Subcellular Site of the Phosphatidylinositol Effect. Distribution on Density Gradients of Labelled Lipids From Resting and Active Sympathetic Ganglia of the Rat¹

Localization of the increased [ 32 P]‐labelling of phosphatidylinositol, caused by arrival of presynaptic impulses in a sympathetic ganglion, was investigated by subcellular fractionation of ganglia that had been labelled before homogenization. Paired superior cervical ganglia were excised from adult rats and incubated with 32 P 1 and [methyl‐ 14 C]choline for 4 h at 37°C. The preganglionic nerve of one ganglion of each pair was stimulated repetitively (10/s) during the last 3 h of incubation. The ganglia were then softened with collagenase, homogenized, and fractionated by density gradient centrifugation. Succinate dehydrogenase served as marker for the distribution of mitochondria on the gradient, and [ 14 C]ACh for synaptosomes. [ 32 P]‐labelling of lipids was measured relative to that of phosphatidylcholine. The average changes in relative labelling that were caused by neuronal activity ranged as follows over the homogenate and the various fractions: phosphatidylinositol (PI), increased 39–75%; phosphatidylethanolamine, decreased 10–20%; diphosphatidylglycerol, not significantly affected. The increase in PI labelling was much greater in the denser fractions, in which synaptosomes and mitochondria were concentrated, than in the less dense fractions. The distribution of the PI effect on the gradient could be reasonably well explained by assuming that synaptosomes and mitochondria both contributed to the increase in PI labelling in proportion to the amount of their respective markers in a fraction. One‐third or more of the total increase could thus be associated with synaptosomes and one‐third or more with mitochondria, although alternative association with other structures could not be excluded. The implications of the inferred association with synaptosomes and mitochondria are discussed, current knowledge of the PI effect caused by impulses entering sympathetic ganglia is summarized, and suggestions concerning its physiological significance are reviewed. It is concluded that PI may have multiple roles in neuronal activity.