Human platelet dense granules contain polyphosphate and are similar to acidocalcisomes of bacteria and unicellular eukaryotes

Inorganic polyphosphate (polyP) has been identified and measured in human platelets. Millimolar levels (in terms of P_i residues) of short chain polyP were found. The presence of polyP of ∼70-75 phosphate units was identified by ³¹P NMR and by urea-polyacrylamide gel electrophoresis of platelet extracts. An analysis of human platelet dense granules, purified using metrizamide gradient centrifugation, indicated that polyP was preferentially located in these organelles. This was confirmed by visualization of polyP in the dense granules using 4′,6-diamidino-2-phenylindole and by its release together with pyrophosphate and serotonin upon thrombin stimulation of intact platelets. Dense granules were also shown to contain large amounts of calcium and potassium and both bafilomycin A₁-sensitive ATPase and pyrophosphatase activities. In agreement with these results, when human platelets were loaded with the fluorescent calcium indicator Fura-2 acetoxymethyl ester to measure their intracellular Ca²⁺ concentration ([Ca²⁺]_i), they were shown to possess a significant amount of Ca²⁺ stored in an acidic compartment. This was indicated by the following: 1) the increase in [Ca²⁺]_i induced by nigericin, monensin, or the weak base, NH₄Cl, in the nominal absence of extracellular Ca² and 2) the effect of ionomycin, which could not take Ca²⁺ out of acidic organelles and was more effective after alkalinization of this compartment by the previous addition of nigericin, monensin, or NH₄Cl. All of these characteristics of the platelet dense granules, together with their known acidity and high density (both by weight and by electron microscopy), are similar to those of acidocalcisomes (volutin granules, polyP bodies) of bacteria and unicellular eukaryotes. The results suggest that acidocalcisomes have been conserved during evolution from bacteria to humans.