Platelet aggregation may not be a prerequisite for collagen-stimulated platelet generation of nitric oxide

By determining the sum of the supernatant concentrations of nitrite and nit rate the stimulated generation of nitric oxide (NO) by human washed platele ts induced by a range of fibrillar collagen concentrations (0.0156-25 mu g ml(-1)) was investigated. Platelet serotonin (5-hydroxytryptamine, 5-HT) ef flux and platelet aggregation were also measured. Under resting conditions (0 mu g ml(-1) collagen) platelet NO release was equivalent to 1.06 +/- 0.1 7 nmol per 10(8) platelets. Maximal NO release. equivalent to 2.1 +/- 0.37 nmol per 10(8) platelets, was observed with only 0.0625 pg ml(-1) collagen (P < 0.02, stimulated vs. resting release), higher collagen concentrations producing no further increases in platelet NO output. By contrast, maximal platelet aggregation and 5-HT efflux did not occur until collagen concentra tions of 2.5 mu g ml-(1) and 10-25 mu g ml(-1) respectively, had been achie ved. L-NAME (1 mmol 1(-1)) and L-NMMA (1 mmol 1(-1)) inhibited stimulated p latelet NO generation by 78 +/- 6% and 72%, respectively. Contrasting with fibrillar collagen, fibrillar beta-amyloid protein had no effect on platele t NO generation, or on 5-HT efflux or aggregation. These data perhaps indic ate that NO generation by human platelets is stimulated by concentrations o f fibrillar collagen insufficient to elicit an aggregatory response. Such a mechanism could operate in vivo to inhibit platelet aggregation which migh t otherwise be induced by low concentrations of circulating agonists. (C) 1 999 Elsevier Science B.V. All rights reserved.