FORMATION OF LACTOSE-RESISTANT AGGREGATES OF HUMAN PLATELETS INDUCED BY THE MISTLETOE LECTIN AND DIFFERENTIAL SIGNALING RESPONSES TO CELL CONTACT FORMATION BY THE LECTIN OR THROMBIN

Human platelets afford a suitable and physiologically relevant model t o study receptor-dependent cell aggregation and ensuing biosignaling r eactions. Since cell surface glycoconjugates can serve as ligands in r ecognitive protein-carbohydrate interactions, it is of interest to inv estigate the reactivity of such epitopes for a plant lectin and the el icited intracellular responses. Therefore, the galactose-specific lect in (Viscum album agglutinin, VAA) was employed as a tool for this purp ose. It was found that VAA induced platelet aggregation at a concentra tion of 2.5 mu g/ml using 2.5.10(8) cells/ml, composed of the formatio n of both lactose-sensitive (Lac(+)) and lactose-resistant (Lac(-)) in tercellular contacts. Lac aggregates were formed only by metabolically active platelets of about 70% of the samples from the group of studie d volunteers. The requirement of metabolic activity for formation of t hese contacts which no longer depend on lectin-ligand recognition was underscored by the lack of their appearance in the presence of metabol ic inhibitors such as nordihydroguaiaretic acid, trifluoperazine, N-et hylmaleimide and menadione. With respect to biosignaling, the effectiv e aggregation of platelets did not affect the basal level of Ca2+ in c ells and reduced the rate of the menadione-dependent generation of H2O 2. In parallel series platelet aggregation induced by bovine thrombin (0.03 U/ml) triggered an increase in the cytoplasmic Ca2+ level and an enhancement of the H2O2 generation. Overall, these results imply meta bolically controlled post-binding reactions which strengthen the lecti n-induced cell association and demonstrate differential responses with respect to the Ca2+ level and H2O2-generation between lectin- or thro mbin-mediated aggregation of human platelets.