Fluorescence of oxidized flavoproteins from perifused isolated pancreatic islets

The Proton Leak of the Inner Mitochondrial Membrane Is Enlarged in Freshly Isolated Pancreatic Islets

In a number of investigations on the mechanism of the metabolic amplification of insulin secretion, differences between the response of freshly isolated islets and of islets cultured for one day have been observed. Since no trivial explanation like insufficient numbers of viable cells after cell culture could be found, a more thorough investigation into the mechanisms responsible for the difference was made, concentrating on the function of the mitochondria as the site where the metabolism of nutrient stimulators of secretion forms the signals impacting on the transport and fusion of insulin granules. Using combinations of inhibitors of oxidative phosphorylation, we come to the conclusion that the mitochondrial membrane potential is lower and the exchange of mitochondrial reducing equivalents is faster in freshly isolated islets than in cultured islets. The significantly higher rate of oxygen consumption in fresh islets than in cultured islets (13 vs. 8 pmol/min/islet) was not caused by a different activity of the F1F0-ATPase, but by a larger proton leak. These observations raise the questions as to whether the proton leak is a physiologically regulated pathway and whether its larger size in fresh islets reflects the working condition of the islets within the pancreas.

What Is the Metabolic Amplification of Insulin Secretion and Is It (Still) Relevant?

The pancreatic beta-cell transduces the availability of nutrients into the secretion of insulin. While this process is extensively modified by hormones and neurotransmitters, it is the availability of nutrients, above all glucose, which sets the process of insulin synthesis and secretion in motion. The central role of the mitochondria in this process was identified decades ago, but how changes in mitochondrial activity are coupled to the exocytosis of insulin granules is still incompletely understood. The identification of ATP-sensitive K⁺-channels provided the link between the level of adenine nucleotides and the electrical activity of the beta cell, but the depolarization-induced Ca²⁺-influx into the beta cells, although necessary for stimulated secretion, is not sufficient to generate the secretion pattern as produced by glucose and other nutrient secretagogues. The metabolic amplification of insulin secretion is thus the sequence of events that enables the secretory response to a nutrient secretagogue to exceed the secretory response to a purely depolarizing stimulus and is thus of prime importance. Since the cataplerotic export of mitochondrial metabolites is involved in this signaling, an orienting overview on the topic of nutrient secretagogues beyond glucose is included. Their judicious use may help to define better the nature of the signals and their mechanism of action.

A Parallel Perifusion Slide From Glass for the Functional and Morphological Analysis of Pancreatic Islets

An islet-on-chip system in the form of a completely transparent microscope slide optically accessible from both sides was developed. It is made from laser-structured borosilicate glass and enables the parallel perifusion of five microchannels, each containing one islet precisely immobilized in a pyramidal well. The islets can be in inserted via separate loading windows above each pyramidal well. This design enables a gentle, fast and targeted insertion of the islets and a reliable retention in the well while at the same time permitting a sufficiently fast exchange of the media. In addition to the measurement of the hormone content in the fractionated efflux, parallel live cell imaging of the islet is possible. By programmable movement of the microscopic stage imaging of five wells can be performed. The current chip design ensures sufficient time resolution to characterize typical parameters of stimulus-secretion coupling. This was demonstrated by measuring the reaction of the islets to stimulation by glucose and potassium depolarization. After the perifusion experiment islets can be removed for further analysis. The live-dead assay of the removed islets confirmed that the process of insertion and removal was not detrimental to islet structure and viability. In conclusion, the present islet-on-chip design permits the practical implementation of parallel perifusion experiments on a single and easy to load glass slide. For each immobilized islet the correlation between secretion, signal transduction and morphology is possible. The slide concept allows the scale-up to even higher degrees of parallelization.

