Pancreatic beta-cells from obese-hyperglycemic mice are characterized by excessive firing of cytoplasmic Ca2+ transients

Enhanced glucose-induced intracellular signaling promotes insulin hypersecretion: pancreatic beta-cell functional adaptations in a model of genetic obesity and prediabetes

Obesity is associated with insulin resistance and is known to be a risk factor for type-2 diabetes. In obese individuals, pancreatic beta-cells try to compensate for the increased insulin demand in order to maintain euglycemia. Most studies have reported that this adaptation is due to morphological changes. However, the involvement of beta-cell functional adaptations in this process needs to be clarified. For this purpose, we evaluated different key steps in the glucose-stimulated insulin secretion (GSIS) in intact islets from female ob/ob obese mice and lean controls. Obese mice showed increased body weight, insulin resistance, hyperinsulinemia, glucose intolerance and fed hyperglycemia. Islets from ob/ob mice exhibited increased glucose-induced mitochondrial activity, reflected by enhanced NAD(P)H production and mitochondrial membrane potential hyperpolarization. Perforated patch-clamp examination of beta-cells within intact islets revealed several alterations in the electrical activity such as increased firing frequency and higher sensitivity to low glucose concentrations. A higher intracellular Ca(2+) mobilization in response to glucose was also found in ob/ob islets. Additionally, they displayed a change in the oscillatory pattern and Ca(2+) signals at low glucose levels. Capacitance experiments in intact islets revealed increased exocytosis in individual ob/ob beta-cells. All these up-regulated processes led to increased GSIS. In contrast, we found a lack of beta-cell Ca(2+) signal coupling, which could be a manifestation of early defects that lead to beta-cell malfunction in the progression to diabetes. These findings indicate that beta-cell functional adaptations are an important process in the compensatory response to obesity.

beta-cell function in obese-hyperglycemic mice [ob/ob Mice]

This review summarizes key aspects of what has been learned about the physiology of pancreatic islets and leptin deficiency from studies in obese ob/ob mice. ob/ob Mice lack functional leptin. They are grossly overweight and hyperphagic particularly at young ages and develop severe insulin resistance with hyperglycemia and hyperinsulinemia. ob/ob Mice have large pancreatic islets. The beta-cells respond adequately to most stimuli, and ob/ob mice have been used as a rich source of pancreatic islets with high insulin release capacity. ob/ob Mice can perhaps be described as a model for the prediabetic state. The large capacity for islet growth and insulin release makes ob/ob mice a good model for studies on how beta-cells can cope with prolonged functional stress.

Pulsatility of insulin release--a clinically important phenomenon

The mechanisms and clinical importance of pulsatile insulin release are presented against the background of more than half a century of companionship with the islets of Langerhans. The insulin-secreting beta-cells are oscillators with intrinsic variations of cytoplasmic ATP and Ca(2+). Within the islets the beta-cells are mutually entrained into a common rhythm by gap junctions and diffusible factors (ATP). Synchronization of the different islets in the pancreas is supposed to be due to adjustment of the oscillations to the same phase by neural output of acetylcholine and ATP. Studies of hormone secretion from the perfused pancreas of rats and mice revealed that glucose induces pulses of glucagon anti-synchronous with pulses of insulin and somatostatin. The anti-synchrony may result from a paracrine action of somatostatin on the glucagon-producing alpha-cells. Purinoceptors have a key function for pulsatile release of islet hormones. It was possible to remove the glucagon and somatostatin pulses with maintenance of those of insulin with an inhibitor of the P2Y(1) receptors. Knock-out of the adenosine A(1) receptor prolonged the pulses of glucagon and somatostatin without affecting the duration of the insulin pulses. Studies of isolated human islets indicate similar relations between pulses of insulin, glucagon, and somatostatin as found during perfusion of the rodent pancreas. The observation of reversed cycles of insulin and glucagon adds to the understanding how the islets regulate hepatic glucose production. Current protocols for pulsatile intravenous infusion therapy (PIVIT) should be modified to mimic the anti-synchrony between insulin and glucagon normally seen in the portal blood.

