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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.

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