A constitutive nitric oxide synthase modulates insulin secretion in the INS-1 cell line

We provide immunocytochemical evidence that the neuronal isoform of constitutive NO synthase (cNOS) is expressed in the rat insulinoma cell line INS-1. Furthermore, using N omega-nitro-L-arginine methyl ester (L-NAME), a pharmacological inhibitor of cNOS activity, we show that this enzyme is implicated in the modulation of insulin secretion in INS-1 cells. Indeed, in the presence of 2.8 mM glucose, L-NAME induced a specific and dose-dependent increase in insulin release, suggesting that cNOS exerts an inhibitory tone on basal insulin secretion. Moreover, L-arginine, the physiological substrate of cNOS, significantly reduced the marked enhancing effect of L-NAME on insulin release and to a lesser extent, at low concentrations, that of 10 mM KCl. L-NAME also potentiated the insulin secretion stimulated by 5.5 and 8.3 mM glucose, but in this case, its effect was not reduced by L-arginine. In conclusion, our data show that the neuronal isoform of cNOS exerts a negative modulation on insulin secretion in INS-1 cells, confirming the previous results obtained in the isolated perfused rat pancreas or pancreatic islets.

A neuronal isoform of nitric oxide synthase expressed in pancreatic beta-cells controls insulin secretion

Evidence is presented showing that a neuronal isoform of nitric oxide synthase (NOS) is expressed in rat pancreatic islets and INS-1 cells. Sequencing of the coding region indicated a 99.8% homology with rat neuronal NOS (nNOS) with four mutations, three of them resulting in modifications of the amino acid sequence. Double-immunofluorescence studies demonstrated the presence of nNOS in insulin-secreting beta-cells. Electron microscopy studies showed that nNOS was mainly localized in insulin secretory granules and to a lesser extent in the mitochondria and the nucleus. We also studied the mechanism involved in the dysfunction of the beta-cell response to arginine and glucose after nNOS blockade with N(G)-nitro-L-arginine methyl ester. Our data show that miconazole, an inhibitor of nNOS cytochrome c reductase activity, either alone for the experiments with arginine or combined with sodium nitroprusside for glucose, is able to restore normal secretory patterns in response to the two secretagogues. Furthermore, these results were corroborated by the demonstration of a direct enzyme-substrate interaction between nNOS and cytochrome c, which is strongly reinforced in the presence of the NOS inhibitor. Thus, we provide immunochemical and pharmacological evidence that beta-cell nNOS exerts, like brain nNOS, two catalytic activities: a nitric oxide production and an NOS nonoxidating reductase activity, both of which are essential for normal beta-cell function. In conclusion, we suggest that an imbalance between these activities might be implicated in beta-cell dysregulation involved in certain pathological hyperinsulinic states.

Preproinsulin I and II mRNA expression in adult rat submandibular glands

Mammalian salivary glands are known to produce a number of biologically active peptides. The aim of this study was to extend our previous results showing the presence of a biologically active insulin-like immunoreactive peptide in rat salivary glands. In rodents, where two nonallelic and functional insulin genes are expressed, the co-expression of both genes seems to be limited to beta-cells of pancreatic islets or to embryologic developmental processes. We have investigated the expression of insulin genes in rat submandibular glands and in a murine immortalized submandibular cell line, SCA-9. For this purpose, total RNAs were isolated and submitted to reverse transcription. The cDNAs obtained were amplified by a nested polymerase chain reaction using rat preproinsulin I and II primers. Our data show that both preproinsulin I and II mRNAs are expressed in adult rat submandibular glands as well as in the SCA-9 cell line. The identification of salivary gland rat preproinsulin I and II was confirmed by direct sequencing. These results provide, for the first time, evidence for the expression of both preproinsulin I and II mRNA in an extra-pancreatic tissue from adult rodents.

Insulinotropic effect of Citrullus colocynthis fruit extracts

Infusions of Citrullus colocynthis Schrad. (Cucurbitaceae) fruits are traditionally used as antidiabetic medication in Mediterranean countries, but to our knowledge no studies have been undertaken so far to determine the possible mechanisms involved in the antidiabetic properties of the fruit. The present study was designed to investigate whether these fruits possess insulinotropic effects. For this purpose, different extracts of Citrullus colocynthis seed components were obtained: RN II (crude extract), RN VI (hydro-alcoholic extract), RN X (purified extract) and RN XVII (beta-pyrazol-1-ylalanine), the major free amino acid present in the seeds. The insulin secretory effects of these different extracts were evaluated in vitro in the isolated rat pancreas and isolated rat islets in the presence of 8.3 mM glucose. All tested extracts, when perfused for 20 min at 0.1 mg/ml, immediately and significantly stimulated insulin secretion. This effect was transient. In addition, the purified extract (RN X) provoked a clear dose-dependent increase in insulin release from isolated islets. Moreover, a significant and persistant increase in pancreatic flow rate appeared during RN VI, RN X and RN XVII perfusions. In conclusion, our results show that different Citrullus colocynthis seed extracts have an insulinotropic effect which could at least partially account for the antidiabetic activities of these fruits.

4-Hydroxyisoleucine: effects of synthetic and natural analogues on insulin secretion

4-Hydroxyisoleucine, a peculiar amino acid extracted from fenugreek seeds and never found in mammalian tissues, exhibits interesting insulinotropic activity. To investigate the structural requirements for this stimulating effect, the insulinotropic activity of the major isomer (2S,3R,4S) of 4-hydroxyisoleucine, in the presence of 8. 3 mM glucose, was compared to that of (1) its minor isomer (2R,3R, 4S) (2) its lactone form, (3) classical structurally related amino acids, and (4) synthetic monomethylated analogues. In the isolated, ex vivo, perfused rat pancreas, only the major isomer of 4-hydroxyisoleucine (200 microM) potentiated insulin release. On incubated isolated rat islets, the threshold concentration for a significant increase (P<0.05) in insulin release was 200 microM for (2S,3R,4S) 4-hydroxyisoleucine, 500 microM for (2S,4R) and (2S,4S) gamma-hydroxynorvalines as well as (2S,3S) and (2S,3R) gamma-hydroxyvalines, and 1 mM or more for other congeners. In conclusion, the insulinotropic properties of 4-hydroxyisoleucine, in the micromolar range, are seen only in the presence of the linear major isoform; they also require carbon alpha in S-configuration, full methylation and carbon gamma-hydroxylation.

4-Hydroxyisoleucine: experimental evidence of its insulinotropic and antidiabetic properties

We have recently shown in vitro that 4-hydroxyisoleucine (4-OH-Ile), an amino acid extracted from fenugreek seeds, potentiates insulin secretion in a glucose-dependent manner. The present study was designed to investigate whether 4-OH-Ile could exert in vivo insulinotropic and antidiabetic properties. For this purpose, intravenous or oral glucose tolerance tests (IVGTTs and OGTTs, respectively) were performed not only in normal animals but also in a type II diabetes rat model. During IVGTT in normal rats or OGTT in normal dogs, 4-OH-Ile (18 mg/kg) improved glucose tolerance. The lactonic form of 4-OH-Ile was ineffective in normal rats. In non-insulin-dependent diabetic (NIDD) rats, a single intravenous administration of 4-OH-Ile (50 mg/kg) partially restored glucose-induced insulin response without affecting glucose tolerance; a 6-day subchronic administration of 4-OH-Ile (50 mg/kg, daily) reduced basal hyperglycemia, decreased basal insulinemia, and slightly, but significantly, improved glucose tolerance. In vitro, 4-OH-Ile (200 microM) potentiated glucose (16.7 mM)-induced insulin release from NIDD rat-isolated islets. So, the antidiabetic effects of 4-OH-Ile on NIDD rats result, at least in part, from a direct pancreatic B cell stimulation.

