Inorganic
polyphosphate (
polyP) has been identified and measured in human platelets. Millimolar levels (in terms of Pi residues) of short chain
polyP were found. The presence of
polyP of approximately 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.