Evidence for the involvement of Na/Ca exchange in glucose-induced insulin release from rat pancreatic islets

A crosstalk between Na⁺ channels, Na⁺/K⁺ pump and mitochondrial Na⁺ transporters controls glucose-dependent cytosolic and mitochondrial Na⁺ signals

Glucose-dependent cytosolic Na(+) influx in pancreatic islet β cells is mediated by TTX-sensitive Na(+) channels and is propagated into the mitochondria through the mitochondrial Na(+)/Ca(2+) exchanger, NCLX. Mitochondrial Na(+) transients are also controlled by the mitochondrial Na(+)/H(+) exchanger, NHE, while cytosolic Na(+) changes are governed by Na(+)/K(+) ATPase pump. The functional interaction between the Na(+) channels, Na(+)/K(+) ATPase pump and mitochondrial Na(+) transporters, NCLX and NHE, in mediating Na(+) signaling is poorly understood. Here, we combine fluorescent Na(+) imaging, pharmacological inhibition by TTX, ouabain and EIPA, with molecular control of NCLX expression, so as to investigate the crosstalk between Na(+) transporters on both the plasma membrane and the mitochondria. According to our results, glucose-dependent cytosolic Na(+) response was enhanced by ouabain and was followed by a rise in mitochondrial Na(+) signal. Silencing of NCLX expression using siNCLX, did not affect the glucose- or ouabain-dependent cytosolic rise in Na(+). In contrast, the ouabain-dependent rise in mitochondrial Na(+) was strongly suppressed by siNCLX. Furthermore, mitochondrial Na(+) influx rates were accelerated in cells treated with the Na(+)/H(+) exchanger inhibitor, EIPA or by combination of EIPA and ouabain. Similarly, TTX blocked the cytosolic and mitochondrial Na(+) responses, which were enhanced by ouabain or EIPA, respectively. Our results suggest that Na(+)/K(+) ATPase pump controls cytosolic glucose-dependent Na(+) rise, in a manner that is mediated by TTX-sensitive Na(+) channels and subsequent mitochondrial Na(+) uptake via NCLX. Furthermore, these results indicate that mitochondrial Na(+) influx via NCLX is antagonized by Na(+) efflux, which is mediated by the mitochondrial NHE; thus, the duration of mitochondrial Na(+) transients is set by the interplay between these pivotal transporters.

Rebaudioside A directly stimulates insulin secretion from pancreatic beta cells: a glucose-dependent action via inhibition of ATP-sensitive K-channels

Recently, we showed that rebaudioside A potently stimulates the insulin secretion from isolated mouse islets in a dose-, glucose- and Ca(2+)-dependent manner. Little is known about the mechanisms underlying the insulinotropic action of rebaudioside A. The aim of this study was to define the signalling system by which, rebaudioside A acts. Isolated mouse islets were used in the cAMP[(125)I] scintillation proximity assay to measure total cAMP level, and in a luminometric method to measure intracellular ATP and ADP concentrations. Conventional and permeabilized whole-cell configuration of the patch-clamp technique was used to verify the effect of rebaudioside A on ATP-sensitive K(+)-channels from dispersed single beta cells from isolated mouse islets. Insulin was measured by radioimmunoassay from insulinoma MIN6 cells. In the presence of 16.7 mM glucose, the addition of the maximally effective concentration of rebaudioside A (10(-9) M) increased the ATP/ADP ratio significantly, while it did not change the intracellular cAMP level. Rebaudioside A (10(-9) M) and stevioside (10(-6) M) reduced the ATP-sensitive potassium channel (K(ATP)) conductance in a glucose-dependent manner. Moreover, rebaudioside A stimulated the insulin secretion from MIN6 cells in a dose- and glucose-dependent manner. In conclusion, the insulinotropic effect of rebaudioside A is mediated via inhibition of ATP-sensitive K(+)-channels and requires the presence of high glucose. The inhibition of ATP-sensitive K(+)-channels is probably induced by changes in the ATP/ADP ratio. The results indicate that rebaudioside A may offer a distinct therapeutic advantage over sulphonylureas because of less risk of causing hypoglycaemia.

