Effects of changes in calcium concentration on basal and stimulated 32P incorporation into phospholipids in rat pineal cells.

The Ca2+ requirement for alpha-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 microM) or the ionophore for divalent cations A23187 (10 microM) elicited large increases in phosphatidic acid, phosphatidyl-CMP, and phosphatidylglycerol labeling while strongly inhibiting the phosphatidylinositol effect. The Ca2+ translocation inhibitor LaCl3 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.