine regulation of intracellular ca2+ homeostasis and insulin secretion

To understand the role of the insulin receptor pathway in beta-cell function, we have generated stable beta-cells (betaIRS1-A) that overexpress by 2-fold the insulin receptor substrate-1 (IRS-1) and compared them to vector-expressing controls. IRS-1 overexpression dramatically increased basal cytosolic Ca2+ levels from 81 to 278 nM, but it did not affect Ca2+ response to glucose. Overexpression of the insulin receptor also caused an increase in cytosolic Ca2+. Increased cytosolic Ca2+ was due to inhibition of Ca2+ uptake by the endoplasmic reticulum, because endoplasmic reticulum Ca2+ uptake and content were reduced in betaIRS1-A cells. Fractional insulin secretion was significantly increased 2-fold, and there was a decrease in betaIRS1-A insulin content and insulin biosynthesis. Steady-state insulin mRNA levels and glucose-stimulated ATP were unchanged. High IRS-1 levels also reduced beta-cell proliferation. These data demonstrate a direct link between the insulin receptor signaling pathway and the Ca2+-dependent pathways regulating insulin secretion of beta-cells. We postulate that during regulated insulin secretion, released insulin binds the beta-cell insulin receptor and activates IRS-1, thus further increasing cytosolic Ca2+ by reducing Ca2+ uptake. We suggest the existence of a novel pathway of autocrine regulation of intracellular Ca2+ homeostasis and insulin secretion in the beta-cell of the endocrine pancreas.

cAMP-dependent mobilization of intracellular Ca2+ stores by activation of ryanodine receptors in pancreatic beta-cells. A Ca2+ signaling system stimulated by the insulinotropic hormone glucagon-like peptide-1-(7-37)

Glucagon-like peptide-1 (GLP-1) is an intestinally derived insulinotropic hormone currently under investigation for use as a novel therapeutic agent in the treatment of type 2 diabetes mellitus. In vitro studies of pancreatic islets of Langerhans demonstrated that GLP-1 interacts with specific beta-cell G protein-coupled receptors, thereby facilitating insulin exocytosis by raising intracellular levels of cAMP and Ca2+. Here we report that the stimulatory influence of GLP-1 on Ca2+ signaling results, in part, from cAMP-dependent mobilization of ryanodine-sensitive Ca2+ stores. Studies of human, rat, and mouse beta-cells demonstrate that the binding of a fluorescent derivative of ryanodine (BODIPY FL-X ryanodine) to its receptors is specific, reversible, and of high affinity. Rat islets and BTC3 insulinoma cells are shown by reverse transcriptase polymerase chain reaction analyses to express mRNA corresponding to the type 2 isoform of ryanodine receptor-intracellular Ca2+ release channel (RYR2). Single-cell measurements of [Ca2+]i using primary cultures of rat and human beta-cells indicate that GLP-1 facilitates Ca2+-induced Ca2+ release (CICR), whereby mobilization of Ca2+ stores is triggered by influx of Ca2+ through L-type Ca2+ channels. In these cells, GLP-1 is shown to interact with metabolism of D-glucose to produce a fast transient increase of [Ca2+]i. This effect is reproduced by 8-Br-cAMP, but is blocked by a GLP-1 receptor antagonist (exendin-(9-39)), a cAMP antagonist ((Rp)-cAMPS), an L-type Ca2+ channel antagonist (nimodipine), an antagonist of the sarco(endo)plasmic reticulum Ca2+ ATPase (thapsigargin), or by ryanodine. Characterization of the CICR mechanism by voltage clamp analysis also demonstrates a stimulation of Ca2+ release by caffeine. These findings provide new support for a model of beta-cell signal transduction whereby GLP-1 promotes CICR by sensitizing intracellular Ca2+ release channels to the stimulatory influence of cytosolic Ca2+.

Arachidonic acid induces mobilization of calcium stores and c-jun gene expression: evidence that intracellular calcium release is associated with c-jun activation

Arachidonic acid (AA) plays a signaling role in the induction of several genes. We previously demonstrated that AA induces c-jun gene expression in the stromal cell line +/+.1 LDA 11 by a signaling pathway involving activation of the c-jun amino-terminal kinase (JNK). This study investigated the role of calcium in AA signaling of c-jun activation in +/+.1 LDA 11 cells. AA (10-50 microM) caused a rapid dose-dependent rise in cytosolic calcium. AA-induced calcium mobilization involved both influx of extracellular calcium and the release of intracellular calcium. The importance of calcium was investigated by variation of the extracellular calcium concentration, chelation of intracellular calcium and by calcium ionophore-induced influx of extracellular calcium. AA-induced c-jun gene expression and increased luciferase activity of a construct containing the high affinity AP-1 binding site was decreased in cells preincubated with the intracellular calcium chelator 1,2-bis(o-aminophenoxy)-eThane-N,N,N',N',-tetraacetic acid tetra(aceToxymethyl-esTer) (BAPTA-AM, 10 microM) prior to stimulation with AA. Similarly, chelation of intracellular calcium decreased AA-induced JNK activation. On the contrary, changes in the extracellular calcium concentration had no effect. Also, ionophore A23187 failed to induce c-jun and JNK activation either alone than in combination with AA. These results suggested that calcium was required for AA-dependent activation of c-jun, but that calcium alone was insufficient to induce activation of c-jun. Thus, release of calcium from intracellular stores is implicated in the signaling pathway of AA-induced c-jun activation in stromal cells.

