Novel effects of a sleep-inducing lipid, oleamide, on Ca2+ signaling in renal tubular cells

The effect of oleamide, a sleep-inducing endogenous lipid in animal models, on intracellular free levels of Ca2+ ([Ca2+](i)) in Madin-Darby renal tubu lar cells was examined using fura-2 as a fluorescent dye. Oleamide (5-50 mu M) increased [Ca2+](i) in a concentration-dependent fashion with an EC50 va lue of 20 muM. The [Ca2+](i) signal comprised an initial rise and an elevat ed phase and was reduced by removing extracellular Ca2+ by 50%. After pretr eatment with 5-50 muM oleamide in Ca2+-free medium, addition of 3 mM Ca2+ i ncreased [Ca2+](i) in a manner dependent on the concentration of oleamide. In Ca2+-free medium, pretreatment with thapsigargin (1 muM), an endoplasmic reticulum Ca2+ pump inhibitor, abolished [Ca2+](i) increases induced by 20 muM oleamide; conversely, pretreatment with 20 muM oleamide reduced 1 muM thapsigargin-induced [Ca2+](i) increases by 50%. Suppression of the activit y of phospholipase C with 2 muM U73122 abolished 20 muM oleamide-induced Ca 2+ release. Collectively, these data demonstrate that oleamide induced sign ificant [Ca2+](i) increases in renal tubular cells by a phospholipase C-dep endent release of Ca2+ from thapsigargin-sensitive stores and by inducing C a2+ entry via store-operated Ca2+ entry. (C) 2001 Wiley-Liss, Inc.