Ouabain suppresses glucose-induced mitochondrial ATP production and insulin release by generating reactive oxygen species in pancreatic islets

We examined the effects of reduced Na(+)/K(+)-ATPase activity on mitochondrial ATP production and insulin release from rat islets. Ouabain, an inhibitor of Na(+)/K(+)-ATPase, augmented 16.7 mmol/l glucose-induced insulin release in the early period but suppressed it after a delay of 20-30 min. Unexpectedly, the ATP content in an islet decreases in the presence of 16.7 mmol/l glucose when Na(+)/K(+)-ATPase activity is diminished by ouabain, despite the reduced consumption of ATP by the enzyme. Ouabain also suppressed the increment of ATP content produced by glucose even in Ca(2+)-depleted or Na(+)-depleted conditions. That mitochondrial membrane hyperpolarization and O(2) consumption in islets exposed to 16.7 mmol/l glucose were suppressed by ouabain indicates that the glycoside inhibits mitochondrial respiration but does not produce uncoupling. Ouabain induced mitochondrial reactive oxygen species (ROS) production that was blocked by myxothiazol, an inhibitor of site III of the mitochondrial respiratory chain. An antioxidant, alpha-tocopherol, also blocked ouabain-induced ROS production as well as the suppressive effect of ouabain on ATP production and insulin release. However, ouabain did not directly affect the mitochondrial ATP production originating from succinate and ADP. These results indicate that ouabain suppresses mitochondrial ATP production by generating ROS via transduction, independently of the intracellular cationic alternation that may account in part for the suppressive effect on insulin secretion.

Metabolic, cationic and secretory response to D-glucose in depolarized and Ca(2+)-deprived rat islets exposed to diazoxide

D-glucose stimulates insulin release from islets exposed to both diazoxide, to activate ATP-responsive K+ channels, and a high concentration of K+, to cause depolarization of the B-cell plasma membrane. Under these conditions, the insulinotropic action of D-glucose is claimed to occur despite unaltered cytosolic Ca2+ concentration, but no information is so far available on the changes in Ca2+ fluxes possibly caused by the hexose. In the present experiments, we investigated the effect of D-glucose upon 45Ca efflux from islets exposed to both diazoxide and high K+ concentrations. In the presence of diazoxide and at normal extracellular Ca2+ concentration, D-glucose (16.7 mmol/l) inhibited insulin release at 5 mmol/l K+, but stimulated insulin release of 90 mmol/l K+. In both cases, the hexose inhibited 45Ca outflow. In the presence of diazoxide, but absence of Ca2+, D-glucose (8.3 to 25.0 mmol/l) first caused a rapid decrease in insulin output followed by a progressive increase in secretory rate. This phenomenon was observed both at 5 mmol/l or higher concentrations (30, 60 and 90 mmol/l) of extracellular K+. It coincided with a monophasic decrease in 45Ca efflux and either a transient (at 5 mmol/l K+) or sustained (at 90 mmol/l K+) decrease in overall cytosolic Ca2+ concentration. The decrease in 45Ca efflux could be due to inhibition of Na(+)-Ca2+ countertransport with resulting localized Ca2+ accumulation in the cell web of insulin-producing cells. A comparable process may be involved in the secretory response to D-glucose in islets exposed to diazoxide and a high concentration of K+ in the presence of extracellular Ca2+.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Effect of ouabain on insulin secretion in the amphibian pancreas

We investigated the influence of ouabain on glucose-induced insulin release from toad pancreatic minces in the same nanomolar range as that of an ouabain-like compound found in human blood. Ouabain increased insulin secretion at basal (2 mM), but not maximally stimulatory (8 mM), glucose levels in a dose-dependent manner up to an optimal concentration of 1 nM, with the values declining progressively thereafter. Ouabain at 3 nM changed the shape of the overall dose-response curve for glucose from sigmoidal to hyperbolic and displaced the optimal insulinotropic glucose concentration from 8 to 2 mM. Preincubation with ouabain (3 nM) followed by glycoside washout potentiated insulin induction at 2 mM, but not at 8 mM glucose, but this same pretreatment followed by incubation in Ca(2+)-free medium depressed insulin release under all conditions, and especially at high glucose; here, however, the preexposure to ouabain partially prevented the drop in insulin secretion at 8 mM glucose. Acetylcholine at 8 microM augmented insulin release at both levels of glucose, and ouabain potentiated this effect synergistically at high, but not low glucose. Ouabain, at physiologic concentrations, thus appears to regulate the effect of secretagogues such as glucose and acetylcholine in amphibians.