Regulation of ATR-dependent DNA damage response by nitric oxide

We have shown that nitric oxide limits ataxia-telangiectasia mutated signaling by inhibiting mitochondrial oxidative metabolism in a β-cell selective manner. In this study, we examined the actions of nitric oxide on a second DNA damage response transducer kinase, ataxia-telangiectasia and Rad3-related protein (ATR). In β-cells and non-β-cells, nitric oxide activates ATR signaling by inhibiting ribonucleotide reductase; however, when produced at inducible nitric oxide synthase-derived (low micromolar) levels, nitric oxide impairs ATR signaling in a β-cell selective manner. The inhibitory actions of nitric oxide are associated with impaired mitochondrial oxidative metabolism and lack of glycolytic compensation that result in a decrease in β-cell ATP. Like nitric oxide, inhibitors of mitochondrial respiration reduce ATP levels and limit ATR signaling in a β-cell selective manner. When non-β-cells are forced to utilize mitochondrial oxidative metabolism for ATP generation, their response is more like β-cells, as nitric oxide and inhibitors of mitochondrial respiration attenuate ATR signaling. These studies support a dual role for nitric oxide in regulating ATR signaling. Nitric oxide activates ATR in all cell types examined by inhibiting ribonucleotide reductase, and in a β-cell selective manner, inducible nitric oxide synthase-derived levels of nitric oxide limit ATR signaling by attenuating mitochondrial oxidative metabolism and depleting ATP.

Fresh and cultured mouse islets differ in their response to nutrient stimulation

Observing different kinetics of nutrient-induced insulin secretion in fresh and cultured islets under the same condition we compared parameters of stimulus secretion coupling in freshly isolated and 22-h-cultured NMRI mouse islets. Stimulation of fresh islets with 30 mM glucose after perifusion without nutrient gave a continuously ascending secretion rate. In 22-h-cultured islets the same protocol produced a brisk first phase followed by a moderately elevated plateau, a pattern regarded to be typical for mouse islets. This was also the response of cultured islets to the nutrient secretagogue alpha-ketoisocaproic acid, whereas the secretion of fresh islets increased similarly fast but remained strongly elevated. The responses of fresh and cultured islets to purely depolarizing stimuli (tolbutamide or KCl), however, were closely similar. Signs of apoptosis and necrosis were rare in both preparations. In cultured islets, the glucose-induced rise of the cytosolic Ca2+ concentration started from a lower value and was larger as was the increase of the ATP/ADP ratio. The prestimulatory level of mitochondrial reducing equivalents, expressed as the NAD(P)H/FAD fluorescence ratio, was lower in cultured islets, but increased more strongly than in fresh islets. When culture conditions were modified by replacing RPMI with Krebs-Ringer medium and FCS with BSA, the amount of released insulin varied widely, but the kinetics always showed a predominant first phase. In conclusion, the secretion kinetics of fresh mouse islets is more responsive to variations of nutrient stimulation than cultured islets. The more uniform kinetics of the latter may be caused by a different use of endogenous metabolites.

Metabolic and Functional Heterogeneity in Pancreatic β Cells

Metabolic and secretory heterogeneity are fundamental properties of pancreatic islet β cells. Emerging data suggest that stable differences in the transcriptome and proteome of individual cells may create cellular hierarchies, which, in turn, establish coordinated functional networks. These networks appear to govern the secretory activity of the whole islet and be affected in some forms of diabetes mellitus. Functional imaging, for example, of intracellular calcium dynamics, has led to the demonstration of "small worlds" behavior, and the identification of highly connected "hub" (or "leader") cells and of follower populations subservient to them. Subsequent inactivation of members of either population, for example, using optogenetic approaches or photoablation, has confirmed the importance of hub cells as possible pacemakers. Hub cells appear to be enriched for the glucose phosphorylating enzyme glucokinase and for genes encoding other enzymes involved in glucose metabolism compared to follower cells. Recent findings have shown the relevance of cellular hierarchy in islets from multiple species including human, mouse and fish, and shown that it is preserved in vivo in the context of the fully vascularized and innervated islet. Importantly, connectivity is impaired by insults, which mimic the diabetic milieu, including high glucose and/or fatty levels, and by the ablation of genes associated with type 2 diabetes risk in genome-wide association studies. We discuss here the evidence for the existence of these networks and their failure in disease settings. We also briefly survey the challenges in understanding their properties.

