Calcium mobilization in density subpopulations of rabbit washed platelets

To estimate the responsiveness of an individual platelet, rather than a group of platelets, we determined the intracellular calcium concentration ([Ca2+]i) of single platelets using digital imaging microscopy after density fractionation. The mean [Ca2+]i of non-fractionated resting single platelets was 74.0 +/- 4.2 nM (n = 71). Immediately after thrombin stimulation (100 mu/ml), [Ca2+]i increased markedly in the entire cytoplasm, reaching maximum levels of 400-1000 nM, and decreased slightly thereafter. These maximum levels varied markedly from cell to cell in non-fractionated platelets. Although there was no significant difference in the resting levels of [Ca2+]i of single platelets even after density fractionation, the peak [Ca2+]i levels after thrombin stimulation (10 and 100 mu/ml) in platelets with higher density ( > 1.062) were significantly higher than those in platelets with intermediate density (1.052-1.061) or lower density ( < 1.051). This higher responsiveness of higher-density platelets in calcium mobilization was accompanied by higher aggregability and higher reactivity in the release reaction. These results suggest that higher responsiveness in higher-density platelets may be explained by the higher reactivity in [Ca2+]i mobilization.