In the present study the modulatory action of platelet-activating factor (PAF) on sphingolipid metabolism in cerebral cortical slices was studied. PAF did not alter the basal levels of either sphingomyelin (SM) or ceramide. However, the SMase-elicited reciprocal alterations in SM and ceramide levels were partially prevented by the PAF treatment. The PAF effect was dose-dependent, with 10 28 m being the lowest effective concentration, and receptor-mediated as it was abolished by WEB 2086, a PAF receptor antagonist. Neither N-oleoylethanolamine (OE, ceramidase inhibitor) or d,l-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP, an inhibitor of glucosylceramide synthase and the formation of 1-O-acyl ceramides) prevented the action of PAF. Therefore, the effect of PAF was unlikely to be dependent upon transformation of ceramides into glycosphingolipids, 1-O-acyl ceramides or sphingosine. Experiments with different labeled compounds ([ 14 C]serine, [ 14 C]arachidonate and phosphatidyl [N-methyl-3 H]choline) were also performed to test whether PAF could affect the resynthesis of SM. Data obtained agree with the idea that selective pools of both choline and ethanolamine phospholipids were used as precursors for the resynthesis of SM elicited by SMase treatment. PAF itself did not evoke any variation in the lipids analyzed but always prevented the SMase-evoked alterations. Together the data suggest the interesting possibility that PAF increases the overall turnover of SM. In summary, the present data demonstrate that PAF is able to regulate the cellular ceramide levels in brain by accelerating the SM cycle.

Ceramide is a lipid second messenger that acts on multiple-target enzymes, some of which are involved in other signal-transduction systems. We have previously demonstrated that endogenous ceramide modifies the metabolism of brain ethanolamine plasmalogens. The mechanism involved was studied. On the basis of measurements of breakdown products, specific inhibitor effects, and previous findings, we suggest that a plasmalogen-selective phospholipase A 2 is the ceramide target. Arachidonaterich pools of the diacylphosphatidylethanolamine subclass were also affected by ceramide, but the most affected were plasmalogens. Concomitantly with production of free arachidonate, increased 1-O-arachidonoyl ceramide formation was observed. Quinacrine (phospholipase A 2 inhibitor) and 1-O-octadecyl-2-O-methyl-rac-glycerol-3-phosphocholine (CoA-independent transacylase inhibitor) prevented all of these ceramide-elicited effects. Therefore, phospholipase and transacylase activities are tightly coupled. Okadaic acid (phosphatase 2A inhibitor) and PD 98059 (mitogen-activated protein kinase inhibitor) modified basal levels of ceramide and sphingomyelinaseinduced accumulation of ceramide, respectively. Therefore, they provided no evidence to determine whether there is a sensitive enzyme downstream of ceramide. The evidence shows that there are serine-dependent and thiol-dependent enzymes downstream of ceramide generation. Furthermore, experiments with Ac-DEVD-CMK (caspase-3 specific inhibitor) have led us to conclude that caspase-3 is downstream of ceramide in activating the brain plasmalogen-selective phospholipase A 2 .

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The purpose of this work was to study whether exercise training induces changes in the lipid profile of rat aorta and nervous system and in the in vitro intrinsic responsiveness of these tissues to endothel in-1 (ET-1) treatment. The exercise program performed successfully produced the characteristic metabolic alterations of the trained state. Exercise training induced a large and significant increase in the levels of both aortic ethanolamine plasmalogens (PlasEtn) and glucosylceramides. In contrast, a decrease of aortic ceramide and cholesterol levels was evoked by exercise training. ET-1 increased PlasEtn content only in sedentary animals. An exercise-induced increase in cerebellum levels of ceramides and ceramide monohexosides was found. The cerebellum ceramide content was increased by ET-1 more noticeably in sedentary rats than in trained animals. In contrast, cerebral cortex was observed to be largely insensitive to both exercise training and ET-1 treatment. It was concluded that exercise training (i) induces changes in both vascular and cerebellar lipid profiles, the former being much more pronounced than the latter, and (ii) diminishes the aortic and cerebellar sensitivity to ET-1 action.