The osmotic sensitivity of isolated and in situ bovine articular chondrocytes

Articular chondrocytes experience changes to matrix hydration during both p hysiological (static load) and pathophysiological (osteoarthrosis. OA) cond itions. Such changes should alter chondrocytes' Volume. which has been show n to modify matrix metabolism. However, the osmometric behaviour of chondro cytes is not well understood. Here. using confocal laser scanning microscop y (CLSM). we have investigated the 'passive' osmotic responses of fluoresce nt-labelled chondrocytes within, and isolated front. the matrix. The volume -regulatory pathways normally activated by cell shrinkage/swelling, were bl ocked by bumetanide/REV5901. respectively. Chondrocytes in Situ were broadl y grouped into superficial (SZ), mid (MZ) and deep (DZ) zones. and there wa s a significant increase in resting cell volume with depth into the cartila ge. Variation in medium osmolarity (range 0-530 mOsm; corresponding to an e xtracellular osmolarity of similar to 150 to similar to 600 mOsm) caused a rapid and Sustained change to in situ MZ chondrocytes' volume. Over the ran ge 180-380 mOsm, the change to in Situ or isolated chondrocytes, volume was similar. For MZ chondrocytes. ideal osmometric (Boyle-van't Hoff) behaviou r was apparent over the extracellular osmolarity range of similar to 250 to similar to 600 mOsm. Chondrocytes within the SZ appeared to be more sensit ive to reduced osmolarity. swelling more for a given reduction in osmolarit y than MZ or DZ chondrocytes, These data show that over wide variations in osmolarity, articular chondrocytes in situ were osmotically sensitive, and for MZ chondrocytes behaved as perfect osmometers with the extracellular ma trix (ECM) not restraining cell volume changes. Changes to matrix hydration may therefore alter passive chondrocytes' volume and unless compensated by volume-regulatory pathways. could lead to changes in cell volume, and henc e matrix metabolism. (C) 2001 Orthopaedic Research Society. Published by El sevier Science Ltd. All rights reserved.