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Biomedical subjects

P Bergsten

Publications and source records attributed to P Bergsten.

At least 37 records · Page 2Linked to original sources
PubMed1994

Synchronous oscillations of cytoplasmic Ca2+ and insulin release in glucose-stimulated pancreatic islets.

The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in single pancreatic mouse islets superfused in a system allowing concomitant recordings of insulin release. When glucose was raised from 3 to 11 mM, [Ca2+]i responded by a transient lowering followed by a rise to an average level of 192 +/- 11 nM. In 77% of the islets the rise was associated with the gradual appearance of oscillations, which were either fast (2-7/min), slow (0.3-0.9/min), or a combination of both types. The characteristics of the fast [Ca2+]i oscillations were those expected from a relationship with the electrical burst activity in islets. Accordingly, in most cases the fast oscillations were remarkably regular. The slow [Ca2+]i oscillations had characteristics similar to the large amplitude ones in individual beta-cells. Whereas glucagon and dibutyryl cAMP could transform slow islet oscillations into fast ones, the alpha 2-adrenergic agonist clonidine had the opposite effect. The rapid islet oscillations were also facilitated by elevated concentrations of extracellular Ca2+. Reinforcing the arguments for [Ca2+]i oscillations as responsible for a pulsatile insulin secretion it was possible to demonstrate that the release of the hormone from single islets is synchronized with the slow [Ca2+]i oscillations.

Animals↗
PubMed1994

Ascorbic acid and insulin secretion in pancreatic islets.

The effect of ascorbic acid on glucose-induced insulin release from single pancreatic islets was measured using a new, ultra-sensitive enzyme-linked immunosorbent insulin assay. Within 20 s ascorbic acid inhibited insulin secretion; inhibition was dose dependent and completely reversible. There was a 50% inhibition of the secretory response with 200 microM ascorbic acid and 90% inhibition with 400 microM ascorbic acid. The decrease in insulin secretion was recorded as a reduction of the amplitudes of the fast insulin transients, which give rise to the oscillatory nature of insulin secretion. The inhibition of glucose-induced insulin release by ascorbic acid was associated with hyperpolarization of the pancreatic beta-cell. Suppression of glucose-induced membrane depolarization was evident after 20 s, was dose dependent, and was completely reversible. The data here may provide the first explanation of why plasma ascorbate concentrations are tightly controlled.

Animals↗
PubMed1994

Glucose induces oscillatory Ca2+ signalling and insulin release in human pancreatic beta cells.

Mechanisms of pulsatile insulin release in man were explored by studying the induction of oscillatory Ca2+ signals in individual beta cells and islets isolated from the human pancreas. Evidence was provided for a glucose-induced closure of ATP-regulated K+ channels, resulting in voltage-dependent entry of Ca2+. The observation of step-wise increases of capacitance in response to depolarizing pulses suggests that an enhanced influx of Ca2+ is an effective means of stimulating the secretory activity of the isolated human beta cell. Activation of muscarinic receptors (1-10 mumol/l carbachol) and of purinergic P2 receptors (0.01-1 mumol/l ATP) resulted in repetitive transients followed by sustained elevation of the cytoplasmic Ca2+ concentration ([Ca2+]i). Periodic mobilisation of intracellular calcium was seen also when injecting 100 mumol/l GTP-gamma-S into beta cells hyperpolarized to -70 mV. Individual beta cells responded to glucose and tolbutamide with increases of [Ca2+]i, manifested either as large amplitude oscillations (frequency 0.1-0.5/min) or as a sustained elevation. Glucose regulation was based on sudden transitions between the basal and the two alternative states of raised [Ca2+]i at threshold concentrations of the sugar characteristic for the individual beta cells. The oscillatory characteristics of coupled cells were determined collectively rather than by particular pacemaker cells. In intact pancreatic islets the glucose induction of well-synchronized [Ca2+]i oscillations had its counterpart in 2-5 min pulses of insulin. Each of these pulses could be resolved into regularly occurring short insulin transients. It is concluded that glucose stimulation of insulin release in man is determined by the number of beta cells entering into a state with Ca(2+)-induced secretory pulses.

Adenosine Triphosphate↗
PubMed1993

Ascorbic acid accumulation and transport in human fibroblasts.

As the initial step in the use of fibroblasts as a model system for 'in situ kinetics', ascorbic acid (vitamin C) accumulation in normal human fibroblasts was investigated for the first time. Ascorbic acid was transported into fibroblasts and accumulated against a concentration gradient up to 20-fold, as measured by h.p.l.c. with coulometric electrochemical detection. Ascorbic acid accumulation was mediated by two concentration-dependent transport activities. The first was a high-affinity activity with an apparent Km of 6 microM and an apparent Vmax. of 203 microM/h, and the second was a low-affinity activity with an apparent Km of 5 mM and an apparent Vmax. of 1.8 mM/h. Both activities were inhibited by metabolic inhibitors and inhibitors of ascorbic acid transport in human neutrophils. The low-affinity transporter could not be accounted for by diffusion. Although the high-affinity transport activity was comparable with that described for human neutrophils, the low-affinity transporter was different. These data provide the first evidence that two-component ascorbic acid transport may be a generalized mechanism for accumulation of this vitamin in humans.

