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1 Division de Nephrologie, Fondation pour Recherches Medicales, Geneva, Geneva, Switzerland
2 Departement de Pathologie, Centre Medical Universitaire, Geneva, Geneva, Switzerland
* To whom correspondence should be addressed. E-mail: Eric.Feraille{at}medecine.unige.ch.
The kidney medulla is physiologically exposed to variations of extracellular osmolality. In response to hypertonic cell shrinkage, cells of the rat kidney medullary thick ascending limb of the Henle's loop undergo p38 kinase-dependent regulatory volume increase (RVI). In the present study, we have investigated the role of the actin cytoskeleton reorganization in this process. Addition of hyperosmotic NaCl or sucrose, which activates MAP kinases and reduces cellular volume, induced a sustained actin polymerization occurring after 10 min and concurrently with RVI. In contrast, hyperosmotic urea, which does not modify MAP kinase activity and cellular volume, did not induce sustained actin polymerization. Fluorescence microscopy revealed that hyperosmotic NaCl and sucrose, but not urea, induced the redistribution of F-actin from a dense cortical ring to a diffuse network of actin bundles. Stabilization of actin filaments by jasplakinolide and inhibition of the generation of new actin filaments by swinholide A prevented RVI, whereas depolymerization of actin filaments by latrunculin B attenuated cell shrinkage and enhanced RVI. These actin-interfering drugs did not alter extracellular regulated kinase and p38 kinase activation under hypertonic conditions. Similar to swinholide A, inhibiting p38 kinase with SB 203580 blunted sustained actin polymerization, actin redistribution and decreased RVI efficacy. We therefore propose that in rat kidney medullary thick ascending limb of the Henle's loop exposed to extracellular hypertonicity, p38 kinase activation induces depolymerization of the F-actin cortical ring and polymerization of a dense diffuse F-actin network that both contribute to increase RVI efficacy.
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