Vanadyl treatment normalizes submandibular salivary gland insulin-like immunoreactivity in streptozotocin-diabetic rats

Rat submandibular salivary glands (SSG) contain a compound displaying insulin-like immunoreactivity (ILI) and various biological activities of insulin. As SSG ILI levels were reported to be increased in streptozotocin (STZ)-induced diabetes but not normalized by a two-week insulin treatment, we decided to check whether another antidiabetic treatment, vanadyl sulphate (VOSO4), was able to regulate SSG ILI concentration. A short term (8 days) i.p. VOSO4 treatment (total dose = 1.3 mmol/kg) of rats made diabetic 8 days earlier by a single i.v. injection of STZ (60 mg/kg BW) was able to induce a long-term (4 weeks) correction of hyperglycemia while weight gain was re-established. In untreated diabetic animals (approximately -25%) and increased (approximately +175%) as compared to normal rats. Both parameters were normalized in VOSO4-treated diabetic rats.

4-Hydroxyisoleucine: a novel amino acid potentiator of insulin secretion

We report the characterization of a new insulinotropic compound, 4-hydroxyisoleucine. This amino acid has been extracted and purified from fenugreek seeds, which are known in traditional medicine for their antidiabetic properties. 4-Hydroxyisoleucine increases glucose-induced insulin release, in the concentration range of 100 micromol/l to 1 mmol/l, through a direct effect on isolated islets of Langerhans from both rats and humans. The stimulating effect of 4-hydroxyisoleucine was strictly glucose dependent; indeed, ineffective at low (3 mmol/l) or basal (5 mmol/l) glucose concentrations, the amino acid potentiated the insulin secretion induced by supranormal (6.6-16.7 mmol/l) concentrations of glucose. In addition, in the isolated perfused rat pancreas, we could show 1) that the pattern of insulin secretion induced by 4-hydroxyisoleucine was biphasic, 2) that this effect occurred in the absence of any change in pancreatic alpha- and delta-cell activity, and 3) that the more glucose concentration was increased, the more insulin response was amplified. Moreover, 4-hydroxyisoleucine did not interact with other agonists of insulin secretion (leucine, arginine, tolbutamide, glyceraldehyde). Therefore, we conclude that 4-hydroxyisoleucine insulinotropic activity might, at least in part, account for fenugreek seeds' antidiabetic properties. This secretagogue may be considered as a novel drug with potential interest for the treatment of NIDDM.

Experimental NIDDM: development of a new model in adult rats administered streptozotocin and nicotinamide

We took advantage of the partial protection exerted by suitable dosages of nicotinamide against the beta-cytotoxic effect of streptozotocin (STZ) to create a new experimental diabetic syndrome in adult rats that appears closer to NIDDM than other available animal models with regard to insulin responsiveness to glucose and sulfonylureas. Among the various dosages of nicotinamide tested in 3-month-old Wistar rats (100-350 mg/kg body wt), the dosage of 230 mg/kg, given intraperitoneally 15 min before STZ administration (65 mg/kg i.v.) yielded a maximum of animals with moderate and stable nonfasting hyperglycemia (155 +/- 3 vs. 121 +/- 3 mg/dl in controls; P < 0.05) and 40% preservation of pancreatic insulin stores. We also evaluated beta-cell function both in vitro and in vivo 4-9 weeks after inducing diabetes. In the isolated perfused pancreas, insulin response to glucose elevation (5-11 mmol/l) was clearly present, although significantly reduced with respect to controls (P < 0.01). Moreover, the insulin response to tolbutamide (0.19 mmol/l) was similar to that observed in normal pancreases. Perfused pancreases from diabetic animals also exhibited a striking hypersensitivity to arginine infusion (7 mmol/l). In rats administered STZ plus nicotinamide, intravenous glucose tolerance tests revealed clear abnormalities in glucose tolerance and insulin responsiveness, which were interestingly reversed by tolbutamide administration (40 mg/kg i.v.). In conclusion, this novel NIDDM syndrome with reduced pancreatic insulin stores, which is similar to human NIDDM in that it has a significant response to glucose (although abnormal in kinetics) and preserved sensitivity to tolbutamide, may provide a particularly advantageous tool for pharmacological investigations of new insulinotropic agents.

Antazoline increases insulin secretion and improves glucose tolerance in rats and dogs

In vivo effects of an imidazoline devoid of alpha2-adrenoceptor antagonistic properties, antazoline, on insulin secretion and glycemia were investigated both in fasted rats and dogs. In both species, antazoline (1.5 mg/kg i.v.) transiently increased insulinemia without affecting basal plasma glucose levels. In contrast, during an i.v. glucose tolerance test, antazoline markedly potentiated insulin release and thus increased the glucose disappearance rate. In rats, during an oral glucose tolerance test, the intragastric administration of antazoline (1.5 mg/kg) clearly enhanced insulin secretion and reduced hyperglycemia. In dogs provided with a venous pancreatico-duodenal bypass, antazoline (0.5 mg/kg i.v.) induced an immediate and transient increase in insulin and somatostatin but not in glucagon pancreatico-duodenal outputs. In conclusion, intravenously and orally administered, the imidazoline antazoline is able to stimulate insulin secretion in vivo and improve glucose tolerance. The imidazoline compounds could therefore have a potential therapeutic relevance as new antihyperglycemic insulinotropic agents.

Mechanisms involved in the effect of nitric oxide synthase inhibition on L-arginine-induced insulin secretion

