Diadenosine pentaphosphate (AP5A) is a diadenosine polyphosphate. Diadenosine polyphosphates (APnAs, n = 3-6) are a family of endogenous vasoactive purine dinucleotides which have been isolated from thrombocytes. APnAs have been demonstrated to be involved in the control of vascular tone as well as the growth of vascular smooth muscle cells and hence, possibly, in atherogenesis. APnAs isolated substances are Ap3A, Ap4A, Ap5A, and Ap6A. APnAs are naturally occurring substances that facilitate tear secretion; they are released from the corneal epithelium, they stimulate tear production and therefore they may be considered as physiological modulators of tear secretion. The APnAs were discovered in the mid-sixties in the course of studies on aminoacyl-tRNA synthetases (aaRS). APnAs have emerged as intracellular and extracellular signalling molecules implicated in the maintenance and regulation of vital cellular functions and become considered as second messengers. Great variety of physiological and pathological effects in mammalian cells was found to be associated with alterations of APnAs. APnAs are polyphosphated nucleotidic substances which are found in the CNS and are known to be released in a calcium-dependent manner from storage vesicles in brain synaptosomes. AP5A is a specific adenylate kinase inhibitor in the hippocampus, decreasing the rate of decomposition of ADP and the formation of ATP; a pathway that influences the availability of purines in the central nervous system. AP5A in nanomolar concentrations is found to significantly stimulate the proliferation of vascular smooth muscle cells. AP5A is a P2X agonist. The activation of nucleotide ion tropic receptors increases intracellular calcium concentration, resulting in calcium/calmodulin-dependent protein kinase II (CaMKII) activation. AP5A is an avid inhibitor of eosinophil-derived neurotoxin (EDN). EDN is a catalytically proficient member of the pancreatic ribonuclease superfamily secreted along with other eosinophil granule proteins during innate host defense responses and various eosinophil-related inflammatory and allergic diseases. The ribonucleolytic activity of EDN is central to its antiviral and neurotoxic activities and possibly to other facets of its biological activity. AP5A have been identified in human platelets and shown to be important modulator of cardiovascular function. AP5A is stored in synaptic vesicles and released upon nerve terminal depolarization. At the extracellular level, AP5A can stimulate presynaptic dinucleotide receptors. Responses to AP5A have been described in isolated synaptic terminals (synaptosomes) from several brain areas in different animal species, including man. Dinucleotide receptors are ligand-operated ion channels that allow the influx of cations into the terminals. These cations reach a threshold for N- and P/Q-type voltage-dependent calcium channels, which become activated. The activation of the dinucleotide receptor together with the activation of these calcium channels triggers the release of neurotransmitters. The ability of Ap5A to promote glutamate, GABA or acetylcholine release has been described. (PMID: 11212966, 12738682, 11810214, 9607303, 8922753, 10094777, 16401072, 16819989, 17721817, 17361116, 14502438).