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

Exploring the Effects of Metabolism-Disrupting Chemicals on Pancreatic α-Cell Viability, Gene Expression and Function: A Screening Testing Approach

Humans are constantly exposed to many environmental pollutants, some of which have been largely acknowledged as key factors in the development of metabolic disorders such as diabetes and obesity. These chemicals have been classified as endocrine-disrupting chemicals (EDCs) and, more recently, since they can interfere with metabolic functions, they have been renamed as metabolism-disrupting chemicals (MDCs). MDCs are present in many consumer products, including food packaging, personal care products, plastic bottles and containers, and detergents. The scientific literature has ever-increasingly focused on insulin-releasing pancreatic β-cells as one of the main targets for MDCs. Evidence highlights that these substances may disrupt glucose homeostasis by altering pancreatic β-cell physiology. However, their potential impact on glucagon-secreting pancreatic α-cells remains poorly known despite the essential role that this cellular type plays in controlling glucose metabolism. In the present study, we have selected seven paradigmatic MDCs representing major toxic classes, including bisphenols, phthalates, perfluorinated compounds, metals, and pesticides. By using an in vitro cell-based model, the pancreatic α-cell line αTC1-9, we have explored the effects of these compounds on pancreatic α-cell viability, gene expression, and secretion. We found that cell viability was moderately affected after bisphenol-A (BPA), bisphenol-F (BPF), and perfluorooctanesulfonic acid (PFOS) exposure, although cytotoxicity was relatively low. In addition, all bisphenols, as well as di(2-ethylhexyl) phthalate (DEHP) and cadmium chloride (CdCl₂), promoted a marked decreased on glucagon secretion, together with changes in the expression of glucagon and/or transcription factors involved in cell function and identity, such as Foxo1 and Arx. Overall, our results indicated that most of the selected chemicals studied caused functional alterations in pancreatic α-cells. Moreover, we revealed, for the first time, their direct effects on key molecular aspects of pancreatic α-cell biology.

The structure of mineral and bone disorders in patients with сhronic kidney disease of the 5th dialysis stage, taking into account the presence or absence of a diagnosis of type 1 diabetes mellitus

BACKGROUND: In patients with end-stage CKD, receiving renal replacement therapy (RRT) with programmed hemodialysis (HD), the severity of complications is associated with metabolic disturbances: accumulation of uremic toxins, nephrogenic anemia, secondary hyperparathyroidism (SHPT), extraskeletal calcification, impaired clearance and rhythm of hormone secretion.

AIM: To evaluate the main biochemical and hormonal parameters, and manifestations of mineral bone disease (MBD) in patients receiving RRT with HD, before and after hemodialysis, taking into account the presence or absence of diabetes mellitus.

MATERIALS AND METHODS: We divided all patients receiving RRT with HD in two groups: #1 (n=24) — patients with DM, #2 (n=16) — patients without DM. All of them had their blood analyzed before and immediately after the HD. Data analysis was performed with the Statistica 13 (StatSoft, USA). A prognostically significant model was considered at p<0.05.

RESULTS: The level of iPTH, both at baseline and after HD, was lower in group #1 (p<0.001). The level of alkaline phosphatase (AP) was significantly higher in group #2 (p=0.012). In both groups before HD, a high incidence of hypocalcemia was detected (according to albumin-corrected calcium in group #1 in 58.3%, in group #2 in 43.7% of cases, p = 0.366) and hyperphosphatemia (in 66.7% and in 43 .7% of cases, respectively, p=0.151). Hypocalcemia after HD in group #1 persisted in 14%, in group #2 — in 20% of cases (p>0.05); hyperphosphatemia in group #1 was completely leveled, in group #2 it persisted in 7% of cases (p=0.417). Prior to the HD session, group #1 had significantly higher levels of RAGE, glucagon, immunoreactive insulin (IRI), cortisol, and glucose than after the HD session (p<0.05). In group #2, after HD, the levels of glucagon, IRI and cortisol significantly decreased (p<0.05), and the level of 3-nitrotyrosine (3-HT) increased significantly (p=0.026). In group #1, fibrocalcinosis of the heart valves according to ECHO and calcification of the arteries of the lower extremities according to ultrasonic doplerography were more common than in group #2 (42% vs 25%, p<0.001 and 75% vs 37.5%, p=0.018, respectively). (χ2)). Compression fractures occurred with the same frequency in both groups (60%). A decrease in bone mineral density (BMD) to the level of osteopenia was noted more often in group #1 (50% vs 18.8%), and osteoporosis was more common in group #2 (68.8% vs 33.3%) (p<0.001, χ2).

