Formation and Destruction of Platelet Aggregates in Viscometric Flow
Even after pseudopodia formation platelets - unlike all other known formed blood elements - remain dispersed in stasis and creeping flow and become aggregated only in the presence of a minimum amount of shearing. The “rheoaggregometer” (Rieger et al., Pflüger’s Archiv, 343, R 33, 1973) allows to measure the minimum shear rates necessary for platelet aggregation (PA), as well as the initial rate and the maximum extent of PA in citrated PRP.PA is quantified photometrically as a function of variable shear rates. The initial rate of PA steadily increases with increasing shear rates up to 460 sec-1. However, the maximal extent of PA (indicating the mechanical integrity of formed aggregates) saturates at about 35 sec-1 and then decreases because of a destruction of formed aggregates and of prevention of further PA. The aggregability of the platelets, as reflected by various degrees of shape changes, is enhanced by a drop of temperature and a rise in pH as well as by the so called aggregating agents (e.g. epinephrine 10-6 up to 10-9 M/l) : consecutively lower shear rates (lower effects of collision) are necessary to induce PA. In citrated PRP stable platelet aggregates are produced only within a defined range of shear rates. Platelet aggregability and aggregate stability are independent variables influenced by different experimental conditions.