5-HT-stimulated arachidonic acid release from labeled phosphatidylinositol in blowfly salivary glands.

In blowfly salivary glands, breakdown of phosphatidylinositol has been linked to the activation of hormone-sensitive Ca2+ channels. Addition of 5-hydroxytryptamine to blowfly salivary glands stimulated the breakdown of phosphatidylinositol prelabeled with 32P or [3H]arachidonic acid. This was associated with a transient accumulation of [3H]arachidonic-labeled diglyceride. There was no appreciable effect of 5-hydroxytryptamine on breakdown of phosphatidylethanolamine or phosphatidylcholine labeled with 32P or [3H]arachidonic acid, indicating that phosphatidylinositol was the immediate source of diglyceride. Extracellular Ca2+ was necessary for [3H]arachidonic acid but not 32P loss from phosphatidylinositol. Addition of arachidonic acid to salivary glands did not stimulate salivary gland secretion or 45Ca flux. In contrast, 5-hydroxytryptamine stimulated both salivary gland secretion and 45Ca flux. These results indicate that, although [3H]arachidonic acid is incorporated into phosphatidylinositol and its release from this phospholipid is increased by 5-hydroxytryptamine, the liberated arachidonic acid does not stimulate salivary gland secretion or 45Ca flux.