1. A constitutive nitric oxide synthase (NOSc) pathway negatively controls L-arginine-stimulated insulin release by pancreatic beta cells. We investigated the effect of glucose on this mechanism and whether it could be accounted for by nitric oxide production. 2. NOSc was inhibited by N omega-nitro-L-arginine methyl ester (L-NAME), and sodium nitroprusside (SNP) was used as a palliative NO donor to test whether the effects of L-NAME resulted from decreased NO production. 3. In the rat isolated perfused pancreas, L-NAME (5 mM) strongly potentiated L-arginine (5 mM)-induced insulin secretion at 5 mM glucose, but L-arginine and L-NAME exerted only additive effects at 8.3 mM glucose. At 11 mM glucose, L-NAME significantly inhibited L-arginine-induced insulin secretion. Similar data were obtained in rat isolated islets. 4. At high concentrations (3 and 300 microM), SNP increased the potentiation of arginine-induced insulin output by L-NAME, but not at lower concentrations (3 or 30 nM). 5. L-Arginine (5 mM) and L-ornithine (5 mM) in the presence of 5 mM glucose induced monophasic beta cell responses which were both significantly reduced by SNP at 3 nM but not at 30 nM; in contrast, the L-ornithine effect was significantly increased by SNP at 3 microM. 6. Simultaneous treatment with L-ornithine and L-arginine provoked a biphasic insulin response. 7. At 5 mM glucose, L-NAME (5 mM) did not affect the L-ornithine secretory effect, but the amino acid strongly potentiated the alteration by L-NAME of L-arginine-induced insulin secretion. 8. L-Citrulline (5 mM) significantly reduced the second phase of the insulin response to L-NAME (5 mM) + L-arginine (5 mM) and to L-NAME + L-arginine + SNP 3 microM. 9. The intermediate in NO biosynthesis, NG-hydroxy-L-arginine (150-300 microM) strongly counteracted the potentiation by L-NAME of the secretory effect of L-arginine at 5 mM glucose. 10. We conclude that the potentiation of L-arginine-induced insulin secretion resulting from the blockade of NOSc activity in the presence of a basal glucose concentration (1) is strongly modulated by higher glucose concentrations, (2) is not due to decreased NO production but (3) is probably accounted for by decreased levels of NG-hydroxy-L-arginine or L-citrulline, resulting in the attenuation of an inhibitory effect on arginase activity.

Vanadyl sulphate differently influences insulin response to glucose in isolated pancreas of normal rats after in vivo or in vitro exposure

The effect of the antidiabetic agent vanadyl sulphate (VOSO4) on the endocrine pancreas function of normal rats was studied using the isolated pancreas preparation. A short-term (8 days) i.p. treatment (15 mg/kg per day) resulted in attenuation of high glucose-stimulated insulin release, at day 9 but also at days 19, i.e., after full recovery of appetite and weight, while blood and pancreas vanadium concentrations were still elevated. Six months of oral VOSO4 treatment (0.75 mg/ml in drinking water) resulted in elevated vanadium concentrations while glucose-stimulated insulin release was attenuated as compared to pair-fed animals. Conversely, when directly perfused in pancreas, VOSO4 potentiated glucose-stimulated insulin release. These apparently opposite effects may be related to the ability of VOSO4 to exert both peripheral insulinomimetic effects-leading to chronic reduction in insulin demand-, and a direct pancreatic insulinotropic activity.

Age-dependent changes in insulin-like immunoreactivity in rat submandibular salivary glands

In recent years, a growing interest had arisen in hormonal factors in salivary glands. We have investigated the changes in the content of an insulin-like immunoreactive (ILI) compound in the submandibular salivary glands of Sprague Dawley rats during physiological aging, in the range 15 days-27 months. The amount of ILI in the submandibular glands of young adult rats was found to be doubled in the post-natal period until the age of puberty and was maintained in senescence. No significant correlation was found between age-dependent variations in ILI levels of submandibular salivary glands and circulating insulin concentrations, further supporting previous indications that ILI is being synthesized in situ. It is possible that ILI could exert paracrine effects within the glands, as regards the development of other glandular structures during the first months of life, as well as the preservation of glandular function in senescent animals as well.

Pancreastatin modulates glucose-stimulated insulin secretion from the perfused rat pancreas

Pancreastatin is known to be produced in islet B cells and to inhibit insulin secretion. In this study, we examined whether the peptide affects insulin secretion from the perfused rat pancreas during raising or lowering the glucose concentration within the physiological range. We found that synthetic porcine pancreastatin (15 nmol L-1) significantly inhibited the first phase (5 min) of insulin release induced by raising the glucose concentration from 4.2 to 8.3 mmol L-1 (P < 0.05) without affecting the second phase. Furthermore, the fall in insulin secretion induced by lowering the glucose concentration from 8.3 to 5.5 mmol L-1 was significantly exaggerated in presence of pancreastatin during the first 5 min (P < 0.05). In contrast, pancreastatin did not affect the inhibition of insulin secretion induced by either the alpha 2-adrenoceptor agonist, clonidine (2 nmol L-1), or the P1-purinoceptor (A1-subtype) agonist, N6 (+)-phenylisopropyladenosine (D-PIA) (1.65 mumol L-1). In conclusion, our results show that pancreastatin is a modulator of the early changes in insulin secretion after increase or decrease of the glucose concentration within the physiological range. This suggests that pancreastatin is a modulator of glucose-stimulated insulin secretion.

Long-term antidiabetic activity of vanadyl after treatment withdrawal: restoration of insulin secretion?

In its vanadate (V5+) or vanadyl (V4+) forms, vanadium has been demonstrated to possess antidiabetic activity. Oral treatment of streptozotocin (STZ)-diabetic animals with either form is associated with correction of hyperglycemia, and prevention of diabetes-induced complications, although weight gain is unaffected. Vanadium treatment of non-diabetic animals lowers plasma insulin levels by reducing insulin demand, as these animals remain normoglycemic. These results suggest that vanadium has in vivo insulin-mimetic or insulin-enhancing effects, in agreement with several in vitro observations. Chronic treatment with vanadium has also been shown to result in sustained antidiabetic effects in STZ-diabetic animals long after treatment has ceased. Thus, at 13 weeks after withdrawal from treatment, corrected animals had normalized glucose and weight gain, and improved basal insulin levels. In addition, near-normal glucose tolerance was found despite an insignificant insulin response. Since vanadium accumulates in several tissue sites (e.g. bone, kidney) when pharmacological doses are administered, it is possible that stored vanadium may be important in maintaining near-normal glucose tolerance at least in the short-term following withdrawal from treatment. Recently, following withdrawal of vanadyl treatment up to 30 weeks, diabetic animals which had remained normoglycemic and had normalized glucose tolerance showed improvements in plasma insulin levels both in the basal state and in response to oral glucose, as compared to those which had reverted to hyperglycemia. The observed significant improvements in insulin capacity over the long-term ( > 3 months) suggests that a restored and/or preserved insulin secretion may be essential for maintained reversal of the diabetic state over a prolonged period after treatment is withdrawn.

Long-term correction of STZ-diabetic rats after short-term i.p. VOSO4 treatment: persistence of insulin secreting capacities assessed by isolated pancreas studies

We have previously shown that 3 week oral VOSO4 treatment of streptozotocin (STZ, 60 mg/kg)-induced diabetic rats was able to correct diabetes for 13 weeks after treatment withdrawal. In the present study, we investigated whether a short-term (8 days) i.p. VOSO4 treatment was similarly able to reverse the diabetic state. Insulin secretory capacities were assessed at distance of treatment using the isolated pancreas preparation. Seven treatment-groups were performed: high dose VOSO4-treated diabetics (HVD, 1.3 mM/kg/8 days), food-restricted diabetics (FRD, food adjusted to HVD levels), low dose VOSO4-treated diabetes (LVD, 0.06 mM/kg/day), insulin-treated diabetics (ID, dose adjusted to normalize glycaemia) and VOSO4 (0.06 mM/kg/day) + insulin (dose adjusted to normalize glycaemia in the presence of vanadium)-treated diabetics (IVD), in addition to the corresponding untreated non-diabetic controls (C) and diabetics (D). Our results indicate that long-term correction of diabetes (a) can be obtained after an 8 day treatment using i.p. VOSO4 in diabetic animals retaining some degree of pancreatic function, (b) is not obtained with insulin treatment or food restriction although the association of VOSO4 and insulin was found beneficial, (c) can be prolonged in some individuals for at least 4 months, i.e. in conditions such that tissue vanadium concentrations had returned to values close to pre-treatment levels, (d) is associated with improved and in some cases normalized insulin secretion from isolated pancreas. The protective or corrective role of VOSO4 on diabetes-related pancreatic alterations, as well as the potential of the VOSO4-insulin association should be further studied in view of the possible use of vanadium derivatives in the treatment of diabetes.