2,4-Dinitrophenol↗
PubMed1993

Glucose-induced cycles of insulin release can be resolved into distinct periods of secretory activity.

The periodic behaviour of glucose-stimulated insulin release was studied using islets isolated from ob/ob-mice and rats. A single islet was perifused at a rate of 150 microliters/min and insulin measured in samples of the medium taken at intervals as short as 3 s. Exposure to 11 mM glucose resulted in intermittent insulin release, implying that previously observed 2-3 min cycles of secretion could be resolved into regularly occurring fast (< 25 s) transients. A further raise of glucose to 20 mM was associated with increase of the amplitudes of the transients leaving their frequencies unaffected. The discovery of the insulin transients raises the question whether there is a quantal release of the hormone in response to glucose-induced rapid oscillations of cytoplasmic Ca2+ in the pancreatic beta-cells.

Activity Cycles↗
PubMed1993

Glucose-induced amplitude regulation of pulsatile insulin secretion from individual pancreatic islets.

The insulin secretory response to glucose was studied in single pancreatic islets isolated from ob/ob mice and rats. The perfusate from an individual islet was collected during 18-s periods and analyzed for insulin with an ELISA technique. Increase of the glucose concentration from 3 to > or = 5.5 mM resulted in pulses of insulin release often originating from the basal level and having a frequency of 0.4/min. Glucose regulation of insulin release from the individual islet was manifested by alterations of the amplitudes of the pulses but not of their frequency. It is concluded that the large amplitude oscillations of cytoplasmic Ca2+ known to occur in the pancreatic beta-cells have their counterpart in pulses of insulin release and that glucose stimulation of the secretory activity may be the result of recruitment of more beta-cells into an oscillatory state.

Animals↗
PubMed1991

Ascorbic acid accumulation in human skin fibroblasts.

The transport and accumulation of ascorbic acid in normal human skin fibroblasts in culture was investigated by using high-performance liquid chromatographic separation and coulometric electrochemical detection. Results measured as picomole ascorbic acid per microgram cell protein were expressed in molar amounts after determining the volume of skin fibroblasts. Confluent fibroblasts contained undetectable amounts of ascorbic acid. On incubation with micromole per liter amounts of ascorbic acid in the medium, cells showed increasing uptake of ascorbic acid with time, accumulating a 15-fold excess in 3.5 h. Kinetic experiments suggested two transport mechanisms, a high-affinity and a low-affinity transport activity. Both transport activities were temperature sensitive and accumulated ascorbic acid against a concentration gradient.

Ascorbic Acid↗
PubMed1991

Ascorbic acid and in situ kinetics: a new approach to vitamin requirements.

Ascorbic acid requirements are based on preventing the deficiency disease scurvy and on urinary excretion of vitamin C. We proposed the first quantitative approach to determining optimal requirements for ascorbic acid and other vitamins, called in situ kinetics. In situ kinetics biochemically is based on the application of Michaelis-Menten reaction kinetics to ascorbic acid-dependent reactions in situ. Clinically in situ kinetics is based on determining vitamin availability to tissues so that cell-specific reactions can occur. The biochemical concepts of in situ kinetics are verified for the first time through studying ascorbic acid regulation of norepinephrine biosynthesis. The principles of in situ kinetics can now be applied to humans and human cells and for determining optimal requirements for ascorbic acid and for other vitamins.

Ascorbic Acid↗
PubMed1990

Millimolar concentrations of ascorbic acid in purified human mononuclear leukocytes. Depletion and reaccumulation.

Ascorbic acid (vitamin C) was found in isolated human mononuclear leukocytes and their purified components in millimolar concentration. Intracellular ascorbic acid was depleted greater than 96% during cell culture and was rapidly reaccumulated after addition of physiologic concentrations of ascorbic acid to the extracellular medium. Purified cells maintained concentration gradients of ascorbic acid as large as 100-fold across the plasma membrane. The ability to vary intracellular ascorbic acid concentrations over such a wide range makes it possible for the first time in these cells to study ascorbic acid function in direct relationship to intracellular concentration.

Ascorbic Acid↗
PubMed1989

Reversal of glucose-induced inhibition of insulin release by dibutyryl cyclic AMP.

Insulin release in response to glucose was measured after culture of islets from ob/obmice in a medium deficient in Ca2+. When present at a concentration of 6 mmol/l, glucose inhibited, insulin release. After activation of the alpha2-adrenergic receptors by 1 mumol/l clonidine also 20 mmol/l glucose became inhibitory. The inhibitory action of glucose on insulin release disappeared in the presence of 1 mmol/l dibutyryl cyclic AMP. Raising the glucose concentration from 3 to 20 mmol/l resulted in increased cytoplasmic Ca2+ after an initial depression. Whereas clonidine removed the Ca2+ increase, this phase was partly restored after simultaneous addition of dibutyryl cyclic AMP. It is concluded that cyclic AMP plays a major role in controlling the balance between the stimulatory and inhibitory components in the glucose action on insulin release.

