Alterations in pancreatic islet phosphate content during secretory stimulation with glucose

Phosphate Dysregulation and Metabolic Syndrome

Phosphorus is one of the most abundant minerals in the human body. It is essential for almost all biochemical activities through ATP formation, intracellular signal transduction, cell membrane formation, bone mineralization, DNA and RNA synthesis, and inflammation modulation through various inflammatory cytokines. Phosphorus levels must be optimally regulated, as any deviations may lead to substantial derangements in glucose homeostasis. Clinical studies have reported that hyperphosphatemia can increase an individual's risk of developing metabolic syndrome. High phosphate burden has been shown to impair glucose metabolism by impairing pancreatic insulin secretion and increasing the risk of cardiometabolic disorders. Phosphate toxicity deserves more attention as metabolic syndrome is being seen more frequently worldwide and should be investigated further to determine the underlying mechanism of how phosphate burden may increase the cardiometabolic risk in the general population.

XPR1 Mediates the Pancreatic β-Cell Phosphate Flush

Glucose-stimulated insulin secretion is the hallmark of the pancreatic β-cell, a critical player in the regulation of blood glucose concentration. In 1974, the remarkable observation was made that an efflux of intracellular inorganic phosphate (P_i) accompanied the events of stimulated insulin secretion. The mechanism behind this "phosphate flush," its association with insulin secretion, and its regulation have since then remained a mystery. We recapitulated the phosphate flush in the MIN6m9 β-cell line and pseudoislets. We demonstrated that knockdown of XPR1, a phosphate transporter present in MIN6m9 cells and pancreatic islets, prevented this flush. Concomitantly, XPR1 silencing led to intracellular P_i accumulation and a potential impact on Ca²⁺ signaling. XPR1 knockdown slightly blunted first-phase glucose-stimulated insulin secretion in MIN6m9 cells, but had no significant impact on pseudoislet secretion. In keeping with other cell types, basal P_i efflux was stimulated by inositol pyrophosphates, and basal intracellular P_i accumulated following knockdown of inositol hexakisphosphate kinases. However, the glucose-driven phosphate flush occurred despite inositol pyrophosphate depletion. Finally, while it is unlikely that XPR1 directly affects exocytosis, it may protect Ca²⁺ signaling. Thus, we have revealed XPR1 as the missing mediator of the phosphate flush, shedding light on a 45-year-old mystery.

Oxygen dependence of glucose sensing: role in glucose homeostasis and related pathology

In glucose homeostasis, glucose concentration is sensed by its metabolism through glucokinase (GCK) and oxidative phosphorylation. Because oxidative phosphorylation is an integral part of the sensory system, glucose sensing is necessarily dependent on oxygen pressure. Much of the dependence on oxygen is suppressed by location of glucose sensing cells in tissues with well-regulated blood flow. In healthy individuals the oxygen dependence is primarily observed in response to transient global hypoxia events such as during birth or transition to high altitude. The GCK sensing system is, however, used to control release of both insulin and glucagon, the preeminant hormonal regulators of blood glucose, as well as glucose sensitive neuronal activity. Suppression of oxygen delivery to glucose-sensing cells or interference with regulation of tissue blood flow by either local or systemic causes, stresses the glucose regulatory system. This is true whether the stress is imposed locally, such as by altered oxygen delivery to the pancreas, or globally, as in pulmonary insufficiency or exposure to high altitude. It may be expected that chronic application of this stress predisposes individuals to developing diabetes. Type 2 diabetes is a broad class of diseases characterized by disturbance of glucose homeostasis, i.e., having either hyperglycemia and/or decreased sensitivity to insulin. Given the role of oxidative phosphorylation in glucose sensing, tissue oxygen deprivation may predispose individuals to developing diabetes as well as contributing to the disease itself. This is particularly true in age-related diabetes because the incidence of vascular insufficiency increases markedly with increasing age. NEW & NOTEWORTHY Glucose sensing requires glucose metabolism through glycolysis and oxidative phosphorylation. Dependence of the latter on oxygen concentration imposes an oxygen dependence on glucose sensing. We have used a validated computational model to quantify that dependence. Evidence is presented that tissue oxygenation plays an important role in predisposition of individuals to developing type 2 diabetes and in progression of the disease.

