Relationship between platelet density and platelet aggregation, ATP release, and cytosolic-free calcium mobilization...

To study the relationship of platelet density and platelet functions, including aggregation, ATP release, and intracellular calcium mobilization.

Platelet density was evaluated by discontinuous gradients of Percoll and cytosolic-free calcium concentration in single platelets was measured by fluorescence and imaging.

Rabbit platelets were assorted into 3 subpopulations using discontinuous gradients of Percoll: high density (HD, > 1.062, 20%-30%), intermediate density (ID, around 1.057, 40%-50%) and, low density (LD, < 1.051, 10%-15%). The sizes of platelets in these subpopulations were widely different (HD: 725 +/- 76; ID: 443 +/- 47; and LD: 307 +/- 46) and well correlated with the density (r = 0.988, P < 0.01). Thrombin 0.5 U-induced aggregations in the 3 subpopulations were 78 +/- 8%, 69 +/- 4%, and 62 +/- 3%, respectively and a difference was found between HD and LD (P < 0.05). The amounts of ATP released during aggregation were 4.7 +/- 0.9, 3.4 +/- 1.0, and 2.6 +/- 0.6 mumol/4 x 10(5) platelets in HD, ID, and LD groups, respectively. Thrombin 150 U-induced [Ca2+]i mobilization in HD (990 +/- 130 nmol L-1) was different (P < 0.01) from those in ID (410 +/- 80 nmol L-1) and LD (400 +/- 40 nmol L-1). Serotonin (5-HT) 3 mumol L-1 also caused [Ca2+]i mobilization in single-washed platelet subpopulations and results were similar to those of thrombin although it did not induce any detectable aggregation or release reaction.

Platelet functions and intracellular calcium concentraotion were increased with platelet density in rabbits.