Platelet adhesion and release: interfacial concentration of released materials.

Platelet deposition on collagen-coated glass and release from adherent platelets were studied in tube flow. Shear rates between 80 and 320 s-1 were evaluated by use of washed porcine platelets radiolabeled with 51Cr and [3H]serotonin in Tyrode solution containing albumin, apyrase, and red blood cells. Cell accumulation was highest at the tube's inlet and was directly proportional to exposure time for all positions up to 20 cm from the entrance and for times up to 800 s. Release was lowest at the inlet, 60%, and increased to 90% downstream. The accumulation and release information served as input to a calculation that yielded ADP surface concentration values at the tube's surface. These ranged from 0.6 microM at 80 s-1 to 1.8 microM at 320 s-1. In a turbidimetric device, ADP concentrations of 0.5, 1.0, and 5.0 microM were required to induce shape change, 10 and 80% aggregation, respectively. Aggregation was also obtained with subthreshold ADP and collagen in combination. We surmise that platelet shape change could have occurred adjacent to the surface in preparation for adhesion, aggregation, and release at the surface, and that ADP and collagen acted synergistically.