Glutamate dehydrogenase: role in regulating metabolism and insulin release in pancreatic β-cells

Regulation of insulin release and glucose homeostasis by pancreatic β-cells is dependent on the metabolism of glucose by glucokinase (GK) and the influence of that activity on oxidative phosphorylation. Genetic alterations that result in hyperactivity of mitochondrial glutamate dehydrogenase (GDH-1) can cause hypoglycemia-hyperammonemia following high protein meals, but the role of GDH-1 remains poorly understood. GDH-1 activity is strongly inhibited by GTP, to near zero in the absence of ADP, and cooperatively activated ( n = 2.3) by ADP. The dissociation constant for ADP is near 200 µM in vivo, but leucine and its nonmetabolized analog 2-amino-2-norbornane-carboxylic acid (BCH) can activate GDH-1 by increasing the affinity for ADP. Under physiological conditions, as [ADP] increases GDH-1 activity remains very low until ~35 µM (threshold) and then increases rapidly. A model for GDH-1 and its regulation has been combined with a previously published model for glucose sensing that coupled GK activity and oxidative phosphorylation. The combined model (GK-GDH-core) shows that GK activity determines the energy state ([ATP]/[ADP][Pi]) in β-cells for glucose concentrations > 5 mM ([ADP] < 35 µM). As glucose falls < 5 mM the [ADP]-dependent increase in GDH-1 activity prevents [ADP] from rising above ~70 µM. Thus, GDH-1 dynamically buffers β-cell energy metabolism during hypoglycemia, maintaining the energy state and the basal rate of insulin release. GDH-1 hyperactivity suppresses the normal increase in [ADP] in hypoglycemia. This leads to hypoglycemia following a high protein meal by increasing the basal rate of insulin release (β-cells) and decreasing glucagon release (α-cells). NEW & NOTEWORTHY A model of β-cell metabolism and regulation of insulin release is presented. The model integrates regulation of oxidative phosphorylation, glucokinase (GK), and glutamate dehydrogenase (GDH-1). GDH-1 is near equilibrium under physiological conditions, but the activity is inhibited by GTP. In hypoglycemia, however, GK activity is low and [ADP], a potent activator of GDH-1, increases. Reducing equivalents from GDH dynamically buffers the intramitochondrial [NADH]/[NAD⁺], and thereby the energy state, preventing hypoglycemia-induced substrate deprivation.

Intracellular alkalinization by phosphate uptake via type III sodium-phosphate cotransporter participates in high-phosphate-induced mitochondrial oxidative stress and defective insulin secretion

Elevated plasma levels of inorganic phosphate (P_i) are harmful, causing, among other complications, vascular calcification and defective insulin secretion. The underlying molecular mechanisms of these complications remain poorly understood. We demonstrated the role of P_i transport across the plasmalemma on P_i toxicity in INS-1E rat clonal β cells and rat pancreatic islet cells. Type III sodium-phosphate cotransporters (NaP_is) are the predominant P_i transporters expressed in insulin-secreting cells. Transcript and protein levels of sodium-dependent phosphate transporter 1 and 2 (PiT-1 and -2), isotypes of type III NaP_i, were up-regulated by high-P_i incubation. In patch-clamp experiments, extracellular P_i elicited a Na⁺-dependent, inwardly rectifying current, which was markedly reduced under acidic extracellular conditions. Cellular uptake of P_i elicited cytosolic alkalinization; intriguingly, this pH change facilitated P_i transport into the mitochondrial matrix. Increased mitochondrial P_i uptake accelerated superoxide generation, mitochondrial permeability transition (mPT), and endoplasmic reticulum stress-mediated translational attenuation, leading to reduced insulin content and impaired glucose-stimulated insulin secretion. Silencing of PiT-1/2 prevented P_i-induced superoxide generation and mPT, and restored insulin secretion. We propose that P_i transport across the plasma membrane and consequent cytosolic alkalinization could be a therapeutic target for protection from P_i toxicity in insulin-secreting cells, as well as in other cell types.-Nguyen, T. T., Quan, X., Xu, S., Das, R., Cha, S.-K., Kong, I. D., Shong, M., Wollheim, C. B., Park, K.-S. Intracellular alkalinization by phosphate uptake via type III sodium-phosphate cotransporter participates in high-phosphate-induced mitochondrial oxidative stress and defective insulin secretion.