CONCLUSION: The low level of PTH in group #1 may reflect the effect of diabetes on calcium-phosphorus metabolism. Patients with DM have an increased risk of renal osteodystrophy with a low bone turnover because of a number of metabolic factors inherent in diabetes. At the same time, the dynamics of phosphorus and calcium indicators during the HD procedure were similar.

Sodium, Glucose and Dysregulated Glucagon Secretion: The Potential of Sodium Glucose Transporters

Diabetes is defined by hyperglycaemia due to progressive insulin resistance and compromised insulin release. In parallel, alpha cells develop dysregulation of glucagon secretion. Diabetic patients have insufficient glucagon secretion during hypoglycaemia and a lack of inhibition of glucagon secretion at higher blood glucose levels resulting in postprandial hyperglucagonaemia, which contributes to the development of hyperglycaemia. Sodium-glucose co-transporter 2 (SGLT2) inhibitors are an efficient pharmacologic approach for the treatment of hyperglycaemia in type 2 diabetes. While SGLT2 inhibitors aim at increasing glycosuria to decrease blood glucose levels, these inhibitors also increase circulating glucagon concentrations. Here, we review recent advances in our understanding of how SGLTs are involved in the regulation of glucagon secretion. Sodium plays an important role for alpha cell function, and a tight regulation of intracellular sodium levels is important for maintaining plasma membrane potential and intracellular pH. This involves the sodium-potassium pump, sodium-proton exchangers and SGLTs. While the expression of SGLT2 in alpha cells remains controversial, SGLT1 seems to play a central role for alpha cell function. Under hyperglycaemic conditions, SGLT1 mediated accumulation of sodium results in alpha cell dysregulation due to altered cellular acidification and ATP production. Taken together, this suggests that SGLT1 could be a promising, yet highly underappreciated drug target to restore alpha cell function and improve treatment of both type 1 and 2 diabetes.

A K ATP channel-dependent pathway within alpha cells regulates glucagon release from both rodent and human islets of Langerhans

Glucagon, secreted from pancreatic islet α cells, stimulates gluconeogenesis and liver glycogen breakdown. The mechanism regulating glucagon release is debated, and variously attributed to neuronal control, paracrine control by neighbouring β cells, or to an intrinsic glucose sensing by the α cells themselves. We examined hormone secretion and Ca²⁺ responses of α and β cells within intact rodent and human islets. Glucose-dependent suppression of glucagon release persisted when paracrine GABA or Zn²⁺ signalling was blocked, but was reversed by low concentrations (1–20 μM) of the ATP-sensitive K⁺ (K_ATP) channel opener diazoxide, which had no effect on insulin release or β cell responses. This effect was prevented by the K_ATP channel blocker tolbutamide (100 μM). Higher diazoxide concentrations (≥30 μM) decreased glucagon and insulin secretion, and α- and β-cell Ca²⁺ responses, in parallel. In the absence of glucose, tolbutamide at low concentrations (<1 μM) stimulated glucagon secretion, whereas high concentrations (>10 μM) were inhibitory. In the presence of a maximally inhibitory concentration of tolbutamide (0.5 mM), glucose had no additional suppressive effect. Downstream of the K_ATP channel, inhibition of voltage-gated Na⁺ (TTX) and N-type Ca²⁺ channels (ω-conotoxin), but not L-type Ca²⁺ channels (nifedipine), prevented glucagon secretion. Both the N-type Ca²⁺ channels and α-cell exocytosis were inactivated at depolarised membrane potentials. Rodent and human glucagon secretion is regulated by an α-cell K_ATP channel-dependent mechanism. We propose that elevated glucose reduces electrical activity and exocytosis via depolarisation-induced inactivation of ion channels involved in action potential firing and secretion.

