Immediate in-vivo effect of corticosterone on glucose-induced insulin secretion in the rat.

The effect of infused corticosterone (300 micrograms/h per kg body wt) on the concentrations of insulin in the plasma of the rat was examined (1) when glucose concentration was basal, (2) at standardized glucose levels attained by modulated glucose infusion and (3) in response to a standard or a modulated glucose pulse. There was no effect of corticosterone on the levels of plasma insulin when the glucose concentrations were either basal or raised in response to the standard pulse of glucose. However, when glucose was infused a significantly reduced plasma level of insulin was detected after 60 min when the glucocorticoid was present and this level remained significantly reduced after the modulated pulse of glucose. Thus the infusion of corticosterone leads to an acute depression of the concentrations of insulin in the plasma and of their response to a glucose pulse only when the hormone acts in the presence of a concentration of glucose in the plasma that is insulin-stimulatory.

Effect of low temperatures on glucose-induced insulin secretion and ionic fluxes in rat pancreatic islets.

The direct effect of cold on the inhibition of B cell secretion is well known in hibernating and experimentally hypothermic mammals. This temperature dependency may result from the inhibition of ion transport across the membranes. In order to verify this hypothesis, ionic effluxes and insulin secretion from rat islets loaded with 86Rb+ and 45Ca+ were measured during perifusion. At 37 degrees C, the rise in glucose concentration from zero to 16.7 mmol/l provoked a rapid decrease in 86Rb+ efflux, an early fall and subsequent rise in 45Ca2+ efflux and a typical biphasic pattern of insulin secretion. At 27 degrees C, glucose induced only a very slight increase in insulin secretion, while the fluxes of radioactive ions were not significantly modified in amplitude but were clearly delayed. At 17 degrees C, no insulin response to glucose was observed and the decrease in K+ conductance indicated by 86Rb+ flux decrease was less temperature-dependent than the movement of Ca2+. After supplementary stimulation with a high extracellular concentration of Ca2+, insulin secretion was enhanced at 27 degrees C and reached levels induced by glucose alone at 37 degrees C. An increase in hormone secretion occurred even at 17 degrees C, but only during a first phase of secretion. Regular increases in temperature potentiated insulin secretion and provoked changes in ionic fluxes which suggest that B cell depolarization (86Rb+ flux decrease) induced by glucose can occur at 15 degrees C but cannot induce the opening of voltage-dependent Ca2+ channels (increase in 45Ca2+ efflux) until temperatures higher than 27 degrees C are reached.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of low temperatures on glucose-induced insulin secretion and glucose metabolism in isolated pancreatic islets of the rat.

The direct effect of hypothermia on the inhibition of insulin secretion may result from inhibition of the availability of energetic substrates and/or the lack of metabolic signals. In order to verify this hypothesis, the insulin secretion and the main metabolic glucose pathways were measured during the incubation of rat islets. In the presence of 16.7 mmol glucose/l and at 37 degrees C, insulin secretion was 925 +/- 119 microU/2 h per ten islets. With the same experimental conditions, glucose utilization, determined as the formation of 3H2O from [5-3H]glucose was 2225 +/- 184 pmol/2 h per ten islets, glucose oxidation measured as the formation of 14CO2 from [U-14C]glucose was 673 +/- 51 pmol/2 h per ten islets, pentose cycle determined as the formation of 14CO2 from either [1-14C]glucose or [6-14C]glucose was 37 +/- 5 pmol/2 h per ten islets; glucose oxidation by the tricarboxilic acid cycle, calculated to be the difference between glucose oxidation and pentose cycle values, was 636 pmol/2 h per ten islets. Hypothermia highly inhibited glucose-induced insulin secretion and glucose utilization. Inhibition of insulin secretion was partial at 27 degrees C since it was 2.5 times lower than that at 37 degrees C, and it was complete at 17 degrees C. Glucose oxidation in the tricarboxilic acid cycle was markedly inhibited by hypothermia since the inhibition coefficient (Q10) between 37 and 27 degrees C was 5. In contrast, glucose oxidation in the pentose phosphate shunt was enhanced at 27 degrees C, reaching 92 +/- 17 pmol/2 h per ten islets, and it was inhibited relatively little at 17 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by corticosterone of calcium inflow and insulin release in rat pancreatic islets.