Muscarinic activation of Ca2+/calmodulin-dependent protein kinase II in pancreatic islets. Temporal dissociation of kinase activation and insulin secretion

We have demonstrated previously that glucose activates the multifunctional Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) in isolated rat pancreatic islets in a manner consistent with a role of this enzyme in the regulation of insulin secretion [Wenham, Landt and Easom (1994) J. Biol. Chem. 269, 4947-4952]. In the current study, the muscarinic agonist, carbachol, has been shown to induce the conversion of CaM kinase II into a Ca(2+)-independent, autonomous form indicative of its activation. Maximal activation (2-fold) was achieved by 15 s, followed by a rapid return to basal levels by 1 min. This response was primarily the result of the mobilization of Ca2+ from intracellular stores since it was not affected by a concentration (20 microM) of verapamil that completely prevented the activation of CaM kinase II by glucose. Surprisingly, carbachol added prior to, or simultaneously with, glucose attenuated nutrient activation of CaM kinase II. This effect was mimicked by cholecystokinin-8 (CCK-8) and thapsigargin, suggesting its mediation by phospholipase C and the mobilization of intracellular Ca2+. In contrast, carbachol, CCK-8 and thapsigargin markedly potentiated glucose (12 mM)-induced insulin secretion. These results suggest that CaM kinase II activation can be temporally dissociated from insulin secretion but do not exclude the potential dependence of insulin exocytosis on CaM kinase II-mediated protein phosphorylation.

Effects of caffeine on intracellular calcium release and calcium influx in a clonal beta-cell line RINm5F

We studied the effects of caffeine on intracellular calcium in a clonal insulin-secreting cell line RINm5F. Caffeine (1-30 mM) induced a dose-dependent intracellular Ca2+ release and Ca2+ influx. Pretreatment with combination of ryanodine (10 microM) and caffeine (10 mM), but not ryanodine alone, abolished subsequent caffeine-induced release of intracellular Ca2+. Pretreatment with another ryanodine receptor blocker procaine (0.1, 0.3, and 1mM) antagonized the caffeine-induced release of intracellular Ca2+. Pretreatment with thapsigargin (2 microM), the endoplasmic reticular Ca2+-ATPase inhibitor, abolished both the caffeine- and arginine vasopressin (AVP)-induced Ca2+ release. AVP induces Ca2+ release by increasing the formation of IP3. Pretreatment with AVP greatly reduced caffeine-induced Ca2+ release whereas pretreatment with caffeine also reduced AVP-induced Ca2+ release. The L-type Ca2+ channel blocker nimodipine (1 microM) inhibited caffeine-induced Ca2+ influx. In addition, depletion of intracellular Ca2+ stores with thapsigargin did not affect caffeine-induced Ca2+ influx. These results suggested that: 1) the ryanodine- and caffeine-sensitive Ca2+ store exists in RINm5F cells, 2) most of these Ca2+ release channels in RINm5F cells are closed in the resting state, since pretreatment with ryanodine alone failed to block the caffeine-induced Ca2+ release, 3) there is a "cross-talk" between the caffeine- and IP3-sensitive pools in RINm5F cells, 4) caffeine increases Ca2+ entry in RINm5F cells through L-type voltage-dependent Ca2+ channels, and 5) the caffeine-induced Ca2+ influx is independent of the Ca2+ release from the intracellular Ca2+ stores.

Diacylglycerol hydrolysis to arachidonic acid is necessary for insulin secretion from isolated pancreatic islets: sequential actions of diacylglycerol and monoacylglycerol lipases

Arachidonic acid has been implicated as a second messenger in insulin secretion on the basis of (1) mobilization of intracellular Ca2+ from the endoplasmic reticulum of islets and (2) amplification of voltage-dependent Ca2+ entry. The insulin secretagogues D-glucose and the muscarinic agonist carbachol both increase unesterified arachidonic acid accumulation in isolated islets. We now show that diacylglycerol, a product of phospholipase C action, is a major source of free arachidonic acid in islets. Diacylglycerol hydrolysis in islets occurs through a two-step process. In the first step, the sn-1 bond of 1-stearoyl-2-arachidonyl-sn-glycerol is hydrolyzed by a diacylglycerol lipase, giving rise to 2-arachidonyl-sn-glycerol. Next, the sn-2 bond of 2-arachidonyl-sn-glycerol is hydrolyzed by a monoacylglycerol lipase, which is the rate-limiting step, releasing unesterified arachidonic acid. Both diacylglycerol lipase and monoacylglycerol lipase are highly enriched in the plasma membrane of beta-cells. Diacylglycerol lipase activity in islet homogenates is selectively inhibited in a dose-dependent manner by the compound RHC-80267, a specific diacylglycerol lipase inhibitor. RHC-80267 inhibits glucose- and carbachol-induced insulin release from intact islets in a dose-dependent manner that parallels its inhibition of diacylglycerol lipase activity. Importantly, RHC-80267, at concentrations that almost completely inhibit diacylglycerol lipase activity and glucose- and carbachol-induced insulin secretion by islets, markedly inhibits glucose- and carbachol-induced increases in islet arachidonic acid levels, as measured by gas chromatography with electron-capture detection of its pentafluorobenzyl esters. RHC-80267 did not significantly affect islet glucose oxidation, phospholipase C, monoacylglycerol lipase, or phospholipase A2. Since glucose and carbachol are known to stimulate phospholipase C, our observations indicate that diacylglycerol is an important source of arachidonic acid and other free fatty acids in islets. Furthermore, production of arachidonic acid from the hydrolysis of diacylglycerol is essential for glucose- and carbachol-induced insulin secretion.

Activation of phospholipase C by heat shock requires GTP analogs and is resistant to pertussis toxin

The heat shock response in mammals consists of a complex array of intracellular reactions initiated by stress, although its regulation is poorly understood. We have investigated the role of transmembrane signal transduction in the response, examining mechanisms involved in the activation of phospholipase C (PLC) by heat shock. In rodent fibroblasts permeabilized with digitonin, heat shock and receptor-mediated PLC activity exhibited a strict GTP analog dependency. This indicates that heat shock-mediated phospholipase activation, in common with receptor mediated stimulation, does not involve direct effects on the phospholipases and suggests the participation of GTP binding (G) proteins in the activation process. When cells were treated with the inhibitor pertussis toxin (PTX), the phospholipases retained their inducibility by heat shock, but became refractory to thrombin treatment, indicating that heat shock may influence PLC activity through a distinct population of G proteins compared to thrombin. The data seem to exclude a role for PTX sensitive G proteins in the production of IP3 after heating and suggest a pathway involving the direct thermal activation of the Gq class of G proteins, which are coupled to the PLC beta 1 isoform.