Alterations of insulin response to different beta cell secretagogues and pancreatic vascular resistance induced by N omega-nitro-L-arginine methyl ester

1. We studied a possible interplay of pancreatic NO synthase activity on insulin secretion induced by different beta cell secretagogues and also on pancreatic vascular bed resistance. 2. This study was performed in the isolated perfused pancreas of the rat. Blockage of NO synthase was achieved with Nw-nitro-L-arginine methyl ester (L-NAME); The specificity of the antagonist was checked by using its D-enantiomer as well as by substitutive treatments with sodium nitroprusside (SNP) as a NO donor in studies of glucose-induced insulin secretion. 3. Arginine (5 mM) induced a monophasic response which was, in the presence of L-NAME at equimolar concentration, very strongly potentiated and converted into a 13 times higher biphasic one. D-NAME (5 mM) was only able to induce a 3 times higher response, but provoked a similar vasoconstrictor effect. 4. The small biphasic insulin secretion induced by L-leucine (5 mM) was also strongly enhanced, by 8 times, in the presence of L-NAME (5 mM) vs 2 times in the presence of D-NAME (5 mM). 5. beta cell responses to KCl (5 mM) and tolbutamide (0.185 mM) were only slight increased by L-NAME (5 mM) to values not far from the sum of the effects of L-NAME and of the two drugs alone. D-NAME (5 mM) was totally ineffective on the actions of both secretagogues. 6. L-NAME, infused 15 min before and during a rise in glucose concentration from 5 to 11 mM, was able in the low millimolar range (0.1-0.5 mM) to blunt the classical biphasic pattern of beta cell response to glucose and, at 5 mM, to convert it into a significantly greater monophasic one. In contrast, D-NAME (5 mM) was unable to induce similar effects. 7. SNP alone at 3 microM was ineffective but at 30 microM substantially reduced to second phase of insulin response to glucose; however, at both concentrations the NO donor partly reversed alterations in insulin secretion caused by L-NAME (5 mM) and restored a biphasic response.

Biological activity of immunoreactive insulin-like activity extracted from rat submandibular gland

Earlier studies indicate the presence of an insulin-like immunoreactivity (ILI) in rat submandibular salivary glands (SSG). Previous observations also showed that streptozotocin (STZ)-induced diabetes was accompanied by an increase in SSG ILI concentrations. In the present work we studied the effect of SSG ILI from normal and STZ diabetic rats (ILI-N and ILI-D, respectively) on insulin receptor binding and function in LMH cell line. ILI-N and ILI-D inhibited 125I-insulin binding to intact cells and wheat germ agglutinin (WGA)-purified insulin receptors with a high affinity. Furthermore, ILI-N and ILI-D activated, although weakly, the beta-subunit autophosphorylation of solubilized and WGA-purified insulin receptors. An ATP hydrolytic activity was present in ILI-N and, to a greater extent, in ILI-D extracts, which can at least in part explain their low potency for activating autophosphorylation and kinase activity of insulin receptors in vitro. However, after ILI treatment of intact cells and immunoprecipitation of insulin receptors, ILI induced a dose-dependent tyrosine phosphorylation of the insulin receptor beta-subunit. Finally, ILI-N and ILI-D stimulated amino acid uptake and lipogenesis in LMH cells. These findings suggest that SSG ILI is biologically active and can participate in metabolic regulations.

Effect of n-hexacosanol on insulin secretion in the rat

n-Hexacosanol, a long-chain saturated fatty alcohol extracted from Hygrophyla erecta Hochr., has been recently shown to exert neurotrophic properties on central neurons and to stimulate phagocytosis in macrophages. The present work was designed to investigate the effects of hexacosanol on stimulated insulin secretion in vivo and in vitro. In anaesthetized rats, hexacosanol (2 mg/kg i.p.) induced a reduction of the insulin response to an intravenous glucose tolerance test (0.3 g/kg) with a consequent increase in hyperglycaemia. In vitro, in the isolated perfused pancreas, hexacosanol at the concentration of 10(-7) M clearly reduced the two phases of glucose-induced insulin secretion. At the higher concentration (10(-5) M), hexacosanol was no longer able to exert an inhibition of glucose-induced insulin release; surprisingly a stimulating effect occurred which was of the same magnitude as in control experiments with Tween alone, at the concentration used to dissolve hexacosanol. In isolated perifused islets, 22 mM glucose-stimulated insulin release was also inhibited by hexacosanol at the concentrations of 10(-9) M and 10(-7) M, but not at 10(-5) M. In contrast, insulin secretion induced by arginine (20 mM) was not affected by the different concentrations of hexacosanol. It is concluded that n-hexacosanol at 10(-9) M and 10(-7) M exerts an inhibitory effect on insulin secretion stimulated by glucose in vivo and in vitro in the rat, suggesting a direct effect on islets of Langerhans.

In vivo and in vitro effects of adenosine-5'-O-(2-thiodiphosphate) on pancreatic hormones in dogs

Adenosine-5'-O-(2-thiodiphosphate) (ADP beta S), a P2y purinergic agonist, has been shown to be a potent insulin secretagogue on the isolated rat pancreas. In the present work the effects of ADP beta S on insulin somatostatin, and glucagon secretions were investigated in dogs. In vivo, in anesthetized fasted dogs, i.v. ADP beta S (0.1 mg kg-1) induced an immediate increase in insulin and somatostatin-like immunoreactivity (SLI) but not in glucagon pancreaticoduodenal outputs. In conscious fasted dogs, i.v. ADP beta S (0.1 mg kg-1) produced an immediate and transient augmentation in plasma insulin levels but not in plasma SLI and glucagon levels. In vitro, the effects of ADP beta S were investigated on the isolated uncinate process of dog pancreas, from normal and alloxan-diabetic animals. In normal uncinate process, in presence of 8.3 mM glucose, ADP beta S (1 microM) stimulated insulin and SLI releases but not glucagon release. On uncinate process from diabetic animals, ADP beta S (1 microM) retained its stimulating effects but the responses were impaired as compared with normal dogs: Insulin response was drastically diminished and SLI response strongly enhanced. In conclusion, ADP beta S is a potent insulin secretory agent in dog. This P2y purinoceptor agonist, which exerts a direct stimulatory effect on pancreatic SLI, is interestingly devoid of direct glucagonotropic properties.

