Bradykinin and phorbol ester but not 5-HT2B receptor activation stimulate phospholipase D activity in the rat stomach fundus

1. Serotonin has been implicated as a mediator involved in migraine headach e, an effect that may involve central 5-HT2B receptor activation. 5-HT2B re ceptor signal transduction is controversial. 2. Rat stomach fundus contraction to serotonin has been used as a model for 5-HT2B receptor activation. Serotonin-induced contractility involves intra cellular calcium release and activation of protein kinase C without stimula tion of phosphoinositide (PI) hydrolysis. 3. Since phospholipase D (PLD) activation results in phosphatidic acid prod uction, which can release intracellular calcium and provide diacylglycerol for PKC activation, the purpose of this study was to determine whether the 5-HT2B receptor coupled to PLD activation using the rat stomach fundus as a model system. 4. Using phosphatidylethanol production to measure PLD activity, both brady kinin (0.01-1 mu M) and phorbol dibutyrate (PDBu, 1 mu M) stimulated PLD ac tivity in rat stomach fundal strips, indicating that this tissue possesses an active PLD system. 5. Under identical conditions, 5-hydroxytryptamine (5-HT) failed to stimula te PLD activity over a concentration range (1 nM-1 mu M) documented to indu ce 5-HT2B receptor-mediated contraction in rat stomach fundus. Thus, the 5- HT2B contractile receptor in rat stomach fundus is not coupled to PLD activ ation, whereas both bradykinin and phorbol ester do couple to PLD.