Relationship between phosphatidylinositol synthesis and recovery of 5-hydroxytryptamine-responsive Ca2+ flux in blowfly salivary glands

Abstract

Each salivary gland contains about 135 pmol of phosphatidylinositol. In glands prelabeled by incubation for 1 h with [32P]Pi or [3H]inositol there was a subsequent breakdown of 80% of the labeled phosphatidylinositol over a 2 h incubation period with 10 .mu.M-5-hydroxytryptamine. There was no detectable decrease either in total phosphatidylinositol based on P analysis by chemical estimation or in the radioactivity of [32P]phosphatidylinositol in salivary glands of flies raised from the larval stage on diets containing[32P]Pi and those phospholipids were uniformly labelled. These results suggest that the pool of phosphatidylinositol involved with Ca2+ gating is a small fraction of the total phosphatidylinositol content. It is this small compartment that is preferentially radioactively labeled during short-term incubations with radioactively labeled precursors. In salivary glands incubated for 2 h with 10 .mu.M-5-hydroxytryptamine there was a marked decrease in the flux of 45Ca2+ across the gland. After removal of the hormone, incubation of salivary glands for 1 h in the presence of 2 mM-inositol, but not choline or ethanolamine, resulted in a recovery of hormone-responsive 45Ca2+ flux. Quantitative studies revealed that < 9 pmol of phosphatidylinositol must be formed to fully restore the 5-hydroxytryptamine-responsive 45Ca2+ flux.