A role for Na/Ca exchange in the pancreatic B cell. Studies with thapsigargin and caffeine

Sodium/calcium (Na/Ca) exchange is thought to play a role in Ca2+ extrusion from the pancreatic B cell. The aim of the present study was to provide direct evidence for such a role. The effect of extracellular Na+ (Na0+) removal on cytosolic free Ca2+ ([Ca2+]i) in single pancreatic B cells was examined using fura 2 and dual wavelength microfluorimetry. Isosmotical replacement of Na0+ by sucrose increased [Ca2+]i in the presence of extracellular Ca2+ but failed to affect [Ca2+]i in the absence of the divalent cation. Thapsigargin (1 microM), an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, induced a transient increase in [Ca2+]i in the presence of Na0+. This increase was enhanced and more sustained in the absence of Na0+. In the absence of Na0+ and the presence of thapsigargin, reintroduction of Na0+ induced a rapid decrease in [Ca2+]i. A similar picture was observed when caffeine (10 mM) was used to release Ca2+ from the endoplasmic reticulum. The decrease in [Ca2+]i induced by Na0+ reintroduction was accompanied by an important increase in 45Ca outflow from perifused islets. In conclusion, this study provides direct evidence that Na/Ca exchange may regulate B cell [Ca2+]i within physiological range.

Na+/Ca2+ exchange in plasma membrane vesicles from a glucose-responsive insulinoma

Plasma membrane vesicles from a glucose-responsive insulinoma exhibited properties consistent with the presence of a membrane Na+/Ca2+ exchange. The exchange was rapid, reversible, and was dependent on the external Ca2+ concentration (Km = 4.1 +/- 1.1 microM). External Na+ inhibited the uptake in a dose-dependent manner (IC50 = 15 mM). Dissipation of the Na+ gradient by 10 microM monensin decreased Na+/Ca2+ exchange from 0.74 +/- 0.17 nmoles/mg protein/s to 0.11 +/- 0.05 nmoles/mg protein/s. Exchange was not influenced by veratridine, tetrodotoxin and ouabain, or by modifiers of cAMP. No effect was seen using the calcium channel blockers, nitrendipine or nifedipine. Glucose had no direct effect on Na+/Ca2+ exchange, while glyceraldehyde, glyceraldehyde-3-phosphate and dihydroxyacetone inhibited the exchange. Na+ induced efflux of calcium was seen in Ca2+ loaded vesicles and was half maximal at [Na+] of 11.1 +/- 0.75 mM. Ca2+ efflux was dependent on [Na+], with a Hill coefficient of 2.7 +/- 0.07 indicating that activation of Ca2+ release involves a minimum of three sites. The electrogenicity of this exchange was demonstrated using the lipophilic cation tetraphenylphosphonium [( 3H]-TPP), a membrane potential sensitive probe. [3H]-TPP uptake increased transiently during Na+/Ca2+ exchange indicating that the exchange generated a membrane potential. These results show that Na+/Ca2+ exchange operates in the beta cell and may be an important regulator of intracellular free Ca2+ concentrations.

Inhibition of Na/Ca exchange in pancreatic islet cells by 3',4'-dichlorobenzamil

Na/Ca exchange may play a role in Ca2+ extrusion from the pancreatic B cell. The role played by the exchanger was examined by characterizing the effects of 3'-4'-dichlorobenzamil on ionic fluxes and insulin release in normal rat pancreatic islet cells. 3',4'-Dichlorobenzamil potently inhibited 45Ca uptake mediated by reverse Na/Ca exchange (IC50: 18 microM) in islet cells. The drug failed to decrease intracellular pH but reduced 86Rb outflow from perifused islets. The effects of glucose and 3',4'-dichlorobenzamil on 86Rb outflow were not additive. The drug potently blocked 45Ca uptake through voltage-sensitive Ca2+ channels (IC50: 7.5 microM). In the presence of extracellular Ca2+ and 3',4'-dichlorobenzamil, glucose lost part of its ability to reduce 45Ca outflow. The drug failed to affect the secondary rise in 45Ca outflow induced by the sugar. In the absence of extracellular Ca2+, 3',4'-dichlorobenzamil induced a delayed inhibition of 45Ca outflow, the effect of the sugar and the drug being not additive. This effect of 3',4'-dichlorobenzamil and its ability to impair the inhibitory effect of glucose were reproduced by the removal of extracellular Na+ and disappeared under the latter experimental condition. 3',4'-Dichlorobenzamil did not affect insulin release in the absence of glucose but significantly increased glucose-induced insulin release when used at a high concentration. It is concluded that 3',4'-dichlorobenzamil is a potent inhibitor of the process of Na/Ca exchange in the pancreatic B cell. Unfortunately, the drug is of poor specificity and blocks, in the same range of concentrations, both K+ channels and voltage-sensitive Ca2+ channels. The data also indicate that glucose inhibits 45Ca outflow from pancreatic islets to a great extent (at least 75%) by inhibiting Na/Ca exchange. The type of Na/Ca exchange that is inhibited by glucose, remains to be elucidated.

