PROTEIN, NOT ADENOSINE OR ADENINE-NUCLEOTIDES, MEDIATES PLATELET DECREASE IN ENDOTHELIAL PERMEABILITY

Platelets and platelet-conditioned medium (PCM) decrease endothelial p rotein permeability in vitro. Adenosine and a >100-kDa protein have pr eviously been implicated as the soluble factors released from platelet s that decrease endothelial permeability. The objective of this study was to further investigate the role of adenosine in this platelet resp onse. Measurements of adenosine and its precursor adenine nucleotides by high-performance liquid chromatography were correlated with the ass essment of permeability by I-125-labeled albumin clearance and electri cal resistance across endothelial cell monolayers derived from the bov ine pulmonary artery. PCM contained micromolar concentrations of AMP, ADP, and ATP, but adenosine was below detectable levels (less than or equal to 0.1 mu M). Adenosine deaminase, an enzyme that converts adeno sine to inactive inosine, or an adenosine-receptor antagonist did not block the platelet- or PCM-mediated decrease in endothelial permeabili ty. A <3-kDa fraction of PCM that contained micromolar concentrations of AMP and ADP did not affect endothelial permeability, whereas a >3-k Da fraction that contained much reduced levels of AMP and ADP signific antly decreased permeability. This activity of PCM was sensitive to in soluble trypsin. This study rules out adenosine and adenine nucleotide s as primary factors in the platelet-induced decrease in endothelial p ermeability and suggests that the active factor is a protein.