Human platelets contain a pool of free zinc in dense granules.

BACKGROUND: Activated platelets release procoagulant factors that include Ca2+ and Zn2+ . Releasable Ca2+ stores have been identified in platelet dense granules and the dense tubular system, but similar stores of free Zn2+ have not been identified.

OBJECTIVES: Guided by studies of platelet Ca2+ , we employed minimally disruptive methods to identify and localize concentrated free Zn2+ in human platelets.

METHODS: Resting platelets from normal donors (NDs), patients with gray platelet syndrome (GPS) lacking α-granules, and patients with Hermansky-Pudlak syndrome (HPS) deficient in dense granules were loaded with cell-permeant fluorescent probes specific to free Ca2+ or Zn2+ . Ion concentrations were detected in fixed cells as bright puncta via high-resolution confocal microscopy. Ions were also directly detected via transmission electron microscopy and energy dispersive X-ray analysis. Levels of total platelet Ca, Zn, and Mg were measured via inductively coupled plasma optical emission spectroscopy.

RESULTS: Fluorescent Zn2+ puncta counts were similar in ND and GPS platelets and markedly lower in HPS platelets, pointing to dense granules as likely reservoirs of free Zn2+ . This localization was supported by direct detection of Ca2+ , Zn2+ , and Na+ in platelet dense granules via transmission electron microscopy and energy dispersive X-ray analysis. Measurements of total platelet Ca, Zn, and Mg via inductively coupled plasma optical emission spectroscopy indicated that free Zn2+ represents a small proportion of total platelet zinc, consistent with the strong affinity of Zn2+ for binding proteins, including several abundant in platelet α-granules.

CONCLUSION: We conclude that normal human platelets contain a pool of free Zn2+ concentrated in dense granules that is available for secretion upon platelet activation and potentially contributes to hemostasis.