Characterization of the process of sodium-calcium exchange in pancreatic islet cells

Na(+)-Ca2+ exchange may play a role in Ca2+ extrusion from the pancreatic B-cell. The characteristics of the process working in its reverse mode were examined in normal rat pancreatic islet cells. Isosmotical replacement of extracellular Na+ by sucrose induced a concentration-dependent increase in 45Ca uptake, displaying a pharmacological sensitivity compatible with an uptake mediated by Na(+)-Ca2+ exchange. Glucose, up to 2.8 mM, stimulated reverse Na(+)-Ca2+ exchange. Likewise, membrane depolarization activated the process but only under raised intracellular Na+ activity. In conclusion, the B-cell Na(+)-Ca2+ exchange displays properties similar to those observed in other cells: reversibility and sensitivity to membrane potential. When working in its reverse mode the exchanger displays a quite large capacity. The role played by the exchanger in the process of insulin release warrants further investigation.

Evidence for a Na+/Ca2+ exchange mechanism in the neuroendocrine cells of the locust corpus cardiacum

The glandular lobe of the locust corpus cardiacum (CC) is a peripheral neuroendocrine gland consisting primarily of neurosecretory cells (NSCs), all of which synthesize, store, and release adipokinetic hormones (AKHs). An influx of extracellular Ca2+ into the NSCs provides an essential trigger for initiating AKH release. In this study we demonstrate that the Na+ gradient across the plasma membrane of these NSCs has a significant influence on the process of AKH release and on Ca2+ fluxes across the membrane. AKHs are released from NSCs when the corpus cardiacum is incubated in Na+-free saline or in K+-free saline with ouabain. The rate of 45Ca2+ efflux from the gland is reduced by 50% when glands are incubated in Na+-free saline compared to normal saline. The amount of 45Ca2+ uptake by Na+-loaded cells is nearly 100% greater in glands exposed to Na+-free saline compared to glands in normal saline. These results are most clearly interpreted by proposing that a Na+/Ca2+ exchange system is present in the NSCs. This system may provide an essential Ca2+ buffering mechanism by extruding Ca2+ from the NSCs following a stimulus-secretion cycle.

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

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

Abnormal calcium handling by perifused pancreatic islets from neonatal streptozotocin diabetic model rats

To elucidate the mechanism of impaired insulin release in case of non-insulin-dependent diabetes (NIDDM), we investigated insulin release and 45Ca++ efflux from perifused islets obtained from neonatal streptozotocin diabetic model rats. The model rats were prepared by the intraperitoneal administration of 65 mg/kg streptozotocin (STZ) to neonatal males. Rats treated with STZ did not differ from controls in body weight from 1 week to 16 weeks. The model rats had significant hyperglycemia both in the fasting state and after intraperitoneal administration of 2 g/kg glucose. Although the diameter of the islets from the model rats was not significantly different from that of controls, immunoreactivity to anti-insulin was slightly diminished, and degranulation was slightly observed in B-cells. Insulin content was reduced to 45.6% of the control. Insulin release from the perifused islets of STZ-treated rats responded little to 16.7 mmol/L glucose, but normally to 20 mmol/L arginine in the presence of 5.5 mmol/L glucose. In experiments to test the 45Ca++ efflux from the perifused islets prelabeled with 45Ca++, a rise of 45Ca++ efflux concomitant with the second phase of insulin release from the islets of the model rats was inhibited although a sharp increase of 45Ca++ efflux concomitant with the first phase of insulin release was maintained. 45Ca++ uptake for 30 minutes was reduced in the islets from the model rats in the basal and stimulated state of insulin secretion although the incremental 45Ca++ uptake was similar. It is possible that the abnormal calcium handling in pancreatic B-cells may be one of the causes of defect in insulin release in our model rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium requirement for insulin release: putative role in regulation of intracellular pH

