Intracellular diadenosine polyphosphates:: a novel second messenger in stimulus-secretion coupling

In pancreatic beta-cells, stimulatory glucose concentrations increase cytosolic diadenosine polyphosphates ([Ap(n)A](i)) to concentrations sufficient to block ATP-sensitive K+ (K-ATP) channels, High-performance liquid chromatography and patch clamp techniques were used to study the 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 parallel, though 30-to 50-fold higher, to changes observed in adenine nucleotides, Other fuel secretagogues, leucine and 2-ketoisocaproate, waise [Ap(n)A](i) as efficiently as 22 mM glucose, Blockade of glycolysis or Krebs cycle decreases glucose-induced [Ap(n)A](i). No significant increase in cytosolic Ap(n)A concentrations is induced by nonnutrient secretagogues or nonmetabolizable nutrient secretagogues. Inorganic pyrophosphatase inhibition with sodium fluoride blocks 22 mM glucose-induced [Ap(n)A](i) increase. Ap(n)A inhibition of K-ATP channel resembles that of ATP in efficacy, but shows clear functional differences, Unlike ATP, Ap(4)A does not restore channel activity after rundown. Furthermore, 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 through pyrophosphatase activation is necessary to induce Ap(n)A synthesis, which in turn constitutes a new, ATP-independent, metabolic regulator of K-ATP channel activity.