Stimulation of phosphoinositide hydrolysis by oxytocin in renal epithelial cells

Recently, it has been reported that oxytocin (OT) produces diuresis by its interaction with OT receptors in the kidney. LLC-PK1 cells have been used as a model system for renal epithelial cells. To determine if OT stimulates receptor-mediated phosphoinositide (PI) hydrolysis in LLC-PK1 cells as it does in nonrenal cell systems, we measured the release of PI hydrolysis products in LLC-PK1 cells by OT and a selective OT agonist (AK-2-60) in the absence and presence of a selective OT antagonist (KB-5-21). In addition, we determined the effect of an increase in osmolality of the incubation medium on OT-stimulated PI hydrolysis in LLC-PK1 cells. The methods involved the incubation of LLC-PK1 cells with [3H]inositol for its incorporation into membrane PI and the measurement of the release of [3H]inositol phosphates in the presence of LiCl which prevents dephosphorylation. The osmolality of the incubation media was increased from 300 mOsmol/kg of H2O to 600, 900 and 1200 mOsmol/kg of H2O by addition of NaCl and urea. In an iso-osmotic incubation medium OT (10(-11) M) produced a greater than 100% increase in PI hydrolysis in LLC-PK1 cells. The OT agonist, AK-2-60, produced a significant increase in PI hydrolysis in LLC-PK1 cells at 10(-8) M concentration. The effects of both OT and its agonist were concentration-dependent and were blocked by the OT antagonist, KB-5-21. An increase in osmolality of the incubation media decreased OT-stimulated PI hydrolysis in LLC-PK1 and abolished completely the effect of OT at 1200 mOsmol/kg of H2O.(ABSTRACT TRUNCATED AT 250 WORDS)