Metabolic amplification of insulin secretion is differentially desensitized by depolarization in the absence of exogenous fuels

OBJECTIVE

The metabolic amplification of insulin secretion is the sequence of events which enables the secretory response to a fuel secretagogue to exceed the secretory response to a purely depolarizing stimulus. The signals in this pathway are incompletely understood. Here, we have characterized an experimental procedure by which the amplifying response to glucose is reversibly desensitized, while the response to α-ketoisocaproic acid (KIC) is unchanged.

MATERIALS/METHODS

Insulin secretion, NAD(P)H- and FAD-autofluorescence, Fura-2 fluorescence and oxygen consumption were measured in perifused NMRI mouse islets. The ATP- and ADP-contents were measured in statically incubated mouse islets. All islets were freshly isolated.

RESULTS

While the original observation on the dissociation between glucose- and KIC-amplification was obtained with islets that had been exposed to a high concentration of the sulfonylurea glipizide in the absence of glucose, we now show that in the absence of exogenous fuel a moderate depolarization, irrespective of its mechanism, progressively decreased the amplification in response to both glucose and KIC. However, the amplification in response to glucose declined faster, so a time window exists where glucose was already inefficient, whereas KIC was of unimpaired efficiency. Measurements of adenine nucleotides, NAD(P)H- and FAD-autofluorescence, and oxygen consumption point to a central role of the mitochondrial metabolism in this process. The desensitization could be quickly reversed by increasing oxidative deamination of glutamate and consequently anaplerosis of the citrate cycle.

CONCLUSION

Depolarization in the absence of exogenous fuel may be a useful model to identify those signals which are indispensable for the generation of metabolic amplification.

Different responses of mouse islets and MIN6 pseudo-islets to metabolic stimulation: a note of caution

MIN6 cells and MIN6 pseudo-islets are popular surrogates for the use of primary beta cells and islets. Even though it is generally agreed that the stimulus-secretion coupling may deviate from that of beta cells or islets, direct comparisons are rare. The present side-by-side comparison of insulin secretion, cytosolic Ca(2+) concentration ([Ca(2+)] i ) and oxygen consumption rate (OCR) points out where similarities and differences exist between MIN6 cells and normal mouse beta cells. In mouse islets and MIN6 pseudo-islets depolarization by 40 mM KCl was a more robust insulinotropic stimulus than 30 mM glucose. In MIN6 pseudo-islets, but not in mouse islets, the response to 30 mM glucose was much lower than to 40 mM KCl and could be suppressed by a preceding stimulation with 40 mM KCl. In MIN6 pseudo-islets, glucose was less effective to raise [Ca(2+)] i than in primary islets. In marked contrast to islets, the OCR response of MIN6 pseudo-islets to 30 mM glucose was smaller than to 40 mM KCl and was further diminished by a preceding stimulation with 40 mM KCl. The same pattern was observed when MIN6 pseudo-islets were cultured in 5 mM glucose. As with insulin secretion memory effects on the OCR remained after wash-out of a stimulus. The differences between MIN6 cells and primary beta cells were generally larger in the responses to glucose than to depolarization by KCl. Thus, the use of MIN6 cells in investigations on metabolic signalling requires particular caution.

Pancreatic β-cell identity, glucose sensing and the control of insulin secretion

Insulin release from pancreatic β-cells is required to maintain normal glucose homoeostasis in man and many other animals. Defective insulin secretion underlies all forms of diabetes mellitus, a disease currently reaching epidemic proportions worldwide. Although the destruction of β-cells is responsible for Type 1 diabetes (T1D), both lowered β-cell mass and loss of secretory function are implicated in Type 2 diabetes (T2D). Emerging results suggest that a functional deficiency, involving de-differentiation of the mature β-cell towards a more progenitor-like state, may be an important driver for impaired secretion in T2D. Conversely, at least in rodents, reprogramming of islet non-β to β-cells appears to occur spontaneously in models of T1D, and may occur in man. In the present paper, we summarize the biochemical properties which define the 'identity' of the mature β-cell as a glucose sensor par excellence. In particular, we discuss the importance of suppressing a group of 11 'disallowed' housekeeping genes, including Ldha and the monocarboxylate transporter Mct1 (Slc16a1), for normal nutrient sensing. We then survey the changes in the expression and/or activity of β-cell-enriched transcription factors, including FOXO1, PDX1, NKX6.1, MAFA and RFX6, as well as non-coding RNAs, which may contribute to β-cell de-differentiation and functional impairment in T2D. The relevance of these observations for the development of new approaches to treat T1D and T2D is considered.

