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.