Mitochondrial phosphate transport during nutrient stimulation of INS-1E insulinoma cells

Here, we have investigated the role of inorganic phosphate (Pi) transport in mitochondria of rat clonal β-cells. In α-toxin-permeabilized INS-1E cells, succinate and glycerol-3-phosphate increased mitochondrial ATP release which depends on exogenous ADP and Pi. In the presence of substrates, addition of Pi caused mitochondrial matrix acidification and hyperpolarisation which promoted ATP export. Dissipation of the mitochondrial pH gradient or pharmacological inhibition of Pi transport blocked the effects of Pi on electrochemical gradient and ATP export. Knock-down of the phosphate transporter PiC, however, neither prevented Pi-induced mitochondrial activation nor glucose-induced insulin secretion. Using (31)P NMR we observed reduction of Pi pools during nutrient stimulation of INS-1E cells. Interestingly, Pi loss was less pronounced in mitochondria than in the cytosol. We conclude that matrix alkalinisation is necessary to maintain a mitochondrial Pi pool, at levels sufficient to stimulate energy metabolism in insulin-secreting cells beyond its role as a substrate for ATP synthesis.

Mitochondrial dysfunction contributes to impaired insulin secretion in INS-1 cells with dominant-negative mutations of HNF-1alpha and in HNF-1alpha-deficient islets

Maturity Onset Diabetes of the Young-type 3 (MODY-3) has been linked to mutations in the transcription factor hepatic nuclear factor (HNF)-1alpha, resulting in deficiency in glucose-stimulated insulin secretion. In INS-1 cells overexpressing doxycycline-inducible HNF-1alpha dominant-negative (DN-) gene mutations, and islets from Hnf-1alpha knock-out mice, insulin secretion was impaired in response to glucose (15 mm) and other nutrient secretagogues. Decreased rates of insulin secretion in response to glutamine plus leucine and to methyl pyruvate, but not potassium depolarization, indicate defects specific to mitochondrial metabolism. To identify the biochemical mechanisms responsible for impaired insulin secretion, we used (31)P NMR measured mitochondrial ATP synthesis (distinct from glycolytic ATP synthesis) together with oxygen consumption measurements to determine the efficiency of mitochondrial oxidative phosphorylation. Mitochondrial uncoupling was significantly higher in DN-HNF-1alpha cells, such that rates of ATP synthesis were decreased by approximately one-half in response to the secretagogues glucose, glutamine plus leucine, or pyruvate. In addition to closure of the ATP-sensitive K(+) channels with mitochondrial ATP synthesis, mitochondrial production of second messengers through increased anaplerotic flux has been shown to be critical for coupling metabolism to insulin secretion. (13)C-Isotopomer analysis and tandem mass spectrometry measurement of Krebs cycle intermediates revealed a negative impact of DN-HNF-1alpha and Hnf-1alpha knock-out on mitochondrial second messenger production with glucose but not amino acids. Taken together, these results indicate that, in addition to reduced glycolytic flux, uncoupling of mitochondrial oxidative phosphorylation contributes to impaired nutrient-stimulated insulin secretion with either mutations or loss of HNF-1alpha.

Is the glucose-induced phosphate flush in pancreatic islets attributable to gating of volume-sensitive anion channels?

D-glucose and other nutrient insulin secretagogues have long been known to induce a transient increase in inorganic phosphate release from pancreatic islets, a phenomenon currently referred to as a "phosphate flush". The objective of this study was to explore the possible participation of volume-sensitive anion channels in such a process. Rat pancreatic islets were preincubated for 60 min in the presence of [32P]orthophosphate and then perifused for 90 min to measure 32P fractional outflow rate and insulin secretion. From minutes 46 to 70 inclusive either the concentration of D-glucose was increased from 1.1 to 8.3 mmol L-1 or the extracellular osmolarity was decreased by reducing the NaCl concentration by 50 mmol L-1. The increase in D-glucose concentration induced a typical phosphate flush and biphasic stimulation of insulin release. Extracellular hypoosmolarity caused a monophasic increase in both effluent radioactivity and insulin output. The inhibitor of volume-sensitive anion channels 5-nitro-2-(3-phenylpropylamino)benzoate (0.1 mmol L-1) inhibited both stimulation of insulin release and phosphate flush induced by either the increase in D-glucose concentration or extracellular hypoosmolarity. It is proposed that gating of volume-sensitive anion channels accounts for the occurrence of the phosphate flush and subsequent stimulation of insulin secretion in response to either an increase in D-glucose concentration or a decrease in extracellular osmolarity.