Cyclic ADP-ribose in insulin secretion from pancreatic beta cells

Inositol 1,4,5-trisphosphate (IP3) is thought to be a second messenger for intracellular calcium mobilization. However, in a cell-free system of islet microsomes, cyclic adenosine diphosphate-ribose (cADP-ribose), a nicotinamide adenine dinucleotide (NAD+) metabolite, but not IP3, induced calcium release. In digitonin-permeabilized islets, cADP-ribose and calcium, but not IP3, induced insulin secretion. Islet microsomes released calcium when combined with the extract from intact islets that had been incubated with high concentrations of glucose. Sequential additions of cADP-ribose inhibited the calcium release response to extracts from islets treated with high concentrations of glucose. Conversely, repeated additions of the islet extract inhibited the calcium release response to a subsequent addition of cADP-ribose. These results suggest that cADP-ribose is a mediator of calcium release from islet microsomes and may be generated in islets by glucose stimulation, serving as a second messenger for calcium mobilization in the endoplasmic reticulum.

Carbachol stimulation of phospholipase A2 and insulin secretion in pancreatic islets

Arachidonic acid has been implicated as a second messenger in insulin secretion by islets of Langerhans. D-Glucose, the major physiological stimulus, increases unesterified arachidonate accumulation in islets. We now show, for the first time, that the muscarinic agonist carbachol, at concentrations which stimulate insulin secretion, causes a rapid and nearly 3-fold increase in arachidonic acid accumulation in islets. The combination of glucose and carbachol has an additive effect on unesterified arachidonate release. There is a large component of secretagogue-induced arachidonate accumulation that is independent of extracellular Ca2+. Carbachol stimulation of arachidonic acid release is mediated by activation of phospholipase A2, as demonstrated by early increases in endogenous lysophosphatidylcholine. In addition to phospholipase A2 activation, carbachol-induced arachidonic acid accumulation also appears to involve diacylglycerol hydrolysis, since the diacylglycerol lipase inhibitor RG80267 partly inhibited arachidonic acid accumulation. In contrast, glucose-induced arachidonic acid accumulation appears to reflect diacylglycerol hydrolysis entirely. Our observations indicate that phospholipase A2 has an important role in muscarinic-induced insulin secretion.

Parallel effects of arachidonic acid on insulin secretion, calmodulin-dependent protein kinase activity and protein kinase C activity in pancreatic islets

A potential role of arachidonic acid in the modulation of insulin secretion was investigated by measuring its effects on calmodulin-dependent protein kinase and protein kinase C in islet subcellular fractions. The results were interpreted in the light of arachidonic acid effects on insulin secretion from intact islets. Arachidonic acid could replace phosphatidylserine in activation of cytosolic protein kinase C (K0.5 of 10 microM) and maximum activation was observed at 50 microM arachidonate. Arachidonic acid did not affect the Ca2+ requirement of the phosphatidylserine-stimulated activity. Arachidonic acid (200 microM) inhibited (greater than 90%) calmodulin-dependent protein kinase activity (K0.5 = 50-100 microM) but modestly increased basal phosphorylation activity (no added calcium or calmodulin). Arachidonic acid inhibited glucose-sensitive insulin secretion from islets (K0.5 = 24 microM) measured in static secretion assays. Maximum inhibition (approximately 70%) was achieved at 50-100 microM arachidonic acid. Basal insulin secretion (3 mM glucose) was modestly stimulated by 100 microM arachidonic acid but in a non-saturable manner. In perifusion secretion studies, arachidonic acid (20 microM) had no effect on the first phase of glucose-induced secretion but nearly completely suppressed second phase secretion. At basal glucose (4 mM), arachidonic acid induced a modest but reproducible biphasic insulin secretion response which mimicked glucose-sensitive secretion. However, phosphorylation of an 80 kD protein substrate of protein kinase C was not increased when intact islets were incubated with arachidonic acid, suggesting that the small increases in insulin secretion seen with arachidonic acid were not mediated by protein kinase C. These data suggest that arachidonic acid generated by exposure of islets to glucose may influence insulin secretion by inhibiting the activity of calmodulin-dependent protein kinase but probably has little effect on protein kinase C activity.

Ca2+ release and InsP3 production in avian uterine cells: effects of PGF2 alpha and AVT

Primary cultures of smooth muscle cells isolated from the shell gland ("uterus") of the domestic hen were permeabilized with digitonin and loaded with 45Ca2+ in the presence of ATP. When these cells were stimulated with prostaglandin F2 alpha (PGF2 alpha), arginine vasotocin (AVT), or D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], there was a rapid, biphasic, and dose-related release of 45Ca2+ from nonmitochondrial pools. 2-Nitro-4-carboxyphenyl-N,N-diphenylcarbamate, an inhibitor of phospholipase C, had no effect on PGF2 alpha - and Ins(1,4,5)P3-promoted 45Ca2+ efflux, whereas it significantly inhibited AVT-stimulated and a stable analogue of GTP-stimulated 45Ca2+ release. In fura-2-loaded intact cells, both PGF2 alpha and AVT increased intracellular Ca2+ levels [( Ca2+]i) in a dose-related manner in the presence of extracellular Ca2+. However, omission of extracellular Ca2+ prevented a PGF2 alpha, but not AVT-induced, rise in [Ca2+]i In D-myo-[3H]inositol-labeled cells, 10 nM AVT caused a rapid, two- to threefold increase in [3H]-Insp3, whereas PGF2 alpha up to 1 microM was infective. Raising PGF2 alpha to 10 microM increased total inositol phosphates by 22% over controls (P less than 0.05). These results point to marked differences in the mechanisms by which AVT and PGF2 alpha regulate [Ca2+]i in uterine smooth muscle cells. It is suggested that the two agonists act in concert to initiate oviposition.

