Thrombocytosis and platelet aggregates in the circulation of adjuvant arthritic rats

Abstract

A marked increase in platelet count was observed in adjuvant-induced polyarthritic rats. Elevation of platelet count was still significant 3 months after onset of disease. Increased number and larger size of platelets reflect an increase in the rate of platelet production. By means of the platelet-count ratio technique, circulating platelet aggregates were demonstrated in rats with severe arthritis. Aspirin treatment prevented thrombocytosis and formation of platelet aggregates.Elevated platelet count is encountered in a number of clinical conditions. A platelet count in excess of normal may occur in association with myeloproliferative disorders or secondary to certain underlying diseases. Among the latter a recent study has shown that thrombocytosis occurs in rheumatoid arthritis ( I ) . In our previous studies (2,3) platelet function abnormalities (decreased bleeding time, increased aggregation caused by various stimuli, and increased susceptibility to formation of thrombus) were estimated in adjuvant arthritic rats. The polyarthritic syndrome induced in rats by the administration of Freund's adjuvant is the most common model for testing therapeutic substances. In contrast to the absence of rheumatoid factors, adjuvant arthritis as a cell-mediated immune reaction is at present the nearest experimental approximation to human rheumatoid arthritis (43).The possibility that the elevated platelet count may be associated with a predisposition to the thrombotic state must be considered. It therefore seemed desirable to investigate the platelet count in established adjuvant arthritis. The observed thrombocytosis and the detection of circulating platelet aggregates in the blood of arthritic rats are considered significant factors in the time-course and vascular complications of this animal disease. METHODSAdjuvant-Induced Polyarthritis in the Rat. Male, Sprague-Dawley (CFY) rats weighing 150-160 grams at the start of the experiment were used. Adjuvant (suspension of 0.5 mg heat-killed Mycobacterium tuberculosis in 0.1 ml paraffinoil) was injected intradermally into the base of the tail. The severity of the arthritis in each paw was subjectively assessed in units of 0.5 on a scale from 0 to 4. Maximal score for an individual rat was 16. Severely arthritic rats were defined as the score/rat > 12.Platelet Count. Platelet count was determined from the arterial blood by the sedimentation method of Bull et al. (6). Four milliliters of blood were drawn into a syringe containing 50 U of heparin, transferred to a tube containing 20 mg of solid EDTA and 0.05 ml of 40% polyvinyl-pyrrolidone, and shaken thoroughly. After standing for 1 hour a 10 &aliquot of platelet-rich plasma (PRP) was diluted with 5 ml of 10%