Synchronization and entrainment of cytoplasmic Ca2+ oscillations in cell clusters prepared from single or multiple mouse pancreatic islets

In contrast to pancreatic islets, isolated beta-cells stimulated by glucose display irregular and asynchronous increases in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)). Here, clusters of 5-30 cells were prepared from a single mouse islet or from pools of islets, loaded with fura-2, and studied with a camera-based system. [Ca(2+)](i) oscillations were compared in pairs of clusters by computing the difference in period and a synchronization index lambda. During perifusion with 12 mM glucose, the clusters exhibited regular [Ca(2+)](i) oscillations that were quasi-perfectly synchronized (Delta period of 1.4% and index lambda close to 1.0) between cells of each cluster. In contrast, separate clusters were not synchronized, even when prepared from one single islet. Pairs of clusters neighboring on the same coverslip were not better synchronized than pairs of clusters examined separately (distinct coverslips). We next attempted to synchronize clusters perifused with 12 mM glucose by applying external signals. A single pulse of 20 mM glucose, 10 mM amino acids, or 10 microM tolbutamide transiently altered [Ca(2+)](i) oscillations but did not reset the clusters to oscillate synchronously. On a background of 12 mM glucose, repetitive applications (1 min/5 min) of 10 microM tolbutamide, but not of 20 mM glucose, synchronized separate clusters. Our results identify a level of beta-cell heterogeneity intermediate between single beta-cells and the whole islet. They do not support the idea that substances released by islet cells serve as paracrine synchronizers. However, synchronization can be achieved by an external signal, if this signal has a sufficient strength to overwhelm the intrinsic rhythm of glucose-induced oscillations and is repetitively applied.

Regulation of intracellular Ca2+ stores by multiple Ca2+-releasing messengers

Although glucose-elicited insulin secretion depends on Ca(2+) entry through voltage-gated Ca(2+) channels in the surface cell membrane of the pancreatic beta-cell, there is also ample evidence for an important role of intracellular Ca(2+) stores, particularly in relation to hormone- or neurotransmitter-induced insulin secretion. There is now direct evidence for Ca(2+) entry-induced release of Ca(2+) from the endoplasmic reticulum in neurons, but with regard to glucose stimulation of beta-cells, there is conflicting evidence about the operation of such a process. This finding suggests that the sensitivity of the Ca(2+) release channels in the endoplasmic reticulum membrane varies under different conditions and therefore is regulated. Recent evidence from studies of pancreatic acinar cells has revealed combinatorial roles of multiple messengers in setting the sensitivity of the endoplasmic reticulum for Ca(2+) release. Here we focus on the possible combinatorial roles of inositol 1,4,5-trisphosphate, cyclic ADP-ribose, and nicotinic acid adenine dinucleotide phosphate in beta-cell function.

Insulin-secreting INS-1 cells generate a novel type of poorly synchronized Ca2+ transients

Pancreatic beta-cells have an intrinsic oscillatory Ca2+ activity supposed to be synchronized among the islets by cytoplasmic Ca2+ transients elicited by nonadrenergic, noncholinergic (NANC) neurons. To improve the understanding of this process, the cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in two insulin-releasing cell lines using dual wavelength microfluorometry and the indicator fura-2. INS-1 cells but not RINm5F cells were found to generate transients of [Ca2+]i in the presence of the Ca2+ channel blocker methoxyverapamil. These transients differed from those occurring in native beta-cells persisting in the presence of thapsigargin or during prolonged exposure to ATP. Moreover, the [Ca2+]i transients were poorly synchronized whether or not the INS-1 cells had physical contact. If appearing in native beta-cells, the type of [Ca2+]i transients now observed may interfere with the coordination of the beta-cell rhythmicity evoked by NANC neurons.

Contribution of the endoplasmic reticulum to the glucose-induced [Ca(2+)](c) response in mouse pancreatic islets

