Effects of lysophosphatidylcholine and arachidonic acid on the regulation of intracellular Ca2+ transport

The influence of an inhibitor of lipoxygenases on the modulation of the plasticity of cholinoreceptors by 15-HETE

The role of acyclic eicosanoids in the modulation of the plasticity of somatic cholinoreceptors by 15(S)-hydroxy-(5Z,8Z,11Z,13E)-eicosatetraenoic acid (15-HETE) was investigated in identified RPa3 and LPa3 neurons of Helix lucorum using the two-electrode voltage clamping technique. It was demonstrated that the inhibitor of lipoxygenases, nordihydroguaiaretic acid (2-20 microM), completely blocks the short- and long-latency effects of 15-HETE (2-20 microM) on the depth of damping of the inward current induced by rhythmic applications of acetylcholine (ACh) to the soma. It is concluded that the short-latency effect of 15-HETE on the plasticity of cholinoreceptors is determined by its inhibition mainly of 5-lipoxygenase, which leads to a reduction in the level of acyclic eicosanoids that are formed under the influence of this enzyme. The potentiation of the effects of the acyclic eicosanoids, whose synthesis is resumed in the second phase, probably underlies the direct long-latency modulatory effect of 15-HETE.

Signaling pathways for sphingosylphosphorylcholine-mediated mitogenesis in Swiss 3T3 fibroblasts

Sphingosylphosphorylcholine (SPC), or lysophingomyelin, a wide-spectrum growth promoting agent for a variety of cell types (Desai, N. N., and S. Spiegel. 1991. Biochem. Biophys. Res. Comm. 181: 361-366), stimulates cellular proliferation of quiescent Swiss 3T3 fibroblasts to a greater extent than other known growth factors or than the structurally related molecules, sphingosine and sphingosine-1-phosphate. SPC potentiated the mitogenic effect of an activator of protein kinase C, 12-O-tetradecanoylphorbol 13-acetate, and did not compete with phorbol esters for binding to protein kinase C in intact Swiss 3T3 fibroblasts. However, downregulation of protein kinase C, by prolonged treatment with phorbol ester, reduced, but did not eliminate, the ability of SPC to stimulate DNA synthesis, indicating that SPC may act via both protein kinase C-dependent and -independent signaling pathways. SPC induced a rapid rise in intracellular free calcium ([Ca2+]i) in viable 3T3 fibroblasts determined with a digital imaging system. Although the increases in [Ca2+]i were observed even in the absence of calcium in the external medium, no increase in the levels of inositol phosphates could be detected in response to mitogenic concentrations of SPC. Furthermore, in contrast to sphingosine or sphingosine-1-phosphate, the mitogenic effect of SPC was not accompanied by increases in phosphatidic acid levels or changes in cAMP levels. SPC, but not sphingosine or sphingosine-1-phosphate, stimulates the release of arachidonic acid. Therefore, the ability of SPC to act an extremely potent mitogen may be due to activation of signaling pathway(s) distinct from those used by sphingosine or sphingosine-1-phosphate.

Arachidonic acid and its acyclic derivatives regulate short-term plasticity of the cholinoreceptors of neurons of the edible snail

The influence of eicosanoids on the dynamics of the extinction of the inward current, induced by repeated iontophoretic applications of acetylcholine to the soma has been demonstrated in identified RPa3 and LPa3 neurons of the edible snail by means of bielectrode recording of the membrane potential. The extracellular action of arachidonic acid (50-100 mumole/l) intensifies the extinction. Quinacrine (100-600 mumole/l), an inhibitor of phospholipase A2, which decreases the content of arachidonic acid in the cells, acts ambiguously. Nordihydroguaiaretic acid (3-10 mumole/l), which is an inhibitor of the lipoxygenase oxidation of arachidonic acid, attenuates the extinction. Indomethacin (10-50 mumole/l), a blocker of cyclooxygenase oxidation of arachidonic acid, does not influence extinction. All of the compounds investigated decrease the amplitude of the inward current induced by acetylcholine. The results obtained make it possible to hypothesize that arachidonic acid and its acyclic metabolites, which are formed as a result of lipoxygenase oxidation, regulate short-term plasticity of the cholinoreceptors of neurons of the edible snail. The cyclic eicosanoids which are formed during the cyclooxygenase oxidation of arachidonic acid do not exert a regulatory effect on the plasticity of the cholinoreceptors.

The catabolism of exogenous lysophosphatidylcholine in isolated perfused rat and guinea pig hearts: a comparative study

Lysophosphatidylcholine (lysoPC) is an arrhythmogenic phospholipid metabolite which accumulates in the ischemic myocardium. Reduced catabolism of lysoPC has been proposed to be one of the biochemical mechanisms responsible for the increase in lysoPC content. In this investigation we compared the microsomal catabolism of exogenous labeled lysoPC in isolated perfused rat and guinea pig hearts. Analysis of the amount of radioactivity in microsomal phosphatidylcholine (PC) and free fatty acid (FFA) was used as an index of the participation in lysoPC clearance by acylation catalyzed by acyl-CoA:lysoPC acyltransferase and deacylation catalyzed by lysophospholipase, respectively. There was no significant difference in the incorporation of radioactivity into rat and guinea pig heart microsomes; however, the patterns of radioactivity in lysoPC metabolites were notably different. Equal participation by deacylation and reacylation was observed in rat microsomes, whereas deacylation was clearly the preferred route for lysoPC clearance in guinea pig microsomes. Modulation of enzyme activity by treatment of the isolated heart with pHMB, a sulfhydryl agent, was used to probe the relationship among acylation, deacylation and the extent of lysoPC clearance. In guinea pig microsomes impairment of lysoPC acylation was not associated with any change in the amount of radioactivity in lysoPC because of a compensatory increase in deacylation. In contrast, impaired deacylation in rat microsomes led to significant elevations in the amount of radioactivity in lysoPC. We conclude, therefore, that in intact perfused rat and guinea pig hearts the relative participation of acylation and deacylation in lysoPC clearance differs. Moreover, we propose that the level of deacylation by lysophospholipase is an important factor in the extent of clearance of lysoPC.