Cell shrinkage triggers the activation of mitogen-activated protein kinases by hypertonicity in the rat kidney medullary thick ascending limb of the Henle's loop - Requirement of p38 kinase for the regulatory volume increaseresponse

The kidney medulla is exposed to very high interstitial osmolarity leading to the activation of mitogen-activated protein kinases (MAPK). However, the respective roles of increased intracellular osmolality and of cell shrinka ge in MAPK activation are not known. Similarly, the participation of MAPK i n the regulatory volume increase (RVI) following cell shrinkage remains to be investigated. In the rat medullary thick ascending limb of Henle (MTAL), extracellular hypertonicity produced by addition of NaCl or sucrose increa sed the phosphorylation level of extracellular signal-regulated kinase (ERK ) and p38 kinase and to a lesser extent c-Jun NH2-terminal kinase with sucr ose only. Both hypertonic solutions decreased the MTAL cellular volume in a dose-and time-dependent manner. In contrast, hypertonic urea had no effect , The extent of MAPK activation was correlated with the extent of MTAL cell ular volume decrease. Increasing intracellular osmolality without modifying cellular volume did not activate MAPK, whereas cell shrinkage without vari ation in osmolality activated both ERK and p38, In the presence of 600 mosm ol/liter NaCl, the maximal cell shrinkage was observed after 10 min at 37 d egrees C and the MTAL cellular volume was reduced to 70% of its initial val ue. Then, RVI occurred and the cellular volume progressively recovered to r each about 90% of its initial value after 30 min. SB203580, a specific inhi bitor of p38, almost completely inhibited the cellular volume recovery, whe reas inhibition of ERK did not alter RVI. In conclusion, in rat MTAL: 1) ce ll shrinkage, but not intracellular hyperosmolality, triggers the activatio n of both ERK and p38 kinase in response to extracellular hypertonicity; an d 2) RVI is dependent on p38 kinase activation.