EFFECTS OF CONCANAVALIN A ON 5-HYDROXYTRYPTAMINE UPTAKE BY RABBIT-BLOOD PLATELETS AND ON THEIR ULTRASTRUCTURE

Effects of concanavalin A (Con A) and other lectins on 5‐hydroxytryptamine (5‐HT) uptake by rabbit blood platelets and on their ultrastructure were studied. Uptake of [3H]‐5‐HT by platelets was decreased by application of Con A, E‐PHA (lectin from Phaseolus vulgaris) and lentil‐PHA (lectin from Lens culinaris), but not by wheat germ agglutinin (WGA). Con A induced specific changes in the ultrastructure of platelets, causing (i) a change in external appearance from a discoid to an irregularly spherical shape, (ii) re‐arrangement of the canalicular system and formation of a concentric structure. These effects of Con A on platelets were antagonized by pretreatment with α‐methyl‐D‐mannoside (a‐MM), a specific inhibitor of Con A binding to glycoprotein. The inhibition of 5‐HT uptake by Con A was antagonized by colchicine, vinblastine and sodium nitroprusside (SNP), but not by cytochalasin B. Theophylline, papaverine and dibutyryl cyclic adenosine 3′,5′‐monophosphate (db cyclic AMP) antagonized the effect of Con A on 5‐HT uptake, but dibutyryl cyclic guanosine 3′,5′‐monophosphate had no effect. Theophylline and db cyclic AMP did not influence the effect of Con A on the ultrastructure of platelets. It is suggested that binding of Con A to specific receptor glycoproteins can inhibit the 5‐HT uptake system of platelets. Microtubules, contractile protein and the membrane adenylate cyclase system of platelets may also be regulatory factors in this mechanism. 1979 British Pharmacological Society