The effects on insulin release of Na+ removal, alteration of extracellular pH, and inhibition of acid extrusion processes were examined using freshly isolated and 46-h cultured islets of Langerhans. Na+ removal inhibited the secretory response to 16.7 mM glucose specifically in fresh islets but stimulated the low-glucose response in cultured islets. These divergent effects of Na+ depletion were perfectly mimicked by amiloride, an inhibitor of Na+-H+ exchange, and could be overcome in a raised HCO-3, pH 8.0 medium. Simply raising extracellular pH at constant HCO-3 had no effect. Na+ removal also inhibited the secretory responses to other nutrient secretagogues such as D-glyceraldehyde and L-leucine but not to agents like 3-isobutyl-1-methylxanthine or 12-O-tetradecanoylphorbol-13-acetate. The results show that Na+ is not itself essential for the secretory process but rather suggest that it plays a permissive role in the regulation of intracellular pH via Na+-H+ exchange. Such a regulation appears important mainly for nutrient-stimulated insulin release, which is associated with the generation of acidic metabolites.

Stimulation of insulin release in isolated rat islets by GIP in physiological concentrations and its relation to islet cyclic AMP content

Several insulinotropic hormones have been shown to increase the level of cyclic AMP in isolated islets. This study was performed to investigate whether gastric inhibitory polypeptide (glucose-dependent insulin-releasing polypeptide) has a similar effect, in particular at concentrations close to the physiological level in blood. Collagenase isolated rat islets were maintained for 24 h in tissue culture (medium 199) and then incubated for 30 min for measurement of insulin release and cyclic AMP content. Glucose-induced (16.7 mmol/l) insulin release was enhanced by gastric inhibitory polypeptide 1-100 ng/ml (0.196-19.6 nmol/l) in a dose-related fashion. The cyclic AMP content was enhanced only by 100 ng/ml. However, when 0.1 mmol/l of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine was present, even 1 ng/ml of gastric inhibitory polypeptide increased both cyclic AMP content and insulin release. Such a concentration of the hormone can be measured in human blood after a meal. In contrast, in freshly isolated islets no effect of the hormone on glucose-induced insulin release or cyclic AMP content could be detected for concentrations ranging from 1 to 100 ng/ml. These findings demonstrate that the hormone sensitivity of isolated islets is markedly enhanced by short-term maintenance in tissue culture. The results suggest that an increase in cyclic AMP is seen in response to gastric inhibitory polypeptide and may be causally related to the insulinotropic effect of the hormone.

Glucagon-like peptide-1 but not glucagon-like peptide-2 stimulates insulin release from isolated rat pancreatic islets

Glucagon-like peptide-1 and glucagon-like peptide-2 are encoded by the m-RNA of pancreatic preproglucagon. They show high conservation in different species and substantial sequence homology to glucagon. Because no definite biological activity of these peptides has been reported, we investigated the effect of synthetic C-terminally amidated glucagon-like peptide-1 [1-36] and synthetic human glucagon-like peptide-2 [1-34] with a free C-terminus on insulin release from isolated precultured rat pancreatic islets in the presence of glucose. Glucagon-like peptide-1 stimulates insulin release at 10 and 16.7 mmol/l glucose in a dose-dependent manner. Significant stimulation starts at 2.5 nmol/l in the presence of 10 mmol/l glucose and near maximal release is observed at 250 nmol/l, with approximately 100% increase over basal at both glucose concentrations. The peptide reaches 63% of the maximal stimulatory effect of glucagon. No stimulation occurs in the presence of 2.8 mmol/l glucose. Glucagon-like peptide-2 has no effect on insulin secretion at any glucose concentration tested. It is concluded that glucagon-like peptide-1, in contrast to glucagon-like peptide-2, exhibits a glucose-dependent insulinotropic action on isolated rat pancreatic islets similar to that of glucagon and gastric inhibitory polypeptide.

Beta-cell cytoplasmic Ca2+ balance as a determinant for glucose-stimulated insulin release

The introduction of new techniques and the access to clonal lines of insulin-secreting cells have enabled re-evaluation of glucose effects on Ca2+ movements in pancreatic beta cells. It became evident that glucose, in addition to stimulating the entry of Ca2+, also promotes active sequestration of the ion in intracellular stores and its extrusion from the beta cells. The balance between these processes will determine the activity of Ca2+ in the cytoplasm and consequently the rate of insulin release. With the demonstration that glucose can not only increase but also lower cytoplasmic Ca2+, it follows that exposure to the sugar under certain conditions results in a paradoxical inhibition of insulin release. In diabetic patients this may be manifest as prompt reduction of circulating concentrations of insulin and C-peptide after an intravenous injection of glucose. The concept of the dual action of glucose might aid in explaining a number of poorly understood phenomena, such as the induction of rhythmic oscillations of the membrane potential of beta cells and the fact that their secretory response is improved by prolonged exposure to glucose and after priming with the sugar.

