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

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Abstract
The Ca2+ requirement for .alpha.-agonist stimulation of 32P incorporation into acidic phospholipids (the phosphatidylinositol effect) of dispersed pineal cells was evaluated by 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 [ethylene glycol bis(.beta. aminoethyl ether)-N,N,N'',N''-tetraacetic acid] (200 .mu.M) or the ionophore for divalent cations A23187 [calcimycin] (10 .mu.M) elicited large increases in phophatidic acid, phosphatidyl-CMP and phosphatidylglycerol labeling while strongly inhibiting the phosphatidylinositol effect. The Ca2+ translocation inhibitor LaCl3 reduced the magnitude of this effect. The phosphatidylinositol effect is 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, phatidylglycerol 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. Changes in membrane Ca2+ availability and/or disposition evidently 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.

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