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Effects of Changes in Calcium Concentration on Basal and Stimulated 32P Incorporation into Phospholipids in Rat Pineal Cells

Abstract: The Ca2+ requirement for α‐agonist stimulation of 32P incorporation into acidic phospholipids (the phosphatidylinositol effect) of dispersed pineal cells was evaluated by means of several different compounds that interfere with Ca2+ disposition. Simple omission of Ca2+ led to slight increases in basal and norepinephrine‐stimulated phosphatidyl‐CMP (CDP‐diacylglycerol) and phosphatidylglycerol labeling without affecting phosphatidylinositol labeling. In the absence of Ca2+, EGTA (200 μM) or the ionophore for divalent cations A23187 (10 μM) elicited large increases in phosphatidic acid, phosphatidyl‐CMP, and phosphatidylglycerol labeling while strongly inhibiting the phosphatidylinositol effect. The Ca2+ translocation inhibitor LaCI3 also reduced the magnitude of this effect. The phosphatidylinositol effect is, however, not induced by increased Ca2+ entry into the cytosol, since A23187 did not mimic the effect of norepinephrine. Under conditions where membrane Ca2+ was lowered, the addition of 1 mM‐inositol greatly reduced phosphatidic acid, phosphatidylglycerol, and phosphatidyl‐CMP labeling with concomitant increases in basal and norepinephrine‐stimulated phosphatidylinositol labeling approaching that observed in the presence of norepinephrine and 2.5 mM‐Ca2+. In the presence of 2.5 mM‐Ca2+, inositol had negligible effects on phosphatidylinositol labeling. It was concluded that changes in membrane Ca2+ availability and/or disposition alter phospholipid metabolism and concurrently reduce the magnitude of the phosphatidylinositol effect, perhaps by making the pool of readily available inositol in pinealocytes rate‐limiting.