We previously found evidence (based on the use of 5HT as a marker) that i.v. injection of a lipopolysaccharide (LPS) into mice induces a rapid accumulation of platelets in liver and lung. Our previous studies lacked measurement of the platelet count itself, but we have now compared the LPS-induced changes in 5HT levels with the change in platelet count. We also examined the effects on the platelet response of some drugs that act on platelets. In mice, sublethal doses of LPS induced parallel decreases in platelets and 5HT in the blood. The 5HT lost from the blood accounted well for the 5HT accumulated in liver and lung. Soon after this accumulation, the levels of platelets and 5HT in the blood recovered in parallel, and these recoveries corresponded well with the decreases in 5HT occurring in liver and lung. Aspirin and dexamethasone were effective at both reducing pulmonary platelet-accumulation and promoting their return to the circulation. By contrast, oestrogen tended to reduce the return of platelets from lung to circulation. Heparin did not inhibit pulmonary platelet-accumulation but it did decrease their return to the circulation. These results suggest that (i) in response to sublethal doses of LPS, platelets translocate into the liver and lung, then return to the circulation; (ii) this platelet response involves mechanisms that can be modified by drugs; and (iii) the use of this platelet response as a tool for drug evaluation might help identify new drugs with therapeutic potential.
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