Corticosterone (0.6 mumol/l) inhibited both 45Ca outflow and insulin release evoked by glucose, the combination of leucine and glutamine, 2-ketoisocaproate, gliclazide or the association of gliclazide and a tumour-promoting phorbol ester in rat pancreatic islets perifused at normal extracellular Ca2+ concentration (1.0 mmol/l). In all cases, the inhibitory action of corticosterone reached statistical significance within 10-22 min of exposure to this steroid and failed to be rapidly reversible. Corticosterone failed to affect basal 45Ca outflow and insulin release. The steroid also failed to affect the inhibitory action of glucose upon 45Ca outflow, as judged from either the glucose-induced early fall in effluent radioactivity from islets maintained at normal extracellular Ca2+ concentration or the steady-state values for 45Ca outflow from glucose-stimulated but Ca2+-deprived islets. Corticosterone caused a modest increase in 86Rb outflow from islets perifused in the presence of glucose (16.7 mmol/l). It is concluded that corticosterone impairs Ca2+ inflow into the islet cells and, by doing so, causes a progressive inhibition of insulin release. The pancreatic B cell might thus serve as a further model for the study of the rapid biological response to steroids, as presumably mediated by alteration in the biophysical properties of the plasma membrane.

Immediate, short- and long-term opposite effects of oestradiol-17 beta on glucose metabolism in rat adipocytes: relationship with the biphasic changes in body weight and food intake.

The changes in the effects of oestradiol-17 beta on body weight, food intake and [1-14C]glucose oxidation in adipocytes were followed in sham-operated, ovariectomized and adrenalectomized-ovariectomized rats to eliminate effects of endogenous progesterone and corticosterone. During the first 5 days oestradiol induced a dramatic fall in food intake and body weight concomitant with a decrease in glucose oxidation by adipocytes, when tested 12 h and 3 days after the beginning of treatment. In-vitro incubations with oestradiol showed that this was a direct effect of this hormone. On the other hand, from days 5 to 14 of treatment, body weight and food intake increased, though they were still lower than in sham-operated controls. On day 14, as values of treated rats tended to reach those of controls, glucose oxidation in adipocytes was stimulated by oestradiol treatment. An insulin effect was still observable and none of these effects was dependent on the adrenal gland. These biphasic changes in the parameters studied could be closely related; moreover, a relationship with other oestradiol actions on metabolism that are known to be corticosterone-dependent could be eliminated.

Relationship between calbindin-D28K levels in the A and B cells of the rat endocrine pancreas and the secretion of insulin and glucagon: influence of vitamin D3 deficiency and 1,25-dihydroxyvitamin D3.

The pancreatic B cell is equipped with specific receptors for 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) and contains vitamin D-dependent calcium binding proteins (calbindin-D). Insulin secretion is impaired by vitamin D deficiency and is restored by 1,25-(OH)2D3 (concomitantly with an improved calcium handling within B cells) but the effect of 1,25-(OH)2D3 on the pancreatic B cell via calbindin-D is unclear. Therefore we examined the relationship between calbindin-D28K or calbindin-D9K and the activity of the endocrine pancreas in normal (N), four week vitamin D-deficient (-D) and one week 1,25-(OH)2D3-replete (+D) rats. Calbindin-D9K was not found in the pancreas, neither in the islets nor in the exocrine part, of any of the groups of rats (N, -D, or+D). Surprisingly, total islet calbindin-D28K content was increased by vitamin D deficiency and partly restored by 1,25-(OH)2D3. Calbindin-D28K immunostaining was observed only on A and B cells in the endocrine part of the pancreas, the greatest staining being found in A cells. This difference in staining density was increased by vitamin D deficiency and decreased by 1,25-(OH)2D3 treatment. In vitro, 1,25-(OH)2D3 also produced a negative influence on calbindin-D28K staining in A cells, as demonstrated using pieces of pancreas incubated with the steroid for 2 h. No significant influence on labeling intensity of B cell calbindin-D28K could be shown. Plasma insulin and islet insulin release in response to 10 mM arginine stimulation were decreased in -D rats and enhanced in +D rats towards N values. In contrast, plasma glucagon and the amount of glucagon secretion, stimulated in vitro by 10 mM arginine or by low (1.7 mM) glucose concentration, was increased in -D rats and attenuated by 1,25-(OH)2D3. Thus there appears to be no relationship between the steady state level of B cell calbindin-D28K and the regulation of insulin secretion by 1,25-(OH)2D3 in vitamin D-deficient rats. However there is a correlation between A cell calbindin-D28K and glucagon secretion, which are both negatively regulated by 1,25-(OH)2D3. The predominance of calbindin-D28K in A cells raises the question as to how A and B cells interact and the role of calbindin-D28K in calcium handling.

