1. The effects of phytohaemagglutinin and of a Ca2+ ionophore (A23187) on glycerolipid metabolism in lymphocytes from pig lymph nodes were compared (a) by studying the incorporation of [32P]Pi and [3H]glycerol, and (b) by following the redistribution of [3H]glycerol among the lipids caused by these agents in pulse-chase experiments. 2. Phytohaemagglutinin only stimulated 32P incorporation into phosphatidylinositol and, to a slight extent, phosphatidate. Removal of most of the extracellular Ca2+ somewhat decreased this response. 3. Ionophore A23187 stimulated the labelling of phosphatidate and phosphatidylinositol with 32P to a much greater extent than did phytohaemagglutinin: the increase in phosphatidate labelling, but not that of phosphatidylinositol, was almost abolished by the removal of extracellular Ca2+. 4. The combined effects of phytohaemagglutinin and ionophore appeared to be additive, rather than synergistic. 5. Treatment with ionophore A23187 somewhat decreased the total incorporation of [3H]glycerol into glycerolipids, possibly because it lowered cell ATP content. In these experiments di- and tri-acylglycerol behaved anomalously, triacylglycerol labelling being suppressed completely, whereas that of diacylglycerol was enhanced. The pulse-chase results revealed that triacylglycerol was converted into diacylglycerol in the ionophore-treated cells, and the availability of this diacylglycerol probably led to the enhanced labelling of phosphatidate and phosphatidylinositol in the these cells. 6. Thus an increase in intracellular Ca2+ concentration appeared to have three effects on glycerolipid metabolism: (a) slight inhibition of some metabolic step preceding phosphatidate synthesis, (b) inhibition of diacylglycerol acyltransferase and (c) activation of a triacylglycerol lipase. 7. In contrast, it seems likely that the only effect of phytohaemagglutinin is to stimulate phosphatidylinositol breakdown. 8. Pig polymorphonuclear leucocytes treated with ionophore A23187 showed metabolic changes that were similar to those demonstrated with lymphocytes. 9. A possible similarity is suggested between Ca2+-stimulated triacylglycerol lipase in lymphocytes and polymorphonuclear leucocytes and previous observations of enhanced triacylglycerol metabolism in stimulated cells whose metabolic functions involve membrane fusion.

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