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Adenine nucleotides have been shown to stimulate insulin secretion by acting on P2 receptors of the P2Y type. Since there have been some discrepancies in the insulin response of different analogues of ATP and ADP, we investigated whether two different types of P2 receptors exist on pancreatic B cells. The effects of alpha,beta-methylene ATP, which is more specific for the P2X subtype, were studied in vitro in pancreatic islets and isolated perfused pancreas from rats, in comparison with the potent P2Y receptor agonist ADPbetaS. In isolated islets, incubated with a slightly stimulating glucose concentration (8.3 mM), alpha,beta-me ATP (200 microM) and ADPbetaS (50 microM) similarly stimulated insulin secretion; by contrast, under a non stimulating glucose concentration (3 mM), alpha,beta-me ATP was still effective whereas ADPbetaS was not. In the same way, in islets perifused with 3 mM glucose, alpha,beta-me ATP but not ADPbetaS induced a partial but significant reduction in the peak 86Rb efflux induced by the ATP-dependent potassium channel opener diazoxide. In the isolated pancreas, perfused with a non stimulating glucose concentration (4.2 mM), ADPbetaS and alpha,beta-me ATP (5-50 microM), administered for 10 min, induced an immediate, transient and concentration-dependent increase in the insulin secretion; their relative potency was not significantly different. In contrast, with a slightly stimulating glucose concentration (8.3 mM), ADPbetaS was previously shown to be 100 fold more potent than alpha,beta-me ATP. Furthermore, at 4.2 mM glucose a second administration of alpha,beta-me ATP was ineffective. In the same way, ADPbetaS was also able to desensitize its own insulin response. At 3 mM glucose, alpha,beta-me ATP as well as ADPbetaS (50 microM) induced a transient stimulation of insulin secretion and down regulated the action of each other. These results give evidence that pancreatic B cells, in addition to P2Y receptors, which potentiate glucose-induced insulin secretion, are provided with P2X receptors, which transiently stimulate insulin release at low non-stimulating glucose concentration and slightly affect the potassium conductance of the membrane.

1. Epidemiological and experimental data have shown that homocysteine may provoke vascular lesions and that moderate homocysteinaemia may constitute an independent risk factor for vascular disease. It is now documented that homocysteine damages human endothelial cells in culture, possibly by producing hydrogen peroxide in an oxygen-dependent reaction. 2. In this study, we have examined the direct effect of this sulphur amino acid on pancreatic vascular resistance. Experiments were performed on the vascular bed of the rat isolated pancreas perfused at constant pressure; thus, any change in pancreatic vascular resistance resulted in a change in the flow rate. D,L-Homocysteine perfused for one hour at three different concentrations (200 microM, 2 mM, 20 mM) did not induce any significant change in the flow rate per se. Homocysteine infusion for 30 min at a concentration of 200 microM or 2 mM abolished the endothelium-dependent vasodilatation induced by acetylcholine (0.05 microM), but did not modify adenosine (1.5 microM)-induced vasodilatation. 3. The effect of D,L-homocysteine (200 microM or 2 mM) cannot be ascribed to a direct antimuscarinic effect since 30 min pretreatment of rat ileum with these concentrations did not significantly change the contractile effect of increasing concentrations of acetylcholine (0.015-15 microM). 4. Preincubation of human umbilical vein endothelial cells with D,L-homocysteine (0.2-5.0 mM) had no significant effect on overall cell number or viability during 18 h of incubation; the endothelial cells exposed to concentrations up to 5 mM exhibited a spindle-shaped, whirled pattern. This pattern was reversed 48 h after the removal of homocysteine. A cytotoxic effect was seen after 18 h incubation in 10 mM D,L-homocysteine. 5. In conclusion, an acute infusion of homocysteine altered acetylcholine endothelium-induced vasodilation, whereas the adenosine vasodilatator effect was insensitive to the deleterious action of homocysteine in vitro.