Effects of tioconazole on parturition and serum levels of 17 beta-oestradiol, progesterone, LH and PRL in the rat.

Tioconazole, an imidazole antifungal agent, was administered orally at 100 mg/kg/day to pregnant rats according to two regimens; in one, treatment started on day 15 post-insemination (p.i.) and in the other it started on day 18 p.i. The first regimen caused a delay in onset of parturition and a prolongation of labour. Serum progesterone was decreased from days 17 to 21 p.i., 17 beta-oestradiol decreased on day 21 p.i., LH increased on day 17 p.i., and the normal surge of prolactin on day 21 p.i. abolished. The parturition disorders disappeared when 17 beta-oestradiol (0.125 microgram/animal/day s.c.) was given with tioconazole from day 15 p.i. In the second regimen, tioconazole treatment advanced by about 24 hours the onset of parturition and the normal fall in serum progesterone and the surge in prolactin. Serum 17 beta-oestradiol was unaffected, but LH was raised on days 19 and 20 p.i. In animals receiving progesterone (2.5 mg/animal/day, s.c.) and tioconazole from day 18 p.i. parturition was no longer advanced. In conclusion, the parturition disorders observed in rats during tioconazole treatment are associated with a modification of progesterone and 17 beta-oestradiol serum levels. These findings have questionable relevance for the human situation as the roles of these steroid hormones in parturition in women are different from those in rats.

Stimulatory effect of 1,25-dihydroxyvitamin D3 on calcium handling and insulin secretion by islets from vitamin D3-deficient rats.

Among the various vitamin D3 metabolites, 1,25-(OH)2D3 is the specific secosteroid hormone that can enhance, in vitro, the weak insulin response to glucose of islets from vitamin D3-deficient rats. Because this potentiating effect is preceded by an increase in Ca2+ handling, several putative sites of action were studied by measuring 45Ca2+ and 86Rb+ (as K+ tracer) efflux during perifusions in the presence of various stimuli known to affect Ca2+ movements in different ways: high glucose without calcium, high calcium without glucose, high potassium, or barium-theophylline without calcium or glucose. The present results show that 1,25-(OH)2D3 may activate Ca2+ handling by at least two mechanisms: (1) an increase of Ca2+ entry via voltage-dependent Ca2+ channels in the experiments in which extracellular Ca2+ was present and where Ca2+ channels were opened; this 1,25-(OH)2D3 influence on Ca2+ channels was not mediated by a possible indirect influence on K+ channels because 86Rb+ fluxes were never observed to be affected by the steroid; (2) an enhancement of 45Ca2+ mobilization from intracellular stores as suggested by barium-theophylline stimulation and probably also via the Ca2+ stimulus. Both of these 1,25-(OH)2D3 influences tended to provide more calcium to the B cell of vitamin D3-deficient rats. But this prerequisite was not sufficient in itself to potentiate the insulin response; indeed, experiments with barium-theophylline suggested that 1,25-(OH)2D3 may also activate a cAMP-dependent exocytosis process.

Progesterone and synthetic steroids produce insulin resistance at the post-receptor level in adipocytes of female rats.

