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

The chondrocyte is the cell responsible for the maintenance of the articula r cartilage matrix. The negative charges of proteoglycans of the matrix dra w cations, principally Na+, into the matrix to balance the negative charge distribution. The Na+,K+-ATPase is the plasma membrane enzyme that maintain s the intracellular Na+ and K+ concentrations. The enzyme is composed of an alpha and a beta subunit, so far, 4 alpha and 3 beta isoforms have been id entified in mammals. Chondrocytes are sensitive to their ionic and osmotic environment and are capable of adaptive responses to ionic environmental pe rturbations particularly changes to extracellular [Na+]. In this article we show that human fetal and adult chondrocytes express three alpha (alpha 1, alpha 2 and the neural form of alpha 3) and the three beta isoforms (beta 1, beta 2 and beta 3) 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 spec ialization in order to maintain the Na+/K+ gradients. Changes in the ionic and osmotic environment of chondrocytes occur in osteoarthritis and rheumat oid arthritis as result of tissue hydration and proteoglycan loss leading t o a fall in tissue Na+ and K+ content. Although the expression levels and c ellular distribution of the proteins tested do not vary, we detect changes in p-nitrophenyl-phosphatase activity "in situ" between control and patholo gical samples. This change in the sodium pump enzymatic activity suggests t hat the chondrocyte responds to these cationic environmental changes with a variation of the active isozyme types present in the plasma membrane.