Thapsigargin (TG), a blocker of Ca(2+) uptake by the endoplasmic reticulum (ER), was used to evaluate the contribution of the organelle to the oscillations of cytosolic Ca(2+) concentration ([Ca(2+)](c)) induced by repetitive Ca(2+) influx in mouse pancreatic beta-cells. Because TG depolarized the plasma membrane in the presence of glucose alone, extracellular K(+) was alternated between 10 and 30 mM in the presence of diazoxide to impose membrane potential (MP) oscillations. In control islets, pulses of K(+), mimicking regular MP oscillations elicited by 10 mM glucose, induced [Ca(2+)](c) oscillations whose nadir remained higher than basal [Ca(2+)](c). Increasing the depolarization phase of the pulses while keeping their frequency constant (to mimic the effects of a further rise of the glucose concentration on MP) caused an upward shift of the nadir of [Ca(2+)](c) oscillations that was reproduced by raising extracellular Ca(2+) (to increase Ca(2+) influx) without changing the pulse protocol. In TG-pretreated islets, the imposed [Ca(2+)](c) oscillations were of much larger amplitude than in control islets and occurred on basal levels. During intermittent trains of depolarizations, control islets displayed mixed [Ca(2+)](c) oscillations characterized by a summation of fast oscillations on top of slow ones, whereas no progressive summation of the fast oscillations was observed in TG-pretreated islets. In conclusion, the buffering capacity of the ER in pancreatic beta-cells limits the amplitude of [Ca(2+)](c) oscillations and may explain how the nadir between oscillations remains above baseline during regular oscillations or gradually increases during mixed [Ca(2+)](c) oscillations, two types of response observed during glucose stimulation.

The ryanodine receptor calcium channel of beta-cells: molecular regulation and physiological significance

The list of Ca(2+) channels involved in stimulus-secretion coupling in beta-cells is increasing. In this respect the roles of the voltage-gated Ca(2+) channels and IP(3) receptors are well accepted. There is a lack of consensus about the significance of a third group of Ca(2+) channels called ryanodine (RY) receptors. These are large conduits located on Ca(2+) storage organelle. Ca(2+) gates these channels in a concentration- and time-dependent manner. Activation of these channels by Ca(2+) leads to fast release of Ca(2+) from the stores, a process called Ca(2+)-induced Ca(2+) release (CICR). A substantial body of evidence confirms that beta-cells have RY receptors. CICR by RY receptors amplifies Ca(2+) signals. Some properties of RY receptors ensure that this amplification process is engaged in a context-dependent manner. Several endogenous molecules and processes that modulate RY receptors determine the appropriate context. Among these are several glycolytic intermediates, long-chain acyl CoA, ATP, cAMP, cADPR, NO, and high luminal Ca(2+) concentration, and all of these have been shown to sensitize RY receptors to the trigger action of Ca(2+). RY receptors, thus, detect co-incident signals and integrate them. These Ca(2+) channels are targets for the action of cAMP-linked incretin hormones that stimulate glucose-dependent insulin secretion. In beta-cells some RY receptors are located on the secretory vesicles. Thus, despite their low abundance, RY receptors are emerging as distinct players in beta-cell function by virtue of their large conductance, strategic locations, and their ability to amplify Ca(2+) signals in a context-dependent manner.

Ca2+ handling of rat pancreatic beta-cells exposed to ryanodine, caffeine, and glucagon

Reported species differences in the stimulus-secretion coupling of insulin release made it important to compare the Ca2+ handling of rat beta-cells with that previously observed in mice. Single beta-cells and small aggregates were prepared from pancreatic islets of Wistar rats, attached to cover slips and then used for measuring the cytoplasmic Ca2+ concentration ([Ca2+]i) with the ratiometric fura-2 technique. Glucose (11 mM) induced slow oscillations of [Ca2+]i similar to those seen in other species, including humans. Comparison of the oscillations in rat beta-cells with those previously described in mouse revealed that there was a slightly lower frequency and an increased tendency to transformation into sustained [Ca2+]i in response to glucagon or caffeine. Ryanodine (5-20 microM) did not affect existing oscillations but sometimes restored rhythmic activity in the presence of caffeine. Stimulation with glucose resulted not only in oscillations but also in transients of [Ca2+]i sometimes appearing in synchrony in adjacent beta-cells and disappearing after the addition of 200 nM thapsigargin or 20 mM caffeine. The frequency of transients recorded in a medium containing glucagon and methoxyverapamil was higher than seen under similar conditions in mouse beta-cells. Although exhibiting some differences compared with mouse beta-cells, rat beta-cells also have an intrinsic ability to oscillate and to generate the transients of [Ca2+] that are supposed to synchronize the rhythmicity of the islets in the pancreas.