Effect of fluoroquinolones on mitochondrial function in pancreatic beta cells

Hyper- and hypoglycaemias are known side effects of fluoroquinolone antibiotics, resulting in a number of fatalities. Fluoroquinolone-induced hypoglycaemias are due to stimulated insulin release by the inhibition of the KATP channel activity of the beta cell. Recently, it was found that fluoroquinolones were much less effective on metabolically intact beta cells than on open cell preparations. Thus the intracellular effects of gatifloxacin, moxifloxacin and ciprofloxacin were investigated by measuring NAD(P)H- and FAD-autofluorescence, the mitochondrial membrane potential, and the adenine nucleotide content of isolated pancreatic islets and beta cells. 100 μM of moxifloxacin abolished the NAD(P)H increase elicited by 20mM glucose, while gatifloxacin diminished it and ciprofloxacin had no significant effect. This pattern was also seen with islets from SUR1 Ko mice, which have no functional KATP channels. Moxifloxacin also diminished the glucose-induced decrease of FAD-fluorescence, which reflects the intramitochondrial production of reducing equivalents. Moxifloxacin, but not ciprofloxacin or gatifloxacin significantly reduced the effect of 20mM glucose on the ATP/ADP ratio. The mitochondrial hyperpolarization caused by 20mM glucose was partially antagonized by moxifloxacin, but not by ciprofloxacin or gatifloxacin. Ultrastructural analyses after 20 h tissue culture showed that all three compounds (at 10 and 100 μM) diminished the number of insulin secretory granules and that gatifloxacin and ciprofloxacin, but not moxifloxacin induced fission/fusion configurations of the beta cell mitochondria. In conclusion, fluoroquinolones affect the function of the mitochondria in pancreatic beta cells which may diminish the insulinotropic effect of KATP channel closure and contribute to the hyperglycaemic episodes.

Acute metabolic amplification of insulin secretion in mouse islets is mediated by mitochondrial export of metabolites, but not by mitochondrial energy generation

OBJECTIVE

The β-cell metabolism of glucose and of some other fuels (e.g. α-ketoisocaproate) generates signals triggering and acutely amplifying insulin secretion. As the pathway coupling metabolism with amplification is largely unknown, we aimed to narrow down the putative amplifying signals.

MATERIALS/METHODS

An experimental design was used which previously prevented glucose-induced, but not α-ketoisocaproate-induced insulin secretion. Isolated mouse islets were pretreated for one hour with medium devoid of fuels and containing the sulfonylurea glipizide in high concentration which closed all ATP-sensitive K(+) channels. This concentration was also applied during the subsequent examination of fuel-induced effects. In perifused or incubated islets, insulin secretion and metabolic parameters were measured.

RESULTS

The pretreatment decreased the islet ATP/ADP ratio. Whereas glucose and α-ketoisovalerate were ineffective or weakly effective, respectively, when tested separately, their combination strongly enhanced the insulin secretion. Compared with glucose, the strong amplifier α-ketoisocaproate caused less increase in NAD(P)H-fluorescence and less mitochondrial hyperpolarization. Compared with α-ketoisovalerate, α-ketoisocaproate caused greater increase in NAD(P)H-fluorescence and greater mitochondrial hyperpolarization. Neither α-ketoacid anion enhanced the islet ATP/ADP ratio during onset of the insulin secretion. α-Ketoisocaproate induced a higher pyruvate content than glucose, slowly elevated the citrate content which was not changed by glucose and generated a much higher acetoacetate content than other fuels. α-Ketoisovalerate alone or in combination with glucose did not increase the citrate content.

