title: The rate of hypo-osmotic challenge influences regulatory volume decrease (RVD) and mechanical properties of articular chondrocytes.
abstract: OBJECTIVE
  Osteoarthritis (OA) is associated with a gradual reduction in the interstitial osmotic pressure within articular cartilage. The aim of this study was to compare the effects of sudden and gradual hypo-osmotic challenge on chondrocyte morphology and biomechanics.
  
  
  METHODS
  Bovine articular chondrocytes were exposed to a reduction in extracellular osmolality from 327 to 153 mOsmol/kg applied either suddenly (<5 s) or gradually (over 180 min). Temporal changes in cell diameter and the existence of regulatory volume decrease (RVD) were quantified along with changes in cortical actin and chromatin condensation. The cellular viscoelastic mechanical properties were determined by micropipette aspiration.
  
  
  RESULTS
  In response to a sudden hypo-osmotic stress, 66% of chondrocytes exhibited an increase in diameter followed by RVD, whilst 25% showed no RVD. By contrast, cells exposed to gradual hypo-osmotic stress exhibited reduced cell swelling without subsequent RVD. There was an increase in the equilibrium modulus for cells exposed to sudden hypo-osmotic stress. However, gradual hypo-osmotic challenge had no effect on cell mechanical properties. This cell stiffening response to sudden hypo-osmotic challenge was abolished when actin organization was disrupted with cytochalasin D or RVD inhibited with REV5901. Both sudden and gradual hypo-osmotic challenge reduced cortical F-actin distribution and caused chromatin decondensation.
  
  
  CONCLUSIONS
  Sudden hypo-osmotic challenge increases chondrocyte mechanics by activation of RVD and interaction with the actin cytoskeleton. Moreover, the rate of hypo-osmotic challenge is shown to have a profound effect on chondrocyte morphology and biomechanics. This important phenomenon needs to be considered when studying the response of chondrocytes to pathological hypo-osmotic stress.
authors.affiliation: Institute of Bioengineering and School of Engineering and Materials Science, Queen Mary University of London, London, United Kingdom.
authors.forename: Z
authors.initials: Z
authors.lastname: Wang
authors.affiliation: Biophysical Engineering Laboratory, University of Pennsylvania, Philadelphia, PA, USA.
authors.forename: J
authors.initials: J
authors.lastname: Irianto
authors.affiliation: Institute of Bioengineering and School of Engineering and Materials Science, Queen Mary University of London, London, United Kingdom.
authors.forename: S
authors.initials: S
authors.lastname: Kazun
authors.affiliation: Institute of Bioengineering and School of Engineering and Materials Science, Queen Mary University of London, London, United Kingdom.
authors.forename: W
authors.initials: W
authors.lastname: Wang
authors.affiliation: Institute of Bioengineering and School of Engineering and Materials Science, Queen Mary University of London, London, United Kingdom. Electronic address: m.m.knight@qmul.ac.uk.
authors.forename: M M
authors.initials: MM
authors.lastname: Knight
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002358:Cartilage, Articular / Q000166:cytology*; D002417:Cattle; D019902:Chondrocytes / Q000166:cytology* / Q000502:physiology*; D009995:Osmosis; D013312:Stress, Physiological
keywords: Actin; Cell mechanics; Chondrocyte; Osmotic pressure; Pipette aspiration; Volume regulation