Inositol 1,4,5-trisphosphate- and arachidonic acid-induced calcium mobilization in T and B lymphocytes

Inositol triphosphate (IP3) formation and increase in intracytoplasmic calcium are mediators of signal transduction in lymphocytes. It has been proposed that IP3 induces Ca2+ release from intracellular stores. It is in order to study the relationship between these two events that we have analyzed the effect of IP3 addition on Ca2+ mobilization in permeabilized resting T and B lymphocytes, EBV-B lymphocytes, and HTLV1-T lymphocytes. IP3 induces a rapid and significant release of Ca2+ from the endoplasmic reticulum in a dose-dependent manner. Ca2+ release is more sensitive to IP3 addition in cycling cells (EBV-B lymphocytes and HTLV1-T lymphocytes) than in resting T and B lymphocytes. Arachidonic acid (AA) induces Ca2+ release from the endoplasmic reticulum (ER) in a manner similar to that of IP3. Neither component has an effect on Ca2+ accumulated in mitochondria, and they have no additive effects suggesting that they act on a similar Ca2+ pool. These results directly demonstrate that in T and B human lymphocytes IP3 mobilizes Ca2+ from ER as in other cellular systems and that other potential second messengers, namely AA, could play a significant role in the internal mobilization of calcium during T and B lymphocyte activation.

Prostaglandin E2 inhibits phosphoinositide metabolism in isolated pancreatic islets

Isolated islets of the rat labelled with myo-[3H]inositol showed decreased accumulation of total inositol phosphates (InsPs) and [3H]polyphosphoinositide hydrolysis in response to glucose after preincubation with prostaglandin E2 (PGE2). The response was concentration-dependent and specific for PGE2. PGE2 did not affect basal [3H]phosphoinositide hydrolysis or InsPs accumulation. Pertussis-toxin pretreatment antagonized the response to PGE2, whereas 8-bromo cyclic AMP was without effect. The PGE2-induced decrease in InsPs may contribute to the suppression of insulin secretion.

Effect of different insulin secretagogues and blocking agents on islet cell Ca2+-ATPase activity

Plasma membrane Ca2+-ATPase activity was measured in rat islet homogenates. The enzyme was inhibited, in a dose-dependent manner, when the islets were preincubated for 5 min with different concentrations of glucose (2 to 16 mM). This inhibition disappeared almost entirely after 15 min incubation, regardless of the glucose concentration in the medium. Simultaneous measurement of insulin in the medium revealed a stimulatory effect of glucose upon insulin secretion. The Ca2+-ATPase activity was also inhibited when the islets were preincubated for 3 min with other stimulators of insulin secretion such as gliclazide (76 microM), tolbutamide (1.5 mM), glucagon (1.4 microM) + theophylline (10 mM) and ketoisocaproic acid (15 mM). Conversely, the activity of the enzyme was significantly enhanced when the islets were preincubated briefly with the insulin secretion blocker, somatostatin (1.4 microM). Neither glucose nor any of the other substances tested when added directly to the enzyme assay medium modified significantly the Ca2+-ATPase activity measured in the islet homogenates. These results would suggest that the activity of the islet plasma membrane is modulated by one or more of the intracellular metabolites produced when the islets are challenged by the insulin stimulator or blocking agents.

Glucose-, calcium- and concentration-dependence of acetylcholine stimulation of insulin release and ionic fluxes in mouse islets

Mouse islets were used to define the glucose-dependence and extracellular Ca2+ requirement of muscarinic stimulation of pancreatic beta-cells. In the presence of a stimulatory concentration of glucose (10 mM) and of Ca2+, acetylcholine (0.1-100 microM) accelerated 3H efflux from islets preloaded with myo-[3H]inositol. It also stimulated 45Ca2+ influx and efflux, 86Rb+ efflux and insulin release. In the absence of Ca2+, only 10-100 microM-acetylcholine mobilized enough intracellular Ca2+ to trigger an early but brief peak of insulin release. At a non-stimulatory concentration of glucose (3 mM), 1 microM- and 100 microM-acetylcholine increased 45Ca2+ and 86Rb+ efflux in the presence and absence of extracellular Ca2+. However, only 100 microM-acetylcholine marginally increased 45Ca2+ influx and caused a small, delayed, stimulation of insulin release, which was abolished by omission of Ca2+. At a maximally effective concentration of glucose (30 mM), 1 microM- and 100 microM-acetylcholine increased 45Ca2+ influx and efflux only slightly, but markedly amplified insulin release. Again, only 100 microM-acetylcholine mobilized enough Ca2+ to trigger a peak of insulin release in the absence of Ca2+. The results thus show that only high concentrations of acetylcholine (greater than or equal to 10 microM) can induce release at low glucose or in a Ca2+-free medium. beta-Cells exhibit their highest sensitivity to acetylcholine in the presence of Ca2+ and stimulatory glucose. Under these physiological conditions, the large amplification of insulin release appears to be the result of combined effects of the neurotransmitter on Ca2+ influx, on intracellular Ca2+ stores and on the efficiency with which Ca2+ activates the releasing machinery.

GTP- and inositol 1,4,5-trisphosphate-induced release of 45Ca2+ from a membrane store co-localized with pancreatic-islet-cell plasma membrane

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], arising from hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], is proposed as the link between membrane-receptor activation and mobilization of Ca2+ from intracellular sites in hormone-secreting cells. The location of Ins(1,4,5)P3-sensitive membranes was investigated in cultured neonatal beta-cells. Membranes were obtained after lysis of cells attached to positively charged Sephadex. After lysis the presence of the enzyme markers 5'-nucleotidase, glucose-6-phosphatase, NADH-cytochrome c reductase, UDP-galactosyltransferase and succinate dehydrogenase indicated the mixed nature of the preparation. After sonication, however, UDP-galactosyltransferase and succinate dehydrogenase activities were undetectable, but 4.8% of total cellular glucose-6-phosphatase and 3.4% of total cellular NADH-cytochrome c reductase remained with 5'-nucleotidase in the preparation, indicating endoplasmic-reticulum association. ATP-dependent 45Ca2+ accumulation was shown in this preparation (410 +/- 24 pmol/mg of protein at 150 nM free Ca2+) and was inhibited by vanadate (100 microM). Ca2+ release was effected by Ins(1,4,5)P3, with half-maximal release at 0.5 +/- 0.14 microM-Ins(1,4,5)P3, t1/2 11.2 +/- 1.1 s. GTP- and guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG)-promoted release of 45Ca2+ was demonstrated in this preparation, but the kinetics of release (half-maximal Ca2+ release at 5.4 +/- 0.7 microM, with t1/2 77.3 +/- 6.9 s, and at 51.1 +/- 4.2 microM, with t1/2 19.0 +/- 2.2 s, for GTP and p[NH]ppG respectively), and the ability of neomycin sulphate to block p[NH]ppG-induced release only, are indicative of separate release mechanisms after treatment with these agents. A close association between plasma membrane and elements of the endoplasmic reticulum is indicated in this model, providing a possible mechanism for local alterations in free Ca2+ in the sub-plasma-membrane region.

