Abstract-Diadenosine polyphosphates have been characterized as extracellular mediators controlling numerous physiological effects. In this study, diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate were isolated and identified in human myocardial tissue. Human myocardial tissue was homogenized and fractionated by affinity chromatography, displacement chromatography, anion-exchange chromatography, and reversed-phase chromatography. In fractions purified to homogeneity, diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate were revealed by matrix-assisted laser desorption/ionization mass spectrometry and ultraviolet spectroscopy. These diadenosine polyphosphates were further identified by enzymatic analysis, which demonstrated an interconnection of the phosphate groups with the adenosines in the 5Ј positions of the riboses. Furthermore, diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate were found in human cardiac-specific granules, and the amount of diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate was estimated in the range of Ϸ500 mol/L. In conclusion, the experiments show that the diadenosine polyphosphates with 2 and 3 phosphate groups occur in human myocardial tissue, and so do the diadenosine polyphosphates with 4 to 6 phosphate groups. After being released by cholinergic stimulation, which is known to affect diadenosine polyphosphate release from secretory granules, diadenosine tetraphosphate, diadenosine pentaphosphate, and diadenosine hexaphosphate activate P2X purinoceptors in vascular smooth muscle; hence, they can act as vasoconstrictors. It may be inferred that the differential action of both predominantly vasodilator and vasoconstrictor diadenosine polyphosphates allow a fine-tuning of myocardial blood flow by locally released diadenosine polyphosphates. [1][2][3][4][5] The actions of the Ap n A within the cardiovascular system are mediated by the various purinoceptor subtypes. So far, 14 mammalian purinoceptor subtypes have been cloned, 6,7 and 6 Ap n As containing 2 to 7 phosphate groups have been identified in humans. 8 -11 The affinities of a given Ap n A to the various purinoceptor subtypes depends on the number of phosphate groups linking both adenosine moieties. 9,12,13 Moreover, the purinoceptor subtypes are very differently distributed within the cardiovascular system. Depending on the purinoceptor subtypes activated in a given tissue, the Ap n A are both vasoconstrictors and vasodilators, 14,15 inhibitors and stimulators of platelet aggregation, 8,9,11 and modulators of cell proliferation. 9,11,16 Given this diversity of Ap n A actions, it is not surprising that the Ap n A actions reported in literature widely differ among various species. Currently, it is difficult to decide to what extent species-dependent differences in Ap n A actions are caused by different purinoceptor distribution and to species-specificity of some of the known purinoceptor subtyp...