Mechanisms of synergism between glucose and cAMP on stimulation of insulin release

The mechanism of synergism between glucose and adenosine 3',5'-cyclic monophosphate (cAMP) on insulin release has been studied. Synergism may result from 1) inhibition of Na+-Ca2+ exchange by glucose and 2) a cAMP-induced sensitization of the release machinery to Ca2+. To distinguish between these two possibilities, isolated rat pancreatic islets were perifused with agents that raise intracellular levels of cAMP [3-isobutyl-1-methylxanthine (IBMX) and forskolin] and others that increase intracellular concentrations of Ca2+ either by blocking Na2+-Ca2+ exchange (ouabain and choline-Ringer solution) or by causing increased Ca2+ influx (KCl, carbachol, and 10 mM Ca2+). The results indicate that both the combination of cAMP and increased Ca2+ influx or blocked Na2-Ca2+ exchange and increased Ca2+ influx potentiated insulin release. When the relative potentiating abilities of cAMP and blocked Na2+-Ca2+ exchange were compared by determining the individual effects of IBMX and 1 mM ouabain (a concentration that causes similar inhibition of 45C2+ efflux as 16.7 mM glucose) in the presence of carbachol, cAMP was only 1.4 times more potent as a potentiating agent than blocked Na+-Ca2+ exchange. The greatest potentiation of insulin release was observed when Na+-Ca2+ exchange was blocked in the presence of increased levels of intracellular cAMP.

Cytosolic free Ca2+ in insulin secreting cells and its regulation by isolated organelles

The role of Ca2+ in secretagogue-induced insulin release is documented not only by the measurements of 45Ca fluxes in pancreatic islets, but also, by direct monitoring of cytosolic free Ca2+, [Ca2+]i. As demonstrated, using the fluorescent indicator quin 2, glyceraldehyde, carbamylcholine and alanine raise [Ca2+]i in the insulin secreting cell line RINm5F, whereas glucose has a similar effect in pancreatic islet cells. The regulation of cellular Ca2+ homeostasis by organelles from a rat insulinoma, was investigated with a Ca2+ selective electrode. The results suggest that both the endoplasmic reticulum and the mitochondria participate in this regulation, albeit at different Ca2+ concentrations. By contrast, the secretory granules do not appear to be involved in the short-term regulation of [Ca2+]i. Evidence is presented that inositol 1,4,5-trisphosphate, which is shown to mobilize Ca2+ from the endoplasmic reticulum, is acting as an intracellular mediator in the stimulation of insulin release.

Role of hexose monophosphate shunt in parathyroid hormone secretion

The metabolism of labeled glucose by collagenase-dispersed bovine parathyroid cells was examined. When the medium calcium ion concentration was increased to 2.0 mM, the rate of 14CO2 release from [1-14C]glucose was increased 169 +/- 45% compared with the rate of 0.5 mM calcium. There was no significant change in the rate of 14CO2 release from [6-14C]glucose by this maneuver. The greatest increase in 14CO2 release and decrease in parathyroid hormone secretion occurred between medium calcium ion concentrations of 0.5-1.5 mM. This difference in the metabolism of glucose represents a true increase in hexose shunt activity because the incorporation of label from either [1-14C]- or [6-14C]glucose into parathyroid tissue lipids was equal. This suggests equilibration of label at the level of triose-phosphates. The increase in hexose shunt activity was not due to a calcium-mediated increase in glucose uptake because calcium changes did not affect 2-[3H]deoxyglucose transport by the cells. Phenazine methosulfate added to cells incubated at 0.5 mM calcium selectively increased hexose shunt activity in a dose-dependent manner (91 +/- 33% overall) and concomitantly inhibited parathyroid hormone secretion 65% overall at 0.5 mM calcium. The compound 6-aminonicotinamide inhibited hexose shunt activity but could not overcome the inhibition of hormone secretion at 2.0 mM calcium. A decrease in protein biosynthesis cannot fully explain the inhibition of hormone secretion by calcium or phenazine methosulfate because [3H]-leucine incorporation into total cell protein was not as affected as secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Distinct effects of various amino acids on 45Ca2+ fluxes in rat pancreatic islets