Semisynthetic derivatives of inositol 1,4,5-trisphosphate substituted at the 1-phosphate group. Effects on calcium release from permeabilized guinea-pig parotid acinar cells and comparison with binding to aldolase A

Derivatives of inositol 1,4,5-(tris)phosphate [Ins(1,4,5)P3] substituted at phosphate 1 were compared with respect to their calcium releasing effect in permeabilized guinea pig parotid acinar cells and to their inhibitory action on aldolase A. sn-Glycero(3)-1-phospho-D-myo-inositol-4,5-(bis)phosphate, but also glycolaldehyde(2)-1-phospho-D-myo-inositol-4,5-(bis)phosphate [GcaPIns(4,5)P2] and its derivative N-octyl-aminoethanol(1)-1-phospho-D-myo-inositol-4,5-(bis)phosphate stimulated calcium release and inhibited aldolase A. The relative efficacy of the different derivatives of Ins(1,4,5)P3 was similar for both effects. N-Hydroxyethyl-2-aminoethanol(1)-1-phospho-D-myo-inositol-4,5-(bis)phosp hate [HeAetPIns(4,5)P2], another derivative of GcaPIns(4,5)P2 was considerably less effective on both parameters than the other Ins(1,4,5)P3 derivatives. Although the concentration leading to half-maximal activation of calcium release varied from 1.7 microM for Ins(1,4,5)P3 to 128 microM for HeAetPIns(4,5)P2, the maximal effect was the same for all derivatives. The results indicate that the 1-phosphate group of Ins(1,4,5)P3 can be modified without or with only minor loss of biological activity. This may be utilized for future studies aiming at elucidating the putative Ins(1,4,5)P3 binding site.

Effects of a phorbol ester and clomiphene on protein phosphorylation and insulin secretion in rat pancreatic islets

The potentiation of glucose-stimulated insulin release induced by 100 nM-12-O-tetradecanoylphorbol 13-acetate (TPA) was inhibited by clomiphene, an inhibitor of protein kinase C (PK C), in a dose-dependent manner. Clomiphene at concentrations up to 50 microM had a modest inhibitory action (27%) on insulin release stimulated by 10 mM-glucose alone, but had no effect on the potentiation of insulin release induced by forskolin. Islet PK C activity, associated with a particulate fraction, was stimulated maximally by 100 nM-TPA. This stimulation was blocked by clomiphene in a dose-dependent manner, with 50% inhibition at 30 microM. Incubation of intact islets with TPA after preincubation with [32P]Pi and 10 mM-glucose to label intracellular ATP resulted primarily in enhanced phosphorylation of a 37 kDa protein (mean value, +/- S.E.M., 36,700 +/- 600 Da; n = 7). This increased phosphorylation was blocked by the simultaneous inclusion of clomiphene. Subcellular fractionation revealed the presence of the 37 kDa phosphoprotein in a 24,000 g particulate fraction of islet homogenates. Neither clomiphene nor TPA affected the rate of glucose oxidation by islets. These results show that the phosphorylation state of a 37 kDa membrane protein parallels the modulation of insulin release induced by TPA and clomiphene and support a role for PK C in the insulin-secretory mechanism.

Dissociation between intracellular calcium mobilization and insulin secretion in isolated rat islets of Langerhans

The neuropeptide bombesin provoked a dose-dependent stimulation of 45Ca2+ efflux from pre-loaded islets of Langerhans. This response occurred rapidly, was not sustained and did not depend on the presence of extracellular calcium, suggesting that it resulted from the mobilization of intracellular calcium stores. Under conditions when large increases in 45Ca2+ efflux were observed, bombesin completely failed to stimulate the rate of insulin secretion. Similar results were also obtained with the muscarinic cholinergic agonist, carbachol. The data suggest that the release of calcium from intracellular pools is not sufficient to induce an increase in insulin secretion in normal islet cells.

Inositol lipids and DNA replication

Control of DNA synthesis by growth factors seems to depend upon the generation of intracellular mitogenic signals, which are responsible for initiating the sequence of events leading to the onset of DNA synthesis. Many growth factors have tyrosine kinase activity suggesting the proteins phosphorylated on tyrosine might be likely candidates as intracellular signals. Other candidates are the calcium and hydrogen ions whose concentrations change dramatically during the action of most growth factors, many of which also stimulate the hydrolysis of inositol lipids. In particular, certain growth factors stimulate the hydrolysis of phosphatidylinositol 4,5-bisphosphate to give the two second messengers diacylglycerol and inositol 1,4,5-trisphosphate (Ins1,4,5P3). The former stimulates protein kinase C, which is responsible for increasing intracellular pH by switching on an Na+-H+ exchanger. The water-soluble Ins1,4,5P3 released to the cytosol can be metabolized along two separate pathways: it can either be dephosphorylated to free inositol or it can be converted into additional inositol polyphosphates such as Ins1,3,4,5P4 and Ins1,3,4P3. These inositol phosphates seem to play a key role in regulating intracellular calcium, with Ins1,4,5P3 functioning to release internal calcium, whereas Ins1,3,4,5P4 may function to regulate the entry of external calcium. There is evidence to suggest that these internal messengers may converge on certain key processes responsible for initiating the programme of cell growth. It is argued that an increase in intracellular calcium might be an important intracellular signal for activating both the transcription of a family of early genes, typified by fos, as well as the enzyme S6 kinase, which phosphorylates the ribosomal protein S6 which may regulate protein synthesis. The increase in pH seems to play a permissive role and may create the necessary ionic milieu for S6 phosphorylation and protein synthesis to occur. The onset of RNA and protein synthesis, which occur within the first few minutes after the arrival of a growth factor, represent the initial events of the programme of cell growth which culminates in DNA synthesis and cell division.

