Rapid ADP-evoked currents in human platelets recorded with the nystatin permeabilized patch technique

Area of Science:

• Cellular physiology
• Ion channel biophysics
• Platelet function

Background:

• Human platelets play a crucial role in hemostasis and thrombosis.
• Agonist-induced calcium (Ca2+) influx is a key event in platelet activation.
• The specific ion channels responsible for early Ca2+ entry remain incompletely understood.

Purpose of the Study:

• To investigate the ion channels involved in agonist-evoked calcium influx in human platelets.
• To characterize the properties of an ADP-activated ion channel.
• To compare the mechanisms of calcium entry induced by ADP and thrombin.

Main Methods:

• Whole-cell patch-clamp recordings from single human platelets.
• Application of agonists (ADP, thrombin) using a "puffer" pipette.
• Ion substitution experiments to determine channel permeability.
• Fura-2 fluorescence measurements to monitor intracellular Ca2+ changes.

Main Results:

• Adenosine diphosphate (ADP) activated a transient inward current with rapid kinetics (tens of milliseconds).
• The ADP-evoked channel is permeable to Na+, K+, and Ba2+, but not Cl-, with a single channel conductance of 11-15 pS.
• Thrombin did not elicit detectable currents under identical conditions, suggesting a different Ca2+ entry mechanism.

Conclusions:

• A novel, rapidly activating ion channel permeable to cations mediates ADP-induced Ca2+ influx in human platelets.
• Early thrombin-evoked Ca2+ entry likely occurs through small conductance channels or an electroneutral pathway, distinct from the ADP-activated channel.
• The characterized ADP-evoked channel possesses the speed necessary to explain rapid Ca2+ influx observed in platelet activation studies.