Effects of Lysophosphatidic Acid, a Novel Lipid Mediator, on Cytosolic Ca 2+ and Contractility in Cultured Rat Mesangial Cells

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Abstract
Lysophosphatidic acid (LPA), the smallest and structurally simplest phospholipid, has the potential to modulate cellular signaling in diverse tissues and cell types, including fibroblasts. In the present study, we assessed the effects of LPA on cultured rat glomerular mesangial cells. Quantitative changes of [Ca2+]i in response to LPA were measured in monolayers of mesangial cells loaded with the fluorescent Ca2+ indicator fura 2. LPA (10 nmol/L to 100 μmol/L) increased [Ca2+]i in a dose-dependent manner and evoked inositol trisphosphate formation. LPA (1 μmol/L to 30 μmol/L) also elicited a marked, albeit transient, contractile response in mesangial cells cultured on collagen gel, as assessed by a decrease in cell surface area (CSA). The contraction persisted for 5 minutes (CSA decreased by 31%), whereupon the mesangial cells gradually relaxed with a consequent increase in CSA. Pretreatment of mesangial cells with isradipine (1 μmol/L), a dihydropyridine-sensitive Ca2+ channel blocker, completely blocked LPA-induced contraction. Isradipine also decreased resting [Ca2+]i levels as well as the peak and the subsequently sustained [Ca2+]i levels induced by LPA, suggesting that the contractile effects of LPA are dependent on Ca2+ entry through voltage-gated Ca2+ channels. Finally, LPA stimulated an increase in both prostaglandin E2 synthesis and cAMP accumulation, indicating that these mediators may modulate the contractile effects of LPA. Our study is the first demonstration that exogenous LPA induces mesangial cell contraction and suggests that the contraction is mediated by mobilization of intracellular Ca2+ by activation of the phosphoinositide cascade as well as by promotion of Ca2+ entry across the plasma membrane.