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.

Mechanisms involved in intracellular calcium mobilization in isolated rat islets of Langerhans

1. The rate of 45Ca2+ efflux from prelabelled rat islets of Langerhans was stimulated by carbachol in a dose-dependent manner. 2. Significant stimulation occurred in the presence of 0.2 microM-carbachol; the response was half-maximal at 3-5 microM and was maximal at 20 microM. 3. Stimulation of 45Ca2+ efflux by carbachol was not dependent on the presence of extracellular Ca2+ and was enhanced in Ca2+-depleted medium. 4. Stimulation of 45Ca2+ efflux by 5 microM-carbachol occurred independently of any change in [3H]arachidonic acid release in prelabelled islets, and probably reflected generation of inositol trisphosphate in the cells. 5. The amphipathic peptide melittin failed to increase islet-cell 45Ca2+ efflux at a concentration of 1 microgram/ml, and caused only a modest increase at 10 micrograms/ml. 6. Despite its failure to increase 45Ca2+ efflux, melittin at 1 microgram/ml caused a marked enhancement of 3H release from islets that had been prelabelled with [3H]arachidonic acid. 7. The stimulation of 3H efflux caused by melittin correlated with a dose-dependent increase in the unesterified [3H]arachidonic acid content of prelabelled islets and with a corresponding decrease in the extent of labelling of islet phospholipids. 8. Combined addition of melittin (1 microgram/ml) and 5 microM-carbachol to perifused islets failed to augment 45Ca2+ efflux relative to that elicited by carbachol alone. 9. The data indicate that melittin promotes an increase in arachidonic acid availability in intact rat islets. They do not, however, support the proposal that this can either directly reproduce or subsequently modify the extent of intracellular Ca2+ mobilization induced by agents that cause an increase in inositol trisphosphate.

Mechanism of arachidonic acid-induced Ca2+ mobilization from rat liver microsomes

Recent evidence indicates that unesterified arachidonic acid functions as a mediator of intracellular Ca2+ mobilization by inducing Ca2+ release from the endoplasmic reticulum of pancreatic islet beta cells in a manner closely similar to that of inositol 1,4,5-trisphosphate. To test the generality and explore the mechanism of this phenomenon we have examined the effects of arachidonic acid on calcium accumulation and release by hepatocyte subcellular fractions enriched in endoplasmic reticulum (microsomes). At concentrations above 0.017 mumol/mg microsomal protein, arachidonate induced rapid (under 2 min) 45Ca2+ release from microsomes that had been preloaded with 45Ca2+. Arachidonate also suppressed microsomal 45Ca2+ accumulation when present during the loading period, as reflected by reduction both of 45Ca2+ accumulation at steady state and of the rate of uptake. Neither the cyclooxygenase inhibitor indomethacin nor the lipoxygenase/cyclooxygenase inhibitor BW755C suppressed arachidonate-induced 45Ca2+ release, indicating that this effect was not dependent upon oxygenation of the fatty acid to metabolites. The long-chain unsaturated fatty acids oleate and linoleate were less potent than arachidonate in inducing 45Ca2+ release, and the saturated fatty acid stearate did not exert this effect. Albumin prevented 45Ca2+ release by arachidonate, presumably by binding the fatty acid. As is the case for inositol 1,4,5-trisphosphate, the ability of arachidonate to induce 45Ca2+ release was dependent on the ambient free Ca2+ concentration. Arachidonate did not influence microsomal membrane permeability or Ca2+-ATPase activity and may exert its effects on microsomal Ca2+ handling by activation of a Ca2+ extrusion mechanism or by dissociating Ca2+ uptake from Ca2+-ATPase activity.

Relationship between cAMP and Ca2+ fluxes in human platelet membranes

The effect of cAMP (which involved a 23 kDa protein phosphorylation) has been studied on the Ca2+ uptake and Ca2+ release from a human platelet membrane vesicle fraction. It was tested in the presence of the catalytic subunit of the cAMP-dependent protein kinase (C Sub). The addition of C Sub increased the steady state level of the Ca2+ uptake into the membrane vesicles. The effect was enhanced when tested in the absence of Ca2+ precipitating agent. The response was proportional to the dose of C Sub. Moreover, the effect varied with the Ca2+ concentration. The effect of C Sub has been tested on the inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release. A phosphorylated state of the 23 kDa protein appeared to be necessary. Indeed, a phosphorylation inhibition prevented the IP3 effect and the addition of C Sub increased the percentage of released Ca2+ (without modification of the time course). However, the C Sub dose-dependent response was not linear. The effect of cAMP on the two functions (Ca2+ uptake and Ca2+ release) appears to be different. Therefore, these results led us to suggest a more complex role of cAMP in the regulation of platelet Ca2+ concentration.