Effects of imidazolines and derivatives on insulin secretion and vascular resistance in perfused rat pancreas

The effects of imidazolines and derivatives were studied on insulin secretion and vascular resistance in the isolated perfused rat pancreas. On insulin secretion, two imidazoline alpha 2-adrenoceptor antagonists, efaroxan (1-100 microM) and RX821002 (10 microM), had a stimulating response; however, idazoxan, like the non-imidazoline alpha 2-adrenoceptor antagonist yohimbine, was ineffective at 10 microM. The oxazoline rilmenidine with alpha 2-adrenergic activity at 10 microM), an imidazoline devoid of alpha 2-adrenergic activity, also had an insulin-releasing effect. On pancreatic vessels, all imidazolines tested (efaroxan, RX821002, antazoline and idazoxan), in contrast to yohimbine, induced vasoconstriction. Rilmenidine did not have a vasoconstrictor effect after blockade of alpha 2-adrenoceptors. Furthermore, the efaroxan-induced insulin release or vasoconstriction was not affected by the blockade of alpha 2- and alpha 1-adrenoceptors. This study shows that imidazolines and derivatives are able to stimulate insulin release and induce vasoconstriction in the rat pancreas. These effects cannot be ascribed to an interaction with alpha-adrenoceptors but may involve different types of imidazoline sites.

Effects of 2-chloropropionate on venous plasma lactate concentration and anaerobic power during periods of incremental intensive exercise in humans

We investigated the effects of a stimulation of pyruvate dehydrogenase (PDH) activity induced by 2-chloropropionate (2-CP) on venous plasma lactate concentration and peak anaerobic power (W an, peak) during periods (6 s) of incremental intense exercise, i.e. a force-velocity (F-v) test known to induce a marked accumulation of lactate in the blood. The F-v test was performed twice by six subjects according to a double-blind randomized crossover protocol: once with placebo and once with 2-CP (43 mg.kg-1 body mass). Blood samples were taken at ingestion of the drug, at 10, 20, and 40 min into the pretest period, at the end of each period of intense exercise, at the end of each 5-min recovery period, and after completion of the F-v test at 5, 10, 15, and 30 min. During the F-v test, venous plasma lactate concentrations with both placebo and 2-CP increased significantly when measured at the end of each period of intense exercise (F = 33.5, P < 0.001), and each 5-min recovery period (F = 24.6, P < 0.001). Venous plasma lactate concentrations were significantly lower with 2-CP at the end of each recovery period (P < 0.01), especially for high braking forces, i.e. 8 kg (P < 0.05), 9 kg (P < 0.02), and maximal braking force (P < 0.05). After completion of the F-v test, venous plasma lactate concentrations were also significantly lower with 2-CP (P < 0.001). The percentage of lactate decrease between 5- and 30-min recovery was significantly higher with 2-CP than with the placebo [59 (SEM 4)% vs 44.6 (SEM 5.5)%, P < 0.05]. Furthermore, W an, peak was significantly higher with 2-CP than with the placebo [1016 (SEM 60) W vs 957 (SEM 55) W, P < 0.05]. In conclusion, PDH activation by 2-CP attenuated the increase in venous plasma lactate concentration during the F-v test. Ingestion of 2-CP led to an increased W an, peak.

Effects of a fenugreek seed extract on feeding behaviour in the rat: metabolic-endocrine correlates

Fenugreek seeds (Trigonella foenum graecum L.) are assumed to have restorative and nutritive properties. The present work was designed to investigate the effects of a fenugreek seed extract on feeding behaviour. Experiments were performed to determine food consumption and motivation to eat as well as metabolic-endocrine changes in chronically treated animals. Male Wistar rats were given the seed extract orally (10 and 100 mg/day per 300 g body weight), mixed together with food, and control animals were monitored in parallel. The results show that chronic oral administration of the fenugreek extract significantly increases food intake and the motivation to eat. The treatment, however, does not prevent the anorexia nor the decreased motivation to eat induced by d-fenfluramine (2 mg/kg, IP). An increase in plasma insulin and a decrease in total cholesterol and very low-density lipoprotein (VLDL)-low-density lipoprotein (LDL) total cholesterol were also observed. In conclusion, chronic administration of a fenugreek seed extract enhances food consumption and motivation to eat in rats and also induces hyperinsulinemia as well as hypocholesterolemia.

Stimulation of insulin secretion and improvement of glucose tolerance in rat and dog by the P2y-purinoceptor agonist, adenosine-5'-O-(2-thiodiphosphate)

1. In vivo effect of a P2y-purinoceptor agonist, adenosine-5'-O-(2-thiodiphosphate) (ADP beta S), on insulin secretion and glycaemia were studied both in rats and dogs. 2. In anaesthetized rats, i.v. administered ADP beta S (0.2 mg kg-1) produced an insulin response dependent on the nutritional state of the animals, since we observed only a transient increase in overnight-fasted rats and a sustained insulin secretion followed by a reduction in plasma glucose levels in fed rats. During an i.v. glucose tolerance test, ADP beta S enhanced insulin release and thus increased the glucose disappearance rate. 3. In anaesthetized fasted dogs, i.v. administered ADP beta S (0.1 mg kg-1) increased pancreaticoduodenal insulin output and slightly decreased blood glucose levels. 4. In conscious fasted dogs, orally administered ADP beta S (0.1 mg kg-1) transiently increased insulinemia and punctually reduced glycaemia. Furthermore, during an oral glucose tolerance test, orally administered ADP beta S at the same dose markedly enhanced insulin secretion and consequently reduced the hyperglycaemia. 5. In conclusion, the P2y-agonist, ADP beta S, is a potent insulin secretagogue in vivo, improves glucose tolerance and is effective after oral administration. Thus, the P2y-purinoceptors of the beta cell may be a target for new antidiabetic drugs.

Effects of a peripheral-type benzodiazepine on glucose-induced insulin secretion

Benzodiazepines, besides interacting with central-type receptors which mediate their well-known pharmacological actions, bind to peripheral-type receptors that are distributed in a variety of peripheral tissues including numerous endocrine organs. The present work was designed to investigate the effects of a selective peripheral-type benzodiazepine, 4'-chlordiazepam (Ro 5-4864), on glucose-induced insulin secretion in vitro. In the rat isolated pancreas perfused with a Krebs-bicarbonate buffer containing 8.3 mM glucose, the drug (10(-6) and 10(-5) M) induced a progressive and significant decrease in insulin release. Concomitantly, it induced a vasodilator response of the pancreatic vascular bed. In rat isolated islets incubated for 1 h in the presence of 15 mM glucose, 4'-chlordiazepam (10(-5) and 10(-4) M) induced a significant and dose-dependent inhibition of insulin release. In contrast, the selective central-type benzodiazepine, clonazepam (10(-6) - 10(-4) M), did not significantly modify glucose-induced insulin secretion. In addition, experiments were performed to test the effect of 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline-carboxamid e (PK 11195), a peripheral non-benzodiazepine ligand proposed as a putative antagonist. This substance did not counteract the inhibitory effect of 4'-chlordiazepam but itself (10(-6) and 10(-5) M) elicited a potent inhibitory effect on insulin secretion. These results show that drugs such as 4'-chlordiazepam and PK 11195 which have a high affinity for peripheral-type benzodiazepine receptors, in contrast to a central-type benzodiazepine agonist, inhibit glucose-induced insulin secretion in vitro.

