Hypertonic inhibition of exocytosis in neutrophils: central role for osmotic actin skeleton remodeling

Hypertonicity suppresses neutrophil functions by unknown mechanisms. We inv estigated whether osmotically induced cytoskeletal changes might be related to the hypertonic inhibition of exocytosis. Hyperosmolarity abrogated the mobilization of all four granule types induced by diverse stimuli, suggesti ng that it blocks the process of exocytosis itself rather than individual s ignaling pathways. Concomitantly, osmotic stress provoked a twofold increas e in F-actin, induced the formation of a submembranous F-actin ring, and ab olished depolymerization that normally follows agonist-induced actin assemb ly. Several observations suggest a causal relationship between actin polyme rization and inhibition of exocytosis: 1) prestimulus actin levels were inv ersely proportional to the stimulus-induced degranulation, 2) latrunculin B (LB) prevented the osmotic actin response and restored exocytosis, and 3) actin polymerization induced by jasplakinolide inhibited exocytosis under i sotonic conditions. The shrinkage-induced tyrosine phosphorylation and the activation of the Na+/H+ exchanger were not affected by LB. Inhibition of o smosensitive kinases failed to prevent the F-actin change, suggesting that the osmotic tyrosine phosphorylation and actin polymerization are independe nt phenomena. Thus cytoskeletal remodeling appears to be a key component in the neutrophil-suppressive, anti-inflammatory effects of hypertonicity.