The effects of three types of amino acids on 45Ca2+ fluxes in rat pancreatic islets have been compared. Alanine, a non-insulinotropic neutral amino acid, transported with Na+, increased 45Ca2+ efflux in the presence or in the absence of extracellular Ca2+, but not in the absence of Na+. Its effects in Na+-solutions were practically abolished by 7 mM-glucose. Alanine slightly stimulated 45Ca2+ influx (5 min uptake) only when Na+ was present. Two insulinotropic cationic amino acids (arginine and lysine) triggered similar changes in 45Ca2+ efflux. They accelerated the efflux in the presence of Ca2+ and inhibited the efflux in a Ca2+-free medium, whether glucose was present or not. In an Na+-free Ca2+-medium, arginine and lysine markedly accelerated 45Ca2+ efflux, but this effect was suppressed by 7 mM-glucose. Arginine stimulated 45Ca2+ influx irrespective of the presence or absence of glucose and Na+. Leucine, a neutral insulinotropic amino acid well metabolized by islet cells, inhibited 45Ca2+ efflux from the islets in a Ca2+-free medium; this effect was potentiated by glutamine. In the presence of Ca2+ and Na+, leucine was ineffective alone, but triggered a marked increase in 45Ca2+ efflux when combined with glutamine. In an Na+-free Ca2+-medium, leucine accelerated 45Ca2+ efflux to the same extent with or without glutamine. Leucine also stimulated 45Ca2+ influx in the presence or in the absence of Na+, but its effects were potentiated by glutamine only in the presence of Na+. The results show that amino acids of various types cause distinct changes in 45Ca2+ fluxes in pancreatic islets. Certain of these changes involve an Na+-mediated mobilization of cellular Ca2+ from sequestering sites where glucose appears to exert an opposite effect.

Removal of extraplatelet Na+ eliminates indomethacin-sensitive secretion from human platelets stimulated by epinephrine, ADP, and thrombin

We have previously observed that removal of extraplatelet Na+ markedly diminishes human platelet aggregation and secretion in response to epinephrine. The present studies demonstrate that this effect of the removal of extraplatelet Na+ on platelet function is not unique to activation of platelets by alpha 2-adrenergic agents but represents a phenomenon also evident for other platelet stimuli. Thus, platelet aggregation and secretion in response to maximal concentrations of ADP and lower concentrations of thrombin (less than 0.04 unit/ml) were also markedly reduced in platelets in "Na+-free" medium, suggesting that these agents share an effector mechanism that is similarly inhibited by the removal of extraplatelet Na+. In contrast, platelet aggregation and secretion in response to higher concentrations of thrombin (greater than or equal to 0.04 unit/ml) and to 0.04-1.0 microM (15S)-hydroxy-11 alpha, 9 alpha-(epoxymethano)prosta-5Z,13E-dienoic acid (U46619), an endoperoxide analog, were identical in control platelets and in those suspended in "Na+-free" medium, indicating that platelets suspended in "Na+-free" medium are functionally intact, at least in response to some stimuli. Furthermore, the observation that U46619 can elicit platelet aggregation and secretion independently of extraplatelet Na+ indicates that the loss of platelet responsiveness to epinephrine, ADP, and low concentrations of thrombin cannot be attributed to a loss of sensitivity to the stimulus-provoked secondary mediator(s) of platelet function, endoperoxides or thromboxane A2. Treatment with indomethacin to block the secondary aggregation and secretion pathways of platelets reduced the aggregatory and secretory responses of control platelets induced by epinephrine, ADP, and low concentrations of thrombin to those characteristic of platelets suspended in "Na+-free" medium. In contrast, indomethacin did not alter the functional responses induced by these agents in platelets suspended in "Na+-free" medium, suggesting that "primary" aggregation is intact but that the "secondary" aggregation and secretion mediated by arachidonic acid metabolites are eliminated by removal of extraplatelet Na+. Consistent with this interpretation is the observation that the indomethacin-insensitive aggregation and secretion induced by U46619 and higher concentrations of thrombin were retained in platelets suspended in "Na+-free" medium. Thus, the responses eliminated by removal of extraplatelet Na+ are those eliminated by treating control platelets with indomethacin, suggesting a strong link between the presence of extraplatelet Na+ and the operation of platelet function mediated by the cyclooxygenase pathway.

