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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.

1. The effects of exogenous adenine nucleotides and structural analogues on the biphasic insulin response to an increase of glucose concentration in the physiological range (from 4.2 to 8.3 mM) were studied in the isolated perfused rat pancreas. Purinoceptor agonists were added either simultaneously or 15 min before increasing glucose. 2. ATP and ADP at 16.5 microM were ineffective per se in the presence of the non stimulatory glucose concentration (4.2 mM) but markedly potentiated the biphasic insulin response to glucose rise in both experimental protocols. 3. Two more stable analogues of ATP and ADP (adenylylimidodiphosphate and alpha, beta-methylene ADP (alpha, beta-MeADP)) at 16.5 microM behaved like the natural compounds: they were ineffective at a glucose concentration of 4.2 mM and potentiated both phases of insulin response to glucose rise. 4. alpha, beta-MeATP added simultaneously with the high glucose concentration, markedly potentiated the first phase of insulin response to glucose rise but did not potentiate the second one. When alpha, beta-MeATP infusion began 15 min before glucose rise, the biphasic response to glucose was not potentiated, in contrast to what occurred with ATP. 5. In the presence of alpha, beta-MeATP, the ATP potentiating effect was unaffected. 6. It is concluded that ATP and ADP, via activation of beta cell P2 gamma purinoceptors, potentiates the biphasic insulin response to an increase of glucose concentration. On the other hand, alpha, beta-MeATP did not behave like natural and other structural analogues of ATP and ADP: this difference appears not to be the consequence of desensitization of beta cell P2 gamma purinoceptors by alpha, beta-MeATP.

1 The effects of three methylene analogues of adenosine 5'-triphosphate (ATP) or 5'-diphosphate (ADP) have been studied on insulin secretion from the isolated perfused pancreas of the rat: 5'-adenylmethylene diphosphonate or beta, gamma-methylene ATP, adenosine 5'-alpha, beta-methylene triphosphate or alpha, beta-methylene ATP and adenosine 5'-alpha, beta-methylene diphosphonate or alpha, beta-methylene ADP. 2 beta, gamma-Methylene ATP did not elicit any increase of insulin release: alpha, beta-methylene ATP and alpha, beta-methylene ADP induced a biphasic stimulation of insulin secretion; this effect was dose-related between 1.65 and 165 mumol/l. Relative potency ATP/alpha, beta-methylene ATP was 1.2 and ATP/alpha, beta-methylene ADP was 0.31. 3 Our results point to the importance of the steric and electronic characteristics of the polyphosphate chain of the analogues of ATP and ADP in inducing an insulin secretory effect. They support the hypothesis of a purine receptor for ATP and ADP.

1 The effects of a 2-substituted analogue of adenosine 5'-triphosphate (ATP), 2-methylthioadenosine triphosphate (2-methylthio ATP) have been studied on insulin secretion and flow rate of the isolated pancreas of the rat, perfused in the presence of glucose (8.3 mM). 2 2-Methylthio ATP (16.5-1650 nM) increased insulin secretion in a biphasic and concentration-dependent manner; the kinetics were comparable to those previously obtained with ATP. A comparison of relative potency between ATP and 2-methylthio ATP showed that 2-methylthio ATP was 45 times more potent than ATP. 3 2-Methylthio ATP also provoked a transient decrease of the flow rate in a concentration-dependent manner but at concentrations (165-825 microM) about 1000 fold higher than those needed to increase insulin secretion. A comparison of relative potency between the natural derivative and 2-methylthio ATP showed that 2-methylthio ATP was only twice as potent as ATP. 4 These and other previous results (with phosphate-modified analogues of ATP) provide evidence for two different types of P2-purinoceptors on endocrine cell and vessel cells of the pancreas. A P2Y subtype, mediating an increase of insulin secretion, is present on the beta cell of the pancreas. A P2X subtype, mediating vasoconstriction, is present on the vascular bed of the rat pancreas.