Animals↗
PubMed1988

Diazoxide unmasks glucose inhibition of insulin release by counteracting entry of Ca2+.

The interaction of diazoxide with the effects of glucose on the insulin-releasing mechanism was analyzed in beta-cell-rich pancreatic islets isolated from ob/ob mice. When added at a concentration of 400 microM to a medium containing 1.28 mM Ca2+, diazoxide converted glucose stimulation of insulin release into inhibition. Further addition of 2 mM theophylline restored the insulin secretory response to glucose. The paradoxical glucose inhibition of insulin release was accounted for by a diazoxide interaction with the entry of Ca2+, unmasking a capacity of the sugar to lower cytoplasmic Ca2+ below its resting concentration.

Animals↗
PubMed1988

Influence of glucose on the sodium content of beta-cell-rich pancreatic islets exposed to sulphonamides and amiloride.

Sodium was measured in beta-cell-rich pancreatic islets isolated from ob/ob-mice starved overnight. Exposure to glucose (5 or 20 mmol/l) resulted in about a 30% reduction of the sodium content whether or not the Na+/K+ pump was inhibited by removal of K+. The glucose effect was not potentiated after amiloride depression of Na+/H+ exchange, and it disappeared when combining removal of K+ with the addition of the hyperglycemic sulphonamide diazoxide (400 mumol/l). Tolbutamide (100 mumol/l) counteracted the reduction of sodium obtained with 5 mmol/l glucose both in the presence or absence of extracellular K+. It is concluded that closure of ATP-regulated K+ channels does not necessarily result in a lowering of the sodium content. The pancreatic beta-cells can be regarded as exceptional among the excitable cells in not responding to their natural physiological stimulus (glucose) with increase of sodium.

Adenosine Triphosphate↗
PubMed1987

Demonstration of glucose inhibition of insulin release in the presence of diazoxide.

beta-Cell-rich pancreatic islets from ob/ob mice were taken for measurements of insulin release in response to glucose after culture in RPMI 1640 medium. The stimulatory effect of 20 mmol/l glucose was converted into an inhibition when the medium was supplemented with 400 mumol/l diazoxide. Glucose inhibition of insulin release was observed when the islets had been cultured in the presence of 1 or 20 mmol/l glucose in media either containing or lacking Ca2+. The data provide further evidence for an inhibitory component in the action of glucose on insulin release, suggesting that glucose stimulation of the Ca2+ efflux is essential for the appearance of this inhibition.

Animals↗
PubMed1987

Significance of the calcium content of mouse beta cells in the preservation of glucose-induced insulin release during culture.

Pancreatic islets containing more than 90% beta cells from obese-hyperglycaemic (ob/ob) mice were cultured for 3 days in different concentrations of Ca2+ and glucose to evaluate the importance of intracellular Ca2+ sequestration in glucose-induced insulin release. The islet contents of calcium (total and exchangeable) and immunoreactive insulin were compared with the insulin secretory response to glucose after culture. The turnover of Ca2+ increased with increasing concentrations of glucose and Ca2+. Islets cultured in the presence of 5.5 mmol glucose/l contained more calcium and insulin than those cultured with 1 or 20 mmol glucose/l. During culture in 20 mmol glucose/l, a lowering of the Ca2+ concentration of the medium from 0.42 to 0.025 mmol/l resulted in a paradoxical increase in intracellular calcium, with improvement of the subsequent secretory response to the sugar. When the islets had been exposed to the calcium channel blocker D-600 during culture in a Ca2+-deficient medium, substantial insulin release was noted from islets containing relatively small amounts of calcium. The results suggest that the well-established role of glucose in maintaining insulin release is associated with an ability of the sugar to stimulate the retention of calcium in beta cells.

Animals↗
PubMed1986

Unmasking of the inhibitory action of glucose on insulin release after alpha 2-adrenergic activation.

Insulin release in response to glucose was measured after culture of islets from ob/ob-mice in a Ca2+-deficient medium. The stimulatory effect of 20 mM glucose disappeared after addition of 1 microM L-epinephrine, and it was reversed into inhibition when the medium contained 0.1 to 10 microM clonidine. Glucose inhibited insulin release also after activation of the alpha 2-adrenoceptors with B-HT 933, whereas blocking of these receptors with idazoxan removed glucose inhibition in the presence of clonidine. It is concluded that alpha 2-adrenergic activation provides an efficient means of unmasking the inhibitory component in the action of glucose on insulin release.

Adrenergic alpha-Agonists↗
PubMed1986

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.

Animals↗
PubMed1986

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.

Animals↗