DIADENOSINE POLYPHOSPHATES IN INSULIN-SECRETING CELLS - INTERACTION WITH SPECIFIC RECEPTORS AND DEGRADATION

A role of diadenosine polyphosphates as second messengers was suggeste d for insulin-secreting cells. It has not yet been investigated whethe r specific receptors for these compounds exist and how these extracell ular compounds and their degradation products may contribute to insuli n release. Specific saturable binding sites for diadenosine polyphosph ates exist in INS-I cells and rat pancreatic islets. In INS-1 cells, t he rank order of diadenosine polyphosphates displacing [H-3]Ap(4)A fro m binding sites was Ap(4)A = Ap(5)A > Ap(3)A = Ap(6)A. Binding was spe cific, since suramin was not able to displace the binding; adenosine, ATP, UTP, alpha,beta-methylene ATP, beta,gamma-methylene ATP, ADP-beta S, 2-methylthio ATP, and pyridoxalphosphate-6-azophenyl-2' 4'-disulfo nic acid (PPADS) were able to displace [H-3]Ap(4)A from its binding si tes. Insulin release was investigated in INS-1 cells. Perifusion exper iments showed an increase in insulin release stimulated by the diadeno sine polyphosphates in the presence of 8.3 mmol/l glucose; in static i ncubations (90 min), however, insulin release was inhibited dose depen dently by the four diadenosine polyphosphates. This discrepancy might be due to the instability of the compounds. [H-3]Ap(4)A was degraded i n 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 (acut e perifusion experiments), and the insulin inhibitory effect (static i ncubation experiments) is due to the production of inhibitory compound s, such as adenosine, in the medium. Small amounts of intact [H-3]Ap(4 )A, but mainly [H-3]ATP, accumulated in the cells within 20 min. The u ptake of labeled compounds is dependent on an intact metabolism and in tact receptor internalization. This data indicates that 1) specific bi ndings sites for diadenosine polyphosphates exist in INS-1 cells and r at pancreatic islets mediating insulin release; 2) the receptors invol ved in INS-1 cells may be diadenosine polyphosphate receptors, albeit others, such as P-2x-receptors, cannot be ruled out; and 3) diadenosin e polyphosphates, and mainly their degradation products in the extrace llular space, are to a high degree accumulated within cells with unkno wn function. Thus, diadenosine polyphosphates are worth being investig ated more closely in physiological and pathophysiological terms.