Constituents in watercress: Inhibitors of histamine release from RBL-2H3 cells induced by antigen stimulation

Histamine release inhibitors in watercress (Nasturtium officinale) were iso lated using a monitoring system with antigen-stimulated RBL-2H3 cells. Of t he 15 compounds isolated, flavonols and megastigmanes significantly inhibit ed histamine release. Two flavonols, 3-O-sophorosides of rhamnetin and rham nazin, were new compounds, To investigate the inhibitory mechanism, the eff ects of rhamnetin, rhamnetin 3-O-sophoroside and an isolated megastigmane g lucoside on the increase in the intracellular free calcium concentration we re examined at a concentration providing 60% inhibition of histamine releas e. The results suggest that these compounds did not affect the calcium infl ux at that concentration. The structure-activity relationships of the megas tigmanes on histamine release were also investigated.