EFFECT OF HEMATOCRIT ON ADENOSINE-DIPHOSPHATE INDUCED AGGREGATION OF HUMAN PLATELETS IN TUBE FLOW

Both chemical and physical effects of red cells are known to play a ro le in the adenosine diphosphate (ADP)-induced aggregation of human pla telets in sheared blood. Using a previously described double infusion technique (Bell et al., 1989a), we studied the effect of increasing he matocrit from 10 to 60% on the rate and extent of platelet aggregation with 0.2 mu M ADP in citrated whole blood undergoing tube flow. Blood and agonist were rapidly mixed in a small chamber and the suspensions flowed through lengths of 1.19 mm-diameter polyethylene tubing at mea n transit times < t > from 0.2 to 42.8 s at a mean tube shear rate < G > = 335 s(-1). Effluent was collected into 0.5% glutaraldehyde, the r ed cells removed by centrifugation through Percoll, and all single pla telets and aggregates in the volume range 1-10(5) mu m(3) counted and sized using an aperture impedance counter. Both the initial rate (over the first 8.6 s) and the extent of aggregation with time increased wi th increasing mean hematocrit up to 35.8%, being significantly greater than in citrated plasma (cPRP). However, at 61.5% hematocrit, the ext ent of aggregation decreased markedly to a level close to that in cPRP . We also studied the effect of washed red cells at 39% hematocrit on the aggregation of washed platelets: in Tyrodes-albumin fibrinogen-fre e suspensions. It had previously been shown that, at < G > greater tha n or equal to 335 s(-1), washed platelets in platelet-rich Tyrodes (PR T) aggregated with 0.7 mu M ADP. We found that red cells markedly incr eased the extent of aggegation from that in PRT, and promoted the form ation of large aggregates, absent in PRT. Spontaneous aggregation in w hole blood or washed cell suspensions in the absence of added ADP at < t > = 42.8 s was < 10% of that in the presence of ADP. The results in dicate that a physical effect of red cells, likely manifested as an in crease in the efficiency of aggregate formation (Goldsmith et al., 199 5), plays an important role at low and normal hematocrits; however, at high hematocrits, particle crowding impedes the formation of aggregat es.