Cellular and Molecular Biology

Sodium transport systems in human chondrocytes I. Morphological and functional expression of the Na+,K+-ATPase a and ß subunit isoforms in healthy and arthritic chondrocytes

E. Trujillo1,2, D. Alvarez de la Rosa1, A. Mobasheri3, J. Avila1, T. González2 and P. Martín-Vasallo1

1Laboratory of Developmental Biology, Department of Biochemistry and Molecular Biology, University of La Laguna, La Laguna, Tenerife, Spain, 2Rheumatology Service, Universitary Hospital of Canarias, La Cuesta, Tenerife, Spain and 3Department of Biomedical Sciences, School of Biosciences, University of Westminster, London, United Kingdom

Offprint requests to: Pablo Martín-Vasallo, Laboratorio de Biología del Desarrollo (LBD), Departamento de Bioquímica y Biología Molecular, Universidad de La Laguna. 38206 La Laguna, Tenerife, Spain. Fax: 34.922.318354. e-mail: pmartin@ull.es


Summary. The chondrocyte is the cell responsible for the maintenance of the articular cartilage matrix. The negative charges of proteoglycans of the matrix draw cations, principally Na+, into the matrix to balance the negative charge distribution. The Na+,K+-ATPase is the plasma membrane enzyme that maintains the intra-cellular Na+ and K+ concentrations. The enzyme is composed of an a and a ß subunit, so far, 4 a and 3 ß isoforms have been identified in mammals. Chondro-cytes are sensitive to their ionic and osmotic environment and are capable of adaptive responses to ionic environmental perturbations particularly changes to extracellular [Na+]. In this article we show that human fetal and adult chondrocytes express three a (a1, a2 and the neural form of a3) and the three ß isoforms (ß1, ß2 and ß3) of the Na+,K+-ATPase. The presence of multiple Na+,K+-ATPase isoforms in the plasma membrane of chondrocytes suggests a variety of kinetic properties that reflects a cartilage specific and very fine specialization in order to maintain the Na+/K+ gradients. Changes in the ionic and osmotic environment of chondrocytes occur in osteoarthritis and rheumatoid arthritis as result of tissue hydration and proteoglycan loss leading to a fall in tissue Na+ and K+ content. Although the expression levels and cellular distribution of the proteins tested do not vary, we detect changes in p-nitrophenyl-phosphatase activity "in situ" between control and pathological samples. This change in the sodium pump enzymatic activity suggests that the chondrocyte responds to these cationic environmental changes with a variation of the active isozyme types present in the plasma membrane. Histol. Histopathol. 14, 1011-1022 (1999)

Key words: Na+,K+-ATPase, Osteoarthritis, Cartilage, Rheumatoid arthritis

DOI: 10.14670/HH-14.1011