Search

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.

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.

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.

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.

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.

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.

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.

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.

The effects of ATP and ADP structural analogues (2-methylthio ATP; alpha,beta-methylene ADP) on somatostatin secretion were tested in dogs. Insulin and glucagon secretion was also evaluated. Our experiments were performed in vivo and in vitro. In vivo, 2-methylthio ATP was infused directly into the pancreaticoduodenal artery of anesthetized dogs and blood was sampled from the pancreaticoduodenal vein. This ATP analogue (approximately 15 microM) immediately induced stimulation of both somatostatin and insulin secretion, which was accompanied by a slight reduction of glycemia. A delayed increase in glucagon output was observed. In vitro, using the isolated perfused dog pancreas uncinate process, alpha,beta-methylene ADP, a stable ADP analogue (16.5 microM), was infused in the presence of a substimulating glucose concentration (4.2 mM). Under these conditions, alpha,beta-methylene ADP immediately induced the stimulation of somatostatin secretion without affecting basal insulin and glucagon secretion. In conclusion our results suggest the presence of P2 purinoceptors on pancreatic somatostatin secreting cells.

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.

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.

The effects of alpha-adrenergic drugs were studied on glucose-induced insulin secretion and effluent flow rate on the same preparation: the isolated perfused rat pancreas. An alpha 1-adrenoceptor agonist, phenylephrine 0.05 microM slightly decreased insulin secretion (-25%); this inhibition was counteracted by an alpha 2-adrenoceptor antagonist, yohimbine 0.6 microM. Phenylephrine evoked a fall in liquid flow rate (-13%) which was reversed by an alpha 1-adrenoceptor antagonist, prazosin 6 microM, but not by yohimbine. An alpha 2-adrenoceptor agonist, clonidine 0.01 and 0.05 microM decreased insulin secretion (-80%). This inhibition was reversed by yohimbine 0.6 and 6 microM respectively. Only the concentration of 0.05 microM clonidine evoked a fall (-25%) in liquid flow rate; this fall was counteracted by yohimbine 0.6 microM. In conclusion our results show that adrenergic inhibition of insulin secretion is mediated only by alpha 2-receptors whereas both types of adrenoceptors are implicated in the vasoconstrictor effect. The insulin inhibitory effect of adrenoceptor agonists is not related to vasoconstriction.