Phospholipase C activation by prostaglandins and thromboxane A₂ in cultured mesangial cells

Phospholipase C activation by prostaglandins (PG) and thromboxane A₂ (TxA₂) was studied in cultured rat and human glomerular mesangial cells, measuring accumulation of radiolabeled inositol phosphates and cytosolic free calcium ([Ca²⁺](i)) with the fluorescent intracellular probe fura-2. Prostaglandin F(2α) (PGF (2α)) and TxA₂ were found to be the major eicosanoids active on this signaling pathway in rat and human cells, respectively, whereas other PG had lesser or no effects. PGF(2α) and TxA₂ rapidly induced accumulation of inositol trisphosphate accompanied by a simultaneous transient rise of [Ca²⁺](i), followed by sustained elevation or, in human cells, by a distinct second increase of [Ca²⁺](i) within 45 s. A minor initial accumulation of inositol monophosphate was followed by marked elevation > 5 min after the early responses. Responses to different eicosanoids were mediated by separate receptors, functionally characterized using receptor antagonists or heterologous desensitization during sequential applications. Protein kinase C activation by serum and phorbol esters potently inhibited inositol phosphate accumulation and/or [Ca²⁺](i) transients, indicating a pathway for a negative feedback on PG-evoked intracellular signals. We conclude that receptor-mediated phospholipase C activation underlies the biological effects of certain eicosanoids on the glomerular mesangium.