Evidence that inositol 1-phosphate in brain of lithium-treated rats results mainly from phosphatidylinositol metabolism

In cerebral cortex of rats treated with increasing doses of LiCl, the relative concentrations of Ins(1)P, Ins(4)P and Ins(5)P (when InsP is a myo-inositol phosphate) are approx. 10:1:0.2 at all doses. In rats treated with LiCl followed by increasing doses of pilocarpine a similar relationship occurs. myo-Inositol-1-phosphatase (InsP1ase) from bovine brain hydrolyses Ins(1)P, Ins(4)P and Ins(5)P at comparable rates, and these substrates have similar Km values. The hydrolysis of Ins(4)P is inhibited by Li+ to a greater degree than is hydrolysis of Ins(1)P and Ins(5)P. D-Ins(1,4,5)P3 and D-Ins(1,4)P2 are neither substrates nor inhibitors of InsP1ase. A dialysed high-speed supernatant of rat brain showed a greater rate of hydrolysis of Ins(1)P than of D-Ins(1,4)P2 and a lower sensitivity of the bisphosphate hydrolysis to LiCl, as compared with the monophosphate. That enzyme preparation produced Ins(4)P at a greater rate than Ins(1)P when D-Ins(1,4)P2 was the substrate. The amount of D-Ins(3)P [i.e. L-Ins(1)P, possibly from D-Ins(1,3,4)P3] is only 11% of that of D-Ins(1)P on stimulation with pilocarpine in the presence of Li+. DL-Ins(1,4)P2 was hydrolysed by InsP1ase to the extent of about 50%; both Ins(4)P and Ins(1)P are products, the former being produced more rapidly than the latter; apparently L-Ins(1,4)P2 is a substrate for InsP1ase. Li+, but not Ins(2)P, inhibited the hydrolysis of L-Ins(1,4)P2. The following were neither substrates nor inhibitors of InsP1ase; Ins(1,6)P2, Ins(1,2)P2, Ins(1,2,5,6)P4, Ins(1,2,4,5,6)P5, Ins(1,3,4,5,6)P5 and phytic acid. myo-Inositol 1,2-cyclic phosphate was neither substrate nor inhibitor of InsP1ase. We conclude that the 10-fold greater tissue contents of Ins(1)P relative to Ins(4)P in both stimulated and non-stimulated rat brain in vivo are the consequence of a much larger amount of PtdIns metabolism than polyphosphoinositide metabolism under these conditions.

GTP mobilization of Ca2+ from the endoplasmic reticulum of islets. Comparison with myo-inositol 1,4,5-trisphosphate

The effect of the guanine nucleotide GTP on Ca2+ release from the endoplasmic reticulum of digitonin-permeabilized islets was investigated. maximal and half-maximal Ca2+ release were observed at 5 microM- and 2.5 microM-GTP respectively. GTP caused a rapid release of Ca2+ from the endoplasmic reticulum, which was complete within 1 min. GTP-induced Ca2+ release was structurally specific and required the hydrolysis of GTP. The combination of maximal concentrations of GTP (10 microM) and myo-inositol 1,4,5-trisphosphate (IP3) (10 microM) resulted in an additive effect on Ca2+ release from the endoplasmic reticulum. GDP (100 microM), which inhibits GTP-induced Ca2+ release, did not affect IP3-induced Ca2+ release. Furthermore, GTP-induced Ca2+ release was not independent on submicromolar free Ca2+ concentrations, unlike IP3-induced Ca2+ release. These observations suggest that mechanistically GTP-induced Ca2+ release is different from IP3-induced Ca2+ release from the endoplasmic reticulum.

Calmodulin inhibits inositol trisphosphate-induced Ca2+ mobilization from the endoplasmic reticulum of islets

IP3-induced Ca2+ release from the endoplasmic reticulum (ER) of islets is believed to be a key intracellular event in glucose-induced insulin secretion. Calmodulin was shown to increase ATP-dependent Ca2+ steady-state and inhibit by 57.2% IP3-induced Ca2+ mobilization from the ER. Conversely, the calmodulin antagonist, N-(6-aminohexyl)-5-chloro-1-naphtalene sulfonamide (W-7), induced Ca2+ release from the ER. The combination of W-7 (100 microM) and IP3 (10 microM), resulted in a greater release of Ca2+ from the ER than either W-7 or IP3 alone. W-7 was shown not to affect the structural integrity of the ER. Our results suggest that IP3-induced Ca2+ release from the ER is regulated by a calmodulin-dependent process.

Glucose-induced phospholipid hydrolysis in isolated pancreatic islets: quantitative effects on the phospholipid content of arachidonate and other fatty acids

Our recent findings indicate that glucose-induced insulin secretion from isolated pancreatic islets is temporally associated with accumulation of substantial amounts of free arachidonic acid and that arachidonate may serve as a second messenger for intracellular calcium mobilization in islets. In an effort to determine the source of this released arachidonate, the endogenous fatty acid composition of phospholipids from islets has been determined by thin-layer chromatographic separation of the phospholipids, methanolysis to the fatty acid methyl esters, and quantitative gas chromatographic analyses. The relative abundance of phospholipids in islets as judged by their fatty acid content was phosphatidylcholine (PC), 0.63; phosphatidylethanolamine (PE), 0.23; phosphatidylinositol (PI), 0.067; phosphatidylserine (PS), 0.049. Arachidonate constituted 17% of the total islet fatty acid content, and PC contained 43% of total islet arachidonate. Islets incubated with [3H]arachidonate in the presence of 28 mM D-glucose incorporated radiolabel into PC with a considerably higher specific activity than that of PE, PS or PI. The total fatty acid content of PC from islets incubated with 28 mM glucose for 30 min was significantly lower than that of islets incubated with 3 mM glucose, and smaller effects were observed with PE, PS and PI. The molar decrement in PC arachidonate was 3.2 pmol/islet under these conditions, which is sufficient to account for the previously observed accumulation of free arachidonate (2 pmol/islet). A sensitive method involving negative ion-chemical ionization-mass spectrometric analyses of the pentafluorobenzyl esters of fatty acids derived from trace amounts of lysophosphatidylcholine (lyso-PC) was developed, and glucose-stimulation was found to reduce islet lyso-PC content by about 10-fold. These findings indicate that the insulin secretagogue D-glucose induces phospholipid hydrolysis in islets and suggest that PC may be the major source of free arachidonate which accumulates in glucose-stimulated islets.

