Background
Zinc (Zn
2+
) is an essential trace element that regulates intracellular processes in multiple cell types. While the role of Zn
2+
as a platelet agonist is known, its secondary messenger activity in platelets has not been demonstrated.
Objectives
This article determines whether cytosolic Zn
2+
concentrations ([Zn
2+
]
i
) change in platelets in response to agonist stimulation, in a manner consistent with a secondary messenger, and correlates the effects of [Zn
2+
]
i
changes on activation markers.
Methods
Changes in [Zn
2+
]
i
were quantified in Fluozin-3 (Fz-3)-loaded washed, human platelets using fluorometry. Increases in [Zn
2+
]
i
were modelled using Zn
2+
-specific chelators and ionophores. The influence of [Zn
2+
]
i
on platelet function was assessed using platelet aggregometry, flow cytometry and Western blotting.
Results
Increases of intra-platelet Fluozin-3 (Fz-3) fluorescence occurred in response to stimulation by cross-linked collagen-related peptide (CRP-XL) or U46619, consistent with a rise of [Zn
2+
]
i
. Fluoresence increases were blocked by Zn
2+
chelators and modulators of the platelet redox state, and were distinct from agonist-evoked [Ca
2+
]
i
signals. Stimulation of platelets with the Zn
2+
ionophores clioquinol (Cq) or pyrithione (Py) caused sustained increases of [Zn
2+
]
i
, resulting in myosin light chain phosphorylation, and cytoskeletal re-arrangements which were sensitive to cytochalasin-D treatment. Cq stimulation resulted in integrin α
IIb
β
3
activation and release of dense, but not α, granules. Furthermore, Zn
2+
-ionophores induced externalization of phosphatidylserine.
Conclusion
These data suggest that agonist-evoked fluctuations in intra-platelet Zn
2+
couple to functional responses, in a manner that is consistent with a role as a secondary messenger. Increased intra-platelet Zn
2+
regulates signalling processes, including shape change, α
IIb
β
3
up-regulation and dense granule release, in a redox-sensitive manner.