Glucagon is a critical regulator of glucose homeostasis. Its major action is to mobilize glucose from the liver. Glucagon secretion from α cells of the pancreatic islets of Langerhans is suppressed by elevated blood sugar, a response that is often perturbed in diabetes. Much work has focused on the regulation of α-cell glucagon secretion by neuronal factors and by paracrine factors from neighbouring cells, including the important islet hormone insulin. In contrast, we provide evidence in support of a direct effect of glucose on α cells within intact rodent and human islets. Notably, our work implicates an α-cell glucose-sensing pathway similar to that found in insulin-secreting β cells, involving closure of ATP-dependent K⁺ channels in the presence of glucose. Furthermore, we find that membrane depolarisation results in inhibition of Na⁺ and Ca²⁺ channel activity and α-cell exocytosis. Thus, we propose that elevated blood glucose reduces α-cell electrical activity and glucagon secretion by inactivating the ion channels involved in action potential firing and secretion.

Elevated glucose levels reduce electrical activity and the release of glucagon via inactivation of ion channels in pancreatic islet cells.

Effect of extracellular sodium removal upon 86Rb outflow from pancreatic islet cells

The present study was undertaken to characterize the effect of extracellular Na+ removal on 86Rb outflow from perifused rat pancreatic islets. Complete Na+ omission inhibited 86Rb outflow whether the islets were perifused in the presence or in the absence of extracellular Ca2+. Ouabain (1 mM) did not reduce the inhibitory effect of Na+ deprivation, whilst diphenylhydantoin (72.9 microM) mimicked the Na+-removal-induced fall in 86Rb outflow. Glucose (16.7 mM) lost its capacity to inhibit 86Rb outflow when the perifusate was deprived of extracellular Na+. These results indicate that Na+ omission reproduces the inhibitory effect of glucose on 86Rb outflow. The reduction in 86Rb outflow recorded after Na+ deprivation could be mediated by an intracellular acidification and/or a decrease in the intracellular Na+ activity. It is tempting to speculate that the capacity of glucose to reduce the B-cell Na+ content may participate in the process by which the sugar decreases K+ permeability.

Characterization of the glucose-induced lowering of sodium in mouse pancreatic beta-cells

The mechanisms for glucose regulation of the sodium content of the pancreatic beta-cells were examined using aggregates of cells prepared from ob/ob-mice of a local colony. Exposure to glucose rapidly resulted in a protracted lowering of the sodium content as estimated with integrating flame photometry. Sodium became decreased after addition of 1 mmol l-1 glucose, and this effect was maximal with 5 mmol l-1 of the sugar. The effects of low glucose concentrations on the sodium content could not be mimicked by the poorly metabolized 3-o-methyl-D-glucose, and it disappeared in the presence of the metabolic inhibitor antimycin A. The significance of the Na/K pump for maintaining low sodium was illustrated by a substantial increase of the element in the presence of ouabain. However, there was no indication that glucose-induced lowering of sodium reflected activation of this pump when measuring the ouabain-sensitive uptake of 86Rb+. Neither bumetanide nor the bromo derivatives of cyclic AMP or cyclic GMP modified the glucose action on the sodium content. In evaluating whether the effect of glucose was mimicked by other inhibitors of the K+ permeability it was observed that 100 mumol l-1 quinine, but not tolbutamide, decreased sodium. It is concluded that the beta-cell is exceptional among excitable cells in responding to its natural physiological stimulant (glucose) by reduction of sodium. Acting in this way glucose facilitates the removal of Ca2+ from the cytoplasm.

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.

Modifications of the pancreatic beta-cell function after lowering their potassium content

beta-cell-rich pancreatic islets from ob/ob-mice were kept for 3 days in a culture medium and analysed for their content of potassium. In a normal ionic milieu intracellular potassium was calculated as 190-260 mM. Whereas this concentration remained essentially unaffected after lowering extracellular K+ to 1.5 mM, further reduction to 0.15 mM depressed islet potassium to 5% or less of its original value. Irrespective of its medium concentration, potassium in the islets increased when glucose was raised from 1 to 20 mM. Depression of the islet potassium was associated with a rise of intracellular calcium. Despite profound depletion of potassium, the beta-cells maintained their insulin content and could still oxidize glucose at a substantial rate. When potassium was suppressed to 25% or less of the original content, the beta-cells responded to glucose with a paradoxical inhibition of insulin release. After 3 days of potassium depletion, exposure to a normal ionic milieu neither restored the intracellular content of potassium nor a stimulated insulin secretory response to glucose.

