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Lysophosphatidic acid (LPA) functions as an extracellular lipid mediator stimulating phospholipase C and affecting the structure of the cytoskeleton in several cell types. In rat glioma C6 cells, LPA mobilizes calcium from intracellular calcium stores and reverts morphological changes induced by elevated cytosolic cAMP-concentrations. Here we show that LPA-stimulation of C6 cells loaded with the calcium-sensitive fluorescent dye indo-1 results in calcium release from a subset of intracellular calcium stores that are not sensitive to the tumor promoter thapsigargin and do not overlap with calcium stores depleted during purinergic receptor stimulation with ATP. Furthermore, depletion of LPA-sensitive calcium stores does not induce capacitative calcium entry from the extracellular space into the cytosol to the same extent as ATP. These results indicate that inositol phosphate signaling induced by LPA or ATP may differ in kinetics or in spatial organisation within the cell. This may represent a possible explanation for the previous observation that only LPA, but not other calcium-mobilizing agonists, reverts cAMP-induced changes in the cytoskeletal organization in C6 cells.

We tested lysophosphatidic acid (LPA) known to induce inositol phosphate generation and calcium signals as well as rearrangements of the cytoskeleton and mitogenic responses in fibroblasts, for its ability to activate phospholipase C in an exocrine cell system, the salt-secreting cells from the avian nasal salt gland. LPA (> 10 nmol/l) caused the generation of inositol phosphates from membrane-bound phosphatidylinositides. The resulting calcium signals resembled those generated upon activation of muscarinic receptors, the physiological stimulus triggering salt secretion in these cells. However, close examination of the LPA-mediated calcium signals revealed that the initial calcium spike induced by high concentrations of LPA (> 10 mumol/l) may contain a component that is not dependent upon generation of inositol (1,4,5)-trisphosphate (Ins(1,4,5)P3) and may result from calcium influx from the extracellular medium induced by LPA in a direct manner. Low concentrations of LPA (< 10 mumol/l), however, induce inositol phosphate generation, Ins(1,4,5)P3-mediated release of calcium from intracellular pools and calcium entry. These effects seem to be mediated by a specific plasma membrane receptor and a G protein transducing the signal to phospholipase C in a pertussis-toxin-insensitive manner. Signaling pathways of the muscarinic receptor and the putative LPA-receptor seem to merge at the G-protein level as indicated by the fact that carbachol and LPA trigger hydrolysis of the same pool of phosphatidylinositol (4,5)-bisphosphate (PIP2) and mobilize calcium from the same intracellular stores.