CONCLUSIONS

In β-cells, mitochondrial energy generation does not mediate acute metabolic amplification, but mitochondrial production of acetyl-CoA and supplemental acetoacetate supplies cytosolic metabolites which induce the generation of specific amplifying signals.

One-step purification of functional human and rat pancreatic alpha cells

Pancreatic alpha cells contribute to glucose homeostasis by the regulated secretion of glucagon, which increases glycogenolysis and hepatic gluconeogenesis in response to hypoglycemia. Alterations of glucagon secretion are observed in diabetic patients and exacerbate the disease. The restricted availability of purified primary alpha cells has limited our understanding of their function in health and disease. This study was designed to establish convenient protocols for the purification of viable alpha cells from rat and human pancreatic islets by FACS, using intrinsic cellular properties. Islets were isolated from the pancreata of Wistar rats or deceased human organ donors. Dispersed islet cells were separated by FACS based on light scatter and autofluorescence. Purity of sorted cells was evaluated by immunocytochemistry using hormone specific antibodies. Relative hormone expression was further determined by quantitative RT-PCR. Viability was determined by Annexin V and propidium iodide staining and function was assessed by monitoring cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) using Fura-2/AM. We developed species-specific FACS gating strategies that resulted in populations consisting mainly of alpha cells (96.6 ± 1.4%, n = 3 for rat; 95.4 ± 1.7%, n = 4 for human, mean ± SEM). These cell fractions showed ~5-fold and ~4-fold enrichment (rat and human, respectively) of glucagon mRNA expression compared to total ungated islet cells. Most of the sorted cells were viable and functional, as they responded with an increase in [Ca(2+)](i) upon stimulation with L-arginine (10 mM). The majority of the sorted human alpha cells responded also to stimulation with kainate (100 μM), whereas this response was infrequent in rat alpha cells. Using the same sample preparation, but a different gating strategy, we were also able to sort rat and human populations enriched in beta cells. In conclusion, we have simplified and optimized a method for the purification of rat alpha cells, as well as established a novel approach to separate human alpha cells using neither antibodies nor dyes possibly interfering with cellular functions.

Dihydroxyacetone-induced oscillations in cytoplasmic free Ca2+ and the ATP/ADP ratio in pancreatic beta-cells at substimulatory glucose

Glucose stimulation of pancreatic beta-cells causes oscillatory influx of Ca2+, leading to pulsatile insulin secretion. We have proposed that this is due to oscillations of glycolysis and the ATP/ADP ratio, which modulate the activity of ATP-sensitive K+ channels. We show here that dihydroxyacetone, a secretagogue that feeds into glycolysis below the putative oscillator phosphofructokinase, could cause a single initial peak in cytoplasmic free Ca2+ ([Ca2+]i) but did not by itself cause repeated oscillations in [Ca2+]i in mouse pancreatic beta-cells. However, in the presence of a substimulatory concentration of glucose (4 mm), dihydroxyacetone induced [Ca2+]i oscillations. Furthermore, these oscillations correlated with oscillations in the ATP/ADP ratio, as seen previously with glucose stimulation. Insulin secretion in response to dihydroxyacetone was transient in the absence of glucose but was considerably enhanced and somewhat prolonged in the presence of a substimulatory concentration of glucose, in accordance with the enhanced [Ca2+]i response. These results are consistent with the hypothesized role of phosphofructokinase as the generator of the oscillations. Dihydroxyacetone may affect phosphofructokinase by raising the free concentration of fructose 1,6-bisphosphate to a critical level at which it activates the enzyme autocatalytically, thereby inducing the pulses of phosphofructokinase activity that cause the metabolic oscillations.

