Solute convection in dynamically compressed cartilage

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Abstract

Chondrocytes depend upon solute transport within the avascular extracellular matrix of articular cartilage for many of their biological activities. Alterations to solute transport parameters may therefore mediate the cell response to tissue compression. While interstitial solute transport may be supplemented by convection during dynamic tissue compression, matrix compression is also associated with decreased diffusivities. Such trade-offs between increased convection and decreased diffusivities of solutes in dynamically compressed cartilage remain largely unexplored. We measured diffusion and convection coefficients of a wide range of solutes in mature bovine cartilage explant disks subjected to radially unconfined axial ramp compression and release. Solutes included 500Da fluorophores bearing positive and negative charges, and 10 kDa dextrans bearing positive, neutral, and negative charges. Significantly positive values of convection coefficients were measured for several different solutes. Findings therefore support a role for solute convection in mediating the cartilage biological response to dynamic compression.

Introduction

Chondrocytes in adult articular cartilage rely upon solute transport through the avascular extracellular matrix (ECM) for their biological activities (Maroudas, 1975). Alterations to interstitial solute transport may therefore affect cell metabolism by changing pericellular concentrations of essential nutrients, growth factors, cytokines, and waste products, as well as secreted or degraded matrix molecules.

Cartilage responses to mechanical loading suggest that solute convection may play important roles in mechanotransduction. Dynamic loading of cartilage can influence cell-mediated remodeling in vivo (Salter et al., 1980), in cartilage explant disks (Sah et al., 1989), and in engineered tissue constructs (Buschmann et al., 1995, Mauck et al., 2000). However, mechanisms through which dynamic compression may affect solute transport are complex, since convection through the cartilage ECM may increase due to fluid flow (Kim et al., 1995), but matrix compaction may also contribute to decreased diffusivity (Quinn et al., 2001). Measurement of solute transport coefficients in compressed cartilage is thus important to understanding cell biological responses (Bonassar et al., 2001).

While solute diffusion in cartilage has received considerable attention (Bergman, 1968, Maroudas, 1970, Burstein et al., 1993, Torzilli et al., 1997, Quinn et al., 2001, Leddy and Guilak, 2003), solute convection in dynamically compressed cartilage is less well characterized. There is a large body of literature on solute convection in cartilage-like gels (Kapur et al., 1997, Johnston and Deen, 1999), and modeling of effects of dynamic compression on solute transport in cartilage is of significant interest (Ferguson et al., 2004, Mauck et al., 2003), but relatively little quantitative data for solute convection are available, and few experimental methods have been developed for quantifying solute convection in cartilage.

A role for solute convection in cartilage interstitial transport may be inferred from previous studies. Methylene blue (319 Da) penetrated more deeply into mobile than immobile porcine knee joints in vivo (Maroudas et al., 1968). Loading at 2.8 MPa and 1 Hz augmented desorption of 66 kDa serum albumin from human femoral head cartilage explants, though adsorption of urea (60 Da) and NaI (125 Da) was largely unaffected (O’Hara et al., 1990). In studies involving electroosmotically induced fluid flows, transport enhancement due to convection increased with solute size and was affected by solute charge (Garcia et al., 1996). These studies indicate that, as seen in cartilage-like gels, solute convection in cartilage is likely affected by steric and hydrodynamic interactions related to solute size, shape, and charge, as well as interactions between solute molecules and the ECM (Deen, 1987).

Our objective was to quantify solute convection coefficients in cartilage explants during ramp compression. An experimental system for cartilage explant mechanical loading and imaging was developed for assessment of interstitial solute concentration and fluid velocity profiles during mechanical compression. A compression protocol was developed to separate and quantify diffusion and convection contributions to solute transport within the same explant. Diffusion coefficients were determined from measurements during 10% and 50% static compression, while convection coefficients were determined from sequential 10–50% ramp compression and 50–10% ramp release. Low loading rates permitted the simplifying assumption of quasistatic mechanics. Relationships between explant compression and solute transport were examined using positive, negative, and neutrally charged 500 Da fluorophores and 10 kDa dextran–fluorophore conjugates. Methods and results may assist in identifying circumstances under which solute convection induced by dynamic compression may increase transport above that due to diffusion alone, and improve understanding of chondrocyte mechanotransduction.

Section snippets

Methods

Osteochondral cores (4 mm diameter) were drilled from fresh 18-month-old bovine humeral heads and mounted in a microtome (RM 2135, Leica, Wetzlar, Germany) where a superficial cartilage layer was removed. Because previous studies have demonstrated diffusivities that varied with depth within cartilage (Leddy and Guilak, 2003), only the relatively homogeneous middle zone was used. A 650μm thick cartilage sheet was then sliced, from which 2 mm explant disks were punched (Stiefel, Coral Gables, FL).

Results

During ramp compression, radial tensile strain increased quasilinearly up to 0.13±0.06(n=108) (Fig. 2b). Maximum fluid velocity was 0.85±0.02μm/s(n=108) at the end of ramp compression (Fig. 2c). During release, fluid intake velocity peaked at 0.20±0.01μm/s(n=108) just after the transition from compression to relaxation. Radial strain at peak ramp compression was within 1% of radial strain measured at 50% equilibrium compression, while radial strain at the end of ramp release was within 2% of

Discussion

Novel methods were developed for quantification of solute convection in dynamically compressed cartilage explants. Methods permitted investigation into effects of solute and matrix characteristics on convection, and to what extent convection may supplement diffusion under the experimental conditions. Measured diffusivities were consistent with previous results (Maroudas, 1970, Burstein et al., 1993, Torzilli et al., 1997, Quinn et al., 2001, Leddy and Guilak, 2003), supporting the accuracy of

Acknowledgements

This research was supported by the Swiss National Science Foundation and by a US National Science Foundation Graduate Research Fellowship. The authors thank Dr. Jean-Manuel Segura for his help with confocal microscopy and Dr. Andreas Peer for his help with molecular chemistry.

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