Effect of Repeated Treatment of Rabbit Platelets with Low Concentrations of Thrombin on their Function, Metabolism and Survival

S ummary . Low concentrations of thrombin (< 0.05 u/ml) cause reversible aggregation of washed rabbit platelets in Tyrode‐albumin solution containing apyrase. The same platelets were aggregated several times by addition of the same concentration of thrombin with washing and resuspension after each aggregation step. Deaggregation of rabbit platelets aggregated by thrombin was enhanced by the addition of prostaglandin E 1 . In three successive aggregations, rabbit platelets prelabelled with [ 3 H]serotonin released approximately 70% of their radioactivity. Platelet granule nucleotides were labelled by in vivo administration of 32 P‐labelled orthophosphate: repeated thrombin stimulation caused release of 60–65% of the labelled nucleotides. The release of 3 H from [ 3 H]serotonin labelled platelets paralleled the release of 32 P‐labelled adenine nucleotides and magnesium. The concentration of platelet calcium did not change significantly during repeated thrombin stimulation. Electron microscopy showed nearly complete degranulation of platelets after three thrombin treatments. The thrice thrombin‐treated platelets were more sensitive to ADP‐induced aggregation than the control platelets. They did not aggregate upon the addition of low concentrations of thrombin but showed the same extent of aggregation as the control platelets when they were exposed to high concentrations of thrombin. Rabbit platelets labelled in vitro by incubation with [ 14 C]adenosine showed a decrease in their metabolic nucleotides ([ 14 C]ATP, [ 14 C]ADP)as a result of repeated thrombin treatment. These platelets showed increased glucose uptake and increased lactate production. The thrombin‐treated platelets (labelled with [ 3 H]DFP in vitro ) survived in the circulation of rabbits for the same time as control platelets.