Expression of carbonic anhydrase V in pancreatic beta cells suggests role for mitochondrial carbonic anhydrase in insulin secretion

Carbonic anhydrase V (CA-V) is a mitochondrial enzyme that provides bicarbonate for pyruvate carboxylase in liver and kidney. In the course of a survey of the tissue distribution of CA-V, we detected intense immunostaining in pancreatic islets when sections from rat and mouse pancreases were reacted with a polyclonal antibody to recombinant mouse CA-V. The distribution and large number of CA-V-positive cells in each islet suggested that they represented beta cells. Double immunofluorescence staining of tissue sections and isolated islet cells showed cellular colocalization of CA-V and insulin, confirming that beta cells contain CA-V. Western blotting of rat islets of Langerhans and primary beta cells showed 33- and 30-kDa polypeptides of precursor and mature CA-V, respectively. The CA-V expression was beta cell-specific since no CA-V immunoreaction was detected in the primary alpha cells. Immunohistochemical staining for CA-I, CA-II, CA-IV, CA-VI, and CA-IX was negative in beta cells, and Western blotting of beta cells also failed to identify any CA in beta cells except CA-V. The specific localization of CA-V in beta cells led us to hypothesize that CA-V may be functionally linked to the regulation of insulin secretion. Consistent with this hypothesis, the CA inhibitor acetazolamide was found to be a strong inhibitor of glucose-stimulated insulin secretion by isolated rat pancreatic islets.

The coupling of metabolic to secretory events in pancreatic islets. Glucose-induced changes in mitochondrial redox state

Mitochondrial NAD+, NADH, NADP+ and NADPH were measured in dispersed pancreatic islet cells incubated in the absence or presence of D-glucose and then exposed for 20 s to 0.5 mg/ml digitonin. The latter treatment resulted in the full release of lactate dehydrogenase without any detectable loss of glutamate dehydrogenase. The permeabilized cells were separated from the incubation medium by centrifugation through an oil layer and their content in pyridine nucleotides measured by a radioisotopic procedure coupled to the classical cycling technique. Relative to basal value, D-glucose, in concentrations of 2.8 and 16.7 mM, caused a concentration-related increase in both the NADH/NAD+ and NADPH/NADP+ ratio. These findings provide the first direct evidence for the induction of a more reduced mitochondrial redox state in glucose-stimulated pancreatic islets.

Hexose metabolism in pancreatic islets: effect of D-glucose on the mitochondrial redox state

The mitochondrial NADH/NAD+ ratio for free nucleotides in rat pancreatic islets was judged from the cell content in L-glutamate and L-alanine, 2-ketoglutarate and pyruvate, and NH4+. At a physiological concentration of D-glucose, such a ratio averaged 9.6 +/- 1.1%. A rise in hexose concentrations, above a threshold value in excess of 5.6 mM, caused a rapid, sustained and rapidly reversible decrease in the mitochondrial NADH/NAD+ ratio. It is speculated that in the process of glucose-stimulated insulin release, the latter change participates in the coupling between metabolic and secretory events by favouring both the activity of key mitochondrial dehydrogenases and the translocation of Ca2+ from the mitochondria into the cytosol.

Substrate-dependent changes in mitochondrial function, intracellular free calcium concentration and membrane channels in pancreatic beta-cells

Microfluorimetric and patch-clamp techniques have been combined to determine the relationship between changes in mitochondrial metabolism, the activity of KATP channels and changes in intracellular free calcium concentration ([Ca2+]i) in isolated pancreatic beta-cells in response to glucose, ketoisocaproic acid (KIC) and the electron donor couple tetramethyl p-phenylenediamine (TMPD) and ascorbate. Exposure of cells to 20 mM glucose raised NAD(P)H autofluorescence after a delay of 28 +/- 1 s (mean +/- S.E.M., n = 30). The mitochondrial inner membrane potential, delta psi m (monitored using rhodamine 123 fluorescence), hyperpolarized with a latency of 49 +/- 6 s (n = 17), and the [Ca2+]i rose after 129 +/- 13 s (n = 5). The amplitudes of the metabolic changes were graded appropriately with glucose concentration over the range 2.5-20 mM. All variables responded to KIC with shorter latencies: NAD(P)H autofluorescence rose after a delay of 20 +/- 3 s (n = 5) and rhodamine 123 changed after 21 +/- 3 s (n = 6). The electron donor couple, TMPD with ascorbate, rapidly hyperpolarized delta psi m and raised [Ca2+]i. When [Ca2+]i was raised by sustained exposure to 20 mM glucose, TMPD had no further effect. TMPD also decreased whole-cell KATP currents and depolarized the cell membrane, measured with the perforated patch configuration. These data are consistent with a central role for mitochondrial oxidative phosphorylation in coupling changes in glucose concentration with the secretion of insulin.