The effects of progesterone, its agonists (progestin RU-5020, glucocorticoid RU-26988) and antagonist (antiprogesterone, anti-glucocorticoid RU-486) were tested on isolated fat cells in vitro. When added to the incubation medium, all four steroids decreased basal glucose oxidation. The inhibitory effect of the steroids appeared early (20 min incubation) and was sustained during a 2-h incubation. The early inhibitory effect was less marked for progesterone agonist RU-5020 than for the other three steroids. When incubation was prolonged for 2 h, the lowest inhibitory effect was observed with progesterone antagonist RU-486. Insulin-stimulated glucose oxidation was inhibited by progesterone, its antagonist RU-486, one of its agonists RU-26988, but not by the other agonist RU-5020. Analysis of the dose response curves showed that progesterone, RU-26988, and RU-486 decreased fat cells' responsiveness and, only for RU-486, sensitivity to insulin. Adipocytes isolated from ovariectomized, progesterone-treated rats showed a decreased maximal response to insulin and decreased insulin sensitivity in opposition to cells incubated directly with the steroid. No inhibition of 125I-labeled insulin binding was seen as an acute or chronic effect of progesterone. It is concluded that progesterone and the studied related steroids decrease glucose oxidation by mechanism(s) distal to insulin binding to its specific receptors.

Effects of constant magnetic fields on rats and mice: a study of weight.

Variations in the growth of animals from exposure to a magnetic field have been reported by several authors. In this study, young rats and mice were exposed daily to constant uniform magnetic fields. The strength of the field was 400, 600 or 800 mT. Rats were exposed for 4 weeks and mice for 250 d or more. Some mice were exposed in a permanent way to a non-uniform magnetic field of 4.6 mT. No significant effect on the growth was observed.

Effects of constant magnetic fields on the B-cells and insulin target cells in the rat.

Young male rats were exposed to constant uniform magnetic fields of 400 and 800 millitesia (mT). No changes were observed in the body and in the pancreas weight, in the pancreatic insulin content and in the in vitro insulin release of Langerhans islets, in glucose and insulin plasma levels. The magnetic fields abolished the insulin effect on glucose uptake of diaphragms, but not on 1-14C-glucose oxidation. In adipocytes 400 mT increased insulin-stimulated 1-14C-glucose oxidation, when expressed as percentage of mean basal oxidation, and 800 mT diminished it.

[In vitro studies of the effect of temperature on insulin secretion in the rat, the active and the lethargic hedgehog (author's transl)]. / Etude in vitro de l'influence de la température sur la secrétion de l'insuline par le pancréas du rat, du hérisson actif et du hérisson en léthargie.

The study of insulin secretion by the technic of perifusion of pieces of pancreas at increasing temperature allowed us to point out that the pancreas of the lethargic hedgehog is more sensitive to temperature than that of the active hedgehog or that of the Rat.

Dynamics of insulin release by perfused edible dormouse (Glis glis) pancreas. Seasonal variations.

In order to characterize seasonal variations of beta cell function in the edible dormouse (Glis glis), the dynamics of insulin release were examined during perfusion of the isolated pancreas. The B cells exhibited biphasic insulin secretion; however, the dynamic response differed from that of the rat in that there was a steady-state second release phase. Glucose-induced insulin release changed according to the seasons. With 16.8 mmol/l glucose, the average insulin release of the late phase was 30.8 +/- 12.6 ng/min in winter, 7.4 +/- 3.2 ng/min in spring, 13.1 +/- 3 ng/min in summer and 23.3 +/- 4.4 ng/min in autumn. The glucose-induced insulin release, expressed as percent of the insulin content of the pancreas, varied according to the season: it represented 2.23 +/- 0.31% in winter, 1.28 +/- 0.10% in spring, 1.56 +/- 0.15 in summer and 2.6 +/- 0.11 in autumn. It is suggested that in spring and summer, the edible dormouse B cell secretion mechanism is less sensitive to glucose than in the other seasons.

Modulation of the direct effect of corticosterone upon glucose-induced insulin secretion of rat isolated islets of Langerhans.