Regulation of immunoreactive-insulin release from a rat cell line (RINm5F)

1. An insulin-producing cell line, RINm5F, derived from a rat insulinoma was studied. 2. The cellular content of immunoreactive insulin was 0.19 pg/cell, which represents approx. 1% of the insulin content of native rat beta-cells, whereas that of immunoreactive glucagon and somatostatin was five to six orders of magnitude less than that of native alpha- or delta-cells respectively. 3. RINm5F cells released 7-12% of their cellular immunoreactive-insulin content at 2.8 mM-glucose during 60 min in Krebs-Ringer bicarbonate buffer. 4. Glucose utilization was increased by raising glucose from 2.8 to 16.7 mM. There was, however, no stimulation of immunoreactive-insulin release even when glucose was increased from 2.8 to 33.4 mM. A small stimulation of release was, however, found when glucose was raised from 0 to 2.8 mM. 5. Glyceraldehyde stimulated the release of immunoreactive insulin in a dose-dependent manner. 6. At 20 mM, leucine or arginine stimulated release at 2.8 mM-glucose. 7. Raising intracellular cyclic AMP by glucagon or 3-isobutyl-1-methylxanthine stimulated release at 2.8 mM-glucose with no additional stimulation at 16.7 mM-glucose. 8. Stimulation of immunoreactive-insulin release by K+ was dose-related between 2 and 30 mM. Another depolarizing agent, ouabain, also stimulated release. 9. Adrenaline (epinephrine) inhibited both basal (2.8 mM-glucose) release and that stimulated by 30 mM-K+. 10. Raising Ca2+ from 1 to 3 mM stimulated immunoreactive-insulin release, whereas a decrease from 1 to 0.3 or to 0.1 mM-Ca2+ lowered the release. 11. These findings could reflect a relatively specific impairment in glucose handling by RINm5F cells, contrasting with the preserved response to other modulators of insulin release.

Selective inhibition of glucose-stimulated insulin release by dantrolene

Dantrolene sodium, which interferes with excitation-contraction coupling by inhibiting the Ca2+ release from sarcoplasmic reticulum in muscle, was used to investigate the role of stored calcium in the stimulation of insulin release by various secretagogues. Insulin release was measured simultaneously with 45Ca2+ uptake or 45Ca2+ efflux from isolated rat pancreatic islets. Glucose-stimulated insulin release was inhibited by dantrolene (10-100 microM) as was glyceraldehyde- or mannose-stimulated release. In contrast, dantrolene failed to inhibit insulin release stimulated by leucine, arginine, ouabain, potassium, or 3-isobutyl-1-methylxanthine. Although dantrolene lowered glucose-stimulated 45Ca2+ uptake, nonspecific blockade of voltage-dependent Ca2+ channels may not be a primary action of dantrolene because K+-stimulated 45Ca2+ uptake was not inhibited. Glucose utilization (3H2O formation) was unaffected by dantrolene, whereas glucose oxidation (14CO2 production) was decreased. In the absence of Ca2+, the glucose-inhibited 45Ca2+ efflux was unchanged. At normal Ca2+, dantrolene inhibited glucose-stimulated 45Ca2+ efflux and veratridine induced insulin release. This suggests an interference with mobilization of beta-cell calcium stores. The selective action of dantrolene on insulin release makes it an interesting tool for further studies on stimulus-secretion coupling.

Fluctuations of calcium, phosphorus, sodium, potassium, and chlorine in single alpha and beta cells during glucose perifusion of rat islets

To study the relationship between islet hormonal secretion and intracellular content of five elements, a rat islet perifusion technique was used in 24 paired experiments. Control and experimental chambers each containing 100 islets, received 2.8 and 16.7 mM D-glucose, respectively. Effluent was collected frequently for hormone measurements. At eight different time intervals form 0--30 min islets were fixed and prepared for scanning electron microscopy. Over 900 unobscured alpha and beta cells were selected by size and shape criteria. Energy dispersive x-ray analysis was applied to each single cell to determine relative content of calcium (Ca), potassium (K), sodium (Na), chlorine (Cl), and phosphorus (P). Experimental chambers exhibited typical acute (0--9 min) and second phase (10--30 min) insulin secretion in association with suppression of glucagon release after 10 min. At 2 min an abrupt upward K spike in both alpha and beta cells was followed at 3--4 min with a 1.5- to 2-fold rise of Ca and a reciprocal decrease in K, Na, Cl, and P. From 3 to 30 min biphasic insulin secretion. Reduced alpha cell calcium after 6 min preceded suppression of glucagon secretion. After 2 min K related inversely to Ca content in both alpha and beta cells. These results could not be reproduced when D-galactose was substituted for D-glucose. We conclude that sequential changes of Ca content that are reciprocally related to K are predictive of beta cell insulin release and suppression of alpha cell glucagon secretion.