Summary
Aggregation of cat platelets in the citrated plasma is examined by means of Born’s
absorptiometer. A marked tendency of the platelets of this species to spontaneous
aggregation necessitated first of all the development of an improved technique of
blood collection.
A hypothesis according to which 5-HT is released from the platelets, explains the
absence of oscillations on the base line of the absorptiometer, the absence of platelet
swelling, when ADP is added, and the effect of stirring on the aggregation curves
in cat PRP. The average volume of cat platelets amounts to 10.46 μ3 when directly fixed in the blood, when fixed from PRP to 12.17 μ3, when fixed from stirred PRP to 13.51 μ3.
In low concentrations (0.3-2 μM) ADP produce reversible aggregation; in narrowly restricted,
individually dissimilar mean concentrations irreversible aggregation in two phases
and in high concentrations, irreversible aggregation in one phase. Like ADP serotonin
produces 2 phase irreversible aggregation in concentrations of 3-10 μM, but unlike
ADP, the aggregation velocity decreases again with high 5-HT concentrations (>100
μM). Adrenaline does not produce aggregation and it is likely that adenosine and adenosine
monophosphate inhibit the aggregation by serotonin but not by ADP. Species differences
in the aggregation of human, rabbit and cat platelets are discussed.