Diadenosine Polyphosphates from Neuro-Secretory Granules: The Search for Receptors, Signals and Function

Abstract

The diadenosine polyphosphates -ApnA- are stored in secretory granules together with ATP and aminergic compounds: with serotonin in the dense granules of platelets, with acetylcholine in torpedo synaptic vesicles and with adrenaline and noradrenaline in chromaffin granules of adrenal medulla. The vesicular content in neural tissues is released to the extracellular media in a Ca2+ dependent way and in equimolecular ratios with respect to the other costored compounds. Cultured bovine neurochromaffin cells, torpedo synaptosomes and rat brain synaptic terminals, show the existence of specific binding sites for diadenosine polyphosphates. Two high affinity binding sites are found in these neural models with Kd values ranging from 0.1 to 0.7 nM for the first site and 5 to 6 nM for the second binding site. Displacement studies with P2-purinoceptor ligands present a particular potency order, suggesting the presence of a new receptor subtype in rat brain synaptic terminals, designated as P2d- The diadenosine polyphosphate receptors in neurochromaffin cells and vascular endothelial cells from adrenal medulla are coupled to Ca2+ release from internal stores and stimulation of Protein Kinase C. These intracellular signals cause the inhibition of catecholamine secretion from bovine chromaffin cells and induce a negative feedback for excitation in rat hippocampus. Both Ap4A and Ap5A evoke a fast rise in the [Ca2+]i in endothelial cells from bovine adrenal medulla, the effect is concentration dependent, and the EC50 is in the range 1–10 μM. The extracellular destruction of adenine dinucleotides is carried out by an ectodinucleoside polyphosphate hydrolase with high affinity, the Km values being close to 2 μM for the different diadenosine polyphosphates. The adenine mononucleotides produced are degraded to adenosine by the ecto-nucleotidases cascade. This nucleoside can be considered as the last extracellular product of purinergic transmission.