As a direct inhibiting effect of corticosterone (0.2 mg/l) has been shown upon 16.7 mmoles/l glucose-induced insulin secretion rate of rat islets of Langerhans, modulations of this inhibition were studied. The inhibitory effect was observed over a wide range of stimulatory glucose concentrations (8.3--33.4 mmoles/l), but not with non-stimulating glucose values (0-4.2 mmoles/l). During the steady-state insulin secretory rate induced by glucose 16.7 mmoles/l, the inhibiting effect needed about fifteen minutes to become statistically significant; it persisted as long as the glucose stimulation was maintained, independently of corticosterone presence. The inhibition during a subsequent incubation without corticosterone after a preincubation period with the steroid and glucose concentrations between 4.2 and 11.1 mmoles/l, appeared only if the glucose concentration during the preincubation period was a stimulating one. A minimal steroid presence time (10 min) appeared to be necessary for the induction of the subsequent inhibitory effect of corticosterone in presence of 16.7 mmoles/l glucose. Our data suggest that the inhibition of insulin secretion is constant over a wide range of glucose concentrations on condition that the hormone presence lasts more than ten minutes and glucose concentration is a stimulating one; then it persists as long as a stimulatory glucose concentration is provided.

Simultaneous determination of insulin secretion and glucose oxidation rates during incubation of isolated rat islets of Langerhans.

Insulin secretion being related to glucose metabolism of the B cell, it seemed of interest to develop a suitable system to measure these two parameters on the same islet pool, in order to overcome the great individual differences observed when these are measured in separate experiments. For this reason, we used an incubation system allowing the measurement of insulin secretion concomitantly with the determination of the corresponding oxidation rate. The results obtained agree well with those described by several authors using separate experimental procedures. Thus, our experimental design appears to be a reliable method for simultaneous determination of insulin secretion and glucose oxidation rates.

Direct effect of corticosterone upon insulin secretion studied by three different techniques.

The immediate effect of corticosterone upon insulin secretion rates estimated by three different techniques (perfusior of isolated rat pancreas and perifusion or incubation of isolated islets of Langerhans) was studied for one hour. Three corticosterone concentrations were used: 0.02, 0.2 or 20 mg/l. With 4.2 mmol/l glucose, corticosterone did not affect insulin secretion, whereas, with a stimulating glucose concentration (16.7 mmol/l), insulin secretion was inhibited by the three corticosterone concentrations tested during incubation experiments, and by only the two physiological ones (0.02 and 0.2 mg/l) during islets perifusion and pancreas perfusion experiments. Moreover the inhibitory effect appeared more rapid with perifused islets than perfused pancreas, where only the second insulin secretory phase was disturbed.

Effect of corticosterone upon insulin biosynthesis and storage by isolated rat Langerhans islets.

A rapid inhibitory effect of corticosterone upon glucose- or leucine-induced insulin release being established, steroid action upon insulin biosynthesis of rat Langerhans islets was studied as a function of time. Islet proinsulin and insulin content was reduced after two hours of incubation in presence of corticosterone (0.2 mg/l) with a 16.7 mmoles/l glucose stimulation. L (-) [4-5(3)H] leucine incorporation into proinsulin and insulin was lowered at all time (points examined: 1, 2 and 3 hours). Incorporation ratio between proinsulin and insulin remained stable, so an interaction at the proinsulin biosynthetic level may be possible. Corticosterone had no effect upon biosynthesis of other islet proteins. Our data suggest that the corticosterone inhibition of insulin biosynthesis is specific.

Short term and direct influence of oestradiol on glucagon secretion stimulated by arginine.

Long-term oestrogen treatments disturb the endocrine pancreas, with associated hypoglucagonaemia and hyperinsulinism. Short-term influences of oestradiol (4 h-8 h) were tested in vivo, and direct effects were studied in vitro with pieces of pancreas or islets of Langerhans. Both models showed a direct and dose-dependent inhibitory effect of oestradiol on the arginine-stimulated secretion of glucagon, related to the short-term hypoglucagonaemia. However, this direct negative effect of oestradiol on A cells occurred in spite of any influence, either on insulinaemia or on the arginine-induced insulin secretion. This supports the hypothesis of a possibility of A cells to function independently of B cells.