Signal transduction in insulin secretion: comparison between fuel stimuli and receptor agonists

The initial events in signal transduction in insulin-secreting cells are summarized in FIGURE 8. Both nutrient stimuli, such as glucose and amino acids and the muscarinic agonist carbachol (carbamylcholine) raise [Ca2+]i. Although the rise in [Ca2+]i precedes the stimulation of insulin release, it is not a moment-to-moment regulator of release. The metabolizable fuel stimuli cause Ca2+ influx through voltage-dependent Ca2+ channels following depolarization of the membrane potential. In contrast, carbachol, which does not depolarize, elicits Ptd Ins 4,5-P2 hydrolysis, a reaction catalyzed by phospholipase C. The generation of Ins 1,4,5-P3 in this instance is Ca2+ independent, but appears to involve a GTP-binding protein. However, this protein is not a substrate for pertussis toxin. The levels of Ins 1,4,5-P3, which releases Ca2+ from an ATP-dependent Ca2+ pool of the endoplasmic reticulum, are increased prior to the rise in [Ca2+]i. The mitochondria may take up Ca2+ after large increases in [Ca2+]i. A previously proposed second messenger, arachidonic acid, is much less selective than Ins 1,4,5-P3 in that it releases Ca2+ from mitochondria as well as from the endoplasmic reticulum in a slow and irreversible manner. As Ins 1,4,5-P3 is also generated during glucose stimulation of islets, albeit in a Ca2+-dependent manner, this metabolite could mediate not only the action of carbachol but also contribute to amplifying the [Ca2+]i rise in response to glucose.

Glucose-induced accumulation of inositol trisphosphates in isolated pancreatic islets. Predominance of the 1,3,4-isomer

Anion-exchange h.p.l.c. analysis of [3H]inositol phosphates derived from glucose-stimulated isolated pancreatic islets that had been prelabelled with myo-[3H]inositol revealed that the predominant inositol trisphosphate was the 1,3,4-isomer [Ins(1,3,4)P3]. The 1,4,5-isomer [Ins(1,4,5)P3] was also detectable, as was a more polar inositol phosphate with the chromatographic properties of inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. Glucose-induced accumulation of Ins(1,3,4)P3 was augmented by Li+ and occurred after maximal accumulation of Ins(1,4,5)P3. These findings suggest a possible role for Ins(1,3,4)P3 or its probable precursor Ins(1,3,4,5)P4 in stimulus-secretion coupling in pancreatic islets.

Analysis of inositol mono- and polyphosphates by gas chromatography/mass spectrometry and fast atom bombardment

The electron ionization spectra of all of the positional isomers of myo-inositol monophosphate and of myo-inositol 1,2-cyclic phosphate were obtained by gas chromatography/mass spectrometry of the pertrimethylsilyl derivatives. The fragmentation pattern of pertrimethylsilyl myo-inositol-1-phosphate was studied using deuterium labeling. The phosphate moiety was found to direct fragmentation to produce fragment ions of useful intensity with specific carbon retention. The spectrum of pertrimethylsilyl myo-inositol-1,4-bisphosphate is also described. An electron impact gas chromatographic/mass spectrometric method for myo-inositol-1-phosphate has been developed, which has a sensitivity to a level of 0.1 pmol. The positive and negative ion fast atom bombardment spectra of myo-inositol hexakis(disodium phosphate) and myo-inositol hexakis(dihydrogen phosphate) are described. The lesser-phosphorylated inositol polyphosphates were also studied, including inositol pentakis and inositol tetrakis(dihydrogen phosphates) as well as D-myo-inositol-1,4,5-trisphosphate and D-myo-inositol-1,4-bisphosphate from human red blood cells. The sensitivity of fast atom bombardment for the measurement of the latter two substances allows their detection to a level of about 10 nmol. The fast atom bombardment spectrum of synthetic myo-inositol 1,2-cyclic phosphate revealed variable amounts of a dimer produced during its preparation.

myo-Inositol 1,4,5-trisphosphate mobilizes Ca2+ from isolated adipocyte endoplasmic reticulum but not from plasma membranes

The effects of myo-inositol 1,4,5-trisphosphate (IP3) on Ca2+ uptake and release from isolated adipocyte endoplasmic reticulum and plasma membrane vesicles were investigated. Effects of IP3 were initially characterized using an endoplasmic reticulum preparation with cytosol present (S1-ER). Maximal and half-maximal effects of IP3 on Ca2+ release from S1-ER vesicles occurred at 20 microM- and 7 microM-IP3, respectively, in the presence of vanadate which prevents the re-uptake of released Ca2+ via the endoplasmic reticulum Ca2+ pump. At saturating IP3 concentrations, Ca2+ release in the presence of vanadate was 20% of the exchangeable Ca2+ pool. IP3-induced release of Ca2+ from S1-ER was dependent on extravesicular free Ca2+ concentration with maximal release occurring at 0.13 microM free Ca2+. At 20 microM-IP3 there was no effect on the initial rate of Ca2+ uptake by S1-ER. IP3 promoted Ca2+ release from isolated endoplasmic reticulum vesicles (cytosol not present) to a similar level as compared with S1-ER. Addition of cytosol to isolated endoplasmic reticulum vesicles did not affect IP3-induced Ca2+ release. The endoplasmic reticulum preparation was further fractionated into heavy and light vesicles by differential centrifugation. Interestingly, the heavy fraction, but not the light fraction, released Ca2+ when challenged with IP3. IP3 (20 microM) did not promote Ca2+ release from plasma membrane vesicles and had no effect on the (Ca2+ + Mg2+)-ATPase activity or on the initial rate of ATP-dependent Ca2+ uptake by these vesicles. These results support the concept that IP3 acts exclusively at the endoplasmic reticulum to promote Ca2+ release.

Inositol 1,4,5-trisphosphate-induced Ca2+ release from permeabilized mastocytoma cells

The effect of inositol 1,4,5-trisphosphate (IP3) on Ca2+ release in the transformed murine mast cells, mastocytoma P-815 cells permeabilized with digitonin was studied. Ca2+ was sequestered by intracellular organelles in the presence of ATP until the medium free Ca2+ concentration was lowered to a new steady-state level. The subsequent addition of IP3 caused a rapid Ca2+ release, which was followed by a slow re-uptake of Ca2+. Fifty percent of the sequestered Ca2+ was released by 10 microM IP3. Maximal Ca2+ release occurred at 10 microM and half maximal activity was at 1.3 microM. These results indicate that IP3 may function as a messenger of intracellular Ca2+ mobilization in mastocytoma cells.