The novel marine natural product plocamadiene A causes histamine release from mast cells of the guinea-pig and rat in vitro.

1. Plocamadiene A is a polyhalogenated monoterpene, (1R, 2S, 4S, 1'E)-2-bromo-1-chloro-4-(2'-chloroethenyl)-1-methyl-5- methylenecyclohexane, isolated from red marine algae of the Plocamium genus. 2. This study examined the activity of plocamadiene A on guinea-pig isolated ileum (GPI) and subsequently on rat isolated peritoneal mast cells. 3. Plocamadiene A (1 micrograms/mL) produced a slow onset, sustained contraction of the GPI. This was insensitive to atropine (1 mumol/L) and tetrodotoxin (1 mumol/L), but was significantly reduced by H1-histamine receptor antagonists including mepyramine (10 nmol/L), chlorcyclizine (10 nmol/L) and diphenhydramine (0.1 mumol/L). The H2-histamine receptor antagonists cimetidine (0.1 mumol/L) and ranitidine (10 nmol/L) potentiated the response to plocamadiene A (1 micrograms/mL). 4. The amplitude of contraction caused by plocamadiene A (1 micrograms/mL) gradually decreased when the compound was applied repeatedly to the GPI for 5 min every 10 min for 150 min. 5. Contractions to plocamadiene A (1 micrograms/mL) were reduced to approximately 66% of the time control in ovalbumin-sensitized GPI challenged with ovalbumin to release endogenous contractile agents. 6. Cromolyn (0.1 mmol/L) inhibited (by 40%) the response in the GPI caused by plocamadiene A (1 micrograms/mL) as compared with time controls. 7. Spectrofluorometric assay suggested that both plocamadiene A (10 micrograms/mL) and compound 48/80 (10 micrograms/mL) caused histamine release from rat peritoneal mast cells in vitro. This release of histamine was reduced by cromolyn (100 micrograms/mL). 8. This study concluded that plocamadiene A releases histamine from mast cells of the GPI and peritoneal mast cells in the rat.