Inositol lipid metabolism in rat hippocampal formation slices: basal metabolism and effects of cholinergic agonists.

Incubation of rat hippocampal formation slices under steady-state conditions with [3H]inositol leads to only three phospholipids becoming labelled: phosphatidylinositol, phosphatidylinositol 4-phosphate, and phosphatidylinositol 4,5-bisphosphate. All three lipids incorporate [32P]Pi into their phosphodiester phosphate group with the polyphosphoinositides also incorporating this tracer into their monoester phosphate groups. As the concentrations of these lipids remain constant during these labelling processes we conclude that the phosphodiester phosphate, the inositol moiety, and the monoester phosphate groups undergo metabolic turnover in hippocampal formation slices incubated in vitro. The rate of incorporation of [3H]inositol into all three inositol phospholipids was stimulated by the addition of methacholine to the medium. Moreover, following steady-state labelling of the inositol lipids with [3H]inositol, methacholine in the presence of 10 mM LiCl caused a transient fall of 13% in the radiochemical concentration of phosphatidylinositol 4,5-bisphosphate after only 30 s stimulation and a fall of 15% in the radiochemical concentration of phosphatidylinositol after 30 min. Concomitantly, there was an approximately stoichiometric rise in the radiochemical concentration of inositol phosphates. Thus, we suggest that methacholine stimulates an inositol phospholipid phosphoinositidase C in rat hippocampal formation slices.