Ca-dependent and Ca-independent histamine release from mast cells induced by active components of compound 48/80

Compound 48/80 and 14C-labelled compound 48/80 were synthesized, and fractionated by thin-layer chromatography into 14 components with various histamine-releasing activities and different Ca++ requirements for their actions. The histamine release induced from rat mast cells in vitro by the most active component, fraction D (molecular weight = 2280, a tridecamer composed of 13 monomer units), was greatly elevated by extracellular Ca++, and was partially reduced by pretreatment of the cells with dinitrophenylated Ascaris antiserum, an IgE. In contrast, the histamine release induced by fraction H (molecular weight = 1580, a nonamer composed of 9 monomer units), was higher in Ca-free medium than in Ca-containing medium, and partially suppressed by pretreatment of mast cells with neurotensin or substance P, both Ca-independent releasers. The binding potencies of the 14C-labelled components estimated at 4 degrees C in the presence of Ca++, where no degranulation of the cells occurs, generally paralleled their histamine-releasing activities. However, Ca++ was inhibitory for the binding of 14C-fraction H. The binding of fraction D to [3H]arachidonic-acid-preloaded mast cells induced a rapid accumulation of the labelled arachidonic acid into phosphatidic acid, phosphatidylinositol and phosphatidylcholine, with concomitant decrease of the labelled arachidonic acid from phosphatidylethanolamine prior to the detectable histamine release.