HISTOLOGY AND HISTOPATHOLOGY

Cellular and Molecular Biology



Review

Ion transport in chondrocytes: membrane transporters involved in intracellular ion homeostasis and the regulation of cell volume, free [Ca2+] and pH

A. Mobasheri1, R. Mobasheri2, M.J.O. Francis3, E. Trujillo4, D. Alvarez de la Rosa4 and P. Martín-Vasallo4

1University Laboratory of Physiology, University of Oxford, and Department of Biomedical Sciences, School of Biosciences, University of Westminster, London, 2United Medical and Dental Schools of Guy's and St Thomas's Hospitals, London, 3Nuffield Department of Orthopaedic Surgery, Nuffield Orthopaedic Centre, Headington, UK and 4Laboratory of Developmental Biology, Department of Biochemistry and Molecular Biology, University of La Laguna, La Laguna, Tenerife, Spain

Offprint requests to: Dr. A. Mobasheri, Lecturer in Cellular and Molecular Biology, Department of Biomedical Sciences, School of Biosciences, University of Westminster, 115 New Cavendish Street, London W1M 8JS United Kingdom. FAX: 44-171-9115087. e-mail: mobasha@wmin.ac.uk

 

Summary. Chondrocytes exist in an unusual and variable ionic and osmotic environment in the extra-cellular matrix of cartilage and are responsible for maintaining the delicate equilibrium between extra-cellular matrix synthesis and degradation. The mechanical performance of cartilage relies on the biochemical properties of the matrix. Alterations to the ionic and osmotic extracellular environment of chondro-cytes have been shown to influence the volume, intracellular pH and ionic content of the cells, which in turn modify the synthesis and degradation of extra-cellular matrix macromolecules. Physiological ion homeostasis is fundamental to the routine functioning of cartilage and the factors that control the integrity of this highly evolved and specialized tissue. Ion transport in cartilage is relatively unexplored and the biochemical properties and molecular identity of membrane transport mechanisms employed by chondrocytes in the control of intracellular ion concentrations and pH is not fully defined and this review focuses on these processes. Chondrocytes have been shown to express voltage and stretch activated ion channels, passive exchangers and ATP dependent ion pumps. In addition, recent studies of transport systems in chondrocytes have demonstrated the presence of isozyme diversity that includes Na+/H+ exchange (NHE1, NHE3), Na+, K+-ATPase (several isoforms) and others each of which possess considerably different kinetic properties and modes of regulation. This multitude of isozyme diversity indicates the highly specialized handling of ions and protons in order to accomplish a fine regulation of their transmembrane fluxes. The complexities of these transport systems and their patterns of isoform expression underscore the subtlety of ion homeostasis and pH regulation in normal cartilage. Perturbations in these mechanisms may affect the physiological turnover of cartilage and thus increase the susceptibility to degenerative joint disease. Histol. Histopathol. 13, 893-910

 

Key words: Chondrocyte, Cartilage, Ion transport, pH regulation, Na+, K+-ATPase, Na+/H+ exchange, Ca2+-ATPase

DOI: 10.14670/HH-13.893