The characterization of mitochondrial permeability transition in clonal pancreatic beta-cells. Multiple modes and regulation

Mitochondrial permeability transition (MPT), which contributes substantially to the regulation of normal mitochondrial metabolism, also plays a crucial role in the initiation of cell death. It is known that MPT is regulated in a tissue-specific manner. The importance of MPT in the pancreatic beta-cell is heightened by the fact that mitochondrial bioenergetics serve as the main glucose-sensing regulator and energy source for insulin secretion. In the present study, using MIN6 and INS-1 beta-cells, we revealed that both Ca(2+)-phosphate- and oxidant-induced MPT is remarkably different from other tissues. Ca(2+)-phosphate-induced transition is accompanied by a decline in mitochondrial reactive oxygen species production related to a significant potential dependence of reactive oxygen species formation in beta-cell mitochondria. Hydroperoxides, which are indirect MPT co-inducers active in liver and heart mitochondria, are inefficient in beta-cell mitochondria, due to the low mitochondrial ability to metabolize them. Direct cross-linking of mitochondrial thiols in pancreatic beta-cells induces the opening of a low conductance ion permeability of the mitochondrial membrane instead of the full scale MPT opening typical for liver mitochondria. Low conductance MPT is independent of both endogenous and exogenous Ca(2+), suggesting a novel type of nonclassical MPT in beta-cells. It results in the conversion of electrical transmembrane potential into DeltapH instead of a decrease in total protonmotive force, thus mitochondrial respiration remains in a controlled state. Both Ca(2+)- and oxidant-induced MPTs are phosphate-dependent and, through the "phosphate flush" (associated with stimulation of insulin secretion), are expected to participate in the regulation in beta-cell glucose-sensing and secretory activity.

Relationships between energy level and insulin secretion in isolated rat islets of Langerhans. Manipulation of [ATP]/[ADP][Pi] by 2-deoxy-D-glucose

Perifusion of islets with nominally phosphate-free buffer containing increasing concentrations of 2-deoxy-D-glucose (2.5 to 10 mM) produced increments in high alpha-ketoisocaproic acid-induced secretion of insulin beyond those observed in the absence of the sugar analogue. 3-O-methyl-D-glucose, a poorly metabolized sugar, was without effect. Insulin release evoked by 40 mM KCl was not altered by 2-deoxyglucose. The concentration of intracellular inorganic phosphate was lower in islets perifused with 2-deoxyglucose and declined to a lower level after addition of 20 mM alpha-ketoisocaproic acid. The enhancement of alpha-ketoisocaproic acid-induced hormone secretion by 2-deoxyglucose was not seen in islets perifused with medium containing 1.5 mM phosphate; instead a small inhibition was observed. It is postulated that conditions which lower intracellular [Pi] facilitate, either directly or indirectly, hormone release although the mechanism of this effect remains to be elucidated.

Relationships between energy level and insulin secretion in isolated rat islets of Langerhans. A study at various pH values

To define better the role of [ATP]/[ADP] in insulin release from pancreatic islets, changes in the adenine nucleotide ratios elicited by alterations in external pH were correlated with the secretion profiles produced by administration of two metabolic secretagogues, 16 mM glucose and 10 mM alpha-ketoisocaproic acid. Experiments were carried out in buffers with and without bicarbonate, in the pH range 6.5-7.7. Insulin release was dependent on pHe irrespective of the secretagogue used. Secretion profiles for alpha-ketoisocaproic acid were the same both with and without bicarbonate; the release was decreased below pH 7.1 but maintained at 7.4-7.7. The same pattern was seen with glucose in media buffered with Hepes. With bicarbonate present, secretion caused by high glucose showed a bell-shaped dependence on [H+], with reductions at the acid and alkaline sides of pH 7.1-7.4. [ATP] and [ADP] were higher when Hepes was the buffer, at all pH values studied. The [ATP]/[ADP] declined with increasing pH under both basal and stimulated conditions; the values were always larger after stimulation although at pH 7.7 with bicarbonate present and glucose as the stimulant the difference was very small. It is concluded that: (i) the [ATP]/[ADP] in pancreatic islets is markedly dependent on pHe; (ii) there is no straight-forward correlation between either [ATP] or the absolute value for [ATP]/[ADP] and insulin secretion; and (iii) a rise in [ATP]/[ADP] is necessary for glucose-stimulated insulin release although it is not always the rate-determining event.

