The effects of purinergic agonists on insulin release are controversial in the literature. In our studies (mainly using INS-1 cells, but also using rat pancreatic islets), ATP had a dual effect on insulin release depending on the ATP concentration: increasing insulin release (EC50 approximately/= 0.0032 microM) and inhibiting insulin release (EC50 approximately/= 0.32 microM) at both 5.6 and 8.3 mM glucose. This is compatible with the view that either two different receptors are involved, or the cells desensitize and (or) the effect of an inhibitory degradation product such as adenosine (ectonucleotidase effect) emerges. The same dual effects of ATP on insulin release were obtained using rat pancreatic islets instead of INS-1 cells. ADPbetaS, which is less degradable than ATP and rather specific for P2Y1 receptors, had a dual effect on insulin release at 8.3 mM glucose: stimulatory (EC50 approximately/= 0.02 microM) and inhibitory (EC50 approximately/= 0.32 microM). The effectiveness of this compound indicates the possible involvement of a P2Y1 receptor. 2-Methylthio-ATP exhibited an insulinotropic effect at very high concentrations (EC50 approximately/= 15 microM at 8.3 mM glucose). This indicated that distinct P2X or the P2Y1 receptor may be involved in these insulin-secreting cells. UTP increased insulin release (EC50 approximately/= 2 microM) very weakly, indicating that a P2U receptor (P2X3 or possibly a P2Y2 or P2Y4) are not likely to be involved. Suramin (50 microM) antagonized the insulinotropic effect of ATP (0.01 microM) and UTP (0.32 microM). Since suramin is not selective, the data indicated that various P2X and P2Y receptors may be involved. PPADS (100 microM), a P2X and P2Y1,4,6 receptor antagonist, was ineffective using either low or high concentrations of ATP and ADPbetaS, which combined with the suramin data hints at a P2Y receptor effect of the compounds. Adenosine inhibited insulin release in a concentration-dependent manner. DPCPX (100 microM), an adenosine (A1) receptor antagonist, inhibited the inhibitory effects of both adenosine and of high concentrations of ATP. Adenosine deaminase (1 U/mL) abolished the inhibitory effect of high ATP concentrations, indicating the involvement of the degradation product adenosine. Repetitive addition of ATP did not desensitize the stimulatory effect of ATP. U-73122 (2 microM), a PLC inhibitor, abolished the ATP effect at low concentrations. The data indicate that ATP at low concentrations is effective via P2Y receptors and the PLC-system and not via P2X receptors; it inhibits insulin release at high concentrations by being metabolized to adenosine.

A role of diadenosine polyphosphates as second messengers was suggested for insulin-secreting cells. It has not yet been investigated whether specific receptors for these compounds exist and how these extracellular compounds and their degradation products may contribute to insulin release. Specific saturable binding sites for diadenosine polyphosphates exist in INS-1 cells and rat pancreatic islets. In INS-1 cells, the rank order of diadenosine polyphosphates displacing [3H]Ap4A from binding sites was Ap4A = Ap5A >Ap3A = Ap6A. Binding was specific, since suramin was not able to displace the binding; adenosine, ATP, UTP, alpha,beta-methylene ATP, beta,gamma-methylene ATP, ADP-betaS, 2-methylthio ATP, and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) were able to displace [3H]Ap4A from its binding sites. Insulin release was investigated in INS-1 cells. Perifusion experiments showed an increase in insulin release stimulated by the diadenosine polyphosphates in the presence of 8.3 mmol/l glucose; in static incubations (90 min), however, insulin release was inhibited dose dependently by the four diadenosine polyphosphates. This discrepancy might be due to the instability of the compounds. [3H]Ap4A was degraded in the extracellular medium to mainly adenosine and low concentrations of ATP, ADP, AMP, and inosine (half-maximal degradation after 25 min). The insulin stimulatory effect is due to the original compounds (acute perifusion experiments), and the insulin inhibitory effect (static incubation experiments) is due to the production of inhibitory compounds, such as adenosine, in the medium. Small amounts of intact [3H]Ap4A, but mainly [3H]ATP, accumulated in the cells within 20 min. The uptake of labeled compounds is dependent on an intact metabolism and intact receptor internalization. This data indicates that 1) specific bindings sites for diadenosine polyphosphates exist in INS-1 cells and rat pancreatic islets mediating insulin release; 2) the receptors involved in INS-1 cells may be diadenosine polyphosphate receptors, albeit others, such as P2X-receptors, cannot be ruled out; and 3) diadenosine polyphosphates, and mainly their degradation products in the extracellular space, are to a high degree accumulated within cells with unknown function. Thus, diadenosine polyphosphates are worth being investigated more closely in physiological and pathophysiological terms.

The role of diadenosine polyphosphates (ApnA, where "A" denotes "adenosine" and "n" denotes the number of phosphate groups "p") as vasoconstrictors of smooth-muscle cells and as blood-pressure regulating and insulin-releasing compounds has been described. It was the aim of this study to investigate whether specific receptors for these compounds, mediating the above mentioned effects, occur in cultured vascular smooth-muscle cells (VSMC) and in endothelium cells, and whether these compounds are degraded during incubation. Saturable binding sites for diadenosine polyphosphate [3H]Ap4A with an extremely quick saturation equilibrium, even at low temperature (4 degrees C), are present in vascular smooth-muscle cells. Diadenosine polyphosphates at micromolar concentrations displaced [3H]Ap4A from binding sites; the ranking order was Ap4A > Ap3A > Ap5A approximately Ap6A. Compounds interacting with purinergic P2X receptors such as suramin, alpha,beta-methylene ATP and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), albeit at high concentrations, displaced [3H]Ap4A from its binding sites. Surprisingly, at low concentrations the compounds tested increased the binding of [3H]Ap4A, which might imply the occurrence of positive receptor cooperativity or inhibition of [3H]Ap4A degradation. By use of thin-layer chromatography it was observed that [3H]Ap4A was quickly degraded (half-life approx. 12 min) in the extracellular medium to (mainly) adenosine and inosine. [3H]Ap4A and its degradation products were quickly taken up by the cells. Degradation can be inhibited by Ap6A, alpha,beta-methylene ATP or PPADS. Rather similar degradation and uptake results were also obtained when endothelium cells were used. These data indicate that specific binding sites for [3H]Ap4A are present in vascular smooth-muscle cells and that diadenosine polyphosphates at physiological concentrations displace binding. The receptors involved might be distinct diadenosine polyphosphate receptors, although the involvement of others, such as P2X receptors, is also possible. Ap4A is quickly degraded in the extracellular space and compounds that inhibit degradation result in an increase in [3H]Ap4A binding. It should be remembered that when diadenosine polyphos-phates are being investigated in physiological and pathophysiological studies of their impact on smooth-muscle cell proliferation and on vasoconstriction (blood-pressure regulation), results obtained from long-term incubations might be critical.