Evidence for a direct inhibitory effect of PYY on insulin secretion in rats

Peptide YY (PYY) has been shown to inhibit stimulated insulin secretion under in vivo conditions in the mouse, the rat, and the dog. In the present study, we investigated the effects of PYY on insulin secretion from the isolated perfused rat pancreas and isolated rat islets. In isolated pancreas perfused in presence of 8.3 mM glucose, PYY at 10(-10) and 10(-9) M, but not at 10(-8) M, inhibited insulin secretion. In the presence of 5.5 mM glucose, PYY (10(-9) M) did not modify basal insulin release but reduced the biphasic insulin response to arginine (10 mM). PYY also markedly reduced the pancreatic vascular flow rate; this effect was observed at all three concentrations tested in a dose-dependent manner. In isolated islets, glucose (15 mM)-stimulated insulin secretion was inhibited by PYY at 10(-7) M. We conclude that in the perfused rat pancreas, PYY inhibits insulin secretion and induces vasoconstriction without a causal relationship. In addition, our results on isolated islets suggest that the inhibitory action of PYY on insulin secretion is exerted through a direct islet action.

Adrenergic inhibition of insulin secretion involves pertussis toxin-sensitive and -insensitive mechanisms

We studied the involvement of Bordetella pertussis toxin (PTX)-sensitive G proteins in the inhibition by adrenaline of insulin secretion from the isolated rat pancreas. The -90% inhibition induced by adrenaline (0.05 microM) was partially abolished after in vivo PTX pretreatment. The residual inhibitory effect of adrenaline in PTX-pretreated rats was suppressed by the alpha 2-adrenoceptor antagonist, yohimbine, but was not modified by the alpha 1-adrenoceptor antagonist, prazosin. Thus, the alpha 2-inhibitory effect of adrenaline on B-cells is mediated by both PTX-sensitive and PTX-insensitive mechanisms.

P2y purinoceptor responses of beta cells and vascular bed are preserved in diabetic rat pancreas

1. To investigate the effect of experimental diabetes on the P2y purinoceptor responses of pancreatic beta-cells and vascular bed, we used adenosine-5'-O-(2-thiodiphosphate) (ADP beta S), a potent and stable P2y agonist. This work was performed in the isolated perfused pancreas of the rat. 2. Diabetes was induced by streptozotocin (66 mg kg-1, i.p.). Five weeks after the induction of diabetes, on the day of pancreas isolation, the animals displayed marked hyperglycaemia (37.6 +/- 2.7 mM). Age-matched rats were used as controls. 3. Insulin response to a glucose stimulation from 5 to 10 mM was completely lost and stimulation of insulin release by the sulphonylurea, tolbutamide (185 microM), was drastically impaired in the diabetic pancreas (maximum responses were 1.5 +/- 0.4 and 7.0 +/- 1.4 ng min-1 for diabetic and age-matched rats respectively). 4. In contrast, in the diabetic pancreas ADP beta S (15 microM), infused in the presence of glucose 5 mM, elicited an immediate and significant insulin release similar to that observed in the age-matched pancreas (maximum responses were 7.6 +/- 1.5 and 6.7 +/- 1.3 ng min-1 respectively). This ADP beta S stimulating effect occurred independently of the glucose concentration (5, 8.3 and 28 mM) in the diabetic pancreas. On pancreatic vascular resistance, ADP beta S induced a similar vasodilatation in diabetic and age-matched rats. 5. In conclusion, ADP beta S retains its insulin stimulatory and vasodilator effects in experimental diabetes; P2y purinoceptors could therefore be considered as a new target for the development of antidiabetic drugs.

Evidence for a direct stimulatory effect of cibenzoline on insulin secretion in rats

The effect of cibenzoline succinate, a new antiarrhythmic agent, was studied on insulin secretion in rats. Experiments were performed both in vivo and in vitro using two preparations: the isolated perfused pancreas and isolated islets. In anaesthetized rats, cibenzoline was able to increase plasma insulin levels and to reduce glycaemia. These effects were observed at 1 mg/kg i.v. in fed rats and at 3 mg/kg i.v. in fasted rats. In the isolated pancreas perfused in the presence of a slightly stimulating glucose concentration (8.3 mM), cibenzoline (2 and 6 microM) elicited a progressive and sustained insulin response in a concentration-dependent manner. In the presence of a non-stimulating glucose concentration (4.2 mM), cibenzoline was ineffective at 2 microM and slightly increased basal insulin release at 6 microM. In isolated islets incubated with 8.3 mM glucose, cibenzoline (6 and 20 microM) caused a concentration-dependent stimulation of insulin release. It is concluded that cibenzoline stimulates insulin secretion by a direct action on pancreatic B cells in rats.

Maturation and insulin-like immunoreactivity in rat submandibular salivary glands: possible implication of G regulatory proteins

Streptozotocin-induced diabetes is accompanied by an increase in insulin-like immunoreactivity concentration in rat submandibular salivary glands. In this study we have examined whether, in normal state, maturation is accompanied by changes in insulin-like immunoreactivity concentration of rat submandibular salivary glands. Insulin-like immunoreactivity concentrations of submandibular salivary glands were significantly higher in 11 months old rats compared with 3.5 months old control animals. A pertussis toxin pretreatment provoked an increase in insulin-like immunoreactivity, suggesting that a pertussis toxin sensitive G-protein is involved in the regulation of insulin-like immunoreactivity in the rat submandibular salivary glands.

Implication of steroid saponins and sapogenins in the hypocholesterolemic effect of fenugreek

The transformation of fenugreek subfractions, rich in steroid saponins, was studied upon their passage through the digestive tract to determine the contribution of saponins and/or diosgenin and other steroid sapogenins to the hypocholesterolemic effect of fenugreek seeds. Feces of alloxan diabetic dogs fed fenugreek subfractions were analyzed, and diosgenin, smilagenin and gitogenin were identified and measured using capillary gas chromatography/mass spectrometry. Our results show that saponins are, in part (about 57%), hydrolyzed into sapogenins in the digestive tract. It appears that saponins may be implicated, alone or together with diosgenin, in the observed hypocholesterolemic effect of fenugreek seeds in diabetic dogs.

Diabetes alters the responses of glucagon secreting cells and vascular bed to isoprenaline and forskolin in vitro in rat pancreas

Diabetes is known to disturb pancreatic glucagon secreting alpha cell function and blood flow control. In a previous study we could show that streptozotocin-induced diabetes suppressed adenosine stimulating effect on glucagon secretion and reduced its vasodilatory properties; since the nucleotide exerts these effects by activation of A2 purinergic receptors known to be positively coupled to adenylate cyclase, we investigated the effect of streptozotocin diabetes on the responses of alpha cells and vascular bed to stimulation of adenylate cyclase through 2 different ways: 1) with isoprenaline by activation of beta adrenergic receptor positively coupled to the enzyme; 2) with forskolin by direct activation of the catalytic unit of adenylate cyclase. In the isolated perfused pancreas of normal rats, isoprenaline (0.01 microM) or forskolin (1 microM) induced a +200 to +300% increase of glucagon secretion and a 20 to 30% increase of pancreatic vascular flow rate. In pancreas of 5-week diabetic rats, alpha cell responses to isoprenaline and forskolin were completely suppressed and the vasodilatory effects of both drugs were significantly reduced (-35 to -50%). Long term in vivo insulin treatment with glycaemia normalization was able to correct both defects. We can conclude that streptozotocin diabetes suppresses glucagon secretion and reduces pancreatic vasodilatation not only in response to activation of receptors positively coupled to adenylate cyclase but also to a direct activation of this enzyme.

