INHIBITION OF PHOSPHATIDYLINOSITOL SYNTHESIS AND THE INACTIVATION OF CALCIUM ENTRY AFTER PROLONGED EXPOSURE OF THE BLOWFLY SALIVARY-GLAND TO 5-HYDROXYTRYPTAMINE

The incorporation of [32P]P(i) into all salivary-gland phospholipds except phosphatidic acid was inhibited by 5-hydroxytryptamine. The accumulation of [32P]P(i) into phosphatidic acid was actually enhanced by 5-hydroxytryptamine. There was an inhibition of labelled inositol incorporation into phosphatidylinositol by 5-hydroxytryptamine, which seems to be mediated by calcium because it was mimicked by the ionophore A23187, but was prevented if glands were stimulated with 5-hydroxytryptamine in the absence of external calcium. Inhibition of synthesis together with stimulation of breakdown will decrease the concentration of phosphatidylinositol, which could account for the inactivation of calcium transport observed at high 5-hydroxytryptamine concentrations. When salivary glands were stimulated with 1μM-5-hydroxytryptamine, there was a rapid increase in the transfer of 45Ca2+ from the medium into the saliva, but with time this transport declined to a low value. If the glands were washed free of 5-hydroxytryptamine and incubated in the presence of 2mM-inositol for 1h, the increase in calcium transport caused by 5-hydroxytryptamine was restored. There was little recovery in the absence of inositol. If glands were stimulated with 5-hydroxytryptamine in the absence of external calcium, a condition which prevents the inhibition of phosphatidylinositol synthesis, calcium transport in response to 5-hydroxytryptamine was greater than in glands preincubated with 5-hydroxytryptamine in the presence of calcium. The inactivation of calcium transport may result from a decrease in phosphatidylinositol concentration. These results support the hypothesis that the hydrolysis of phosphatidylinositol plays some role in either the opening or closing of calcium 'gates'.