Bioenergetic response of pancreatic islets to stimulation by fuel molecules

The relationship of fuel-stimulated insulin secretion and the beta-cell bioenergetic state was investigated in isolated rat islets. In islets perifused with 5 mmol/L glucose to maintain a high basal energy state, stimulation by 9 to 28 mmol/L glucose increased the [ATP]/[ADP] and [GTP]/[GDP]. The rise in the former occurred prior to, or coincident with, the onset of insulin secretion and was dependent on glucose concentration. The increase in the latter appeared to lag behind the alteration in the [ATP]/[ADP] and achieved statistical significance after 30 minutes of incubation. Addition of 20 mmol/L alpha-ketoisocaproic acid, a powerful secretagogue, also caused a rise in the [ATP]/[ADP]. By contrast, 20 mmol/L lactate, which affected insulin secretion only minimally, failed to alter nucleotide concentrations. These data support the hypothesis that an increase in the islet energy state is a metabolic signal linking fuel metabolism with initiation of insulin secretion.

alpha-Glycerophosphate shuttle in a clonal beta-cell line

It has been proposed that the alpha-glycerophosphate (alpha-GOP) shuttle plays a crucial role in regulation of glycolysis in beta-cells by linking reoxidation of cytosolic NADH to formation of ATP in the electron transport chain (J. Biol. Chem. 265: 8287, 1981). Direct evidence for this suggestion is still lacking, however. In this work the operation of the alpha-GOP shuttle was investigated in the insulin-secreting cell line HIT-T15. The constituent enzymes of the pathway were found to be present in HIT cells. Flavin-linked alpha-GOP dehydrogenase was associated with the mitochondrial fraction, whereas NAD+-dependent alpha-GOP dehydrogenase was localized in the cytosol. In the presence of amobarbital (used to preserve the function of the alpha-GOP shuttle under conditions where oxidation of NADH by the respiratory chain was blocked), glucose increased insulin secretion, O2 consumption, and the cell [ATP]/[ADP] when compared with amobarbital alone. These results indicate that the alpha-GOP shuttle contributes to ATP generation in HIT cells and that its activation may be necessary for the initiation of insulin secretion by glucose.

Net fluxes of orthophosphate across the plasma membrane in human red cells following alteration of pH and extracellular Pi concentration

Even though net fluxes of Pi (orthophosphate) across the cell membrane may be important in clinical disorders involving the abnormal extracellular Pi concentration, in acid-base disturbances, and in the responses of some cells to hormones, relatively few studies have been made of these fluxes, owing to the complexities of interpretation. Here we have studied net fluxes in response to changes in extracellular pH and Pi concentration in the simple case of the human red cell. The permeability of the cell membrane to net Pi fluxes was described in terms of a first-order rate constant, epsilon. By means of a mathematical model, it was possible to discriminate between transmembrane Pi movement, net intracellular generation or consumption of Pi by organic phosphates, and extracellular generation of Pi from the cells lysing during the experiment. We show that net Pi influx into the cell during experimental alkalosis was probably driven by net consumption of Pi by organic phosphates, and that this was reversed during acidosis. Inhibition of net Pi influx by 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonate (SITS) suggests that, like Pi self-exchange, net influx is at least partly mediated by the band 3 transport protein. Unexpectedly, epsilon increased from 2 h-1 at extracellular pH 7.4 to approx. 7 h-1 at pH 7.8. From the value of epsilon at pH 7.4, we conclude that the apparent buffering or regulation of steady-state Pi concentrations, previously reported in red cells in vitro, was not an artifact of intracellular generation of Pi from organic phosphates.

Glucose-induced changes in cytosolic ATP content in pancreatic islets

The cytosolic and mitochondrial contents in ATP, ADP and AMP were measured in islets incubated for 45 min at increasing concentrations of D-glucose and then exposed for 20 s to digitonin. The latter treatment failed to affect the total islet ATP/ADP ratio and adenylate charge. D-Glucose caused a much greater increase in cytosolic than mitochondrial ATP/ADP ratio. In the cytosol, a sigmoidal pattern characterized the changes in ATP/ADP ratio at increasing concentrations of D-glucose. These findings are compatible with the view that cytosolic ATP participates in the coupling of metabolic to ionic events in the process of nutrient-induced insulin release.