Effects of pancreastatin on pancreatic hormone secretion in the dog

In the anaesthetized dog, porcine pancreastatin (98 pmol/min) was infused for 10 min into the pancreaticoduodenal artery either alone or during infusion of glucose. Blood was sampled from the pancreaticoduodenal vein. We found that pancreastatin inhibited pancreatic insulin output only under normoglycaemic conditions. Furthermore, pancreastatin significantly stimulated pancreatic glucagon and somatostatin outputs both during normo- and hyperglycaemic conditions. Our results show that pancreastatin has the capability to affect directly the three pancreatic hormone secretions in dogs.

Diabetes and impaired response of glucagon cells and vascular bed to adenosine in rat pancreas

Previous studies have shown that adenosine, by activation of purinergic A2-receptors, stimulates glucagon secretion and increases vascular flow rate in isolated perfused pancreases from nondiabetic rats. Because alpha-cell function and blood flow control are known to be disturbed in diabetes, we investigated whether adenosine was still effective in streptozocin-induced diabetic (STZ-D) rats. Our experiments were performed on isolated perfused rat pancreases. Whereas, in normal rats, adenosine (1.65 microM) induced a 200% increase in glucagon output and a 25% rise in the pancreatic vascular flow rate, in rats diabetic for 5-6 wk, this nucleoside was ineffective on glucagon secretion, and its vasodilatory effect was strongly reduced. Long-term in vivo insulin treatment that reversed high glycemia levels was able to restore in large part both adenosine effects. In contrast, a short-term in vitro pretreatment with insulin was unable to restore the nucleoside effects. We conclude that STZ-D suppresses the stimulatory effect of adenosine on alpha-cells and strongly reduces its vasodilator properties; these abnormalities may be corrected in large part by long-term insulin treatment with normalization of glycemia.

Hypersensitivity to arginine of both B and D pancreatic cells in adult streptozotocin-diabetic rats

Insulin and pancreatic somatostatin secretions were studied after stimulation with an arginine infusion (5 mmol/l) in isolated perfused pancreata of adult streptozotocin-diabetic rats. In the presence of 2.8 mmol/l glucose, arginine clearly stimulated insulin and somatostatin secretions in diabetic rats, whereas it was ineffective in normal rats. Thus, not only the B-cells, but also the D-cells of the pancreas from streptozotocin-diabetic rats are hypersensitive to arginine. The infusion of insulin (4 U/l) did not modify this hypersensitivity of the D-cells to arginine in pancreata of streptozotocin-diabetic rats.

Involvement of a central nervous pathway in yohimbine-induced insulin secretion

Yohimbine hydrochloride, an alpha 2-adrenoceptor antagonist, was administered (3.3 mg/kg i.v.) to anesthetized normal dogs provided with a T-shaped catheter inserted in the pancreaticoduodenal vein. The effects on blood glucose levels and pancreatic hormones were investigated. We show that yohimbine induced an immediate and pronounced stimulatory effect on insulin secretion accompanied by a clear decrease in blood glucose levels. Yohimbine also stimulated the pancreatic secretion of somatostatin and glucagon. However, the secretion kinetics were not the same for the three hormones: the stimulation was rapid and immediate for insulin and somatostatin, whereas it was progressive for glucagon. All these stimulatory effects were suppressed by propranolol, thus implicating beta-adrenergic mechanisms. Bilateral cervical vagotomy markedly reduced the immediate effect of yohimbine on insulin secretion, suggesting that a central neural pathway was implicated. In contrast, the progressive elevation in glucagon secretion was not decreased by vagotomy. Our results suggest that yohimbine stimulates, at least in part, insulin secretion by blocking central alpha 2-adrenoceptors.

Effect of pertussis toxin on A1-receptor-mediated inhibition of insulin secretion

Previous studies have provided evidence for the presence on B cell membrane of adenosine receptors (P1-purinoceptors) of the A1-subtype which inhibit insulin secretion. In this work we have investigated the implication of a guanosine triphosphate (GTP) binding protein (G protein) in the A1 purinoceptor-induced inhibition of insulin secretion from the isolated perfused pancreas of the rat. A group of rats was treated with pertussis toxin (10 micrograms kg-1, i.v.) 28 h prior to pancreas extirpation. This treatment totally abolished the 50% decrease in insulin secretion induced by (+)-N6-phenylisopropyl adenosine (1.65 microM), a P1-purinoceptor agonist. These results indicate that the A1-receptor-mediated inhibition of insulin secretion involves a pertussis toxin-sensitive G protein.

Effects of 2-methylthio ATP on insulin secretion in the dog in vivo

The effects of 2-methylthio ATP, an ATP analogue that is more specific for the P2Y receptor, were investigated on insulin secretion in the anesthetized dog in vivo. 2-Methylthio ATP was infused directly into the pancreaticoduodenal artery for 15 min. The infusion was performed so as to obtain a pancreaticoduodenal artery blood level of about 15 microM. 2-Methylthio ATP induced an immediate and significant stimulation of insulin secretion measured from the pancreaticoduodenal vein by means of a T-shaped catheter. After the infusion was stopped, the secretion of insulin progressively decreased and at 30 min was close to basal values. The stimulation of insulin secretion induced a transient but significant reduction of peripheral venous blood glucose levels.

Effect of insulin on basal pancreaticoduodenal output of somatostatin in normal and diabetic dogs

The effect of insulin on basal pancreaticoduodenal output of SRIH was investigated in vivo and compared in normal and alloxan diabetic dogs. The experiments were performed on anesthetized dogs having a T-shaped catheter inserted into the pancreaticoduodenal vein just at the exit of the pancreas for blood sampling. In normal dogs, an insulin infusion (1 IU/kg for 20 min) or an iv insulin injection (0.2 IU/kg over 30 sec) produced, before any change in glycemia, an immediate reduction of the venous pancreaticoduodenal output of SRIH. Then pancreaticoduodenal output of SRIH rose close to starting values and decreased again when blood glucose level became very low. In alloxan-diabetic dogs, insulin infusion (1 IU/kg for 20 min) also induced an immediate inhibitory effect on pancreaticoduodenal SRIH output; the effect was more transient and from 20 min, unlike in normal dogs, an increase in pancreaticoduodenal output of SRIH was observed. In conclusion, exogenous insulin induces an immediate reduction in pancreaticoduodenal SRIH secretion both in normal and diabetic dogs, independently of basal blood glucose level and before any change in glycemia. In contrast, the delayed effect is different: SRIH secretion is reduced in normal dogs, whereas it is enhanced in diabetic dogs.

