Mechanical loading of articular cartilage is believed to play an important role in the maintenance of tissue health and homeostasis. The application (and removal) of mechanical loads alters the osmotic environment of the chondrocytes through expression (and recovery) of interstitial water from the tissue. A growing body of literature has addressed the response o f chondrocytes to osmotic stress, but there is a dearth of information on how this response is affected by the presence of inflammatory cytokines. IL-1 is one of the primary catabolic cytokines involved in osteoarthritis (OA). IL-1 signaling in chondrocytes is poorly understood, but is thought to involve changes in the concentration of intracellular calcium ([Ca²⁺]_i) and F-actin. Therefore the objective of this dissertation was to examine the early events in IL-1 signaling in chondrocytes and to address the ability of IL-1 to modulate the response of chondrocytes to osmotic stress.

Isolated and in situ chondrocytes were exposed to IL-1 and changes in [Ca²⁺]_i and F-actin were monitored. A majority of chondrocytes responded to IL-1 by mobilizing Ca²⁺ by a mechanism involving phospholipase C and release of Ca²⁺ from intracellular stores. The response was more robust in embedded chondrocytes than in isolated cells, and in deep zone vs. surface zone cells. IL-1 increased the content of F-actin in both isolated and in situ chondrocytes by a mechanism involving activation of Rho GTPases.

IL-1 inhibited Ca²⁺ and altered volume adaptation in response to both hyperand hypo-osmotic stress. IL-1 slowed the rate and extent of cell shrinking after hyper-osmotic stress by a pathway involving Ca²⁺ mobilization and activation of Rho GTPases. In response to hypo-osmotic stress, IL-1 treated chondrocytes showed normal swelling, but impaired volume regulation and F-actin reorganization.

Finally, functional expression of transient receptor potential vanilloid-4 (TRPV4), an osmotically-sensitive Ca²⁺ channel was confirmed in porcine cartilage. Activation of TRPV4 mobilized Ca²⁺ and enhanced regulatory volume decrease (RVD) after cell swelling. In IL-1 treated cells, TRPV4 activation restored normal RVD after cell swelling.

In summary, the results of this study suggest that IL-1 may influence the ability of chondrocytes to respond to osmotic stress secondary to cartilage compression.