In vitro exhaustion of pancreatic beta-cells

To learn more about possible limited beta-cell secretory capacity and factors essential for insulin release, a perifusion system was applied that allowed the in vitro study of insulin secretion from isolated pancreatic islets for more than 6 h. Islets isolated from rats were stimulated with various glucose concentrations (7.5, 16.7, and 30 mM), alpha-ketoisocaproate (30 mM), and 30 mM glucose plus 1 mM 3-isobutyl-1-methylxanthine for several hours in Krebs-Ringer-bicarbonate buffer (KRB) or RPMI 1640. Islets showed "exhaustion" with all stimulatory conditions used when KRB was the perifusion medium. This was not prevented by addition of amino acids, phosphate, myo-inositol or arachidonic acid. With RPMI 1640 as the basal medium, exhaustion was not seen at 7.5 mM but was readily approached at higher glucose concentrations. It is possible that the exhaustion phenomenon observed here is due to a depletion of a readily releasable insulin pool.

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.

Glucose modulates Mg2+ fluxes in pancreatic islet cells

Magnesium, the most abundant intracellular divalent cation, is an essential cofactor for many enzyme systems, but it remains unknown as to whether variations in the cytoplasmic concentration of ionized Mg2+ directly control cellular processes. Experiments with adrenal medullary cells made 'leaky' by exposure to high electric fields provided evidence that Mg2+ could influence hormone release not only by competing with Ca2+ for entry into the cell, but also at intracellular sites controlling exocytosis. A similar conclusion was reached for insulin release in a study using isolated rat islets also subjected to high voltage discharges. There is no experimental evidence, however, that physiological stimuli influence Mg2+ movements in intact secretory cells. We report here that 28Mg2+ fluxes in pancreatic islet cells are markedly modified by glucose, the physiological stimulus of insulin release, but not by its non-insulinotropic analogue, 3-O-methylglucose.

The stimulus-secretion coupling in glucose-induced insulin release xliv. A possible link between glucose metabolism and phosphate flush

Above a threshold of 3.0-4.2 mmol/l, D-glucose provoked a transient increase in 32P fractional outflow rate from rat pancreatic islets prelabelled with 32P-orthophosphate. Nutrients which stimulate insulin release in the absence of glucose, alpha-ketoisocaproate and L-leucine, also provoked a phosphate flush. No flush occurred in islets exposed to non-insulinotropic nutrients (L-glutamine and and L-lactate) or non-nutriet secretagogues (arginine, tolbutamide, theophylline). A late increase in 32P fractional outflow rate was observed in Ca2+ deprived islets stimulated with BaCl2 and theophylline. The occurrence of a phosphate flush did not appear to be attributable to changes in insulin release, cyclic AMP content, membrane polarisation, K+ conductance, or reduced pyridine nucleotide content. The 32P response to glucose was slightly decreased in the absence of extracellular Ca2+ or HCO3-, markedly impaired in the absence of K4, and virtually abolished in the presence of menadione (10 mumol/l). It is proposed that the occurrence of a phosphate flush is linked to the metabolism of nutrient secretagogues, possibly via an increase in O2 uptake and the production rate of NAD(P)H and ATP.

A new hypothesis for alloxan diabetes

A new hypothesis ("Pi-pH hypothesis") for alloxan diabetes is presented. It is based upon data from our own studies and from the literature. The following data and interpreatations are assumed to be of special importance for the B-cytotoxicity of alloxan: Inhibition of a mitochondrial sulfhydryl dependent transport system for inorganic phosphate (Pi) leading to increased concentration of Pi and decreased pH in the cytosol, and to inhibition of NAD-dependent oxidations and oxidative phosphorylation; mitochondrial lesion because of altered localization and concentration of Pi; inhibited synthesis and glucose induced release of insulin, at least partly due to a fall in intracellular pH; and finally necrosis because of absent mitochondrial function. An inverse relationship between Pi and pH may exist in the B-cells; alloxan sensitivity being associated with high Pi and low pH. Alloxan antagonism may be due to induction of low Pi and high pH in the cytosol. The selectivity of the B-cell for alloxan is believed to be associated with its free permeability for glucose.