Trifluoperazine inhibits phosphoinositide hydrolysis as a histamine H1-receptor antagonist

In human astrocytoma cells (1321N1), trifluoperazine potently inhibited histamine-induced phosphoinositide hydrolysis, while it slightly inhibited carbachol-induced phosphoinositide hydrolysis. Trifluoperazine as well as diphenhydramine inhibited [3H]mepyramine binding in astrocytoma cell membranes with Ki values of 0.052 microM and 0.005 microM, respectively. However, trifluoperazine only slightly inhibited [3H]quinuclidinyl benzilate binding with a Ki value of 3 microM. These results indicate that trifluoperazine specifically inhibits histamine-induced phosphoinositide hydrolysis as a histamine H1-receptor antagonist in human astrocytoma cells.