In vivo study of glucose-induced somatostatin secretion: comparison in normal and alloxan-diabetic dogs

This work was undertaken to study the effect of glucose on pancreaticoduodenal and peripheral venous somatostatin-like immunoreactivity (SLI) levels in dogs. Our experiments were performed in normal and alloxan diabetic dogs, conscious or anesthetized. The response of somatostatin was studied following intravenous (0.2 g/kg) or oral (1 g/kg) glucose administration. SLI levels were assayed in peripheral venous blood and in superior pancreaticoduodenal venous blood. An interplay of the cholinergic nervous system was challenged both after oral and intravenous glucose load by a prior administration of atropine sulfate (0.2 mg/kg i.v.). Our results show that (a) peripheral venous SLI levels do not reflect pancreatic D-cell activity in alloxan diabetic as in normal animals. (b) Increase of peripheral venous SLI level after oral glucose is under cholinergic nervous system control. (c) In alloxan diabetic dogs, the response of pancreaticoduodenal venous SLI to intravenous glucose was decreased, whereas peripheral SLI response to oral glucose was increased.

Alpha 2-adrenergic potentiation of adenosine-stimulating effect on glucagon secretion

Previous studies from our laboratory showed 1) that adenosine (1.65 microM), a substance released by tissues in energy-deficient states, stimulated glucagon secretion by activation of A2 purinergic receptors, and 2) that this effect was potentiated by a low substimulating concentration of epinephrine through activation of alpha-adrenergic receptors. The present work was undertaken to assess the subtype of alpha-adrenergic receptor involved in this potentiation. Therefore, we used adrenergic blockers and agonist drugs more specific for alpha 1- or alpha 2-adrenergic receptors. The potentiating effect of epinephrine (0.01 microM) on glucagon secretion induced by adenosine (1.65 microM) was not prevented by an alpha 1-adrenergic blocker, prazosine (6 microM), but was suppressed by an alpha 2-adrenergic blocker, yohimbine (0.6 microM). The implication of alpha 2-adrenergic receptors in the potentiating effect was confirmed by the use of selective alpha 1- or alpha 2-adrenergic agonist drugs. Indeed, clonidine (0.01 microM), an alpha 2-agonist, ineffective per se, potentiated, whereas phenylephrine (0.01 microM), an alpha 1-agonist, had no effect on glucagon secretion induced by adenosine. We conclude that the potentiation by epinephrine of adenosine-induced glucagon secretion is mediated by alpha 2-adrenergic receptor activation. A potentiation between the effects of A2 purinergic and alpha 2-adrenergic agonists may be of physiological relevance in stressful energy-deficient states, when an increase in glucagon secretion is necessary.

Epinephrine potentiates adenosine-stimulating effect on glucagon secretion

The aim of the present work is to investigate a possible interaction on glucagon secretion between adenosine, a compound released by tissues in energy-deficient states, and epinephrine, the hormone of stress largely implicated in such conditions. The study was performed using the isolated perfused rat pancreas in presence of a physiological glucose concentration (5 mM). Epinephrine administered at a low concentration (0.01 microM) was ineffective on glucagon secretion, and adenosine at 1.65 microM was previously shown to be moderately stimulating. This nucleoside alone induced a transient increase of glucagon secretion rate that peaked at 300% of basal value at 2 min; in presence of epinephrine (ineffective per se) the rise induced by the nucleoside alone was doubled. This potentiating effect was not observed with the neurotransmitter norepinephrine at the dose tested. Propranolol (1 microM) did not alter the potentiating effect of epinephrine but this effect was completely suppressed by the alpha-blocker, phenoxybenzamine (6 microM). In conclusion epinephrine potentiates an adenosine-stimulating effect on glucagon secretion; this effect seems more specific for the adrenal medulla hormone epinephrine, since norepinephrine at the same dose is ineffective; it is mediated via alpha-adrenergic receptors. It is attractive to speculate that epinephrine and adenosine act in potentiating synergism on glucagon secretion; this might be of physiological importance during stressful energy-deficient situations.

Absorption of an aqueous solution of a new synthetic somatostatin analogue administered to man by gavage

To determine the local gastrointestinal absorption of a new synthetic somatostatin analogue (SMS 201-995 = Sandostatin), an intestinal tube was passed in eight healthy volunteers and on different days an aqueous solution was administered at four different locations: stomach, proximal duodenum, ligament of Treitz and jejunum. In a follow-up study, an oro-ileal tube was passed in six of the original volunteers and the drug solution was administered in to the terminal ileum. The aqueous solution of SMS was rapidly absorbed from the gastrointestinal tract after local application, and it was well tolerated. Absorption of the drug from the different sites was comparable, although there was a tendency to decreased peptide absorption after ileal administration. Absorption of the drug was quite variable between the subjects and the different locations. The dose-corrected systemic availability relative to subcutaneous administration in another study was 0.28%. However, significant plasma SMS concentrations were achieved, suggesting that oral delivery of the polypeptide may eventually be possible for long-term treatment of a variety of disorders.

Effect of circulating somatostatin on exocrine pancreatic secretion in conscious dogs

We determined the effects of exogenous somatostatin-14 (100 and 200 ng/kg/h; mimicking postprandial somatostatin concentrations) on pancreatic responses to a background infusion of secretion in combination with graded doses of CCK-8 in conscious dogs with chronic gastric and duodenal fistulas. The lower dose of somatostatin-14 (S-14), which produced S-14 plasma levels lower than measured after a meal, did not change basal or stimulated pancreatic secretion. The upper dose of S-14, which produced plasma S-14 concentrations slightly above the postprandial range, caused inhibition of pancreatic fluid and protein secretion to low doses of CCK-8 (p less than 0.05). The inhibition was surmountable with higher doses of CCK-8. We interpret these data as indicating that circulating S-14 is not an important hormonal regulator of exocrine pancreatic secretion.

Antidiabetic effects of subfractions from fenugreek seeds in diabetic dogs

We have previously shown that the antidiabetic property of fenugreek seeds (Trigonella foenum graecum L.) is associated with the defatted seed material which is rich in fibers, saponins, and proteins. In the present work this defatted preparation was divided into two subfractions: subfraction "a" which contains the testa and endosperm and is rich in fibers (79.6%); and subfraction "b" which contains the cotyledons and axes and is rich in saponins (7.2%) and proteins (52.8%). We investigated the effects of each subfraction on hyperglycemia and the levels of pancreatic hormones when chronically administered to alloxan-diabetic dogs. Each subfraction was studied separately and was given to the dogs per os (mixed with the two daily meals), in addition to the insulin treatment (which was kept the same throughout the experiment) for a period of 21 days. The addition of subfraction "a" to insulin treatment resulted in a clear decrease of hyperglycemia and glycosuria accompanied by a reduction of the high plasma glucagon and somatostatin levels in diabetic dogs. The treatment also decreased the hyperglycemic response to the oral glucose tolerance test. In contrast the chronic administration of subfraction "b" had no effect on hyperglycemia or on the levels of pancreatic hormones in diabetic dogs. Our results show that the antidiabetic properties of fenugreek seeds are contained in the testa and endosperm. Although this subfraction is rich in fibers (high viscosity; 115 cP), it is not possible to exclude the existence of one or more unknown active pharmacological compounds in this subfraction of the seed.