5-Methyltryptamine decreases net accumulation of ³²P into the polyphosphoinositides from [γ-³²P]ATP in a cell-free system from blowfly salivary glands. Activation of breakdown of the newly synthesized [³²P]polyphosphoinositides

Incubation of blowfly salivary gland homogenates with 30 μM [γ-³²P]ATP resulted in a rapid, Mg²⁺-dependent, synthesis of [³²P]polyphosphoinositides and [³²P]phosphatidic acid. 5-Methyltryptamine, in the presence of 10 μM guanosine 5'-(3-O-thio)trisphosphate, reduced the net accumulation of ³²P label into phosphatidylinositol-4,5-P₂ and phosphatidylinositol-4-P by 35 and 20%, respectively. 5-Methyltryptamine did not affect synthesis of [³²P]phosphatidic acid. Phosphorylation of polyphosphoinositides was not affected by 5-methyltryptamine. In membranes labeled in vitro with [γ-³²P]ATP, 5-methyltryptamine stimulated a rapid breakdown of the [³²P]polyphosphoinositides. These results indicate that in blowfly salivary gland homogenates, hormone stimulates breakdown of the newly synthesized polyphosphoinositides. In the presence of hormone, the rate of polyphosphoinositide synthesis does not compensate for the rate of polyphosphoinositide degradation.