INTRACELLULAR DIADENOSINE POLYPHOSPHATES - A NOVEL 2ND-MESSENGER IN STIMULUS-SECRETION COUPLING

In pancreatic beta-cells, stimulatory glucose concentrations increase cytosolic diadenosine polyphosphates ([Ap(n)A](i)) to concentrations s ufficient to block ATP-sensitive K+ (K-ATP) channels, High-performance liquid chromatography and patch clamp techniques were used to study t he metabolic pathways by which pancreatic beta-cells synthesize Ap(n)A and the mechanism through which Ap(n)A inhibit K-ATP channels, Ap(n)A show a glucose- and time-dependent cytosolic concentration increase p arallel, though 30-to 50-fold higher, to changes observed in adenine n ucleotides, Other fuel secretagogues, leucine and 2-ketoisocaproate, w aise [Ap(n)A](i) as efficiently as 22 mM glucose, Blockade of glycolys is or Krebs cycle decreases glucose-induced [Ap(n)A](i). No significan t increase in cytosolic Ap(n)A concentrations is induced by nonnutrien t secretagogues or nonmetabolizable nutrient secretagogues. Inorganic pyrophosphatase inhibition with sodium fluoride blocks 22 mM glucose-i nduced [Ap(n)A](i) increase. Ap(n)A inhibition of K-ATP channel resemb les that of ATP in efficacy, but shows clear functional differences, U nlike ATP, Ap(4)A does not restore channel activity after rundown. Fur thermore, these compounds do not compete with each other for the same site, These features suggest a prominent role for Ap(4)A in beta-cell function, comparable to ATP, We conclude that nutrient metabolism thro ugh pyrophosphatase activation is necessary to induce Ap(n)A synthesis , which in turn constitutes a new, ATP-independent, metabolic regulato r of K-ATP channel activity.