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 tubular cells was examined using fura‐2 as a fluorescent dye. Oleamide (5–50 μM) increased [Ca²⁺]_i in a concentration‐dependent fashion with an EC₅₀ value of 20 μM. The [Ca²⁺]_i signal comprised an initial rise and an elevated phase and was reduced by removing extracellular Ca²⁺ by 50%. After pretreatment with 5–50 μM oleamide in Ca²⁺‐free medium, addition of 3 mM Ca²⁺ increased [Ca²⁺]_i in a manner dependent on the concentration of oleamide. In Ca²⁺‐free medium, pretreatment with thapsigargin (1 μM), an endoplasmic reticulum Ca²⁺ pump inhibitor, abolished [Ca²⁺]_i increases induced by 20 μM oleamide; conversely, pretreatment with 20 μM oleamide reduced 1 μM thapsigargin‐induced [Ca²⁺]_i increases by 50%. Suppression of the activity of phospholipase C with 2 μM U73122 abolished 20 μM oleamide‐induced Ca²⁺ release. Collectively, these data demonstrate that oleamide induced significant [Ca²⁺]_i increases in renal tubular cells by a phospholipase C‐dependent release of Ca²⁺ from thapsigargin‐sensitive stores and by inducing Ca²⁺ entry via store‐operated Ca²⁺ entry. Drug Dev. Res. 54:40–44, 2001. © 2001 Wiley‐Liss, Inc.