The relationship of calcium to receptor controlled stimulation of phosphatidylinositol turnover. Effects of acetylcholine, adrenaline, calcium ions, cinchocaine and a bivalent cation ionophore on rat parotid gland fragments

The possibility that Ca²⁺ ions are involved in the control of the increased phosphatidylinositol turnover which is provoked by α adrenergic or muscarinic cholinergic stimulation of rat parotid gland fragments was investigated. Both types of stimulation provoked phosphatidylinositol breakdown, which was detected either chemically or radiochemically, and provoked a compensatory synthesis of the lipid, detected as an increased rate of incorporation of ³²P(i) ³²P(i), phosphatidylinositol. Acetylcholine had little effect on the incorporation of labelled glycerol, whereas adrenaline stimulated it significantly, but to a much lower extent than ³²P incorporation: this suggests that the response to acetylcholine was entirely accounted for by renewal of the phosphorylinositol head group of the lipid, but that some synthesis de novo was involved in the response to adrenaline. The responses to both types of stimulation, whether measured as phosphatidylinositol breakdown or as phosphatidylinositol labeling, occurred equally well in incubation media containing 2.5 mM Ca²⁺ or 0.2 mM EGTA [ethanedioxybis(ethylamine)tetra acetic acid]. Incubation with a bivalent cation ionophore (A23187) led to a small and more variable increase in phosphatidylinositol labeling with ³²Pi, which occurred whether or not Ca²⁺ was available in the extracellular medium: this was not accompanied by significant phosphatidylinositol breakdown. Cinchocaine, a local anesthetic, produced parallel increases in the incorporation of P(i) and glycerol into phosphatidylinositol. This is compatible with its known ability to inhibit phosphatidate phosphohydrolase and increase phosphatidylinositol synthesis de novo in other cells. The results indicate that the phosphatidylinositol turnover evoked by α adrenergic or muscarinic cholinergic stimuli in rat parotid gland probably does not depend on an influx of Ca²⁺ into the cells in response to stimulation. This is in marked contrast with the K⁺ efflux from this tissue, which is controlled by the same receptors, but is strictly dependent on the presence of extracellular Ca²⁺. The Ca²⁺ independence of stimulated phosphatidylinositol metabolism may mean that it is controlled through a mode of receptor function different from that which controls other cell responses. Alternatively, it can be interpreted as indicating that stimulated phosphatidylinositol breakdown is intimately involved in the mechanisms of action of α adrenergic and muscarinic cholinergic receptor systems.