Nos experiences effectuees sur le pancreas isole et perfuse du rat montrent que: 1. En l'absence de glucose, le pancreas secrete seulement une tres petite quantite d'insuline: c'est la secretion basale. — 2. Quand on utilise des concentrations croissantes de glucose, le debit d'insuline augmente en fonction de la concentration de glucose (suivant une courbe sigmoide). — 3. Le tolbutamide seul, en l'absence de glucose, augmente legerement la secretion basale. — 4. Le tolbutamide potentialise les effets insulino-secreteurs de concentrations croissantes de glucose. En presence de tolbutamide, la quantite supplementaire d'insuline secretee s'eleve en meme temps que la concentration de glucose, jusqu'a 2 g/1; a 3 g/1 elle diminue. — 5. A forte concentration (100 mg/1), l'effet potentialisateur du tolbutamide sur l'insulino-secretion due au glucose (1.5 g/1) se prolonge bien au dela de l'arret de l'infusion du sulfamide. A faible concentration (5 mg/1) il cesse des que l'infusion est supprimee. — 6. Le glibenclamide (a faible concentration: 1 μg/1), non seulement potentialise l'action du glucose, mais aussi prolonge son action stimulante sur la cellule beta, longtemps apres la suppression du sulfamide hypoglycemiant. — 7. Dans nos conditions experimentales, l'administration repetee de tolbutamide, en presence de glucose, ne modifie pas la reponse insulino-secretoire du pancreas a ce sulfamide.

Studies of the role and nature of the cholinergic receptors acting on the secretion of isolated perfused rat pancreas have shown the following: The infusion of acetylcholine at a dose of 2.5 μM in the presence of a concentration of glucose of 1.5 g/1, provoked a first phase of immediate and important stimulation of the secretion of insulin; this initial peak of insulin secretion was followed by a second phase during which a new less intense stimulation occurred; the latter was followed by an inhibition appearing at a time that depended on the pancreas used. At a dose of 0.5 μM of acetylcholine, the first phase of stimulation always appeared; during the second phase some pancreases were inhibited, others remained stimulated. — The peak of insulin secretion obtained by stimulation with acetylcholine during the first phase was dose related. — Eserine intensified the effects of acetylcholine. — The presence of glucose was essential for the insulinsecretory action of acetylcholine. The muscarinic nature of the cholinergic receptors implicated in the secretion of insulin was demonstrated by the use of: — Atropine which completely blocked the effects of acetylcholine, — Muscarine which produced the same effects as acetylcholine on our pancreas preparation, effects which were equally inhibited by atropine. The cholinergic receptors of the endocrine beta cell of the islet of Langerhans of the pancreas are therefore of the muscarinic type.

Des perfusions de doses non hypertensives d'adrenaline (0,5 μg/kg/min) ont ete effectuees pendant 2 a 6 heures chez le chien par la voie de l'artere gastroepiploique droite, branche de l'artere pancreatico-duodenale. Dans certaines experiences le pancreas a ete maintenu intact; dans d'autres la portion splenique ainsi que le ≪processus uncinatus≫ ont ete extirpes. Des biopsies pancreatiques ont ete pratiquees a divers stades de la perfusion. Les cellules des ilots et particulierement les cellules beta ont ete trouvees gravement lesees; cependant le tissu exocrine etait relativement conserve. La glycemie s'est elevee pendant la perfusion puis est redescendue vers la normale. Les jours suivants 35 pour 100 des animaux porteurs seulement de la portion duodenale du pancreas ont presente un diabete persistant; 65 pour 100 ont gueri. La dihydroergotamine s'oppose a l'action nocive de l'adrenaline pour les cellules des ilots. La noradrenaline a paru moins nocive que l'adrenaline. Ces faits sont discutes en ce qui concerne la pathologie du diabete et en particulier pour l'interpretation du diabete concomitant de pheochromocytomes.

Two series of experiments with the isolated perfused rat pancreas were performed in parallel. The conditions differed only with respect to temperature, which was 37.5 degrees C in one series and 28 degrees C in the other. The lowering of the temperature decreased insulin secretion induced by glucose as well as the insulin response to tolbutamide and acetylcholine. Unlike insulin, glucagon secretion was not significantly modified by hypothermia. Our results suggest that the mechanisms involved in glucagon and insulin secretion are different.

The stimulating action of tolbutamide on insulin secretion presents two components; the first is independent of the presence of glucose in the medium which perfuses the beta cells; the second is, on the contrary, dependent upon glucose. D-mannoheptulose and diazoxide permit the dissociation of these two components; the antagonistic effect of the first exerts itself uniquely on that part of the stimulating action of tolbutamide which is gluco-dependent; on the contrary, the antagonistic effect of the second exerts itself on both phases of the stimulating action of the sulfonamide.