Fluorimetry of mitochondria in cells vitally stained with DASPMI or rhodamine 6 GO

The fluorescent dyes DASPMI and rhodamine 6 GO specifically stain mitochondria in living cells. Dye concentrations from 10(-8) to 5 X 10(-6) mole l-1 can be used. Excitation and emission spectra, and quantum efficiency of DASPMI depend on solvent characteristics. Spectra of mitochondria in living cells correspond to those in phospholipids (excitation around 470 nm, emission 560-570 nm). Fluorescence intensity of DASPMI is a measure for the energization of mitochondria, as revealed by in vitro studies. In living cells uptake of the dye is strongly influenced by inhibitors of oxidative phosphorylation (i.e. by oligomycin, FCCP). Distribution of fluorescence intensity indicates an intracellular gradient in energy load of endothelial cells. Single mitochondria exhibit oscillations in fluorescence. Mitochondria loaded with DASPMI release the dye suddenly into the cytoplasm on treatment with FCCP. Cyanide and antimycin however, do not diminish fluorescence in vivo under optimal nutritional conditions, while they do so in mitochondrial suspension, indicating different mitochondrial behaviour in vivo and in suspension.

Stimulaiton of insulin biosynthesis in isolated mouse islets by L-leucine, 2-aminonorbornane-2-carboxylic acid and alpha-ketoisocaproic acid

An immune binding technique was used for measuring the effects of certain amino acids on the rate of insulin biosynthesis. [13H]phenylalanine served as the radioactive precursor for insulin synthesized by isolated mouse pancreatic islets. L-Leucine was found to stimulate the insulin biosynthesis and this effect was observed already at a physiologic concentration in contrast to the much higher concentrations needed to stimulate insulin secretion in vitro. Furthermore, it was found that 2-aminonorbornane-2-carboxylic acid and alpha-ketoisocaproic acid shared with glucose and L-leucine the ability to stimulate insulin biosynthesis. In contrast, L-alanine, L-arginine adn D-leucine had no stimulatory effect in the absence of glucose, while in the presence of 5 mM glucose L-arginine decreased and L-alanine increased the incorporation rate of tritiated phenylalanine. The fact that many of those compounds which stimulated insulin biosynthesis have also been shown elsewhere to be metabolized by the B-cells supports the view that the rate of insulin biosynthesis may be substrate dependent.

Effects of alpha-ketomonocarboxylic acids upon insulin secretion and metabolism of isolated pancreatic islets

In perifused isolated pancreatic islets alpha-ketoisocaproic acid (KIC) or alpha-ketocaproic acid (KC) induced a high insulin secretion rate and a steep increase of the fluorescence of reduced pyridine pyridine nucleotides [NAD(P)H] which fell again to almost prestimulatory levels 6 min after medium change. Insulin release in response to alpha-ketooctanoic (KO) acid started slowly and was accompanied by a decrease of the NAD(P)H-fluorescence trace. Beta-phenylpyruvate which is known to initiate insulin release also caused a fluorescence decrease. Alpha-keto-isovaleric (KIV) acid or pyruvate had no significant effects upon insulin secretion or NAD(P)H-fluorescence. In contrast to l-leucine, l-norleucine or l-valine did not enhance insulin release or fluorescence of NAD(P)H. KIV, alpha-keto-beta-methylvaleric acid (KMV), KIC and KC raised the production their corresponding amino acids by islet cells. From these results it is concluded that alpha-ketomonocarboxylic acids as such trigger insulin release by acting upon receptor sites which differ from those occupied by amino acids.