Glucose-induced reduction of the sodium content in beta-cell-rich pancreatic islets

The sodium contents of beta-cell-rich pancreatic islets from ob/ob-mice were measured with an integrating flame photometer. After washing to an apparent steady state with different types of ice-cold media, islets incubated in the absence of glucose contained 79-108 mmol sodium kg-1 dry weight. Exposure to glucose resulted in 25% reduction of the islet content of sodium. This effect became manifest in the presence of 5 mM glucose, there being no additional reduction with a further increase of glucose to 20 mM. Depression of Na+ activity may partially explain why glucose, under certain conditions, can lower cytoplasmic Ca2+ and even inhibit insulin release.

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.

A new method for the simultaneous estimation of phosphate efflux and influx during glucose stimulation of isolated pancreatic islets

Isolated rat pancreatic islets were prelabeled with [33Pi] and then incubated with basal (2.8 mM) or stimulatory (16.7 mM) glucose in the presence of [32Pi]. Subsequent changes in islet [33P] and [32P] were utilized as respective indices of net efflux and influx. During the initial eight min, (the period usually spanning the first phase of stimulated insulin secretion) efflux was significantly greater with 16.7 than 2.8 mM glucose whereas the lesser amount of phosphate influx did not differ in the two systems. During the subsequent seven min (a time usually associated with the onset of the second phase of stimulated insulin secretion), efflux was dampened in the presence of 16.7 mM glucose and Pi influx significantly exceeded the 2.8 mM glucose values. Thus, acute stimulation with glucose effects an initial phosphate depletion in pancreatic islets as efflux exceeds influx and repletion occurs thereafter as efflux is attenuated and influx is enhanced. These oscillations in islet phosphate may contribute to the biphasic pattern of glucose-stimulated insulin release.

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.

Tolbutamide perifusion of rat islets. Sequential changes in calcium, phosphorus, sodium, potassium, and chlorine in single beta cells

Fluctuations of calcium, phosphorus, sodium, potassium, and chlorine in beta cells were followed during rat islet perifusion with tolbutamide and related to insulin secretion. In 24 paired experiments two chambers containing 100 islets were perifused with buffered medium containing 4.2 mM glucose alone or with added tolbutamide (200 micrograms/ml). Effluent was collected frequently for insulin determinations. At eight different time intervals from 0 to 20 min islets were acutely fixed, prepared for scanning electron microscopy and beta cells in islet tissue were identified. Element content in 480 single cells was measured by energy dispersive x-ray analysis. Tolbutamide elicited typical monophasic insulin release that exceeded control islet secretory rates from 2 to 6 min with a peak value at 3 min. This pattern was preceded by monophasic calcium accumulation in beta cells that abruptly rose 150% above control cells at 1 min and declined to base line by 4 min. The rapid ascent of calcium was associated with significant depressions of sodium and potassium content without alterations of cell phosphorus. Chlorine fell at 2 min and then rose greater than 50% above control cells at 4 min. After 6 min insulin secretion and element content remained near control levels. We conclude that monophasic calcium accumulation in beta cells is the earliest, most predictive event of islet insulin secretion after a tolbutamide stimulus. Oscillations of beta cell sodium and potassium reciprocally relate to calcium, and an elevation of chlorine content is a relatively late phenomenon in the stimulus-secretion coupling process.

Activation, but not inhibition, by glucose of Ca2+-dependent K+ permeability in the rat pancreatic B-cell

The effect of glucose on the Ca2+-activated K+ permeability in pancreatic islet cells was investigated by measuring the rate of 86Rb efflux, 45Ca efflux and insulin release from perifused rat pancreatic islets exposed to step-wise increased in glucose concentration. When the glucose concentration was raised from intermediate (8.3 or 11.1 mM) to higher values, a rapid and sustained increase in 86Rb outflow, 45Ca outflow and insulin release was observed. Likewise, in the presence of 8.3 or 16.7 mM glucose, tolbutamide increased 86Rb and 45Ca efflux, as well as insulin release. In the two series of experiments, a tight correlation was found between the magnitude of the changes in 86Rb and 45Ca outflow, respectively. It is concluded that, at variance with current ideas, glucose does not inhibit the response to cytosolic Ca2+ of the Ca2+-sensitive modality of K+ extrusion. On the contrary, as a result of its effect upon Ca2+ handling, glucose stimulates the Ca2+-activated K+ permeability.